Study on geomechanical stability of the aquifer-caprock system during CO_2 sequestration by coupled hydromechanical modelling

摘要

Carbon dioxide (CO_2) sequestration in deep aquifers is one of the options for reducing the emission of greenhouse gases to the atmosphere. It is efficient to inject super critical CO_2 into an aquifer if the geomechanical stability of the aquifer-caprock system can be maintained during the injection. An axisymmetric horizontal aquifer-caprock system is modeled including hydromechanical coupling in order to study influence of various factors - such as injection rate, permeability and stiffness of the caprock and aquifer - on the stability and especially the integrity of the caprock. Here the local safety factor is considered as an index of caprock integrity. It is shown that the effective stress state of the caprock close to the injection well could become critical for shear failure quite early in the injection. A buildup of gas pressure in the aquifer causes caprock deformation, inducing an increase in deviatoric stress within the caprock. Larger injection rates and lower aquifer permeability amplify such phenomena, increasing the risk of losing caprock integrity. Increased aquifer stiffness, in contrast, restricts the caprock deformation, and lowers the risk.