Discrete element modeling modeling of fault mechanics and permeability evolution for gas production and storage: Effect of shale content and distribution

The presence of clay in fault zones has a dramatic effect on both fault mechanics and permeability. Quantification of this effect relies on small scale laboratory experiments or indirect observations from hydrocarbon fields, for which systematic variation of clay content, clay distribution, and deformation conditions can be problematic. Two-dimensional discrete element models of fault zones were deformed in simple shear to assess the effect of shale on failure criteria and sealing capacity using a systematic series of model simulations. Comparison between the evolution of shear stress and development of microcracks shows that changes in shear stress are correlated with alignment of microcracks, resulting in fractures with similar orientations as R- and Y-shears that can accommodate local deformation. Both shale content and distribution have a marked effect on the failure criteria of all fault models, i.e. with increasing volumetric shale content cohesion decreases up to a factor ~20 and friction coefficients drops below ~0.1 for some distributions. A tradeoff between permeability reduction due to the presence of low permeable clay and permeability increase due to local failure of fault rock determines the location of spill points and integrity of fault seals during gas storag