Investigating the effects of stress creep and effective stress coefficient on stress-dependent permeability measurements of shale rock

Abstract Rock permeability is an essential factor for production prediction and economic evaluation, which is often measured based on core samples in laboratory. However, several phenomena such as stress creep and path-dependent stress still present difficulties in correctly measuring and interpreting permeability under varying compaction stresses for core measurements. In this study, an improved stress-dependent permeability model is proposed to consider the effect of time-dependent compaction behavior on permeability measurements by incorporating the stress creep mechanism. Besides, how to correctly interpret stress-dependent permeability results with effective stress coefficient is introduced in detail. A coupled flow-geomechanics model is also developed to investigate the impact of effective stress coefficient on permeability change and cumulative production. Due to the stress creep phenomenon, rock permeability could be significantly overestimated if not enough measurement time is provided. The creep strain is highly related to the content of clay and kerogen. This is because the increasing creep strain over time is mainly from the closure of pre-existed micro-fractures and the compression of Nano-pores inside the soft materials, such as clay and kerogen. The creep strain model and improved stress-dependent permeability model were validated by comparing with multiple experimental data. Effective stress coefficient cannot be simply assumed one for organic-rich or clay-rich shale reservoirs, especially when pore pressure is changed to alter net effective stress in permeability measurements. A large effective stress coefficient amplifies the sensitivity of pore pressure change on the variation of both effective stress and permeability. The simulation results show a larger effective stress coefficient could provide higher cumulative production because of stronger compression from effective stress. The improved knowledge of stress creep and effective stress coefficient enables the development of best practices for permeability measurement of tight rock and provides more accurate permeability results for reservoir management and field development.