ABSTRACT: Cement paste is used in several areas of civil and petroleum engineering. In oil/gas wells, it forms the cement sheath that provides mechanical support to wells and contributes to fluid isolation in different formations. Evaluation of the stress state in order to preserve well integrity during, for example, a casing test, requires simulations of hydration and loading history from placement until final hydration and also the knowledge of poroelastic parameters of the cement paste at every degree of hydration. Conventional laboratory experiments, such as drained or undrained isotropic compression, are extremely time-consuming and costly, and they are practically impossible to be performed at every degree of hydration. Micromechanical model and homogenization method are usually used to estimate the properties of porous materials. The poroelastic parameters of a class G cement paste were simulated assuming a microstructure in which the C-S-H HD cover some clinker phases and are covered by C-S-H LD. The results are validated on static and dynamic undrained oedometric modulus as well as the static oedometric Skempton coefficient.

ABSTRACT: Special rock mechanical characteristics of natural gas hydrate (NGH) sediments lead to the inapplicability of traditional well stability models. In this paper, a new model to calculate the collapse pressure and fracture pressure of a borehole in NGH formation is studied based on the laboratory experiment results in published literature, mainly considering the effect of the saturation of NGH. Amended Mohr-Coulomb failure criterion is applied to calculate the collapse pressure while the fracture pressure is calculated combining Griffith criterion. Case study of this model is carried out and sensitivity analysis is performed to reveal the effects of original rock cohesion, Biot coefficient, frictional angle and pore pressure. The results show that the collapse pressure increases, while the fracture pressure decreases, with the decrease of NGH saturation. The collapse pressure decreases with the increase of the original rock cohesion and the decreases of Biot coefficient and pore pressure at any saturation. however, it decreases with the increase of the frictional angle in low saturation region while shows opposite trend in high saturation region. The fracture pressure increases with the increases of the frictional angle and the original rock cohesion and the decreases of Biot coefficient and pore pressure.

ABSTRACT: As new development moves into the vicinity of large granite boulders, new factors including vibrational loads caused by construction equipment or seismic shaking, loss of downslope support, and human activity begin to play a role in the stability of precariously balanced boulders. This study presents a practical reliability-based approach, using a Monte Carlo simulation, nine simple field measurements, and three “in house” estimates to determine the stability of a granite boulder, with respect to both sliding and rocking motion. The addition of a contact percentage parameter practically augments the widely accepted joint roughness coefficient (JRC) to account for the three-dimensional effects of real natural rock discontinuities. The Monte Carlo simulation allows for the uncertainty of the field measurements and rock strength properties, obtained using traditional geologic field tools, to be incorporated into the determination of the reliability, or probability of boulder movement. The method of analysis transforms the complexity of estimating potential boulder movements into a simple procedure, which can be conducted using a typical spreadsheet allowing for immediate implementation into geological/geotechnical engineering practice. A case study site is also presented, for which an active mitigation approach using rock bolts is employed.

ABSTRACT: In order to generate a full-scale, real world data set for calibration purposes for the rockfall simulation program RAMMS::ROCKFALL, we conducted extensive single block induced rockfall experiments with blocks of 200/800/2670 kg (~440/1760/5890 lb). With a fully four-dimensional trajectory reconstruction, we are able to retrieve the full set of parameters of interest on the single block rockfall trajectories such as translational velocity vectors, angular velocities, impact duration and forces, ballistic jump heights and lengths. This invaluable information can now be used for calibration purposes of the rockfall simulation code kernel, matching simulation performance to experimental results. Here, we present experimental data of the wheel shaped norm test rock EOTA 221/780kg , its reconstruction methodologies and subsequent calibration routines, with the aim to reduce the risks of inaccurate modelling caused by insufficiently calibrated models.RAMMS::ROCKFALL is the rockfall module in the RAMMS-Software suite ( RA pid M ass M ovement S imulation). It applies a novel contact-algorithm to model rockfalls, as opposed to most other rockfall simulation programs relying solely on restitution coefficients. The model accounts for real rock shapes, computing their runout trajectory over 3D terrain, including their jump heights, velocity, rotational velocity, rotational and total kinetic energy. All possible modes of rockfall motion (jumping, rolling and sliding) are deterministically simulated. Dynamics of single trajectories can be individually inspected or sets of multiple trajectories can be statistically analyzed.

ABSTRACT: This paper deals with the problem of the dissolution of soluble underground rocks and the geomechanical consequences such as subsidence, sinkholes and underground collapse. In this paper, the rock dissolution and the induced underground cavities are modeled using a Diffuse Interface Model. We describe briefly the method. We used to perform the transition (upscaling) from a multiphysics problem formulated at the microscopic scale level (pore-scale) to the macroscopic scale level (Darcy-scale). Rock material considered in this paper is gypsum, despite that the developed method is also suitable for more soluble rocks. The mechanical consequences of dissolution are analyzed for two theoretical configurations, i.e., lens and pillar. 1. INTRODUCTION Many problems in geomechanics such as subsidence, sinkholes and collapses, are related to the dissolution of soluble rocks. For example rock dissolution may create underground voids of large sizes, leading to a potential risk of instability or collapse, as illustrated in Fig. 1. Since dissolution of porous rocks may cause catastrophic damages, it is a major concern in geomechanics field. In many cases, dissolution is driven by an under saturated fluid flow. For instance, the subsurface water flow or hydraulic conditions through soils and rocks determines the onset conditions of geomechanical instability. Moreover, the natural or man-made hydraulic condition may evolve with time and change in space. Dissolution is also used intensively, for example in case of solution mining of salt. This industrial process extracts underground salt, by injection of fresh water through an injection well and extraction of the saturated brine at an extraction well. This process is very suitable in case of thin salt layer located at great depth. The multi-scale and multiphysics features of rock dissolution problem raise many questions. The first of them concerns the accuracy needed in the description of solid-liquid interface recession at the macro-scale level (Darcy-scale). To achieve this goal, a precise mathematical formalization of physicochemical and transport mechanisms at the micro-scale level is required. The second one is linked to the description of dissolution at large spatial scale (in situ scale, site scale). The third one deals with strong physical couplings with other processes, in particular, mechanical behaviors of rocks.

ABSTRACT The aim of this paper is the proposition of a new approach and a specific workflow to characterize and model the natural fractures using as case study the Cambro-Ordovician reservoir analog in the Mouydir basin, Algeria. The fracture analysis is based on two approaches. The first approach is the analysis of global fractures that affect the area of study. The fracture map is generated from the combination of curvature attribute, illumination attribute, geological maps, satellite images, and digital elevation models. The outcomes are the determination of fracture sets, fractures length's distributions, correlation coefficients, power law coefficients, and fractal dimensions. The second approach is the analysis of the fractures affecting the basement and the different Cambro-Ordovician units, by using the same input data and determining the same parameters and outputs. The 3D deterministic fracture models for each unit are built to show the fractures distribution in space and their superposition helps to determine their origin, their relationship, their kinematics, which illustrate the impact of the basement's fractures on the sedimentary cover.

ABSTRACT: The fractal dimension is applied to verify whether the 2-D fracture networks that affect the basement and the Cambro-Ordovician reservoirs analog in north Hoggar shield have fractal dimension. The Ajjers, In-Tahouite, and Tamadjert units compose the Cambro-Ordovician reservoirs analog. They are characterized by stiff tectonic style, showing dense fracture network that affects all the Paleozoic series. The statistical analysis of the length distribution of fracture in the Cambro-Ordovician reservoir analog indicates several fault sets orientation where the fracture length follows a power-law distribution. The fractal dimension Dm is estimated using the center distribution and box-counting algorithms. According to geometry and structure, the fractures can be divided into Major and Minor fracture where the fractal analysis is based on several approaches; analyzing the fractures by sets at different scales. All these approaches show that the fractures affecting the basement and the Cambro-Ordovician units in the area of study are fractal where their fractal dimension D m oscillates between 1 and 2.

ABSTRACT: In view of the complicated flow mechanism of shale gas flow, multi-scale media and multiple pressure systems of organic matter/inorganic matrix pores/natural fractures and discrete hydraulic fractures are established to accurately describe the gas flow in shale reservoir. The mathematic model considers mechanisms of non-equilibrium desorption of the adsorbed gas, the viscous flow and Knudsen diffusion of the free gas, and the influence of inorganic matrix pores, natural fractures and hydraulic fractures pressure system on shale deformation. A fully coupled solid-fluid coupling model is established, and it is solved with finite element method. It is found from the results that the pressure drop in natural fractures and matrix pores is the main influence factor that causes stress sensitivity during production, and natural fractures play a more important role. With the increase of stimulated reservoir volume, the stress sensitivity is more significant, especially in the early production stage. The proposed fully coupled solid-fluid model of shale gas flow have the ability to study the influence of solid deformation on different pore pressure systems, which consider the non-linear flow mechanisms. The model is also of great value for predicting the shale gas production more accurate.

ABSTRACT: The state of stress in the cement of a completed well's annulus is a critical parameter in evaluating well integrity. This state of stress controls opening of permeable micro-annuli between the cement, formation rock, and/or casing as a result of the varying conditions of the well caused by fluid injection, production, thermal, and chemical changes. This state of stress also controls failure of the casing, such as by externally driven crushing or internally driven rupture. The first step in determining this state of stress is to estimate the effective stress exerted by the cement immediately after it cures, when it has transitioned from a liquid to a solid. This point in time, represents the initial condition for all failure and leakage analyses. In this work, we evaluate tensile annulus opening in a cemented wellbore by high-pressure fluid injection as an example illustrating the importance of considering the initial state of stress. We also present a work flow in Abaqus that allows for modelling the expected initial stress condition in a cemented wellbore.

ABSTRACT: Drilling boreholes that weave a complex three-dimensional (3D) geometry has been made possible over the last decades by the emergence of a new generation of steering tools that provide virtually continuous control of the borehole trajectory. These directional tools, referred to as rotary steerable systems (RSS), led to the development of new technologies for horizontal drilling and extended-reach boreholes. Here, a model is proposed to revisit a phenomenon whose full consequences might have been overlooked: the influence of borehole overgauge on the directional tendency of drilling systems. The model in this paper is an extension to an analytical model of propagation based on delay differential equations. The introduction of stabilizer undergauge (or borehole overgauge) in the model breaks its inherent linearity; the relative deflection of the stabilizers solves a linear complementary problem (LCP). Following the non-linearity of the model, critical bifurcations can occur for some drilling systems, which could impair their response in the field or limit their controllability. Indeed, the results of the model, confirmed by simulations of borehole propagation, illustrate that for some combinations of weigh on bit (WOB), inclination, and force at the RSS, the drilling system might experience bifurcations that significantly limit steering capabilities. This phenomenon is most noticeable for flexible systems.

ABSTRACT: Sorptive rocks like coal and shale are considered transversely isotropic for purposes of flow and geomechanical modeling. The factors responsible for anisotropy in rocks are either its inherent rock structure or, stress-induced. In this study, we present an experimental procedure to quantify and characterize anisotropy of these rocks with changes in pore pressure. First, this study uses a model, developed previously to characterize total anisotropy of rocks, to carry out a comparative study of San Juan coal, Indonesian coal and Barnett shale for variations of anisotropy with changes in pore pressure. This is followed by presenting a modification to the model in order to characterize the two components of anisotropy, namely, rock structure anisotropy due to anisotropy in pore structure and the fabric of rock, and stress-induced anisotropy, which depends on the rate of stress changes around the rock and its sorption behavior. The results suggested that rock structure anisotropy does not vary significantly with changes in pressure and the stress induced anisotropy dominates the variation of the total rock anisotropy. The model presents a valuable tool to evaluate the changes in rock anisotropy with depletion for coal and shale reservoirs, adding to the fundamental knowledge of anisotropic character of rocks in the field of geo-mechanics

ABSTRACT: The thermodynamic properties of frozen soil are extremely affected by temperature. Besides the stress field, the temperature field cannot be ignored in polar drilling operations. Based on the correlation of thermodynamic properties of frozen soil with temperature, a thermal -flow-solid coupling numerical model of wellbore stability, which considering phase transformation, is established in this paper. The influence of drilling fluid temperature on the plastic yielding area around the wellbore during the drilling process in the frozen soil was analyzed. The results show that the temperature difference between the drilling fluid and the formation would seriously affect the distribution of the plastic yield area around the wellbore. When the drilling fluid temperature is greater than 0°C, as the drilling fluid temperature increases, the plasticity range will increase nonlinearly. When the drilling fluid temperature is less than 0°C, the weakening of the elastic parameters caused by the increase of the frozen soil temperature will reduce the plastic yielding area, which is beneficial to the stability of the wellbore. The study of drilling fluid temperature on the wellbore stability in the frozen soil provides a theory for the selection of drilling fluid temperature in the polar drilling operation.

ABSTRACT: CCS is a well-studied technology to efficiently reduce anthropogenic CO 2 from the atmosphere, which is believed to be one of the main factors contributing to global warming. In this paper, we investigate possible changes in the mechanical properties of a shale caprock, when in contact with injected CO 2 . Through diffusion, and concentration gradient, the injected CO 2 mixes with the caprock pore fluid, modifying its pH from basic to acid. Different experiments are performed to demonstrate the effect of CO 2 on mechanical properties of the North Sea Draupne shale caprock. The sample was exposed to CO 2 -sat brine for about 20 days under a confining pressure of 20 MPa and a pore pressure of 10 MPa. The poroelastic properties such as Young's modulus, Poisson's ratio, bulk modulus as well as the strength were then measured. Even if a slight increase is observed on the strength, this early-age effect of CO 2 on Draupne shale seems very little and is within the experimental error. The results of P-wave velocity measurements reveal a possibility that CO 2 may be coming out of solution during the test. In order to address this possibility, the experimental conditions are being reviewed and continuous measurements (every minute) of Young's modulus and Poisson's ratio are performed on the sample from consolidation to failure, through the flowing of CO 2 -undersaturated brine.

ABSTRACT: Accurately estimating diffusion coefficient of coal is of great significance for coalbed gas production planning. However, the most commonly used approach (written as infinite series) to inverse D may result in erroneous estimation due to assuming a constant surface concentration in solving the Fick diffusion model. This study first conducted a succession of experiments on coal-gas (CH 4 and CO 2 ) ad/desorption, and on the basis of Fick diffusion model, both an analytical approach and a numerical approach were proposed to inverse D in coal. The inversion result shows D is not a constant, it increases with pore pressure decreasing. The discrepancy resulted from using distinct inversion approaches varies with the pore pressure changing. It implies using the analytical approach to inverse D will underestimate the gas diffusivity of coal in some extend. Assuming a constant surface concentration will introduce some unpredictable deviation, even some unacceptable error. Finally, a dimensionless processing for Fick diffusion model was proposed to easy the numerical approach to inverse gas diffusion coefficient. This work is expected to make a clear evaluation on the influence of holding a constant surface concentration in solving Fick diffusion model and suggest a high-accuracy and efficient approach to estimate gas diffusion coefficient and model gas transport behaviors in coal matrix.

ABSTRACT: In this paper, the main results of a series of laboratory tests, on a tight sand reservoir belonging to the Lajas formation are presented as a part of a geomechanical characterization. Almost eight years ago, the first well was drilled on such formation on the Neuquén basin in Argentina. Its relevance has been raised during the last years due to its constant growth production, becoming the most important gas tight reservoir in the basin. Lajas formation is a Jurassic age tight sand, with low porosities and permeabilities between 0.1 md to 0.0001 md.

ABSTRACT: A fluid-driven fracture propagation model was presented to investigate the mechanical stability of natural fracture (NF) when hydraulic fracture (HF) approaching. The induced stresses generated by the approaching HF were semi-analytically calculated based on the complex variable method in theory of elasticity. The pore pressure effect caused by fracturing fluid leak-off was simulated by adopting the dual porosity medium theory. The critical failure conditions of NF were determined by the maximum tensile criterion and the Barton-Bandis criterion. The sensitivity of the stability of NF to approaching angle, approaching distance, in-situ stress anisotropy, injection rate, fracturing fluid viscosity, and surface roughness was analyzed in detail. Simulation results reveal that the shearing-mode failure of NF is obviously easier than the opening-mode failure, and shear slip zone is much larger than tensile failure zone. As HF approaches NF non-orthogonally, the induced debonding process is unstable, and the debonding zone is asymmetric with respect to the center of NF. The tensile-induced expansion zone is primarily located in the portion of NF ahead of the HF tip, however the shear-induced slip zone can even occur on NF behind of the HF tip. Induced stresses alone have a negligible effect on the stability of NF that is unconnected to HF. The instability of NF dominates the propagation trajectory of subsequent HF. A more suitable arrested/crossing condition by extending Gu-Weng criterion was established to predict the path of HF propagation.

ABSTRACT: A fully coupled thermal-hydraulic-mechanical (THM) model was developed to investigate the heat extraction process of an artificial enhanced geothermal system (EGS). The random fracture network in the stimulated geothermal reservoir was generated by the fractal theory. The local thermal non-equilibrium (LTNE) theory was adopted to simulate the heat exchange between the rock matrix and the injection cold fluid. Temperature-dependent fluid thermodynamic properties and pressure-dependent fracture/pore permeability were also incorporated into the thermo-poroelastic coupling model by some empirical formulas. The proposed multiphysics model was validated by several analytical solutions. The evolution of temperature, effective stress and reservoir permeability during heat extraction was analyzed in detail. The sensitivity of heat production performance to heat convection coefficient, injection rate, injection temperature, fracture network morphology was discussed. Results indicate that at the early stage, the interconnected large-scale fractures around wellbore dominate the mass and heat transport. Some scattered small-scale fractures contribute to uniformly propelling the cooling of the heat rock mass. The change in effective stress associated with the thermo-poroelastic effect may induce fracture shear dilation and pore expansion, resulting in the permeability enhancement of the overall reservoir. In some regions, where the temperature drop is insignificant and the pore pressure decreases, thereby inducing compressive stress to make fractures closure. It is more practical to use the LNTE theory to analyze the heat extraction process in fractured geothermal reservoirs. The heat extraction efficiency, thermal breakthrough time and service-life of an EGS are seriously affected by the hydraulic conductivity and connectivity of fracture networks under constant injection rates and injection temperatures. Generating a complex and scattered fracture network but without preferential channels is conducive to extracting more heat from geothermal reservoirs.

ABSTRACT: The fractures in the carbonate reservoirs have a high degree of development, and the natural fractures are susceptible to unstable cracking due to construction such as drilling and reservoir reconstruction. Based on data feedback during drilling and fracturing construction, combined with the advantages of neural network algorithm in data fault tolerance and prediction, and the ability to extrapolate more data based on existing data, the basic principles of neural network are established. The multi-layer back-propagating neural network uses the existing drilling and fracturing data as training camp samples to train the neural network, and then uses the trained network to predict the samples. Through the analysis of neural network, the integration of drilling and fracturing data and the instability of formation fractures are correlated, so that the cracks of natural fractures and cavities in the near-well formation can be analyzed through drilling data and fracturing data.

ABSTRACT: One of the most challenging issues in reservoir modeling relies on the development of proper numerical schemes for coupling flow and geomechanics with ability to handle highly heterogeneous coefficients with complex spatial distributions while preserving local conservation properties and computational efficiency. Among the class of iterative schemes where hydrodynamics is solved first, we may highlight the fixed strain and stress split. While the fixed strain presents only conditioned stability, the unconditional stable fixed stress split is more efficient, since the source term involving the time-derivative of the total mean stress admits a much slower characteristic time scale compared to the other

ABSTRACT: The rockfalls are recognized as the main cause of accidents at underground mining activities and are denominated as Fall of Ground. Aiming to reach a high level of risk management, real rockfalls examples will be expose as the developed methodology for risk prevention at the Mineração Serra Grande, located in the city of Crixás, in the state of Goiás, in central Brazil, and operated by AngloGold Ashanti. The result has shown the possibility to identify instability potential targets and by applying abacuses and modeling, is possible to manage these locals aiming to minimize the risks caused by rockfalls. The attenuation rule model proposed for this mine presents a good equivalence of 93% with the correlation coefficient. Defining the minimum vibration level for forming instable blocks is possible to determine the blast influence, turning possible to manage the project from an efficient and safe perspective. Therefore, this article aims to contribute for mineral operations management on reducing rockfalls hazards.