A critical review of experimental and theoretical studies on shale geomechanical and deformation properties, fluid flow behavior, and coupled flow and geomechanics effects during production

Abstract

Flow in shale differs substantially from that in conventional reservoirs due to unfavorable reservoir features such as ultra-low permeability and very poor porosity. Shale also exhibits considerable anisotropy and heterogeneity, with clay laminae and bedding angle being the primary variables influencing anisotropy. As a result, shale is subjected to high stress sensitivity and deformation during depressurization, which affects fluid flow. Furthermore, non-Darcy flow mechanisms exist thereby making flow in shale a complex phenomenon. Nonetheless, many studies have recently focused on the geophysical and geomechanical characterization of shale. Various works have equally examined the complexity of fluid flow. Numerous studies were particularly interested in the influence of non-linear flow parameters and stress sensitivity on apparent permeability, intrinsic permeability, and porosity during pressure depletion. However, relatively few works, mostly theoretical, have been carried out on coupled flow and geomechanical reactions. This review thus includes a report on fluid flow and geomechanical characterization of shale formation, as well as an identification of the factors that influence rock deformation and fluid flow during production. The review showed that flow regimes are predominantly dependent on pore pressure and pore size, whereas flow regimes regulate apparent permeability. For example, at low pressures and pore radius less than 10 nm, flow regims were found to significantly increase the apparent permeability. However, at higher bulk modulus (>10 GPa), pore radius has essentially no impact, hence, pore pressure becomes the dominant factor influencing flow. In addition, the review shows that during depressurization, geophysical metrics are more sensitive to pressure changes than geomechanical properties. Finally, some results in literature revealed that the impact of geomechanical characteristics on cumulative production can be ignored in competent formations with high Young's modulus (about 6 × 10⁶-10 × 10⁶ psi). In conclusion, recovery from shale could be optimized by integrating experimental studies with hydromechanical models during initial reservoir studies.