Abstract
Long-term pore pressure depletion significantly alters reservoir stresses, which are known to have a substantial impact on permeability in fractured reservoirs. Increased effective stresses resulting from depletion often induce a decrease in permeability. The opposite has been observed in some reservoirs with an organic rock matrix that exhibits strong sorption-mechanical coupling. With depletion, adsorbed gas desorbs from micropores resulting in shrinkage of the rock matrix, relaxation of effective stresses, and opening of fractures. In addition, reservoir depletion results in an increased stress anisotropy, which may lead to potential reactivation of critically oriented natural fractures and shear failure. The objective of this study is to develop a reservoir simulator with a full poromechanical coupling accounting for sorption-induced change of stresses, shear failure, fines production, and their effect on permeability. This paper aims to estimate the influence of the various mechanical and transport parameters affecting reservoir permeability and to predict its evolution during reservoir depletion. We compare two natural gas reservoirs with strong (San Juan coal basin) and weak (Barnett shale formation) sorption-mechanical coupling. The results of the study highlight the interplay between mechanical moduli, swelling isotherm parameters, fracture compressibility, and rock strength in determining their impact on fracture permeability evolution during depletion. We show that simple stress-dependent permeability models cannot capture permeability evolution in the presence of shear failure and fines production. A modified permeability equation is introduced to describe fines migration and shear dilation. Numerical simulation confirmed that desorption-induced strains in shales may induce changes of horizontal stresses of several MPa. These changes of stress may have a minor effect on permeability but can significantly affect horizontal stress anisotropy and should be considered while planning refracturing.
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