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: Fractures shows strong stress sensitivity when the effective stress changes. Oil production causes the decrease of pore pressure and then leads to the change in subsurface stress. Re-open or closure of fractures bears significant influence on the fluid flow and oil production. The purpose of this work is to develop a general coupling between fluid flow equation and deformation of fractured media. To quantify the deformation of fracture during production, the geomechanical part used a porous elastoplastic damage model as its basis to investigate relationships between fracture dynamic change and pore pressure vary, and a numerical solution for the distribution of continuum damage variables is obtained by using the finite element method, and permeability of fractures was mathematically expressed with the damage variables. A model involving water-oil flow and fracture deformation was built to verify the coupled geomechanics-flow model and illustrate the nature of flow in a dynamic fractured formation. The corresponding damage evolution during production as well as the distribution of fracture permeability and formation pressure were obtained in the simulation. The results suggest that fractures gradually close and permeability decrease during production which can negatively impact the productivity. An uncoupled reservoir simulation may overestimate the oil production capacity of fractured reservoirs.

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 essential task for management of rock related risks in mines is to continuously confirm that the mine is safe. For small scale, day to day hazards observational methods comprising planned triggers, responses and reviews is formalized and well-practiced but for large scale mine scale instability, large seismic events or inrushes the analysis and decision-making process during design and operations is harder to prescribe. Often, only the measurement and observation tasks at a mine are properly documented, and the ensuing analysis and critical remedial actions are not prescribed. In some mines it is unclear who is responsible for maintaining the ‘mine stability safety case’ and team members simply assume that the work to confirm the mine is safely stable is being undertaken by others. Too often, comprehensive databases of real time measurements are ignored by engineers until after a serious event.

ABSTRACT: In this study, bump potential in the vicinity of a fault zone associated with a longwall mining excavation is evaluated with respect to the different stages of shear strain in the normal fault through the use of a numerical modeling technique. The influence of stored and dissipated energy resulting from fault deformation and timing of longwall excavation on the potential for a bump adjacent to the fault was also investigated by the numerical model. While faults are commonly simulated as a single planar or nonplanar interface for a safety or stability analysis in underground mining excavation, the real 3D structure of a fault is often very complex with different branches that activate at different times. Furthermore, these branches are zones of nonzero thickness where material continuously undergoes damage even during interseismic periods. The FLAC3D model successfully demonstrates and creates the 3D fault zone as a normal type structure in the entire thickness of the model. The normal fault structure and its displacement result in the induced deformation in the model. Several panels of a longwall excavation are virtually placed and excavated adjacent to the fault. The characteristics of stored and dissipated energy associated with the panel excavations are examined and observed at the different stages of shear strain in the fault to evaluate bump potential. Depending on the shear strain in the fault, the energy characteristics adjacent to the longwall panels present different degrees of bump potential, which is not possible to capture by conventional fault simulation using an interface.

ABSTRACT: The elastic models combined with the Mohr-Coulomb criterion are frequently used to assess the wellbore stability problem. However, the elastic models are generally too conservative to predict the minimum wellbore pressure. In addition, the Mohr-Coulomb criterion underestimates the strength of the rock in neglecting the strengthening effect of the intermediate stress. In this study, the Mogi-Coulomb criterion which is a triaxial failure criterion is used to investigate the wellbore stability analysis. The rock is modeled with a strain hardening plastic material. The wellbore stability problem is formulated under undrained conditions. Large strain deformations are assumed to occur in the plastic region whereas small strain deformations are prevalent in the elastic region. The stresses in the elastic region are derived using the classical Kirsch equations. By adopting an associated flow rule assumption, plastic strains increments are expressed in the plastic region. Using the proposed approach, semi-analytical solutions of stresses in the plastic region are obtained by solving a set of three differential equations. The minimum wellbore pressure is determined by applying an appropriate controlled-deformation borehole criterion. The numerical results from the case example has enabled to plot the distribution of the stresses and the wellbore pressure around the wellbore. The curves have the possibility to show the stresses variation from the plastic region to the elastic region. The current model has the advantage to predict less wellbore pressure in comparison with elastics models. Moreover, it also accurately accounts the strengthening effect of the intermediate stress on the strength of the rock.

ABSTRACT: The deformation stability and control theory based on viscoelastic, viscoplastic and viscodamage models is developed to simulate time-dependent deformation and quantitatively evaluate the long-term stability of gas storage caverns in salt rock. A quantitative criterion for the long-term stability evaluation in the spatial-temporal non-equilibrium evolution process of gas storage caverns is established based on the T~?E curve, where T is time and ?E is plastic complementary energy (PCE). The T ~ ?E curve provides a unified quantitative criterion of stability evaluation under various external disturbance factors. The driving force of the spatial-temporal non-equilibrium evolution process is revealed and its engineering value is demonstrated especially for reinforcement design. PCE is the quantitative indicator of instability. Over force is the internal effective driving force of unrecoverable time-dependent deformation or damage evolution during the spatial-temporal non-equilibrium evolution process. Deformation stability and control theory is implemented by finite element method to analyze the time-dependent deformation, long-term stability and local failure of gas storage caverns in salt rock. It is indicated that deformation stability and control theory proposes a more effective stability evaluation criteria of gas storage caverns in salt rock.

ABSTRACT: This paper presents the results of a 2018 NIOSH (National Institute for Occupational Safety and Health) study to evaluate longwall-induced subsurface deformations and permeability changes and their effects on shale gas well integrity and safety under shallow cover. A study site was selected over a southwestern Pennsylvania coal mine, which extracts 457-meter-wide longwall faces under 147 meters of cover. One in-place inclinometer (IPI) well and three permeability monitoring wells were drilled and installed over a 38-m by 84-m centers abutment pillar. In addition to the inclinometer well and monitoring wells, surface subsidence measurements and underground coal pillar pressure measurements were conducted as the first 457-meter-wide longwall panel on the south side of the abutment pillar mined by. Comparisons of a series of FLAC3D finite difference simulation results and the surface, subsurface, and underground instrumentation results show that the measured IPI casing deformations are in reasonable agreement with those predicted by the FLAC3D models, and that the measured surface subsidence and pillar pressure are in excellent agreement with those predicted by the 3D models. Measured permeability changes were incorporated into a preliminary calculation to evaluate the effect of a hypothetical shale gas well leak on longwall ventilation, which indicates negligible impact after completion of first panel mining. Results from this 2018 research clearly indicate that under shallow cover, the measured horizontal displacements within the abutment pillar are at least one order of magnitude higher than those measured in a 2017 study under deep cover and slightly higher than those measured in a 2014 study under medium cover. INTRODUCTION Due to a recent shale gas boom, approximately 1,500 unconventional shale gas wells have been drilled through active and future coal reserves in Pennsylvania, West Virginia, and Ohio over the past 15 years. These shale gas wells have penetrated many mineable coal seams, in particular, the Pittsburgh Seam. These unconventional gas wells, whether tapped into the Marcellus or Utica formations, contain very high gas pressure. Strata deformations associated with longwall extractions could induce high stresses and deformations in the shale gas well casings, which may compromise the mechanical integrity of the production, intermediate, and coal protection casings (Figure 1). Damaged well casings could potentially introduce high-pressure, high volume explosive gas into underground mine workings, particularly during an extended well shut-in period, which could seriously jeopardize underground miners’ safety and health.

ABSTRACT: Casing deformation in shale gas wells during multi-fracturing has seriously affected the efficiency and safety of shale gas production in China. Firstly, the characteristics of casing deformation in Weiyuan shale gas field were statistically analyzed, and main influence factors were obtained. Second, the influence of the induced stress from crack is analyzed, and a stress analysis model for casing-cement-formation system stress field nearby wellbore during fracturing is established. Finally, the loads near wellbore in shale gas wells is studied, incorporating variation fracturing methods and operation parameters. The results show that the casing deformation in shale gas wells mainly occurs near the heel of the horizontal section, and shear failure is the main type of casing deformation; The initiation and expansion of crack can lead to the accumulation of non-uniform stress nearby wellbore during fracturing. Further, this will induce formation slip and cause the shear and collapse failure of casing; Accurate identification of faults, natural cracks and improved fracturing technology can alleviate the stress concentration of the casing and reduce the risk of casing deformation failure. This study further reveals the mechanism of casing deformation during multi-fracturing in shale gas wells, and proposes fracturing measures for casing integrity. The results demonstrated in this paper can provide the theoretical basis for preventing casing deformation during fracturing in shale gas wells. 1. INTRODUCTION With the development of unconventional gas reservoirs for shale gas in the world, Significant progress in the exploration and development of shale gas has been made in China recent years, including the construction of the Changning-Weiyuan shale gas field of CNPC and the Chongqing Fuling shale gas field of Sinopec. However, in the national shale gas demonstration area of the Changning-Weiyuan, southwestern Sichuan Basin, since the exploration and development in 2013, some shale gas wells have had some problems, such as poor cementing seal integrity and serious casing deformation. Difficulties in the entry of casings in horizontal well sections, especially in the process of fracturing stimulation and transformation, the impact of casing deformation on the smooth entry of bridge plugs has caused some reservoir sections to be forced to abandon, and even the bridge plugs cannot be drilled smoothly. The emergence of these problems has affected the improvement of single well production in shale gas horizontal wells.

ABSTRACT: In this paper, we integrate hydraulic fracture and geomechanical modelling with microseismic data to quantify deformation and stress field evolution as a predictive tool, as well as to provide a calibration and microseismic workflow. Our approach utilizes simulated fractures as well as moment tensor solutions to represent localized discrete rupture zones. The geomechanical algorithm evaluates the Green's functions for each rupture, and subsequently calculates the co- and post-seismic deformation using linear superposition. These data are then combined with dynamic parameter analysis to define an integrated view of the reservoir. We apply the approach on a data set from a hydraulic fracture stimulation in the Marcellus and show that integrating fracture models with geomechanical simulation can forecast zones of microseismic activity, identified by regions of high deformation and high Coulomb stresses. We compare dynamic parameter analysis from the observed microseismic data with geomechanical predictions from elastic and visco-elastic models and note the behavioral sensitivity of dynamic parameters to the geomechanical Earth model. The visco-elastic models better represent the dynamic or collective behavior of the microseismicity while maintaining reasonable model rigidity to produce consistent stress drops. Furthermore, the elastic models are sufficiently accurate to forecast potential volumes and timing of microseismicity.

ABSTRACT: We experimentally investigate the effect of the pore fluid on AE in four sandstone plugs cored from a homogeneous slab. These plugs were fully saturated with brine, mineral oil, and hexane, while one of them remained dry. AE level was highest in the hexane-saturated sample, smaller in the oil-saturated sample, and even smaller in the brine-saturated sample. It was even smaller in the dry sample. It appears that in the liquid-saturated plugs, the AE level is monotonically related to the acoustic impedance of the pore fluid – the smaller the impedance the higher the AE. It does not appear to be related to the viscosity of the pore fluid.

ABSTRACT: With the shale gas boom over the past decade, many shale gas wells have been drilled through the Pittsburgh coal seam to the Marcellus shale formation. The wells located in longwall barrier pillars are influenced by longwall mining, and their stability has been a safety concern for both the mining and gas industries. This study presented a case in which two longwall panels in the Pittsburgh seam were mined near two shale gas wells in a barrier pillar between the two longwall bleeders. The first longwall panel was mined before the gas wells were drilled and installed in the barrier pillar adjacent to the longwall setup bleeders. The second longwall setup bleeders were developed later on the other side of the barrier pillar, and the second longwall panel was subsequently mined. The stability of the gas wells was evaluated using a numerical modeling approach. The model was set up based on site-specific mining conditions and overburden geology and included sufficient details to simulate the gas well casings, mining sequence, and longwall retreating. The modeling procedures produced realistic results of stresses and deformations around the barrier pillar and in gas well casings, which were verified by in-mine observations.

ABSTRACT: Reservoir compaction and surface subsidence are significant issues for shallow coal seam gas plays, and regulatory authorities can mandate subsidence predictions as a License-To-Operate requirement. It is therefore important to accurately determine the compaction behavior of the coal under the expected in-situ conditions as the reservoir depletes. This paper reports the results of methane pore pressure depletion tests under approximate uniaxial-strain and in-situ stress conditions on whole core coal samples from a coal seam gas reservoir. Test results showed that there is a substantial increase in the compressibility of the core material when the effects of methane desorption are included. The effective uniaxial-strain compaction coefficient C m show a clear increase in compressibility with decreasing pore pressure, and an apparent acceleration of the increase below 1.2 MPa pore pressure. For the tested samples the Cm values began at 1.5 to 4.4x10 -4 /MPa and increased to peak values of 5.1 to 11x10 -4 /MPa. The compressibility excluding the desorption effects was generally below 2x10 -4 /MPa and constant or decreasing with decreasing pore pressure. These data agree with previously reported results on coal from other locations and clearly show the importance of including methane desorption effects in reservoir compaction and subsidence calculations for coal seam gas fields.

ABSTRACT: It is well recognized that injecting CO 2 in methane-saturated coal seams can initially decrease the injectivity, followed by a permeability rebound as gas pressure increases. Studies showed that such permeability change is related to the adsorption-deformation coupling which is governed by both the poromechanical properties and the adsorption characteristics of coals. The adsorption stress derived from Coussy's macroscale theory is now understood as the cause of such adsorption induced straining. This work continues to explore such phenomenon along this line. The key outcomes of the previous analytical findings based on theory of mixture are summarized and the various options for poroelastic and adsorption models are listed. Predictions from these models are compared with the recent experimental data on coals subjected to CO 2 and CO 2 -CH 4 injections. Parametric studies are conducted to illustrate the versatility of the theory in capturing a wide range of adsorption-induced swelling behaviors of porous materials.

ABSTRACT: Mining-induced seismic hazards continue to pose significant challenges for ground control at deep hard-rock underground mines. In the tradeoff between full production and absolute safety, the seismic hazards at deep mines can only be routinely assessed and then selectively mitigated, rather than predicted with full confidence or avoided completely. The improved ore-extraction rate and expansion of mine-wide developments at Alamos’ Young Davidson (YD) Mine in recent years have positioned the mine as a top gold producer of its kind in North America, along with increased seismic activity. This paper describes the proactive measures taken against seismic hazards that have been incorporated in the current YD Ground Control Management Plan. First, the use of seismic monitoring system and seismic analytical platform to identify seismic hazard zones is illustrated. Next, stress modeling as an indicative tool to further examine stope blasting-triggered seismic risk is discussed, and the assessment is enhanced by the use of 3D Velocity Model to closely monitor the rock mass fracturing process in the form of seismic clustering. Finally, an empirical approach specially developed for Canadian deep mines, coupled with site-specific field observations, is employed to justify the support design for large-span intersection developments with a focus on mitigating dynamic loading hazards.

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: 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.

ABSTRACT: Self-propped fracture is an important flow channel in the hydraulic fracturing process of shale. In this study, we have evaluated the flow capacity in the self-propped fracture and revealed its flow characteristics. On the basis of considering the plasticity characteristics, the elastic-plastic deformation model of self-propped fracture is established. Then, through the deformation model solution, the contact state of the fracture surface is analyzed, the deformation and stress of the fracture are calculated, and the model of fracture flow channel under different closure stresses is established. Finally, the three-dimensional incompressible Navier-Stokes equation is used to describe the flow, calculate the fracture conductivity, and analyze the flow characteristics. Compared with the traditional elastic model, the established model has a more accurate contact area of fracture under high closure stress. The size and shape of the contact area have an important influence on flow velocity, flow path and flow pressure difference. The “strip” contact is easier to form “dominant channels” than the “point” contact. In this study, a numerical simulation method is established to predict the self-propped fracture conductivity and reveal the flow characteristics. The results can provide a new approach for the study of self-propped fracture conductivity in deep shale reservoir.

ABSTRACT: The ability to monitor mining-induced ground deformation is an integral part of a Ground Control Management Plan (GCMP) in any underground mine. The integration of such monitoring into an early warning program, coupled with an instantaneous response plan for potentially hazardous conditions, contributes to a safer working environment. There is now a user-friendly monitoring device which allows any worker in the area of a ground movement hazard to be immediately alerted by color changing LED lights. Using this system, no special engineering skills or knowledge are required to recognize the indication of a potential hazard. The system is tamper proof and can only be replaced once the area is declared safe by a competent professional. The movement of the rock mass exceeding a pre-determined limit is accurately quantified by these devices. Therefore, the in situ ground movement monitoring can be used to calibrate numerical modeling and to evaluate designed support efficiency. This better understanding will enable operations to make informed decisions regarding the selection of support systems and the effect of mining rate and excavation proximities

ABSTRACT: The combination of active ultrasonic monitoring and Digital Image Correlation (DIC) was used to detect and characterize crack initiation and growth in pre-cracked rock specimens. Uniaxial compression tests were conducted on prismatic Lyon Sandstone specimens with one oriented flaw. During the experiments, the cracking process was continuously monitored by measuring surface displacements using DIC. Compressional wave pulses were also recorded continuously during the loading to detect signatures of the cracking phenomena and growth using three pairs of transducers. The ultrasonic measurements allowed for the detection of crack initiation at the tip of the pre-existing flaw. The normalized wave amplitude was observed to decrease at initial loading stages because of micro-cracking followed by a sharp decrease in the rate of amplitude reduction due to tensile crack initiation. Digital image correlation (DIC) was able to accurately determine crack initiation, propagation, and its type. The experiment results demonstrated that the combination of these two methods provided a reliable crack characterization technique.