Optimization of CO2 Storage and Leakage through Caprock Fracturing using Coupled Flow-Geomechanics-Fracturing Simulation

Abstract

Summary Geologic CO2 storage (GCS) has been recognized as one of the major mitigation strategies for reducing atmospheric CO2 emission. During CO2 injection into the storage reservoirs, pressure increase induces stress changes in and around the injection reservoir, which may cause various geomechanical hazards such as caprock failure, ground surface uplifting, and induced seismicity. Caprock failure could result in creating a leakage pathway for CO2 to leak from the storage reservoir to shallower aquifers and even to the atmosphere. Prior studies related to CO2 storage and leakage have focused exclusively on the issues of modeling these processes. However, there is little investigation into how caprock fracturing and associated leakage may affect the storage performance at a site. As a result, there is limited knowledge on how to control injection wells to optimize storage in presence of such risks. We focus on the CO2 storage optimization and leakage mitigation problem due to injection-induced changes in pressure and stress. The objective is to minimize the CO2 leakage through caprock fracturing given a fixed amount of total CO2 injection. To this end, we develop a workflow for optimizing CO2 storage under the leakage risk by incorporating coupled flow-geomechanics-fracturing simulation into a gradient-based optimization algorithm. We build a heterogeneous 3D coupled multiphase flow and geomechanics model for CO2 injection using the CMG GEM package, where the Barton-Bandis Model is used to model the fracturing of caprock during injection period. We quantify the CO2 leakage metric as the total amount of CO2 leaking out of the injection reservoir into shallower aquifers through fractures in the caprock caused by pore pressure increase. An optimization formulation is established to minimize the total CO2 leakage amount by finding optimal well control schedules with a fixed amount of total CO2 injection. We show that flow-only simulation has limited application to the caprock fracturing problem. For a fixed total CO2 injection into an aquifer with heterogenous flow properties, the proposed optimization algorithm finds the injection schedule that distributes the pressure in the reservoir to minimize the risk of CO2 leakage. While a flow-only modeling approach cannot be used for injection optimization under leakage risk, the coupled modeling approach can model the fracturing process and estimate the corresponding leakage rates. The use of coupled flow-geomechanics simulation enables CO2 leakage modeling and provides a tool for optimization of storage efficiency while minimizing the risk of caprock integrity failure and CO2 leakage.