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.

ABSTRACT: The degree of certainty in pore pressure estimates from resistivity data is examined through a case study interpreted by eight trained analysts. The relevant data included resistivity, gamma ray, density, and mud weight as a function of depth. A shallow kick pressure and several deeper MDTs were provided for calibration. While the models and tools are quite simple, the shale pressures are derived from several measurements and decisions. The results from each analyst's interpretation were compiled and combined with some theoretical analysis to estimate the uncertainty attached to components used to estimate shale pore pressure. Two sources of variation dominate the uncertainty in estimating pore pressure from resistivity: trust that sand pressure calibration points are representative of shale pressures, and choice of the normal compaction trend.

ABSTRACT: A key to the success of long-term storage of CO 2 in depleted oil or gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. The integrity of this "bounding seal" system is affected by various mechanical, chemical and thermal forces that act during initial exploration, development and oil production operations, during CO 2 injection operations, and during the subsequent CO 2 storage phase. This paper provides a review of the geomechanical factors affecting the hydraulic integrity of the bounding seals for a depleted oil or gas reservoir slated for use as a CO 2 injection zone. Equations are given which are helpful for identifying the key parameters that govern these geomechanical factors, and further enable first-order estimates of the risks that they pose to bounding seal integrity. The results of this review are compiled into a table that summarizes key geomechanics-related risks, the mechanisms associated with these risks, and approaches to assess and mitigate them. Where possible, examples are given where these mechanisms have affected oil and gas field operations. 1. INTRODUCTION In order to achieve significant reductions in the atmospheric release of anthropogenic greenhouse gases, the implementation of technologies to capture carbon dioxide (CO 2 ) and store it in geological formations will be necessary. Deep saline aquifers have the largest potential for CO 2 sequestration in geological media in terms of volume, duration and minimum or null environmental impact [1]. The first commercial scheme for CO 2 sequestration in an aquifer is already in place in the Norwegian sector of the North Sea, where 10 6 tonnes of CO 2 are extracted annually from the Sleipner Gas Field and injected into the 250 m thick Utsira aquifer at a depth of 1000 m below the sea bed [2]. In light of the economic benefits of enhanced oil recovery (EOR) derived from CO 2 injection in oil reservoirs [3], these types of reservoirs will be attractive CO 2 injection targets and, most likely, CO 2 storage in depleted oil and gas reservoirs (or in conjunction with EOR) will be implemented before CO 2 storage in aquifers. An advantage of CO 2 storage in depleted oil or gas fields is the fact that much of the infrastructure for fluid injection (e.g., wellbores, compressors, pipelines) is already in place. The Weyburn CO2 Monitoring and Storage Project in Saskatchewan, Canada [4] is an example of a large-scale application of EOR operations using anthropogenic CO 2 , in which the oil reservoir is being evaluated for subsequent use as a long-term storage zone. A key to the success of long-term storage in depleted oil and gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. Although the initial integrity of this "bounding seal" system is governed by geological factors, it is ultimately affected by various mechanical, chemical and thermal forces that act during exploration, development, oil production and secondary recovery (waterflooding) operations, during CO 2 injection, and during the subsequent CO 2 storage phase. This paper contains a review of the geomechanical factors that pose risks to the integrity of the bounding seal system.

ABSTRACT: It is well-established that stresses within and outside a petroleum reservoir change as a result of pore pressure depletion. This Paper describes possible consequences of these stress alterations, for reservoir compaction, for compaction drive, for permeability, for reservoir monitoring and for reservoir simulation. The analytical model of Rudnicki [6] is used as a platform for discussing the effects of stress changes within limits of linear poroelasticity. Beyond elasticity, discussion is based on discrete element modeling of a depleting reservoir, permitting a dynamic description of failure processes, and on controlled laboratory experiments with synthetic rocks manufactured under stress. 1. INTRODUCTION During the last decade, there has been an increasing consciousness within the petroleum industry that stress changes associated with reservoir depletion have an essential impact on field performance. Traditionally, pore pressure reduction has been thought to create essentially uniaxial (vertical) compaction. Through their analysis of stress evolution during depletion of the Ekofisk field in the North Sea, Teufel et al . [1] came to the conclusion that the reservoir did not follow the expected stress path. Later, evidence has been presented that several fields do not behave according to the uniaxial strain hypothesis [2, 3]. The stress path obviously affects geomechanical behavior, i.e. reservoir compaction and associated surface subsidence. It does however also have a profound impact on petroleum recovery, directly through compaction drive, and indirectly through permeability alteration. Obviously, the stress changes may also lead to changes in seismic velocities or seismic attenuation, hence affecting time-lapse (4D) seismic response. Furthermore, issues like borehole stability during drilling, casing collapse and particle production are strongly linked to the stress path of the reservoir and the overburden. In this Paper, the main ambitions are to demonstrate which parameters that have the strongest impact on the stress path, and to elucidate the effects of stress path both on geomechanical, on fluid flow, and on monitoring aspects of a depleting reservoir. 2. RESERVOIR MECHANICS IN THE POROELASTIC LIMIT 2.1. Reservoir stress path The reservoir stress path may be defined through the parameters [4] (available in full paper) where the subscript j denotes the 3 principal earth stresses and p f denotes the reservoir pore pressure. The traditional assumptions made by the industry have been (i) that the reservoir compacts uniaxially with zero lateral strain, and (ii) that the full weight of the overburden is carried by the reservoir at all times. From linear poroelastic theory, it then follows that (available in full paper) These assumptions may be relevant if the lateral extent of the reservoir is much larger than its vertical dimension, if there is negligible contrast between the mechanical properties of the reservoir and the surrounding rocks, and if the reservoir is not tilted with respect to the principal stress directions within the Earth (assumed vertical - horizontal). Notice that an underlying assumption here is that the reservoir is a homogeneous entity or an isolated compartment within a heterogeneous field. These ad hoc assumptions may however be relaxed.

ABSTRACT: A new way of 4D interpretation of depleting reservoirs has been developed, which makes use of timeshifts measured in the overburden to evaluate the condition of the reservoir. This in contrast to conventional 4D interpretation in which overburden timeshifts are treated as a complication that needs to be corrected for. The new interpretation method is based on the observation in geomechanical modelling that thinner, stiffer or undepleted parts of the reservoir act as stress attractors, which "arch away" overburden load from thicker, more compressible or strongly depleted parts of the reservoir. These stress redistribution effects are present over significant distances in the overburden and cause timeshifts in the overburden, which can be an order of magnitude larger than what is found in the reservoir. In fact, in depleting sandstone reservoirs without significant saturation changes/waterdrive the overburden timeshifts are often the only measurable effects. The overburden timeshifts can be interpreted with the aid of geomechanical modelling. It appears that typical reservoir conditions like sealing faults, non depleted compartments, GWC/OWC, give very specific "timeshift fingerprints" in 4D. This new way of 4D interpretation by combining geomechanics and geophysics opens completely new avenues in reservoir surveillance. 1. INTRODUCTION Since its introduction in the late eighties (Nur 1989, Wason et al. 1989) time-lapse seismic has had large impact on surveillance of reservoirs with strong waterdrive or efficient waterflooding: saturation changes were found to lead to significant density and velocity changes in the reservoir and to become clearly visible in 4D as areas of larger amplitudes or as areas with positive time shifts between base and monitor survey. As such time-lapse seismic appeared to be an excellent tool to track waterdrive or water flooding and to identify pockets of hydrocarbons bypassed by the water. Many positive reports can be found in publications, e.g. Koster et al. 2000. In reservoirs with weak waterdrive or less efficient waterflooding however, it appeared difficult to discriminate saturation effects from depletion or injection effects, which affect velocities in the reservoir as well (Landrø et al. 2001). As the latter effects were governed by effective stress changes in the reservoir, coupled geomechanical and flow modelling were introduced in 4D seismic; first with one way coupling (e.g. Vidal et al. 2002 for gas storage), later also with two way staggered coupling, e.g. in the Belridge field by Minkoff et al. 2004. The stress effects on velocity appeared to be often large when measured on core, but almost negligible when measured in the field for depletion conditions - see Stammeijer et al. 2004. In injection conditions stress effects were far better visible (Par et al. 2000, Landrø 2001, Digranes et al. 2002). Nes et al. 2000 gave a possible explanation for the overrated core results by pointing at core damage effects and providing data from synthetic core representative for "virgin" reservoir conditions that showed much lower stress velocity dependence. Also dispersion effects and geometry upscaling effects are mentioned as cause of the discrepancy between field and core. The bottomline is that stress velocity effects are not only poorly understood, but also might be very small for many of the depleting reservoirs.

ABSTRACT: Here we present the results of a geo-mechanical study combined with a new way of 4D interpretation, applied to a depleting offshore gas condensate field. The 4D interpretation uses the predicted stress changes from the geo-mechanical study, by bringing them to the seismic domain in the form of 'synthetic' time-shifts. It is demonstrated that, by comparing measured time-shifts with these 'synthetic' time-shifts, discrepancies in geomechanics (e.g. in the rock properties and formation stiffness contrasts) and reservoir dynamics modeling (e.g. undepleted pockets and activity of waterdrive) become much better traceable. The rock properties and the pressure distributions were then iteratively calibrated and harmonized with the measured 4D and other field observations (GPS and compaction-log). The applied workflow reduced uncertainties in forward predictions of stress changes from the geo-mechanical model in and around the compacting reservoir, and enabled the design of a new strategy for sand management in the field. 1. INTRODUCTION In petroleum engineering, a geo-mechanical field evaluation focuses on understanding the subsurface mechanical in-situ conditions and the changes in the subsurface induced by pressure depletion or pressure maintenance. Given the structural complexity of certain fields, numerical modeling techniques are nowadays a preferred tool to predict the induced changes in stresses, strains, properties and failure condition of the rock formations. Top-seal integrity. Subsidence or uplift of the seabed. Fault slip. Borehole instability. Risk of sand or fines production. Completion failure. Undepleted reservoir pockets. Sealing or non-sealing faults. Depending on the problem(s) , anticipated the geo-mechanical forward model predictions are used to address specific aspects that impact the economy of the field at different stages. In the e a geo-mechanical evaluation focuses on e.g.: At the e a geo-mechanical assessment looks at e.g.:However, the use of geo-mechanical modeling also provides a new opportunity [1, 2], by incorporating geo-mechanical results of the whole earth [3] in the time-lapse seismic (4D) monitoring of producing oil and gas fields. It can enhance the current reservoir technologies to better detect e.g.: Here we present the workflow of a geo-mechanical study combined with this new way of 4D interpretation, applied to an offshore gas condensate field [4]. 2. GEO-MECHANICS AND 4D The structural geometry of the field, the initial stress state, the formation rock properties and the pressure depletion (or injection) scenario from the dynamic reservoir simulator, are the primary unknowns in the geo-mechanical field equations for calculating production induced stress changes (¿ s ). 2.1. Compaction and Subsidence The subsurface is subjected to the total stress ( s ) of the overburden and the horizontal tectonic stresses, which are carried in the rocks by effective stress ( s eff ) on the grain-to-grain contacts and the fluid pressure ( p ) in the pores: s eff = s - ap I (1) where, a and I , denote the coefficient of Biot and the unit tensor, respectively.

ABSTRACT ABSTRACT: A parametric study has been done using a fully coupled single-phase geomechanics-fluid flow simulator to determine the effect of coupling deformation and fluid flow on reservoir response during production. We have compared these results to standard reservoir simulation and reservoir geomechanical models to see the effects of coupling and under what conditions coupling is important. . Standard reservoir simulation and geomechanical modeling approaches miss the dynamic, implicit coupling between fluid pressure and rock deformation, between reservoir deformation and overburden/sideburden deformation, and between stress and permeability. For example, standard reservoir simulation predicts that a low compressibility in a highly compressible reservoir will yield the same drawdown and production history as a highly compressible fluid in a low compressibility reservoir provided the total compressibility of the two systems is the same. Fully coupled modeling shows that the two systems can have vastly different drawdown and production histories due to the coupled response of the fluid-reservoir-overburden system. Our modeling also shows that geomechanical models of the reservoir can often be misleading because they utilize an "uncoupled" fluid pressure distribution to calculate reservoir and overburden deformations. In addition, fully coupled modeling shows that in many reservoirs, Geertsma's approximations of uniaxial strain do not apply. Modeling shows that reservoir stress paths during drawdown arc strongly affected by ratios of over-burden and reservoir moduli and by the ratio of reservoir thickness to lateral extent. Also, coupled modeling shows that lateral fluid pressure variations in the reservoir can lead to near wellbore stress paths approaching isotropic stress conditions in many reservoirs

ABSTRACT : A displacement-discontinuity program incorporating a frictionless, laminated overburden with "homogeneous stratifications" has recently been developed. This program, LAMODEL, simulates the overburden as a stack of homogeneous isotropic layers with frictionless interfaces, and with each layer having the identical elastic modulus, Poisson's Ratio, and thickness. This formulation does not require specific material properties for each individual layer, and yet it still provides a realistic suppleness to the overburden that is not possible with the classic, homogeneous isotropic elastic overburden. The program calculates stresses and displacements at the seam level and at requested locations in the overburden or at the surface. Both linear elastic and non-linear seam materials can be used; and free surface effects, uneven topography effects, multiple seams, and multiple mining steps can be simulated. As part of the initial investigation and validation of this new laminated formulation, the program is used to model the underground stresses and displacements, the topographic stresses and the inter-seam interactions from two field studies. The results from the model are compared with the field measurements and with previous results from an elastic homogeneous isotropic analysis. Ultimately, it is shown that the many features of the new program coupled with the laminated overburden model help provide realistic stress and displacement calculations for various mining situations. 1 INTRODUCTION The initial mathematical models relating the mechanical behavior of the overburden to mining in seam type deposits were first presented in the early sixties (Berry, 1960; Berry and Sales, 1961, 1962; and Salamon, 1964a, 1964b, 1965a, and 1965b). In the initial publication by Berry (1960), the closed-form equations for the displacements and stresses surrounding a single thin opening or slit in both an infinite two-dimensional homogeneous isotropic elastic plane and a similar half-plane were derived. In this initial work, Berry was primarily interested in solving for the vertical displacements at the surface for the purpose of subsidence prediction. However, he found that his initial numerical results for an elastic medium did not match the observed subsidence. In the next papers by Berry and Sales (1961, 1962), the assumption of an isotropic medium was replaced with the assumption of a transversely isotropic medium, and the math was extended to the three- dimensional solution around a thin rectangular opening. Using this new, non-isotropic, formulation and optimized values for the necessary elastic constants, Berry and Sales were able to show very good correlation between calculated and measured subsidence. In the paper by Salamon (1964a), Berry and Sales's work is extended in several directions. Salamon introduces the idea of a "face element" which is essentially used to discretize the seam into "small" two-dimensional, horizontal areas for implementation of a numerical three-dimensional formulation. (This face element is essentially the same as the presently known displacement-discontinuity (DD) variation of the boundary-element method.) Salamon then goes on to provide the essential mathematical formulas for applying the face element method to both an infinite space and an infinite half- space using four kinds of elastic media; homogeneous isotropic, homogeneous transversely isotropic, frictionless laminated, and multi-membrane.

ABSTRACT: A network of flood control measures for two communities in the coal mining region of southeastern Kentucky has involved the construction of several hundred meters of high flood wall and the relocation of several bridges. A thick cover of weak alluvial soils at the sites made H-piles an economically viable means of foundation support for these structures. All piles were fitted with driving points and driven to refusal in sandstone, siltstone, and shale of Pennsylvanian Age. The refusal criteria used for all H-piles was five blows, with the selected hammer, per 6.35 millimeters of penetration. Altogether, 3,850 piles, which varied from 4.9 to 19.7 meters in length, were driven at the two major projects described in this paper. The first project is located at Pineville, Kentucky and the second project is at Harlan, Kentucky. Some of the geotechnical considerations and problems related to the pile foundations that were constructed at these sites are presented. Preconstruction foundation exploration data, design values, and assumptions are compared with the actual performance during construction. The sub-surface exploration consisted of standard penetration test, Shelby tube, and cone penetrometer borings through the overburden and wire-line coring in the rock. Laboratory test data for the rock is briefly discussed. The design axial load capacities, which were computed using 6 different dynamic design formula, are compared with the actual capacity measured by two field load tests on the Pineville project. The relative conservatism of these formula versus the measured capacity is examined. Particular focus is placed on the pile blow count data and actual refusal depths versus the top of rock elevation predicted from the foundation exploration. Variations in hammer energies, pile sections, driving points, and foundation conditions are described. A histogram of the penetration into rock for the first contract follows a Gausian curve. This was used to predict the distribution of penetration into rock for the second contract. The validity of this approach to predicting penetration into similar foundation conditions is discussed. 1 INTRODUCTION The 2,095 square kilometer Upper Cumberland River Basin in southeastern Kentucky has a long history of destructive flooding. The record flood in 1977 led to construction of projects to alleviate this problem. Two of these projects, the Pineville Floodwall and the Harlan Floodwall involved the used of H-piles driven to refusal in weathered shale as support for T-walls, closure structures, bridge abutments, pump stations, and gate wells. Both projects were built immediately adjacent to the Cumberland river channel and were tied in to high ground on either end. The Harlan project is located in Harlan County at approximately river kilometer 1,113. Construction was performed under one contract that began in August 1992 and was completed in January, 1996. In most geologic settings driving an end bearing pile into rock is reasonably straight forward. Provided, the pile tip is protected from damage, the refusal criteria is reasonable, and appropriate equipment, in good working order is used, a pile can be emplaced that will sustain its design load.