Sealing Efficiency of Caprock in Fractured Tight Reservoirs Considering Multi-Migration Mechanism Simulated using Improved EDFM

Abstract

Abstract Comprehensive flow mechanisms play a crucial role in the transport and distribution of carbon dioxide in the subsurface during geological storage. However, limited researchers have developed models for evaluating the efficiency of the caprock while considering multiple transport mechanisms. Furthermore, most of the models established by scholars do not consider the influence of complex fractures. In this paper, a numerical model for CO2 storage in a composite caprock considering gas adsorption/desorption, diffusion, hydration reaction, and stress sensitivity of fractures is developed to evaluate the leakage risk of the caprock during CO2 storage in tight reservoirs. An improved Embedded Discrete Fracture Model (EDFM) is introduced to describe the transfer flow between matrix-fracture, fracture-fracture, and fracture-well. The effects of caprock permeability, CO2 storage rate, fracture number, and fracture half-length on CO2 storage efficiency are analyzed. The results show that decreasing the caprock permeability, CO2 storage rate, fracture half-length and fracture number can enhance caprock sealing efficiency and safety during long-term storage. Among them, the CO2 storage rate had the most significant influence on the security of the caprock layer for long-term storage, and the ratio of escape could reach 29.20% after 900 years of injection at 60t/d, while that of 20t/d was only 2.62%. The permeability of the caprock have a more significant influence on the security of long-term storage, and the escape ratio of high caprock permeability (690nD) can reach 11.52%. The fracture half-length and the number of fractures have less influence on the security of the caprock for long-term storage, and the escape ratio remains between 6.50% and 7.30%. Hence, this work provides a theoretical basis for the safety evaluation of CO2 geological storage with time.