Skip Nav Destination
Filter
Filter
Filter
Filter
Filter

Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004
Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004

Update search

Filter

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- EISBN
- ISSN
- EISSN
- Issue
- Volume
- References
- Paper Number

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- EISBN
- ISSN
- EISSN
- Issue
- Volume
- References
- Paper Number

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- EISBN
- ISSN
- EISSN
- Issue
- Volume
- References
- Paper Number

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- EISBN
- ISSN
- EISSN
- Issue
- Volume
- References
- Paper Number

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- EISBN
- ISSN
- EISSN
- Issue
- Volume
- References
- Paper Number

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- EISBN
- ISSN
- EISSN
- Issue
- Volume
- References
- Paper Number

### NARROW

Format

Subjects

Date

Availability

1-20 of 488

Keywords: Upstream Oil & Gas

Close
**Follow your search**

Access your saved searches in your account

Would you like to receive an alert when new items match your search?

*Close Modal*

Sort by

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004

Paper Number: ARMA-04-478

... Artificial Intelligence pressure condition neural network correlation net pressure mean effective stress pore pressure prediction shale sandstone overpressure machine learning

**Upstream****Oil**&**Gas**Muderong Shale compaction Dewhurst Reservoir Characterization pore pressure...
Abstract

ABSTRACT: A methodology for remote pore pressure prediction using seismic attributes is presented, based on rock physics responses to various overpressuring mechanisms. A series of laboratory acoustic tests were performed on reservoir sandstones and shales, simulating normal compaction, disequilibrium compaction, fluid expansion and tectonic mechanisms of overpressuring. Sandstones showed lower V p /V s ratios during fluid expansion overpressuring as compared to both normal and disequilibrium compaction. For shales on a tectonic stress path, velocities and elastic constants all increase with increasing mean effective stress. P- and S-wave anisotropy are initially high and diverge with increasing mean effective stress. Seismic attributes of these laboratory waveforms were derived to verify which attributes were most sensitive to changing pore pressure conditions. Positive correlations between effective stress and several instantaneous seismic attributes were established allowing direct mapping of seismic attribute changes into definite values of effective stress. This methodology has been tested on a 3D seismic dataset from the Northwest shelf of Australia and shows good agreement with both the distribution of normally pressured and overpressured wells as well as the magnitude of the overpressures present. 1. INTRODUCTION Pre-drill prediction of overpressure is usually achieved through manipulation of seismic data, founded on the empirical relationship between effective stress and seismic velocity [1,2,3]. Velocity-based methods usually require a normal compaction trend and deviations from this normal compaction trend are taken to be indicative of the presence of overpressure. However, velocity-based pore pressure prediction methods are not reliable under all geological conditions. The relationship between stress and seismic P-wave velocity is, in general, non-unique because P-wave velocity is affected by other factors such as lithology and stress history. The reliability of estimated P-wave seismic velocities from surface measurements also decreases with target depth, signal to noise ratio and structural complexity for example. Consequently reliable remote prediction of abnormal pore pressure requires research into alternative approaches. The application of seismic attributes to predict overpressure based on VSP analysis was proposed by [4]. They conducted an analysis on field data and concluded that instantaneous seismic attributes are sensitive to both variations in lithology and pore pressure. Consequently they suggested that sequence attributes may be more relevant for direct detection of overpressured areas, although the methodology they proposed was qualitative in nature. However, our understanding of factors controlling seismic attribute response at the field scale is limited, but this can be enhanced through the derivation of seismic attributes on core samples at ultrasonic frequencies under controlled stress and pore pressure conditions. Previous work has shown that stress path is an important control on ultrasonic response in sandstones [5]. These authors subjected reservoir rocks to pressure conditions simulating normal compaction, disequilibrium compaction and fluid expansion and showed that velocity response and V p /V s ratios were stress path dependent. The full elastic tensor and anisotropy parameters for a shale were derived by [6] under low stress conditions and this work is expanded here to higher stress levels and anisotropic stress states.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004

Paper Number: ARMA-04-456

... Reservoir Characterization overburden correlation structural geology strength reservoir geomechanics

**Upstream****Oil**&**Gas**stress ratio in-situ stress ratio core plug liquid limit mTVD Poisson contamination friction angle sonic log compressive strength effective stress ratio...
Abstract

ABSTRACT: Strength and deformation properties of clays are known to correlate with a standard geotechnical index parameter, the Plasticity Index, IP, where these correlations are commonly used for characterising top soil or in the case of the offshore oil industry seabed analysis for foundations of offshore structures. By their nature, these studies are generally limited to the top 100+ m of the overburden. From a theoretical standpoint there is nothing to hinder the extension of such methods to greater depths other than the effects of diagenesis. This hypothesis is tested using drill cuttings from two North Sea fields of Tertiary age to characterise the overburden sequence. The results of the study are encouraging when compared with more conventional techniques using borehole log correlations suggesting that the plasticity index may offer a complimentary method for charactering overburden sequences for the purposes of borehole stability and reservoir compaction-seafloor subsidence. 1. INTRODUCTION Direct determination of strength and deformation properties of overburden clay and shale by standard rock mechanics testing are time consuming. Furthermore, rock mechanics testing implies that the relevant formations are cored; unfortunately the coring process is expensive, often difficult to perform and may jeopardize the drilling operation. Nonetheless, knowledge of the strength and deformation of the overburden may be vital to the field development, e.g. well planning and wellbore stability, reservoir compaction and seafloor subsidence. Drill cuttings are generally not used as a source for rock property evaluation partly because most laboratory techniques require pieces of core material larger than 5 mm which are of good to high quality and partly because, in many cases, drill cuttings are not preserved for further analysis once a lithological description has been completed. In those cases where drill cuttings are used the strength and stiffness properties are generally determined indirectly either from indentation or resonant acoustic wave techniques [1]. Strength and deformation properties of clays are known to correlate with a standard geotechnical index parameter, the Plasticity Index, I P , where these correlations are commonly used for characterising top soil or in the case of the offshore oil industry seabed analysis for foundations of offshore structures. By their nature, these studies are generally limited to the top 100+ m of the overburden. To date, there appears to be no documentation on the use of such correlations for evaluating the properties of 1000+ of overburden. Moreover, from a theoretical standpoint there is nothing to hinder the extension of such methods to greater depths other than the effects of diagenesis. In order to test this hypothesis, drill cuttings from two North Sea fields of relatively young geological age (lower Paleocene & younger) were used in a study to characterise the overburden sequence. The results of the study were compared with more conventional techniques using borehole log correlations. 2. GEOLOGICAL SETTING Field 'A' is a deep water gas field situated 120 km west of the Norwegian coast (Kristiansund) and Field 'B', an offshore oil field, is situated 185 km west of Stavanger.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper presented at the Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 5–9, 2004

Paper Number: ARMA-04-489

... dimension Mansfield sandstone borehole wall reservoir geomechanics experiment specimen

**Upstream****Oil**&**Gas**porosity grain contact Reservoir Characterization borehole breakout sandstone far-field stress fracture-like breakout far-field state breakout length breakout tip stress...
Abstract

ABSTRACT: Laboratory experiments were conducted to study the micromechanics leading to fracture-like breakouts in vertical holes drilled into Mansfield sandstone and the relationship between their geometric characteristics and the applied far-field state of stress (s h , s H , s v ). Fracture-like breakouts are tabular, long, and very narrow, originating and extending along the s h springline, where the maximum compressive stress concentration occurs. Several series of tests revealed a clear correlation between the applied stress regime and breakout length, indicating a potential use of these breakouts as stress magnitude indicators. The average breakout width in all the tests is constant regardless of the far-field stress conditions, suggesting that this is a material property related to the lower porosity narrow band developed ahead of the breakout tip. The latter is viewed as a zone of localized compaction formed by the debonding of weakly-sutured grains and their repacking as a result of the high stress concentration along s h springline. Fracture-like breakouts are produced by the removal, with the help of the circulating drilling fluid, of loosened intact and cracked grains from within this compaction band. 1. INTRODUCTION Cross sections of deep vertical holes drilled into the Earth's crust are often found to have elongated as a result of the formation of two diametrically opposed compressive failure zones. These zones are commonly termed 'stress-induced breakouts'. The occurrence of such breakouts was observed in the field [1,2,3], and their occurrence verified in laboratory experiments conducted on crystalline and carbonate rocks [4,5,6]. Breakout orientation in vertical-boreholes is typically along the diameter aligned with the direction of the minimum horizontal in situ stress, called the s h springline, where the highest stress concentration occurs. Hence, stress-induced breakouts have been utilized as a reliable indicator of the in situ stress direction. Breakouts in crystalline and carbonate rocks were found to be typically broad at the borehole wall and short in depth, resembling dog-ears. Micromechanical studies revealed that dog-eared breakouts are formed by episodic spalling of thin rock flakes bounded by tightly spaced dilatant extensile microcracks that are simultaneously subparallel to the borehole wall and to the maximum far-field horizontal stress, s H . Laboratory studies in granites and limestones have also revealed a direct correlation between breakout dimensions (breakout length, and angular span at the borehole wall) and the far-field principal stress magnitudes [5,6]. An important practical application of this finding was the field use of logged angular spans of breakouts at the borehole wall together with the true-triaxial rock strength (Available in full paper) Figure 1. Micrographs of Mansfield sandstone texture. (a) Back scattered electron (BSE) image, showing subrounded quartz grains (Q) as the dominant constituent mineral, and some clay (altered mica) (C). Grains are bonded together primarily by suturing (indicated by arrows) over narrow contact areas. (b) Same field of view as (a) but with Cathodo-luminescence. Note the quartz overgrowth (QO), which is not recognizable in (a). (c) Thin-section image showing sutured grain contacts (indicated by arrows). criterion to constrain the maximum horizontal in situ stress, s H , magnitude at Cajun Pass and KTB scientific deep wellbores [7,8].

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-502

... drilling fluid chemistry hydraulic fracturing drilling fluid selection and formulation

**Upstream****Oil**&**Gas**concentration pressure drop drilling fluids and materials drilling fluid property drilling fluid formulation fracture growth Fluid Dynamics coefficient slurry particle...
Abstract

ABSTRACT: In order to evaluate the growth of fractures for cuttings re-injection, a solid transport model is included in a solidfluid coupled hydraulic fracturing simulator. The fracture geometry is a critical factor affecting the safety of the re-injection operation, and solid particle flow in the fractures is known to have a dominant effect on the fracture propagation. To improve the accuracy of the simulation, the finite element method (FEM) is introduced for modeling the particle motion in the fracture fluid. In the model, opening of the fracture, interaction between multiple particles, and change in viscosity by the solid concentration are taken into account. Numerical examples shown here reveal that the fracture geometry is highly dependent on the concentration of the solid due to the change of gravity and slurry viscosity. The injected solid concentration is one of the few controllable parameters, thus the results suggest the feasibility of geometry control. INTRODUCTION Hydraulic fracturing technique is widely used in the petroleum industry for stimulating wells. Another application of the technology is drill cuttings reinjection, in which huge fractures are created in formations around wellbores to contain the slurrified solid waste produced by the drillings [1]. The major concern of cuttings re-injection is a breakthrough of fracture into adjacent formations and surfaces. If a fracture propagates into usable aquifers, petroleum reservoirs, or the surface or seabed, it can cause the grave environmental pollution and operational risk. Although this operation requires careful design of the fracture growth, there are few controllable factors, and those that are controllable are also restricted by operation margins. A numerical study using a solid transport model in the fracture shows that the solid concentration of the injected cuttings slurry influences the fracture growth significantly through the leak-off character of the formation [2]. The authors have developed another numerical simulator of hydraulic fracturing, in which the true three-dimensional geometry and interaction of multiple fractures are considered [3]. The solid transport model is added to the fully coupled model of fluid flow in the fracture and opening of the fracture in an elastic medium. For the cuttings slurry problem, we need an accurate solution for the case of a high concentration of solid particles. In this paper, we demonstrate a solid transport model that considers the effect of the fracture wall and in the interaction of multiple particles. Furthermore, some numerical results for different concentrations of injected solids are shown to exhibit the effect of this parameter on the final geometry of the fracture. The slurry viscosity and vertical pressure gradient can be manipulated by varying the solid concentration in the slurry, so the fracture growth is controllable by this parameter. NUMERICAL MODELING A fully coupled model of a hydraulic fracturing simulator is developed for the design of well stimulation in complicated stress state and well and fracture geometries [3]. The coupled solution of the fluid pressure and fracture opening is computed using the displacement discontinuity (DD) methodfor solid, and the finite element method (FEM) for Newtonian or non-Newtonian fluid.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-462

... operator to make a risk-based decision on the final casing design. Based on the results, the operator chose an aggressive casing design in order to reach the target sands with the optimum borehole size and a minimum number of casing strings. Reservoir Characterization

**Upstream****Oil**&**Gas**...
Abstract

ABSTRACT ABSTRACT: Wellbore stability and lost circulation problems cost companies millions of dollars in drilling downtime each year. However, these are costs that can be minimized by the proper planning in the pre-drill stages. This paper presents a case history of using a geomechanical model to optimize the casing design of a deviated wellbore and prevent problems that were experienced in offset wells of West Delta block 83, Gulf of Mexico (GOM). Casing points and mud weights were adjusted based on the geomechanical model to meet the challenges of this specific well. Uncertainties in the geomechanical model were evaluated using quantitative risk assessment to determine the confidence level in different casing plans. The operator chose an aggressive casing design in order to reach the target sands with the optimum borehole size and a minimum number of casing strings. To mitigate the risk, a 7-inch contingency liner was added to the drilling plan and AFE. The well was drilled successfully, however, the aggressive casing design could not be achieved and the 7-inch liner was needed. The geomechanical model provided information critical to making informed decisions in the planning and drilling process and the well was completed successfully with minimal wellbore stability problems. 1. INTRODUCTION Traditional approaches to casing design and mud weight selection have typically been based on predicted pore pressures and empirically determined fracture gradients. The resulting mud weights and casing points are then adjusted based on drilling experience in offset wells, often with little understanding of the reasoning behind the adjustments. It is now commonly acknowledged that the adjustments are necessary because the mud weights required to prevent both wellbore failure and lost circulation are highly dependent on wellbore trajectory and the regional stress state. When the risk of wellbore stability problems or lost circulation events are likely, it is important to establish the limits of a safe operating mud window by utilizing a geomechanical model that incorporates the pore pressure, the stress magnitudes and orientations, the rock strength, and the well trajectory. This paper presents a case history of using a geomechanical model to optimize the design of a deviated wellbore in West Delta block 83, GOM to prevent similar problems to those in offset wells. Data from two offset wells was used to develop and validate the geomechanical model. Casing points and mud weights were then adjusted based on the model to meet the challenges of this specific well. The upper bound mud weight was based on the least principal stress, and the lower bound mud weight was based on both the pore pressure and the mud pressure required to keep wellbore instability (breakouts) to an acceptable level. Uncertainties in the geomechanical model were then evaluated using quantitative risk assessment to determine the confidence level in different casing plans, which allowed the operator to make a risk-based decision on the final casing design. Based on the results, the operator chose an aggressive casing design in order to reach the target sands with the optimum borehole size and a minimum number of casing strings.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-483

... shortcomings. hydraulic fracturing reservoir geomechanics Reservoir Characterization Wellbore Design friction PFC Lac du Bonnet granite strength synthetic rock Rock mechanics dilation compressive strength tensile strength agreement

**Upstream****Oil**&**Gas**Bonnet granite Laboratory Test...
Abstract

ABSTRACT: In laboratory tests, the onset of dilation occurs at stress levels far below the peak strength but yielding of the laboratory specimen is not synonymous with the onset of dilation, and is seldom measured or reported in traditional laboratory testing. In field tests, the on-set of dilation is often associated with stress-induced extension fracturing. The displacements associated with these stress-induced fractures, cannot be replicated using traditional constitutive modelling and associated or non-associated flow rules. In this paper a methodology is developed for modeling dilation using the Particle Flow Code ( PFC ) that captures many of the observations reported in conventional laboratory test results. The findings from this research show that clumped-particle geometry provides the best agreement with laboratory test results for both tensile and compressive loading paths. 1 INTRODUCTION Experience with underground excavations at depth indicates that one of the most significant phenomena observed in brittle rocks is extensile fracturing. This fracturing occurs as a result of tangential stress concentrations. Direct observation of brittle rock failure around underground openings reveals that this extensile fracturing exhibits significant dilation (Fig. 1). A detailed description of the spalling process observed around a circular test tunnel was given by Martin et al. [1] and Lajtai [3] showed that in laboratory samples the brittle failure process resulted in the opening of fractures. In materials such as metals and clays, yielding can occur without significant volume change. However, in brittle rocks on the boundary of underground openings overstressing results in the development of micro- and macro-cracks. In the mining industry, the process is often referred to as 'spalling' or 'dog-earing'. In the petroleum industry, the problem is often cast as 'well-bore breakouts'. One of the early descriptions in civil engineering was given by Terzaghi [2] and referred to as 'popping rock'. Modeling of this process has always been challenging and has received a lot of attention in the mining, nuclear waste and petroleum industries since the 1950's. With the advent of modern computers, both continuum mechanics and traditional fracture mechanics approaches have been used to model this fracturing process [3-6]. The use of continuum mechanics to (available in full paper) Fig. 1: Dilation associated with stress-induced fracturing observed in a 600-mm-diameter borehole. simulate a fracturing process that results in an open rough fracture, as described by Lajtai [3] and shown in Fig. 2, is extremely problematic as the displacement field across the fracture in a continuum must be continuous. But if the fracture is open, this requirement cannot be satisfied. In traditional fracture mechanics, the fracture has zero width, again suggesting that this approach is not applicable for representing a process that results in open fractures. In all these approaches specific flow rules are required to capture the displacement field. In continuum mechanics an associated or non-associated flow rule is assumed. For the fracture mechanics approach the control of the fracture growth is related to the fracture toughness ( KIC ) [4, 6-8]. In both approaches there are fundamental shortcomings.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-426

... dielectric permittivity of the earth material can be found through the amplitudes of the propagation vectors of the material's electric fields, if the reflectances of the s- and p-wave components of the transmitted signal R s and R p can be determined. log analysis

**Upstream****Oil**&**Gas**pixel...
Abstract

ABSTRACT: The mechanical properties of an earth material are related to it's dielectric permittivity, or the per-unit dimensional extent of which an electric charge distribution in a material can be polarized or distorted by the application of an electric field. The activation of an electric charge distribution within the material is dependent on the composition of the material, and the electrical resistances of the material's components. Material composition is related to the material's mechanical properties. Dielectric permittivity can be measured through the earth material's absorption of wave energy. The degree of absorption is applied to calculating the material's physical properties such as density, porosity, and also the material's shear strength values at varying depths in terms of cohesion and internal friction angle. Dielectric permittivity is also used to calculate the moisture content and porewater pressures. The level of the phreatic surface is inferred through the permittivity in relation to the degree of absorption of wave energy, the porosity, and the degree of saturation 1. INTRODUCTION Geotechnical data was collected at a copper sulphide tailings basin, primarily through cone penetrometer testing and logging. Field data was gathered in September and October of 1997. Included was data pertaining to tailings density (¿), cohesion (c), angle of internal friction (F), and the depth to the phreatic surface (l). Multispectral imagery of the site was acquired in April of 1999. This imagery had a ground resolution of one meter, and was in 8-bit format. For each test site, pixels were analyzed on the imagery and geotechnical parameters were calculated from the pixel values, which represent the degree of wave signal remission. Results were then compared with the field data. 2. DETERMINATION OF DIELECTRIC PERMITTIVITY The relative dielectric permittivity e r of the material is determined through signal reflectance at varying frequencies. By observing that the earth material to be studied is a dielectric material, and assuming negligible refraction losses of waves hitting earth material, one can ideally assume that reflectance is complementary to absorbance, or [1]: A(f) + R = 1 (1) where A(f) = frequency-dependant attenuation or signal decay (absorption), and R is the signal reflectance (or radiance), and is comprised of wave components: R = .5(R s + R p ) (2) R s and R p are the reflectance/remittance of s- and p-waves of a transmitted wave signal. The value of R can be found, in terms of percent reflectance, by examining the digital number of a single picture element (pixel) of an image, which merely represents the strength of the remitted signal. Most imaging systems use 8-bit binary data formats, which means that pixel values range from 0 (no signal remission) to 2 8 , or 256. The pixel's brightness value, or digital number (DN) will fall within this range for 8-bit data, 256 will be the brightest possible value of any pixel in the image. The dielectric permittivity of the earth material can be found through the amplitudes of the propagation vectors of the material's electric fields, if the reflectances of the s- and p-wave components of the transmitted signal R s and R p can be determined.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-494

... drawdown reservoir geomechanics Saturation profile fluidized solid phase perforation Reservoir Characterization sand production solid phase matrix erosion activity saturation

**Upstream****Oil**&**Gas**sand matrix porosity wellbore deformation plastic shear strain permeability...
Abstract

ABSTRACT: This paper presents a fully coupled reservoir-geomechanics model with erosion mechanics to address wellbore instability phenomena associated with sand production within the framework of mixture theory. A Representative Elementary Volume (REV) is chosen to comprise of five phases, namely solid grains ( s ), fluidized solids ( fs ), oil fluid ( f ), water ( w ) and gas ( g ). The particle transport and balance equations are written to reflect the interactions among phases in terms of mechanical stresses and hydrodynamics. Constitutive laws (mass generation law, Darcy's law, and stress-strain relationships) are written to describe the fundamental behaviour of sand erosion, fluid flow, and deformation of the solid skeleton respectively. Subsequently, the resulting governing equations are solved numerically using Galerkin's method with a generic nonlinear Newton-Raphson iteration scheme. Numerical examples in a typical light oil reservoir are presented to illustrate the capabilities of the proposed model in the absence of the gas phase. It is found that there is an intimate interaction between sand erosion activity and deformation of the solid matrix. As erosion activity progresses, porosity increases and in turn degrades the material strength. Strength degradation leads to an increased propensity for plastic shear failure that further magnifies the erosion activity. An escalation of plastic shear deformations will inevitably lead to instability with the complete erosion of the sand matrix. The self-adjusted mechanism enables the model to predict both the volumetric sand production and the propagation of wormholes, and hence instability phenomena in the wellbore. 1. INTRODUCTION The production of formation sand has plagued the oil and gas industry for decades because of its adverse effects on wellbore stability and equipment, while it has also been proven to be a most effective way to increase well productivity. When hydrocarbon production occurs from shallow and geologically young (or so-called unconsolidated / weakly consolidated) formations that have little or no cementation to hold the sand particles together, the interaction of fluid pressure and stresses within the porous granular material can lead to the mechanical failure of the formation and unwanted mobilization of sand. It has been reported that 10%- 40% sand cuts normally stabilize in time to levels less than 5% in heavy oil reservoirs [1], while an average of 40% productivity increase was achieved through sand management in light oil reservoirs [2]. When sand is produced from reservoir formations, it can cause a number of problems. These include the instability of wellbores, the erosion of pipes, the plugging of production liners, the subsidence of surface ground, and the need for disposal of sand in an environmentally acceptable manner. Each year, these issues cost the oil industry hundreds of millions of dollars. Furthermore, sand production and control becomes extremely crucial in offshore operations where a very low tolerance to sand production is allowed. Hence, it is imperative to find an efficient computational model that has the predictive capability to assist field operators to understand this unique process. The ultimate goal is to design an economical well-production strategy in which sand production and operating costs may be reduced to some extent with maximum hydrocarbon productivity.

Proceedings Papers

Kevin J. Smart, David A. Ferrill, Darrell W. Sims, Nathan M. Franklin, Goodluck I. Ofoegbu, Alan P. Morris

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-470

... Modeling & Simulation information constitutive relationship reservoir geomechanics geomechanical model prediction

**Upstream****Oil**&**Gas**geomechanical modeling boundary condition contour plot differential stress Drilling extension evolution exploration Reservoir...
Abstract

ABSTRACT: A gap exists in the oil and gas industry between exploration, and subsequent drilling and production. Geologic and geophysical data yield geometric and kinematic information rather than the stress and rock properties required by reservoir and production engineers. Numerical geomechanical modeling can bridge the gap by coupling physically realistic and mechanically rigorous analyses that yield testable predictions. Stratigraphic and structural data sets based on seismic and well data yield 3D geologic framework models (GFM). The GFM can be restored to an undeformed configuration that serve as templates for construction of geomechanical models. When coupled with realistic rock properties, this mechanically valid forward model can be used to validate the geometric and kinematic restoration. The complete stress, strain, and pore pressure distribution from the geomechanical model can be superimposed on realistic geometries. For drilling or reservoir performance applications, further simulations can address evolution of field-scale pore pressure and stress distributions during production. The models also provide context for focused well-bore scale simulations. These analyses offer detail comparable to measurements needed for reservoir simulations and can address issues such as well bore stability and formation damage. This integrated geomechanical approach provides complete description of deformations along with testable predictions of stress orientation and magnitude, fluid flow, rock properties, and sites of localized failure. 1. INTRODUCTION In the oil and gas industry, a gap exists between the early phase of exploration, which is driven primarily by the fields of geology and geophysics, and the subsequent drilling and production phases where engineering plays the primary role. The gap is due in part to the large differences in scale that are inherent to the industry (basin versus field versus reservoir versus well bore). In addition, geologic and geophysical data yield primarily geometric and kinematic information, rather than the stress and rock properties that are required by reservoir and production engineers. For example, the standard three-dimensional (3D) seismic data sets lead to stratigraphic and structural interpretations that are macroscale, geometric descriptions. These data and their interpretations contain explicit or implicit information on the temporal and spatial evolution of geologic structures (i.e., kinematics). Extensive small-scale data (e.g., pore pressure and in situ stress distributions) become available only after production drilling commences. Computational tools such as numerical geomechanical modeling offer an opportunity to bridge the gap by coupling physically realistic and mechanically rigorous numerical analyses with existing geometric and kinematic data to yield testable predictions. In this paper, we demonstrate how modern structural analysis can be coupled with geomechanical modeling to solve important problems in exploration and production that range from field scale stress and pore pressure prediction to borehole collapse and formation damage. 2. APPROACH Our workflow incorporates traditional tools such as seismic interpretation and well data, if available, to generate stratigraphic and structural data sets that form the basis of a 3D geologic framework model (GFM). The GFM represents the present day state of knowledge for the problem of interest and serves as a starting point for further investigation.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-479

... Simulation stochastic process plane Artificial Intelligence

**Upstream****Oil**&**Gas**fracture intensity polygon tract 17 hydraulic fracturing fracture system intensity fracture orientation poisson line tessellation tract 49 modeling poisson plane algorithm fracture Massachusetts Institute...
Abstract

ABSTRACT: This paper presents a three-dimensional, geometric-mechanical, hierarchical, stochastic model of natural rock fracture systems. In the model, fracture systems are generated through superposition of hierarchically related sets, created via stochastic methods that reflect inherent relationships between fracture system geometry and underlying geologic mechanisms. The model employs Poisson plane and line processes as well as random spatial rotation and translation to represent fracture orientations, intensity, and relations to major geologic structures. The model is implemented in the computer program GEOFRAC, which incorporates algorithms for representation of fracture systems in different geologic settings, including folds, faults, and central structures. Application of the model to the Permian reservoir in the Yates field in Texas includes geomechanical analysis of fracture evolution, development of case-specific algorithms for fracture intensity modeling based on rock properties, and numerical simulations of fracture sets related to regional depositional trends and reservoir anticlinal structure. 1. INTRODUCTION Natural rock fracture systems are three-dimensional (3D) networks of multiple interconnected fractures that evolve under time-and-space-variant geologic stresses. Since field sampling methods of fractures are typically one-dimensional (logs, cores) or two-dimensional (outcrop maps), there is usually great uncertainty about the 3D fracture system geometry. A 3D model [1], which continues a long tradition in 3D fracture system modeling at the Massachusetts Institute of Technology [2, 3, 4, 5], accounts for that uncertainty through geology-based mathematical and numerical algorithms. The geometric-mechanical model explores the inherent relations between the 3D geometry of fracture systems and the underlying geologic mechanisms. Poisson plane and line processes [6, 7] and random spatial rotation and translation represent orientations and intensity within a fracture set. The UNIX-based C++ code GEOFRAC implements the 3D stochastic model and incorporates routines for generation of fracture systems via superposition of hierarchically related fracture sets in different geologic settings. This paper presents the fundamentals of the 3D model, and its application to the fracture system in the petroleum reservoir of the Yates field in Texas. 2. FRACTURE SET MODELING A Poisson plane network; Subdivision of planes into fractured and intact areas through Poisson line tessellation and random marking of polygons; Random 3D translation and/or rotation of fractured polygons. In the 3D model, fractures are convex polygons that are randomly generated as members of fracture sets through three stochastic processes (Figure 1): A fracture set is generated in a modeling volume enclosed by representative surfaces, e.g. bedding planes, structural boundaries, datum planes, and the ground surface. The three stochastic processes reproduce fracture orientations and intensity as they vary within a fracture set, as follows. 2.1. Modeling of stress field orientation: primary stochastic process The primary stochastic process (Figure 1a): a homogeneous, anisotropic, Poisson plane network [2, 6], represents stress field orientation. The mean orientation of a fracture set is specified in polar coordinates (azimuth, T, and latitude, F) in a global frame of reference (OXYZ), the axes of which coincide with relevant global directions (Figure 2).

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-503

... objective with these measurements is to provide a better understanding of the initiation of the brittle failure process in a fractured rock mass.

**Upstream****Oil**&**Gas**Ã¤spÃ¶ diorite strength skb report Reservoir Characterization stress magnitude shear zone experiment pillar stability...
Abstract

ABSTRACT: The Swedish Nuclear Fuel and Waste Management Co. is carrying out the Äspö Pillar Stability Experiment at the 450-m-level of the Äspö Hard Rock Laboratory. The major objectives of this experiment are to demonstrate our understanding of brittle failure (spalling) in a fractured rock mass, and the effect of confinement (backfill) on the brittle failure process,. During the experiment displacements, temperature, and acoustic emission events will be monitored through a loading cycle that starts in prepeak, i.e., elastic range and ends in the post-peak range. The rock mass that will be studied is a 1-m-wide pillar between two 1.8- m-diameter vertical cylindrical boreholes. The stresses in the pillar are controlled by the geometry of the experiment drift, the spacing of the boreholes and finally heating of the surrounding rock. The pillar response has been predicted by several numerical codes and the results from those codes will be compared to each other and the actual measured results. 1. INTRODUCTION The Swedish Nuclear Fuel and Waste Management Company (SKB) is responsible for the disposal of spent nuclear fuel in Sweden. The fuel is to be placed in copper canisters that will be deposited in vertical 8-m-deep 1.8-m-diameter boreholes at 400- 700 m depth in crystalline rock. This will result in the formation of approximately 4500 rock mass pillars surrounding the emplacement boreholes. The stability of pillars in the mining industry is traditionally carried out using empirical methods. It is unknown if these methods are suitable for the design of a borehole emplacement pillar. Hence, SKB is conducting the Äspö Pillar Stability Experiment (APSE) to: 1) demonstrate our current capability to predict brittle failure (spalling) in a fractured rock mass, 2) demonstrate the effect of ackfill (confining pressure) on the brittle failure response, and 3) compare the 2D and 3D mechanical and thermal predicting capabilities of xisting numerical models. The rock mass that will be studied in the experiment is a 1-m-thick pillar between two vertical boreholes with the same geometry as the deposition holes described above but spaced only 1 m apart. Previous in-situ work on brittle failure focused on massive intact granite [1] The Äspö Hard Rock Laboratory (HRL) is located in fractured rock typically found in the Scandinavian Shield, and hence provides unique opportunities to study brittle failure in a wide range of in-situ conditions. The APSE will be carried out 450m below the ground surface and at this depth the rock mass response around tunnels at Äspö HRL due to the excavationinduced stresses is essentially elastic. This provides a unique opportunity to monitor the rock mass behaviour from the elastic state through the transitional zone characterized by increased micro cracking into the final brittle failure, spalling, stage. The monitoring during this process will provide an indication of the volumetric change of the rock mass during the spalling process. The objective with these measurements is to provide a better understanding of the initiation of the brittle failure process in a fractured rock mass.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-464

... diameter of 1.5 inches, and a specified loading rate [3]. We refer to these tests as 'hole collapse' tests because we do not measure deformations in the hole but simply measure the collapse pressure of the sample. orientation collapse strength peak cyclic load

**Upstream****Oil**&**Gas**cyclic...
Abstract

ABSTRACT: Cyclic loading (fatigue) tests were performed on thick-wall hollow cylinders of reservoir rock material. The number of cycles to failure follows a log relationship with the peak cyclic load, for cycles less than 100 and peak loads less than 87% of the base hole collapse strength. For peak loads between 75% and 87% of the base hole collapse strength, the samples do not fail within 100 cycles, but they are weakened. These findings are applied to injection well design using a coupled 3D non-linear finite element model that includes the wellbore, casing, cement and perforations, and which drives a more detailed perforation submodel. The amount of failed and plastically-strained material surrounding each perforation of interest is compared to the amount revealed in a finite element simulation of the hollow cylinder tests. The non-linear elasto-plastic behavior of the rock is obtained from the results of triaxial compression tests. The analyses show that certain well and perforation orientations will be vulnerable to sanding, but that other well and perforation orientations are expected to remain stable even after repeated injection and shut-down cycles. 1. INTRODUCTION Wells used for water injection in oil reservoirs are often cased and perforated. If the rock is not too weak, the injection wells can be left with open perforations and with no sand control installed. When under injection, the stress level on the perforations may be insufficient to fail the rock. This is because the wellbore pressure is greater than the reservoir pressure (the drawdown is negative). However, injection wells are often shut down, either for routine maintenance or due to unexpected circumstances. This can result in flow into the well (a positive drawdown), which increases the effective stress loading on the perforations. Flow into the well can occur due to dynamic effects immediately after shut-down, and can also occur due to crossflow (flow from a higher-pressured formation to a lower-pressure formation, using the wellbore as a flow conduit). In addition, multiple cycles of injection and shut-down may occur during a well's lifetime. Many materials are known to weaken with cyclic loading. We aimed to answer these questions: Does the rock next to a perforation weaken due to cyclic loading? How can we determine this? And how do we apply the findings to injection well design for a particular field of interest? One of the best methods for simulating the behavior and stability of perforations in the laboratory, without using an actual shaped-charge perforator, is to use thick-wall hollow cylinders loaded on the outer surfaces [1-3]. Although such test samples are subjected to axisymmetric loading, which is usually an unrealistic approximation, they do include the correct geometry and they also include the effects of non-linear behavior and plastic deformation that could occur around perforations in-situ. For our hollow cylinder tests we use a hole size of 0.5- inch and an outer diameter of 1.5 inches, and a specified loading rate [3]. We refer to these tests as 'hole collapse' tests because we do not measure deformations in the hole but simply measure the collapse pressure of the sample.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-433

... station 3200

**Upstream****Oil**&**Gas**History closure effective viscoplastic strain viscoplastic strain creep time-dependent deformation principal stress excavation station 2800 station 2400 soft rock deformation history field measurement deformation 1. INTRODUCTION The time-dependent...
Abstract

ABSTRACT: The phenomenon of time-dependent deformation is commonly observed in underground openings where mining excavations are the main sources of mechanical loads. To predict the long-term deformations in soft rocks, a viscoplastic law has been implemented in a two-dimensional finite element program. A special time marching scheme is adopted to avoid numerical instability. A case study of a potash mine in Canada has been conducted to predict the long-term closures, as well as to analyze the stress distributions around the openings. The simulation results of displacement histories are closely matched with the field data. Comparing the deformations in average, the predicted values are higher than the field values. The maximum discrepancy is less than 20%. From a practical point of view, the predicted values are considered to be acceptable as heterogeneities and unknown discontinuities always exist in the mine. From overall evaluation, it suggests that model can be used as a practical tool for underground mine design of potash deposits. 1. INTRODUCTION The time-dependent behavior such as creep and relaxation of geomaterials is of great importance for the design of deep underground mines in soft rocks. For example, the deformation process of a mined-out room in potash mine may continue for several months and even years after excavation to the extent of complete closure of the openings. The engineer is faced with the problem of adequately predicting the ultimate status of openings based upon evaluations of time-dependent analysis and observations made during the excavation period. A finite element program VISROCK, which implements the elastic viscoplastic model [1], has been developed in CANMET-MMSL, Natural Resources Canada to serve this purpose for salt mines and other soft rock structures. Potash and rock salts are rate sensitive materials, which creep under sustained load, and they can yield excessively if the stress state is beyond a certain threshold. The general time-dependent behavior in creep of a rock sample can be divided into three stages, i.e., primary creep at an exponentially decaying rate, secondary creep at a constant rate, and tertiary creep at an accelerating rate leading to failure. A simple rheological model capable of considering the primary, secondary and tertiary creeps can be represented by a combination of ideal elements [2]. The basic deformation elements include a spring accounting for elastic behavior, a dashpot and a slider accounting for viscous and plastic behavior. Tertiary creep is modeled by assuming that strength (cohesion and friction angle for geomaterials) degrades with increasing viscoplastic strain rate and is herein regarded as a drop in the strength of the slider. A case study of a potash mine in Saskatchewan, Canada has been conducted to predict the long-term closures, as well as to analyze the stress distributions around the openings. Mining sequence of five openings at a depth of 960 metres is simulated. The deformation histories from the analysis are verified by those of the field data over a period of 2820 days after the mine was excavated.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-495

... shows that the fracture geometry is often significantly underestimated without considering the complete fracture mechanics.

**Upstream****Oil**&**Gas**calculation TSO sand control bottom hole pressure contour hydraulic fracturing analytical solution fracture geometry Simulation proppant...
Abstract

ABSTRACT: Adequate determination of the propped fracture geometry is critical for Frac Pack hydraulic fracture designs and analyses. Unfortunately the fracture geometry is difficult to determine in complex reservoirs, primarily because of the difficulty to determine the post tip screenout (TSO) fracture geometry. Two-dimensional analysis or even more sophisticated computer simulations typically only model the fracture process up to the onset of TSO, and do not calculate the fracture geometry beyond the onset of TSO. This paper presents results of three-dimensional hydraulic fracturing simulator calculations of the fracture geometry beyond the onset of TSO, and as TSO followed by fracture re-growth occurs. Comparative analyses of the three-dimensional simulations indicate that fracture lengths can be substantially underestimated if the three-dimensional analysis is not considered. This leads to an incorrect and non-conservative estimate of the propped fracture geometry-both in length and in width. 1. INTRODUCTION During the past decade, hydraulic fracturing that creates the onset of fracture tip screenout (referred to as "TSO") has been recognized as an effective technology to enhance formation conductivity in high permeability reservoirs. Modeling the TSO mechanism is complex, and generally two-dimensional and even pseudo three-dimensional models do not represent these fracturing features adequately. These analyses calculate fracturing only up to the onset of TSO and then assume that the fracture length is fixed; i.e. no further fracture growth occurs. They assume that proppant packing occurs (the fracture width increases) without further fracture growth, until all proppant is injected and pumping stops. In many cases this is likely not an adequate assumption. For example, Bai et. al. (2003) using a three-dimensional fracturing simulator (Clifton, 1989) presented numerical results showing that fracture propagation can continue after the initiation of the first TSO. This understanding is very important, and allows improving the assessment of Frac Pack propped fracture efficiency. As background, it is noted that before the Frac Pack treatment is initiated, a fracture calibration test is usually conducted by injecting a quantity of proppant-free fracturing fluid. This is done primarily to estimate the formation leakoff coefficient. Based on the calibration fracture results (and using an estimate of other required properties), the initial pad (without proppant) volume and slurry schedule to be used are then generated, usually using a two-dimensional or a pseudo three-dimensional computer simulator. The time between the calibration fracture and the Frac Pack treatment is usually hours to a day, and no time is allowed between the clear fluid pad injection and the fluid-proppant slurry injection. Since the two-dimensional or pseudo three-dimensional modeling used in the Frac Pack design does not generally estimate beyond the onset of TSO, actual fracturing mechanics may not be adequately represented. This paper uses a fully three-dimensional simulator to calculate fracture growth beyond the onset of TSO and continuing as TSO occurs, and then as re-growth occurs which is often followed by another TSO and again fracture re-growth (Bai, et. al. 2003). The paper shows that the fracture geometry is often significantly underestimated without considering the complete fracture mechanics.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-471

...]. water management

**Upstream****Oil**&**Gas**hydraulic fracturing displacement potential enhanced recovery geothermal reservoir Heat Extraction integral equation formulation stress distribution extraction fluid flow waterflooding fracture surface heat source thermal stress conduction equation...
Abstract

ABSTRACT: Poro-mechanical, thermal, and chemical processes can play a significant role when developing enhanced geothermal systems. These processes occur on various time scales and the significance of their interaction varies with the problem of interest. Of particular importance is the thermo-mechanical coupling during injection operations (time scale of months/years). In fact, the phenomena of the variation of injectivity with injection water temperature and reservoir seismicity can be attributed to thermal stresses. In this paper a three-dimensional integral equation formulation is presented for calculating thermally induced stresses associated with cooling of a fracture in a geothermal reservoir. The procedure is then implemented in a computer program and is used to treat the problem of injection into an infinite fracture. The thermally induced stresses are calculated using actual field data for an injection experiment. The resulting calculations are found to be consistent with those based on a semi-analytical solution as well as field observations. 1. INTRODUCTION Thermally-induced stresses significantly contribute to seismicity in petroleum and geothermal fields [1, 2]. The variation of injectivity with injection water temperature and reservoir seismicity in geothermal fields have been attributed to thermally-induced stresses. It has been has found that half the earthquakes in The Geysers field seem to be associated with cold water injection [2]. The mechanism by which seismicity occurs is well understood namely, shear slip on natural fractures resulting from a reduction in effective stress acting across the fracture. The magnitude of the thermal stresses associated with advective cooling has been estimated analytically [3] using an axisymmetric model of injection into a planar reservoir and a 1D heat flow in the rock mass. It has been shown that one- and two-dimensional heat flow models underestimate heat transfer to the fluid from the crack [4]. Thus, rock cooling and the associated thermal stresses should be studied using three-dimensional heat transfer and stress models. This requires coupling a 3D heat flow model to a 3D elasticity model. A reason for ignoring the three-dimensional nature of heat conduction in the reservoir is the difficulty in treating the infinite geothermal reservoir geometry by numerical discretization. However, it has been demonstrated [4] that by using 3D Green's function for heat conduction and the integral equation formulation the need for discretizing the 3D reservoir is completely eliminated. In this paper we present a 3D integral equation formulation for calculating thermally induced stresses associated with cooling of a planar fracture in an infinite reservoir. A brief presentation of the fluid flow/heat transfer model is also provided for the sake of completeness. Additional details regarding the heat transfer modeling can be found in [4]. 2. FLUID FLOW & HEAT TRANSFER A schematic view of heat extraction from a fracture or a fracture zone in rock is illustrated in Figure 1. With only a few exceptions such as the finite element solution by [5-7] and the boundary element model in [8], the heat conduction in the reservoir is typically modeled as one-dimensional heat flow perpendicular to the fracture surface [9-11].

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-457

... drilling operation mainbore stability Wellbore Design wellbore stability lateral wellbore Directional Drilling orientation junction contour plot Papanastasiou wellbore integrity

**Upstream****Oil**&**Gas**junction area Mise stress multilateral junction junction angle wellbore wall...
Abstract

ABSTRACT: Because the integrity of the wellbore plays an important role in many well operations, this work focused on studying the complex problem of the stability of multilateral junctions at different orientations in a three-dimensional anisotropic in-situ stress field, based on finite element three-dimensional modeling using commercial software. Stress-displacement analysis in steady-state was coupled with transient phenomena to compute stress behaviors and changes in pore pressure due to disturbances created by drilling. This coupled approach allows for the inclusion of some time dependent processes and the non-linear processes that influence the behavior of the system compounded by rock, fluids contained in the rock, and in-situ stresses. This paper shows the results of stress-displacement analysis of multilateral junctions. For the first time, using the "stress cloud" concept, critical areas regarding failure in the junction area were identified. In order to propose strategies to optimize drilling and completion design of multilateral wells, this analysis included variations of geometry (angles between the lateral and the mainbore), placement, and orientation of the junction. The results demonstrated that the most stable junction, independently of the depth of its placement, is with the lateral wellbore axis oriented parallel to the maximum principal in-situ stress (sH). Similarly, based on the mechanical response of rock, the results also demonstrated that junctions of multilateral wells should be placed as close as possible to the hydrocarbons zones. The significance of this work rests on the reliability that this kind of approach gives for simulating the phenomena encountered in wellbore stability analysis of multilateral junctions in most of the possible cases in oilfield applications, regarding stability. 1. INTRODUCTION Wellbore stability analysis has been the subject of study and discussion for a long time. The integrity of the wellbore plays an important role in many well operations during drilling, completion, and production. Problems involving wellbore stability occur principally through changes in the original stress state due to removal of rock, interactions between rock and drilling or completion fluids, temperature changes, or changes of differential pressures as draw down occurs. For the particular drilling case, support provided originally by the rock is replaced by hydraulic drilling fluid pressure; this creates perturbation and redistribution of stresses around the wellbore that can lead to mechanical instabilities. These instabilities cancause lost circulation or hole closure in the case of tensile or compressive failure respectively. In severe situations, hole closure can cause stuck pipe and loss of the wellbore. These events lead to an increase of drilling costs. Although there exists a significant amount of information related to wellbore stability, most of the information addresses the study of stability in the vicinity of the wellbore for a single hole. When two holes interact, the interference that a lateral hole causes on the stresses around the mainbore is particularly interesting. However, information about research conducted in a multilateral scenario where two holes interact is limited. Aadnoy and Edland [1] and Aadnoy and Froitland [2] investigated the effect of wellbore geometry on the stability of multilateral junctions using the elasticity theory.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-490

... viscosity Reservoir Characterization

**Upstream****Oil**&**Gas**constitutive equation Fluid Dynamics platen experiment application strain rate deformation migration Geophy power law localization Redistribution matrix rheology steel platen liquid pressure equation 1. INTRODUCTION There is...
Abstract

ABSTRACT: A simple approximation of the rheological constitutive equations for isotropic poro-viscoelastic material is proposed. The model is developed by modifying Biot's theory in the transient viscoelsatic regime. This is achieved by considering a liquid-filled spherical shell consisting of Maxwell viscoelastic material as the simplest representation of the poro-viscoelastic materials subjected to large volumetric strains. It is shown that in transition to viscous asymptotic, the stress-strain relations deviates from the canonic poroelastic form and Biot's constants become time-dependent. The proposed model is tested using the published experimental data on the deformation of partially melted rocks at elevated PT conditions. Experimental results are usually interpreted in terms of the power law viscous materials. However, in this work we consider the effect of strain damage on viscosity by treating the latter as a dynamic time-dependent parameter; with the variation rate proportional to the second invariant of strain rate. By taking healing into account, the dynamic power law viscosity has constant asymptotic at a given strain rate. The proposed rheological model is implemented in a 2D FEM code and used to study the formation of partial melt in biaxial tests. It is found that the numerically calculated stress-strain curves demonstrate maxima similar to those found in experiments. Also, the computed pattern of melt redistribution and strain localization at the contact with a stiff spacer is qualitatively similar to the experimental observations. The results also indicate that the matrix sensitivity to damage affects the scale of strain localization and liquid re-distribution. Additionally, the problem of the liquid migration in a folding of poro-viscoelastic layer was considered. 1. INTRODUCTION There is increasing interest in using damage theory in the formulation of geomechanics problems [1-3]. This is in response to the need to model the onset and accumulation of micro-cracks in the process of mechanical rapture of elastic materials. Healing of micro-scale damage is also possible. This occurs in the form of recrystallization when a fluid phase is present or due to thermally activated processes of grain boundary migration and dislocation motion. This approach can potentially provide an alternative to the traditional method of tracking crack propagation using the stress intensity analysis or classic theory of the plastic deformations. The concept of evolution of distributed damage has been used by Lyakhovsky et al. (2001) [2] to describe the non-stationary nature of the effective elastic moduli of porous and nonporous elastic geomaterials undergoing deformation. Such models produce a very realistic portrait of spatial strain localization in the elastic crust underlain by a flowing viscoelastic mantle. Spatial dynamics of the rheological parameters of viscous and viscoelastic materials lead to strain localization and formation of localized rapture zones similar to the cracks in elastic solids. Description of the compliance (viscosity) tensor as a dynamic parameter exposed to strain-weakening and thermally activated healing was suggested by Sleep (2001) [4]. At a constant strain rate, the conventional power law strain-rate dependent viscosity becomes the asymptote of the dynamic power law (for isotropic deformation). For geomechanical applications of the dynamic power law formalism, we use a simplified variant of the poro-viscoelasticity theory.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-484

...]. Reservoir Characterization wellbore integrity reservoir geomechanics wellbore pressure management

**Upstream****Oil**&**Gas**annular pressure drilling experiment drill-bit far-field stress condition fracture-like borehole breakout Mansfield sandstone borehole wall breakout length fracture-like...
Abstract

ABSTRACT: We conducted laboratory experiments aimed at studying the effect of several drilling variables on the shape, size, and failure mechanism of borehole breakouts in Mansfield sandstone. The objective was to acquire a better understanding of the counterintuitive long and narrow fracture-like breakouts observed in this sandstone, and to examine whether the dimensional characteristics can be controlled by manipulating the drilling operation. Initial tests conducted with two common types of drill-bits proved inconclusive as to whether an affect on breakout characteristics exists. Larger-diameter boreholes produced longer breakouts, suggesting that field-scale wellbore drilling could induce breakouts approaching several meters in length. Boosting the drilling-fluid flow rate was found to cause significant breakout lengthening under high far-field stress conditions. On the contrary, drill-bit penetration rate increases shortened the breakout length under the same far-field stress conditions. Heavier drilling-fluids were found to improve borehole stability by yielding shorter breakouts due to a thin "mud cake" deposited on the borehole wall and along the breakout. Despite the varying drilling conditions, in most cases the breakout width remained surprisingly constant. 1. INTRODUCTION Stress-induced breakouts in vertical holes are the product of compressive failure at the borehole wall around the points of intersection with the diameter aligned with the least horizontal principal far-field stress (s h springline), where the maximum tangential stress concentration occurs [1]. Bell & Gough [2] were among the first to discover this in the field, when they realized that the average breakout orientation obtained from four-arm dipmeter logs in a large number of oil wells in Alberta, Canada coincided with the presumed far-field s h direction. This finding led to the growing use of borehole breakouts as indicators of the in situ principal stress azimuths. Laboratory drilling investigations under critical far-field stress conditions conducted in crystalline and fine-grained sedimentary rocks produced dog eared breakouts (Figure 1a), comparable to those observed in the field [3-7]. These breakouts resulted from dilatant tensile intra- and intergranular microcracks that develop in the zone of the maximum tangential stress along the s h springline. The microcracks were sub-parallel to the borehole wall and to the maximum far-field principal stress (s H ). In addition, a distinct correlation was discovered between breakout dimensions and the magnitude of the far-field s H (when the other two principal stresses are kept constant) suggesting a potential additional use of breakouts as crustal stress magnitude indicators. More recently, experimental studies conducted in high-porosity (25%) Berea sandstone revealed a significantly different type of breakout that varied in both size and shape from that observed in previously tested rocks [8]. The breakouts were so unusually long and extremely narrow that they resembled fractures. Counterintuitively, these fracture-like breakouts were perpendicular to the s H direction (Figure 1b). The completely different borehole breakout shape appeared to be the result of a previously unrecognized failure mechanism. Further borehole drilling experiments in St. Peter, (available in full paper) Figure 1. (a) Typical dog eared breakout in Westerly granite. (b) Typical fracture-like breakout in Mansfield sandstone. Mansfield, and Aztec sandstone, which varied in porosity between 10-26%, have also yielded fracture-like breakouts [9-11].

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-480

... Characterization

**Upstream****Oil**&**Gas**strength PFC model laboratory itasca consulting group inc stress-deformation curve particle peak strength mechanical behavior Rock mechanics discontinuous rock mass geometry fluid modeling Wellbore Design Simulation Deformation property investigation...
Abstract

ABSTRACT: The description of the behavior of rock masses is usually based on the behavior of intact rock samples studied in the laboratory. One of the difficulties in describing the rock mass behavior is assigning the appropriate constitutive model. With the progress in discrete element software, this limitation may be overcome as the user is not required to prescribe a constitutive model for the rock mass. Instead the mirco-scale properties of the intact rock and joints are defined and the macro-scale response results from those properties and the geometry of the problem. In this paper a rock mass is simulated using a discrete element model and the results compared to the traditional model where the constitutive model is prescribed. The discrete element model gives very contrasting results compared to the traditional model. 1. INTRODUCTION The international community has been investigating the suitability of deep geological disposal facilities for nuclear waste since the early 1970's. These investigations have mainly focused on issues related to geochemistry, geo-hydrogeology, geology and geomechanics. In the Scandinavian countries of Finland and Sweden, their nuclear waste programs have progressed rapidly and these countries are now in the site selection stage. As a result, their focus is on the site evaluation to ensure the reliability of the selected site for final waste disposal. It must be realized that much of the data collected from site investigations will be obtained from boreholes, and field mapping. Since the nuclear waste repositories are deep seated, the extent of information on which the site evaluation will be based, is limited. However, the data collected from the site investigations must be adequate for a complete site description, which is built up of models related to geology, rock mechanics, thermal properties, transport properties, hydro-geology, hydro-geochemistry, and surface ecosystems. One of these models is the Rock Mechanical Descriptive Model, developed for any site in hard crystalline rock. This descriptive model involves the characterization of rock mass by means of both the empirical relationships and a theoretical approach based on numerical modeling [1]. During the site investigation phase, various scenarios will have to be evaluated that require knowledge of the rock mass response. Extensive work was carried out in early 1990's to investigate the formation and extent of the excavation disturbed zone around underground openings, i.e. the rock mass response at low confinement. However, the evaluations of most of the scenarios require knowledge of the confined rock mass response. Dr. E. Hoek in his keynote address at the U.S. Rock Mechanics Symposium (1999) noted that quantifying the post-peak response of rock masses was the biggest challenge facing the rock mechanics community. While the behavior of intact rock samples has been studied extensively in the laboratory and using numerical simulations, little progress has been made pertaining to rock masses. At present, the rock mass strength and deformation characteristics are evaluated using empirical methods such as the rock mass classification systems of Bieniawski's RMR [2] and Barton's Q-values [3].

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper Number: ARMA-04-497

...: Navier Equation: (available in full paper) Diffusion equation for pore pressure p: (available in full paper) Diffusion equation for temperature T: (available in full paper)

**Upstream****Oil**&**Gas**fundamental solution boundary element method thermo-poroelastic mixed boundary element model...
Abstract

ABSTRACT: Drilling of a borehole in fluid saturated rocks exerts significant influence on its surrounding reservoir. As have been observed extensively, stresses, pressure and/or thermal loadings on the borehole wall will lead to great change of stress field in the rock. As a result, stabilities of the borehole wall and pre-existing fractures will be at risks. In this paper, a mixed boundary element model (BEM) is developed to solve these problems within the framework of thermo-poroelasticity. The model is applied to borehole stability and fracture problems in high temperature subsurface environments. Instability of borehole wall in naturally fractured rocks can be simulated and predicted by studying the potential of crack initiation and growth. This model provides a flexible tool to deal with two-dimensional transient thermal-poroelastic problems. 1. INTRODUCTION Coupled thermal and poromechanical processes play an important role in many geomechanics problems such as borehole stability analysis and studies of initiation and propagation of hydraulic fractures. Thermal effects, as well as poromechanical effects, can greatly change the stresses and pore pressure fields around an underground opening. This is due to the fact that thermal loading induces volumetric deformation because of thermal expansion/contraction of both the pore fluid and the rock solid. A volumetric expansion can result in significant pressurization of the pore fluid. In order to take into account the influence of temperature gradients on pore pressure and stresses, it is necessary to use a non-isothermal poroelastic theory, or thermo-poroelasticity. However, many problems formulated within the framework of thermo-poroelasticity are not amenable to analytical treatment and need to be solved numerically. The boundary element method (BEM) has proven suitable for the poroelastic and thermoelastic problems [e.g., 1, 2]. The advantage of the method is that it reduces the problem dimensionality by one thereby reducing the computational efforts significantly. In this paper, a two-dimensional transient indirect BEM is developed to solve coupled thermo-poroelastic problems. The indirect BEM has two sub-formulations, namely, displacement discontinuity (DD) method and fictitious stress (FS) method. The DD method is particularly suitable for crack-shaped problem. A mixed FS-DD model is developed to take advantage of the strengths of both FS and DD methods. The model is applied to some thermo-poroelastic problems in borehole stability and hydraulic fracturing. 2. THERMO-POROELASTICITY Thermo-poroelasticity is developed on the basis of poroelasticity and thermoelasticity. It couples the time-dependent processes of fluid diffusion and heat diffusion to mechanical behavior. Constitutive equations for this theory were first introduced by Palciauskas and Domenico [3] by extending the classic Biot's poroelastic theory [4] for the non-isothermal case. This theory was later established by other investigators, e.g., McTigue [5], and Coussy [6]. The governing equations for thermo-poroelasticity can be found in the works of McTigue [5] which consist of constitutive equations, transport laws and balance laws. From the governing equations, the field equations can be derived for temperature, displacement, and pore pressure: Navier Equation: (available in full paper) Diffusion equation for pore pressure p: (available in full paper) Diffusion equation for temperature T: (available in full paper)