Numerical Simulation of CO2 Storage in the Soft Sediments of Depleted Reservoirs in the Gulf of Mexico

Abstract

Abstract Depleted oil and gas reservoirs are among the most popular formations for large-scale geologic CO2 storage. While extensive research and characterization have been conducted on the geological structure and physical properties of many of these fields, the lack of field and core sample data related to reservoir flow and mechanical properties has hindered the assessment and modeling of CO2 storage, especially in offshore areas. In this study, we developed a coupled flow-geomechanical numerical model to evaluate the potential for storing CO2 in the soft sediments of the West Delta field of the Gulf of Mexico. The poroelastic parameters of the reservoir rocks were measured in the laboratory and the geological model of the reservoir was constructed based on the available seismic and well logs. The process of reservoir depletion was simulated to consider the effect of porosity reduction and permeability change. Pore volume multipliers were employed at the boundary to reach realistic pressure in the reservoir after depletion. A modified Cam-Clay model was used as the mechanical failure criterion to trace the porosity and formation subsidence. The results showed that the porosity reduced somewhat during the depletion and then increased during the CO2 injection. The porosity, however, did not fully recover after the injection process was completed. The maximum subsidence of the storage formation top reached almost 24 cm at the well location once the depletion completed. The pressure buildup at fault zones varied based on the vicinity of faults to the injection well, fault dip, dip direction, and CO2 injection rate. Sensitivity analysis using Fault Slip Potential (FSP) showed that various geomechanical parameters such as SHmax orientation, friction coefficient and dip of the fault influenced the probability of fault slip markedly. Assuming that faults are sealed, the pressure perturbation required to slip one of the major faults in the reservoir volume studied was lower than the calculated pressure buildup after the CO2 injection. The other two major faults are not predicted to have the potential for activation in any scenario. Therefore, it is advisable to conduct additional mechanical characterization particularly focusing on in-situ stress orientation and the transmissibility of faults as further work. This precautionary measure aims to mitigate the risk of fault activation during or after the injection phase.