Shale Matrix Permeability Evolution During Reservoir Depletion: Fluid Dynamic and Poroelastic Aspects

Abstract

The evolution of reservoir permeability is of fundamental importance in reservoir engineering. However, many of the commonly used concepts in reservoir may not be directly applicable to unconventional reservoirs due to their extremely low (and highly anisotropic) permeability, extremely small pore throat sizes. Therefore, the transport processes of compressible micro- to meso-porous rocks such as shales require a critical revision of the classical concepts. Due to the higher compressibility of shale reservoirs as compared to conventional reservoirs, some processes have to be considered as coupled such as the transition from Darcy-flow to slip-flow and the stress sensitivity of the permeability to pore throat compressibility, which is a poroelastic effect. We also develop a detailed description of the coupling between slip-flow and the stress sensitivity in unconventional reservoirs, and interpret experimental observations in light of this description. We characterize the transport properties of shales in a manner that includes a zero-effective-stress permeability coefficient, a stress sensitivity coefficient, an effective stress coefficient and slippage as a function of effective stress. We model single-phase matrix gas permeability during reservoir depletion for the Eagle Ford and Marcellus shale. This case study shows the significant influence of slip-flow, which can lead up to a two times higher permeability when the pore pressure decline below 10 MPa. Furthermore, considering the coupling between slip flow and poroelasticity in the permeability model, the permeability can be 50% higher for selected samples.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015