GEOMECHANICAL STATE OF ROCKS WITH DEPLETION IN UNCONVENTIONAL COALBED METHANE RESERVOIRS

AN ABSTRACT OF THE DISSERTATION OFSUMAN SAURABH, for the Doctor of Philosophy degree in Engineering Science, presented on August 30, 2019, at Southern Illinois University Carbondale.TITLE: GEOMECHANICAL STATE OF ROCKS WITH DEPLETION IN UNCONVENTIONAL COALBED METHANE RESERVOIRSMAJOR PROFESSOR: Dr. Satya HarpalaniOne of the major reservoir types in the class of unconventional reservoirs is coalbed methane. Researchers have treated these reservoirs as isotropic when modeling stress and permeability, that is, mechanical properties in all directions are same. Furthermore, coal is a highly sorptive and stress- sensitive rock. The focus of this dissertation is to characterize the geomechanical aspects of these reservoirs, strain, stresses, effective stress and, using the information, establish the dynamic flow/permeability behavior with continued depletion. Several aspects of the study presented in this dissertation can be easily extended to shale gas reservoirs. The study started with mechanical characterization and measurement of anisotropy using experimental and modeling work, and evaluation of how the sorptive nature of coal can affect the anisotropy. An attempt was also made to characterize the variation in anisotropy with depletion. The results revealed that the coals tested were orthotropic in nature, but could be approximated as transversely isotropic, that is, the mechanical properties were isotropic in the horizontal plane, but significantly different in vertical direction. Mechanical characterization of coal was followed by flow modeling. Stress data was used to characterize the changes in permeability with depletion. This was achieved by plotting stress path followed by coal during depletion. The model developed was used to successfully predict the permeability variation in coal with depletion for elastic deformations. As expected, the developed model failed to predict the permeability variation resulting from inelastic deformation given that it was based on elastic constitutive equations. Hence, the next logical step was to develop a generalized permeability model, which would be valid for both elastic and inelastic deformations. Investigation of the causes of coal failure due to anisotropic stress redistribution during depletion was also carried out as a part of this study. It was found that highly sorptive rocks experience severe loss in horizontal stresses with depletion and, if their mechanical strength is not adequate to support the anisotropic stress redistribution, rock failure can result. In order to develop a generalized permeability model based on stress data, stress paths for three different coal types were established and the corresponding changes in permeability were studied. Stress path plotted in an octahedral mean stress versus octahedral shear stress plane provided a signal for changes in the permeability for both elastic as well as inelastic deformations. This signal was used to develop a mechanistic model for permeability modeling, based on stress redistribution in rocks during depletion. The model was able to successfully predict the permeability variation for all three coal types. Finally, since coal is highly stress- sensitive, changes in effective stresses were found to be the dictating factor for deformations, changes in permeability and possible failure with depletion. Hence, the next step was to develop an effective stress law for sorptive and transversely isotropic rocks. For development of an effective stress law for stress sensitive, transversely isotropic rocks, previously established constitutive equations were used to formulate a new analytical model. The model was then used to study changes in the variation of Biot’s coefficient of these rocks. It was found that Biot’s coefficient, typically less than one, can take values larger than one for these rocks, and their values also change with depletion. The study provides a methodology which can be used to estimate the Biot’s coefficient of any rock. As a final step, preliminary work was carried out on the problem of under-performing coal reservoirs in the San Juan basin, where coal is extremely tight with very low permeability. An extension of the work presented in this dissertation is to use the geomechanical characterization techniques to unlock these reservoirs and improve their performance. The experimental data collected during this preliminary study is included in the last chapter of the dissertation

Sensitivity analysis of modeling parameters that affect the dual peaking behaviour in coalbed methane reservoirs

Coalbed methane reservoir (CBM) performance is controlled by a complex set of reservoir, geologic, completion and operational parameters and the inter-relationships between those parameters. Therefore in order to understand and analyze CBM prospects, it is necessary to understand the following; (1) the relative importance of each parameter, (2) how they change under different constraints, and (3) what they mean as input parameters to the simulator. CBM exhibits a number of obvious differences from conventional gas reservoirs, one of which is in its modeling. This thesis includes a sensitivity study that provides a fuller understanding of the parameters involved in coalbed methane production, how coalbed methane reservoirs are modeled and the effects of the various modeling parameters on its reservoir performance. A dual porosity coalbed methane simulator is used to model primary production from a single well coal seam, for a variety of coal properties for this work. Varying different coal properties such as desorption time ( ÃÂ), initial gas adsorbed (Vi), fracture and matrix permabilities (kf and km), fracture and matrix porosity ( ÃÂf and ÃÂm), initial fracture and matrix pressure (to enable modeling of saturated and undersaturated reservoirs), we have approximated different types of coals. As part of the work, I will also investigate the modeling parameters that affect the dual peaking behavior observed during production from coalbed methane reservoirs. Generalized correlations, for a 2-D dimensional single well model are developed. The predictive equations can be used to predict the magnitude and time of peak gas rate

Modeling Methane Emissions and Ventilation Needs by Examination of Mining Induced Permeability Changes and Related Damage to Ventilation Controls

Understanding methane emissions in underground coal mines is critical for a safe and productive mine. In addition to reasonable estimation of initial coalbed reservoir parameters, it is also crucial that changes in effective stress due to mining and pore pressure reduction are taken into account due to their effects on porosity and permeability. Primary parameters for estimation of emissions or modeling of the mining environment for this purpose are porosity and permeability which can change dramatically as a result of stress redistribution associated with mining and gas desorption from a large coal volume. These parameters affect the emission rates and ventilation requirements, as well as water inflow into the working environment. Stopping leakage, on the other hand, is a secondary stress dependent factor in estimation of emissions, as convergence of the roof and floor strata, compromising the integrity of the stopping, may result in leakage, making prediction of ventilation requirements difficult. This paper aims to examine the effects of porosity and permeability changes of the coal seam on methane emissions in an underground continuous miner section. The models were developed and executed in a dynamic fashion to simulate an advancing section. Through this process, the changes of effective stress in coal, particularly their change paths, on porosity and permeability were incorporated into the models and methane emissions, concentrations, air requirements, water inflow and possible leakage from the stoppings were investigated using a conventional coalbed methane reservoir model.2009892

A fully coupled hydro-mechanical model for the modeling of coalbed methane recovery

Most coal seams hold important quantities of methane which is recognized as a valuable energy resource. Coal reservoir is considered not conventional because methane is held adsorbed on the coal surface. Coal is naturally fractured, it is a dual-porosity system made of matrix blocks and cleats (i.e fractures). In general, cleats are initially water saturated with the hydrostatic pressure maintaining the gas adsorbed in the coal matrix. Production of coalbed methane (CBM) first requires the mobilization of water in the cleats to reduce the reservoir pressure. Changes of coal properties during methane production are a critical issue in coalbed methane recovery. Indeed, any change of the cleat network will likely translate into modifications of the reservoir permeability. This work consists in the formulation of a consistent hydro-mechanical model for the CBM production modeling. Due to the particular structure of coal, the model is based on a dual-continuum approach to enrich the macroscale with microscale considerations. Shape factors are employed to take into account the geometry of the matrix blocks in the mass exchange between matrix and fractures. The hydro-mechanical model is fully coupled. For example, it captures the sorption-induced volumetric strain or the dependence of permeability on fracture aperture, which evolves with the stress state. The model is implemented in the finite element code Lagamine and is used for the modeling of one production well. A synthetic reservoir and then a real production case are considered. To date, attention has focused on a series of parametric analyses that can highlight the influence of the production scenario or key parameters related to the reservoir

Numerical simulation of ground surface subsidence due to coal-bed methane extraction

Coal bed methane (CBM) has gained significant attention as a source of natural gas. CBM recovery is achieved through either primary production or enhanced CBM production, the later of which remains at an infant stage. Primary CBM extraction involves production of CBM reservoir fluids using production wells to facilitate pressure drawdown within the targeted formation. De-pressurization is required to release adsorbed methane within the interior surface of the coal matrix. However, de-pressurization can cause compaction within the CBM reservoir, especially in the vicinity of production wells. This, in turn, can lead to ground surface subsidence. The objective of this project is to develop a semi-analytical solution to explore ground surface subsidence above CBM extraction wells. To achieve this, an existing analytical solution, for ground surface subsidence above a cylindrical uniform pressure change, is extended to allow for a non-uniform pressure distribution using the principle of superposition. The non-uniform effective pressure to drive the semi-analytical solution for ground surface subsidence is derived from a numerical fluid flow model describing water and methane production from a CBM formation, also developed as part of this project. The numerical fluid flow model describes two-phase fluid flow (gas and water) in porous media in conjunction with non-equilibrium gas adsorption and stress dependent porosity and permeability. The resulting set of partial differential equations is solved using the method of lines by discretising in space using finite difference and then solving the resulting set of coupled non-linear ordinary differential equations (ODE) using MATLAB's ODE solver, ODE15s. The numerical fluid flow model was verified by comparison with published modeling results from the literature. As a further verification, the model's ability to simulate field production and pressure data was demonstrated using field data from a CBM case study in the US. The potential role of initial water saturation on ground surface subsidence was investigated by studying the associated spatial distributions of fluid pressure. It was found that, for a given time, the mean fluid pressure within the reservoir reduces with increasing initial water saturation. However, the spatial distribution of fluid pressure, for a given volume of produced gas, was found to be insensitive to initial water saturation. This can be attributed to the fact that the volume of water stored in the cleats of the coal-bed is very small as compared to the volume of gas stored within the coal matrix. Consequently, the presence of water in the cleats was found to have no influence on ground surface subsidence for a given gas production volume. It was also found that ground surface subsidence for a given gas production volume is insensitive to initial coal permeability and cleat volume compressibility. A simplified analytical solution for ground surface subsidence was derived assuming that the pressure distribution within the reservoir is uniform. Sensitivity analysis showed that the simplified analytical solution is effective at predicting ground surface subsidence for a given gas production volume, predicted by the numerical model, for all of the scenarios studied. This suggests that pressure distribution within a CBM reservoir is not important for determining ground surface subsidence in this context