ABSTRACT: During production from gas reservoirs the formation pressure decreases, leading to increased levels of effective stress. As a result, the reservoir rock is compressed and deformed, leading to displacement of overlaying formations, which can propagate all the way to the ground surface. In unconventional reservoirs such as coalbeds, the production of gas is associated with desorption of gas from the coal matrix, which induces relatively large volumetric changes in addition to the pressure decrease. While reservoir models consider this phenomenon when investigating the variation of the fracture aperture and permeability, mechanical models usually either neglect or oversimplify its influence on the bulk volumetric response of the coal and consequently the displacement of the ground surface. In order to address this issue, a finite element model has been constructed, in which a horizontal poro-elastic hydrogeological profile is subjected to depressurisation in combination with a time-dependent Richard’s equation and linear elasticity. The effect of sorption-induced shrinkage is included in the model using the definition of and the concurrent effects of shrinkage on both permeability and bulk deformation. The results presented in this paper show that implementation of sorption-induced shrinkage can have a significant impact on the modelled geomechanical response of the coalbed and the estimated ground surface subsidence.

ABSTRACT: Accurate measurements of matrix storage and transport properties in unconventional tight oil/shale gas systems are of fundamental importance to informing key business decisions as stress-sensitive matrix response can predominate on controlling later life field performance. We have developed novel laboratory stress application and flow systems/techniques for measuring the effects of a non-hydrostatic stress field (maximum overburden stress orientated perpendicular to sedimentary layering) on storage and transport properties of anisotropic shale formations. Measured pore volume compressibility and stress sensitive, bedding-parallel permeability exponents in Mancos shale samples indicate an approximately 50% reduction in both storage and transport multiplier magnitudes when measured under uniaxial strain boundary conditions, as opposed to the more routinely applied hydrostatic “net confining stress” approach. Various “uniaxial strain correction factors” are outlined for converting hydrostatic measurements to their equivalent uniaxial values, more representative of the u reservoir condition.

ABSTRACT: A persistent increase in the permeability of a rock mass caused by transient stress disturbances, when they exist, can explain the change in groundwater level as well as the petroleum production by seismic waves in the far-field. However, it has yet to be clarified whether the transient stress disturbances induce an increase or decrease in the rock permeability. To clarify the effects of transient stress disturbances on the rock permeability, permeability measurements were carried out on intact and triaxially fractured Shikotsu welded tuff, both before and after the occurrence of axial stress or pore pressure transient disturbances. Based on the experimental results, both types of stress disturbances showed an increasing effect on the permeability for fractured rocks. However, only the pore pressure disturbances showed an increasing effect for intact rocks. It can be estimated that rock masses consisting of glassy pyroclastic rock may exhibit an increase in the permeability through disturbances in the transient stress.

ABSTRACT: Longwall mining causes the strata overlying the coal seam to fracture, break and collapse into the void space resulting from the coals extraction. Estimating the height of longwall mining induced strata fracturing, fracture connectivity and the height of complete groundwater drainage is a contentious issue and there seems to be some misconception and misunderstanding among the practicing engineers and groundwater modelers. This knowledge gap has put extra demand for rigorous study and the development of more sophisticated and reliable assessment tools. The extent of mining induced subsurface deformation, fracturing, surface subsidence, and aquifer interference is largely controlled by local lithological conditions, mining methods and mine layouts. Thus it is imperative to develop methods/tools that, unlike empirically based strata desaturation height estimation methods, can properly consider all factors and accurately predict strata deformation and subsequent fracturing (particularly connective fracturing) in varied hydrogeologic environment to assess the impact of mining on groundwater. This paper describes the use of monitoring data from a mine site in Queensland and a Distinct Element model to assess strata fracturing during longwall extractions.

ABSTRACT: The fracture energy of the shale possesses a significant anisotropy due to the shale bedding and has great effects on the fracturing path. To investigate these effects, three types of the shale specimens are applied in fracturing test with three-point bending. The Digital Imaging Correlation (DIC) and Acoustic Emission (AE) methods are also integrated to analyze the damage processes. According to the AE energy, the damage process of the shale can be divided into three parts using the proportion between the loading value and the peak value of the specimen. In Part I, the sample does not damage. In Part II, the damage begins to appear in the sample, and the work of the external forces is mainly contributed to create the new cracks in the rock layers and to promote the opening and dislocation of the layer interfaces. In Part III, the cracks develop because the damages in the shale are accumulated to an extreme value. Fracture energy anisotropy extent of the shale has been obtained from these partitions, which can be further applied to optimization of the perforation and hydraulic fracturing direction.

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: It is commonly assumed that the dimensions of an induced microseismic cloud around an Enhanced Geothermal System (EGS) well delineate the “stimulated volume” in which permeability has been increased. This interpretation derives from a self-propping fracture model, wherein shear slip results in some permanent increase of hydraulic aperture. Or, more simply, earthquakes equal permeability. In contrast, here, we interpret microearthquakes (MEQs) directly in terms of the stress and pressure conditions that control earthquake triggering. In many cases, triggering is dominated by fluid pressure changes within a fracture, although increasingly poroelastic and earthquake interaction effects are seen as important.

ABSTRACT: Excavation in the complex geological structure often accompanies with water-inrush accidents, which is considered to be resulted from mass migration and loss in fractured rock in the seepage process. When granular matters migrate and lose, the grain size distribution (GSD) changes over time, it will in turn affect the migration and loss of fine matters. In this paper, the authors carried out seepage tests for rock granular matters considering mass migration and loss, discussed the GSD variation and derived a new GSD expression. The results indicate that (1) all the rock matters with different sizes change in the seepage process, no matter what the samples’ Talbot power exponent (TPE) is and what the compression condition is; (2) the residual mass ratios of samples without initial compression and the ones with initial compression of 10mm have the same variation tendency for rock matters with sizes of 0~10mm and 15~25mm, while they have the different variation tendencies for samples of 10~15mm; (3) the GSD varies from mass migration and loss in the seepage process; (4) the varied GSD obeys the like-Talbot continuous grading expression. The work can be applied as a technical reference for a potential seepage instability or water inrush disaster.

ABSTRACT: The capillary pressure curve is a property of rocks whose determination requires elaborate petrophysical measurements. The utility of the capillary pressure curve can be extended to the evaluation of the fluid flow properties of rocks which provides another method for characterization of rocks. In this work we study 1-, 2- and 3-parameter capillary curves models through their effect on the imbibition curves. The 3-parameter model is the usual Van Genuchten family of capillary curves. Each model produces imbibition curves with specific characteristics, encapsulating the behavior of the rock during absorption with increasing detail. Hence, the characteristics of the imbibition curves are associated with suitable regimes of the parameters of each model, thereby providing enough classifying information to determine a suitable capillary curve from imbibition data through back analysis. In order to prove the effectiveness of the methodology we have performed an imbibition test on sandstone and compared it against a series of produced results from finite element analysis for all types of capillary curve models. The degree of coincidence with the experimental imbibition curve is discussed for each model showing the fine differences between the models.

ABSTRACT: We measure acoustic travel-times in Green River shale both parallel and perpendicular to bedding to observe the effects on petrophysical properties of interactions with water (fracture fluids) for different durations. X-ray diffraction shows the samples dominantly comprise carbonate and quartz. Scanning electron microscopy shows morphology of the minerals and pore network of the shale. Horizontal and vertical helium permeability measurements are conducted to analyze permeability evolution under different stress conditions. The samples record very low permeabilities at recreated confining stresses – indicating the need to be stimulated. Permeability decreases with increasing confining stress. Permeability increases with increasing pore pressure due to gas slippage effects. Increasing effective stress generally decreases the permeability. Acoustic travel-time measurements show that compressional and shear wave velocities increase with confining stress. Shear, Young's and bulk modulus all increase with confining stress with the more rigid samples exhibiting higher fracture conductivity. Compressional and shear wave velocities decrease as the shale is exposed to water and quartz dissolves from the pore structure, reducing moduli and acoustic velocities of the samples that exhibit lower fracture conductivity. Measured porosities suggest greater accessibility of the pores from a direction parallel to bedding.

ABSTRACT: Characterization of rock permeability change during the fracture network evolution is critical to the design and assessment of geological storage facilities. Under geo-storage conditions, rock fractures propagate during damage processes and re-bond during healing. However, natural heterogeneities in rocks significantly reduce experimental repeatability. Moreover, healing is a time-consuming process and requires high pressure or temperature to achieve. In this study, we resort to the 3D printing technology for specimen preparation so that complex rock microstructure changes are precisely controlled. We validate the specimen preparation by comparing experimental permeability test results with analytical solutions. Channel networks with different tortuosity, porosity, fracture branch, and structural anisotropy are designed to mimic various microstructure changes in rocks. To fabricate fractured rocks, we also embed heterogeneous fracture networks into 3D printed specimen and modify fracture geometry to account for anisotropic microstructural changes. Experimental findings revealed the evolution trend of permeability with these structural characterization parameters. They also match well with those obtained from computational fractured rock models under identical hydro-mechanically coupled conditions. This study is expected to provide new insights into the dependence of rock permeability on its microstructures under complex geo-storage conditions.

ABSTRACT: Acoustic properties of dry or saturated porous media can be studied numerically by a combined use of Lattice Spring Models (LSM), Lattice Boltzmann models (LBM), and coupled LBM-LSM models. The methodology is briefly described and it is systematically applied to a series of Fontainebleau sandstones whether they are dry or saturated with incompressible or compressible fluids. The porous structure of these sandstones is measured with Computed Micro Tomography (CMT). The velocities of P-waves and S-waves are determined. While the simulation results are in good agreement compared with the existing methods and experimental data, the velocities are always slightly overestimated; the origin of this bias is tentatively studied.

ABSTRACT: Systematic calculations have been done in order to analyze the response of a medium with a hilly surface to explosions created below the ground surface when a cavity can be present. Random surfaces with a given correlation function are studied. The influence of the roughness and of the correlation length of the surface fluctuations is analyzed on the signal reflected by the cavities.

ABSTRACT: Acidizing and wormholing in carbonate reservoirs is studied through the analysis of the linear acidizing problem theoretically and experimentally. Experimentally, linear acidizing tests were performed in Mons chalk, a high porosity analogue of North Sea reservoir chalk. In the experiments the critical injection velocity for wormhole formation at minimum acid injection was obtained. Theoretically acidizing was analyzed by formulating the problem as reactive moving boundary Stefan type problem. Wormholing was analyzed through a linear stability analysis from the equilibrium state of uniform dissolution. The results of the stability analysis determined the critical injection velocity for wormholing and size effect with respect to the specimen diameter.

ABSTRACT: Darcy's law does not always govern flow in shale matrices. Rather, a considerable portion of the flow is dominated by non-Darcy flow regimes. Therefore, modeling permeability of shale matrices is a challenge because at each pressure different proportion of the rock are experiencing different flow regimes. In this study, a new apparent permeability model is presented to describe the combined Darcy and non-Darcy flow of gases at different pressures. This model is then, combined with pore size distribution data (PSD) of shale to establish different proportion of pore sizes of rock experiencing different flow regimes at different pressures. The combined approach of permeability model and pore size distribution data used to estimate the apparent permeability of the shale matrix at each pressure has been named as Permeagram approach in this paper. Finally, the Permeagram is coupled with constitutive equations applicable for shale rocks to formulate a coupled stress-dependent permeability model for shale. The coupled model was validated against permeability data of Marcellus shale with different stress variations. The modeled data was in close agreement with the experimental data and this validation step further solidifies the robustness of the proposed model.

ABSTRACT: We conducted laboratory experiments that demonstrate how an increase in fluid pressure can induce “stick-slip” events along a preexisting fault. Our sample is a 760 x 356 x 76 mm poly(methyl methacrylate) (PMMA) plate with a saw cut simulated fault deformed in a biaxial loading apparatus. Water was injected directly into the simulated fault through a 3 mm diameter hole drilled in the sample. During experiments, normal stress was held constant, while shear stress and water pressure varied to generate dynamic slip events. Fluid-induced slip events were found to have similar slip rates, total slip time, and total displacement as events resulting solely from an increase in externally applied shear stress. Slip measurements reveal that shear-triggered events initiated close to the application of shear stress. Fluid-induced events often initiated in the center of the fault, near where the fluid was injected. We employed fluid pressure measurements from secondary monitoring wells to study the fluid behavior along the fault. These early experiments resulted in slip events induced within seconds of fluid injection, but further monitoring of the fluid on the fault suggests that delayed seismicity may be possible due to the time required for fluid pressure to diffuse along the fault.

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: Injection of fluid into the deep subsurface can at times generate measurable or even damaging earthquakes, but often they only produce aseismic deformations along faults. The relationship between injected pressure and these aseismic deformations is a fundamental point in the estimation of how the crust responds to fluid injection and the associated induced seismic hazard. Here, we use data-driven hydromechanical modeling of fluid injection to show how the fault permeability enhancement, the stress perturbation and the fault structure have an important effect on the fluid pressure diffusion, the growth of aseismic fault slip and the seismicity triggering. Through our investigations, we find that the competition between the fluid pressurization and mechanical stress transfer associated both to pressurization and co-seismic rupture is a viable mechanism for seismic events triggered during multiple fault activation. Our in-situ experiments and models clearly indicate that fluid injection induces significant aseismic slip with associated stress perturbation through slow failure and permeability increases that contribute to trigger micro-seismicity within the fault volume, especially at fault intersections at a distance from injection.

ABSTRACT: u biogenic coalbed methane (iBCBM) reservoirs aim to economically produce microbially stimulated methane. Over the years, researchers have concentrated on optimizing the associated microbiology. However, flow and geomechanical reservoir parameters that govern its technical feasibility have not been investigated. Also, most experimental studies used coal fines, providing larger surface areas, thus resulting in high gas production rates. To date, bioconversion of intact coal cores has not been reported. This paper presents a two-pronged study, investigating bioconversion potential of intact cores held under ideal replicated u conditions, and reports the variation in permeability of the coal-core, pre- and post-bioconversion. The results showed that methane production over sixty days was significantly less, ~10 std. cu ft (scft), compared to 2000 scft reported for coal fines. Furthermore, the permeability decreased significantly post-bioconversion. Both findings would severely impact the production potential of iBCBM reservoirs. The decrease in permeability post-bioconversion suggests that successive treatment-production cycle(s) may not be possible. Any commercial application of the technology would require some means to artificially enhance the reservoir permeability.

ABSTRACT: Coupled fluid flow and solid deformation in porous media modelling is essential in a wide variety of applications. It is also crucial to provide an appropriate discretization for these coupled problems. Therefore, in this study, we provide comparisons between a classical continuous Galerkin (CG), and a new approach, enriched Galerkin (EG) to solve such problem. Even though both the EG and CG methods deliver approximately the same solution for the Terzaghi's 1-D model, the CG method may produce spurious pressure and volumetric strain oscillations where a vast difference of conductivity property is observed. On the contrary, the EG method results demonstrate a smooth transition across the mentioned interfaces. These phenomena lead to different reservoir compaction and conductivity effects, which are induced by volumetric strain, on flux approximation in heterogeneous, both porosity and permeability, materials. These impacts are exemplified, and the differences between the EG and CG methods grow larger in softer materials. Although both methods maintain global mass conservation, the EG method guarantees a local mass conservation property by the construction. The EG method, however, demands more computational resources in terms of both degrees of freedom and number of iterations than the CG method.