Abstract
Bulk geomechanical properties in depleting coal seam gas (CSG) reservoirs are crucial to engineering design related to wellbore stability, drilling, and completion strategy. In this paper, a finite element model is presented to systematically predict the changes in bulk coal properties as a consequence of changes in net effective stress. The model includes the effects of depletion-induced shrinkage, and the implications for mechanical integrity and transport properties in reservoirs are discussed. The approach for estimating the bulk geomechanical properties of coal is based on a finite element method and discrete fracture network (FEM-DFN) model capable of capturing both two- and three-dimensional phenomena. The methodology employees a unit volume of coal and systematically applies fracture networks of increasing complexity for which the bulk properties are estimated. The end goal of the larger project that frames this work is to model fracture networks parametrically within three classifications of ‘linear-2’, ‘blocky-3’, and ‘blocky-4’, and examine their variation from ‘base-1’ without fractures. A measure of fracture density is used in this current work to compare the results with existing correlations. In future work, coal cleat intensity (e.g., P21, P32) and dispersion will be used to generalise the results. The FEM-DFN model is used to calculate bulk coal properties, which provides an improved understanding of the behaviour of coal as a function of the net effective stress. The results found that the stiffness of coals decreases with the presence of fractures. Furthermore, the effect of 30%, 60%, and 100% depletion are described via modelling of the pore pressure variation, matrix shrinkage, and changes in fracture aperture/compressibility. In this current work, a depletion of 50% is set, with the sensitivity of this parameter set for future work. By numerically subjecting a unit volume of coal to controlled stresses the measured change in bulk properties including their directional dependence is highlighted. The knowledge of the variation in these properties is useful to the life-cycle decision-making in drilling and completions as well as reservoir modelling. The novelty of the proposed methodology is the use of advanced numerical techniques to systematically assess the bulk mechanical properties of coal with the inclusion of depletion effects and discrete fracture networks. In addition to life-cycle decision-making, stimulation design, the prediction of compaction and subsidence, and reservoir management can all benefit from the knowledge gained in this study.
