Along with the recent development in unconventional resource extraction techniques, geomechanics has become an important discipline to help us understand and assess the risk created by reservoir stimulation and reservoir performance optimization. At the AGS, we use a finite element modelling technique to study the responses of subsurface structures to external stimulation in various geomechanical/geological situations.
Thermal recovery of highly viscous bitumen usually requires the injection of high-temperature, high-pressure steam into the reservoir. During the injection phase, significantly increasing temperatures and pore pressures cause volumetric dilation of the reservoir due to thermal and poro-elastic effects. This volumetric change stresses the overlying geological formations and results in surface heave. The containment of the reservoir fluids consequently depends on the geomechanical properties of the reservoir and surrounding rock. We analyze the response of the overburden to steam-assisted gravity drainage (SAGD) operation in different hypothetical, but geologically reasonable, scenarios using the finite difference method (FDM) for reservoir heat and fluid flow simulation and finite element method (FEM) for geomechanical modelling.
Researchers hypothesize that fluid injected into the subsurface can flow into faults and, in certain situations, lead to fault slipping by reducing effective confining stress and shear friction. We build a fully coupled hydromechanical finite element model with a hypothetical fault to study fault slipping in response to the fluid flow in the fault. Shear friction stress, contact pressure, pore pressure propagation, and slipping distance are carefully investigated to provide an understanding of fault movement and in situ stress perturbation in response to subsurface fluid injection.
Animation 1: Temperature change in a reservoir during SAGD operation over a ten-year production period. In a SAGD pad with 10 pairs of injection and production wells, the injected high-temperature, high-pressure steam produces a steam chamber that grows over time, leading to subsurface structure deformation and surface heave. To better illustrate the process, the deformation is exaggerated 200 times in this animation. In this animation, red indicates higher temperatures.
Animation 2: Pore pressure change in a reservoir during SAGD operation. The injected steam can increase pore pressure in the reservoir. Combined with thermal expansion, high-pressure steam results in less confining stress and expansion of the reservoir due to poro-elastic effects. In this animation, red indicates higher pore pressure.
Animation 3: Fault sliding in response to fluid injection. The fluid injected into the subsurface occasionally finds its way to a critically stressed fault. The increasing pore pressure reduces the shear friction force and results in fault slipping. In this animation, red and blue indicate structure movement (along the fault) in opposite directions.