Study on the ratio of pore-pressure/stress changes during fluid injection and its implications for CO2 geologic storage

The success of fluid injection into geological formations, which is the main operation during both carbon dioxide (CO2) geologic storage and wastewater injection, is contingent on the geomechanical integrity of the site. A key task that allows us to evaluate the risk of geomechanical failure is the precise prediction of pore-pressure buildup and subsequent change in the state of stresses during and after the fluid injection. Contrary to traditional approaches, where total stresses are assumed to remain constant, recent studies have ascertained that total stresses in fact change in every direction as fluid extraction/injection disturbs the pore-pressure field and causes deformations. In this study, we conduct an in-depth investigation of the ratio of change in total stress to that in pore-pressure, ∆σ/∆P, which has been denoted in the literature as the pore-pressure/stress coupling. We employ a numerical simulation method that couples single-phase fluid flow in porous media with poroelasticity to explore the spatiotemporal evolution of the ∆σ/∆P ratio for various conditions. These numerical experiments allow us to examine how different material properties and structural geometries would influence the evolution of ∆σ/∆P in both vertical and horizontal directions. These ratios of pore-pressure/stress changes exhibit different spatiotemporal evolutions depending on key factors that include the hydraulic boundary condition, Biot's coefficient, Poisson's ratio, and the hydraulic diffusivity of both the injection zone and caprock. On the basis of observations, we suggest firsthand guidelines for analytically determining the ratio of pore-pressure/stress changes, ∆σ/∆P. Finally, we use examples and case studies to illustrate how the ∆σ/∆P ratio can be incorporated into an analytic calculation for determining a maximum sustainable pressure limit.