Critical Parameters for Caprock Tensile Failure Induced by CO2 Injection Into Aquifers

Summary CO2 geological storage, where supercritical CO2 is injected into underground porous formations for long-term sequestration, is one of several mitigation strategies that have been proposed to control the level of CO2 in the atmosphere. CO2 can be injected into several formation types; deep saline aquifers are the most promising mainly because of their large storage capacity. However, safe and effective CO2 geological storage requires an understanding of the potential failure mechanisms that may lead to a loss of containment. In fact, injection of a fluid into an aquifer increases pore pressure above the initial value and this may trigger unwanted geomechanical effects such as fault reactivation or microseismicity. Additionally, the caprock overlaying the aquifer must be able to withstand the expected pressure increase without fracturing. While previous work has explored the mechanism underlying caprock tensile fracturing, an investigation was carried out to understand which parameters are more susceptible to trigger this mode of failure. This study presents comprehensive sensitivity analyses to highlight the relative contribution of different parameters to caprock tensile fracturing. A simple quasi-2D model comprising an aquifer, caprock and monitoring layer was first set up using the compositional reservoir simulator GEM. A total of 22 parameters among aquifer properties, caprock properties, simulation options and injection parameters were then selected as input variables, and simulations were run by varying one parameter at a time. The target variable was chosen as the maximum CO2 mass that could be injected before tensile fractures start opening in the caprock. Findings were then summarized in a tornado plot. For the range of values considered, the aquifer matrix permeability, caprock Young modulus and number of injection wells were found to have the highest impact on the target variable. Furthermore, results showed that higher quantities of stored CO2 can be achieved without caprock tensile failure in systems where the aquifer has a high Young modulus and Poisson ratio and the caprock has a low Young modulus and Poisson ratio. While this research focused on a simple model, these findings can help refine the current criteria for the selection of aquifers suitable for CO2 geological storage