Geomechanical Aspects of CO2 Storage under Fracturing and Thermal Conditions

Abstract

One of the most important concerns with respect to the long term CO2 storage is that stress changes caused by injection could lead to spontaneous fracturing or reactivation of fracture networks which could potentially provide pathways for CO2 migration through previously impermeable rocks. For a rigorous study of the process, a coupled flow and geomechanical model of the formation should be employed and must be improved for the purpose of accurately predicting the location and shape of an induced fracture. The main purpose of this work is to i) improve the modeling technique for fracture propagation within a coupled flow and geomechanical model, ii) model and design injection schemes for CO2 sequestration projects considering the associated geomechanical mechanisms, for different geological settings, and iii) investigate the caprock integrity under upward fracture propagation with this model. Studying the thermal issues of CO2 injection and the effects of injection temperature on fracture pressure, injectivity and the pace of fracture propagation is also included in this work. Our results show that injection above the fracture pressure will have the potential to increase the well injectivity but also the possibility of fracturing the caprock. The degree of vertical propagation will strongly depend on the caprock stress state, mechanical properties and temperature changes. Injection capacity with cold CO2 injection could be significantly lower than expected, and it may be impractical to avoid induced fracture development. At small enough injection rates, fracture propagation is controlled primarily by the injection temperature, and is accelerated as injection temperature decreases. As a result, spontaneous fracturing is expected to take place in most CO2 storage projects in vertical wells with injection temperature below reservoir temperature, unless the injection rates are impractically low. This is an important finding which will also have consequences for caprock integrity. The final contribution of this work is the development of a method for improving the estimation of fracture length and development of a simplified algorithm capable of tracking the position of fracture tip on a sub-grid scale. The proposed simplified method improves the prediction of the fracture length based on estimation of tip position in a fixed grid block.