The Mechanical Response and Sealing Characteristics of Caprock During CO2 Injection in Depleted Reservoirs
Authors: Ren Qianqian , Yang Bing , Huang Hai , Chen Junbin , Zheng Yong , Zhou Dongqiang 
Published: August 2024

ABSTRACT:
Carbon Capture and Storage (CCS) is globally recognized as the most effective method for achieving the ‘dual-carbon’ goals. The depleted oil reservoirs in the Ordos Basin of China have significant potential for carbon storage. In the process of CO2 geological burial, caprock, serving as a natural barrier for CO2, is crucial element in preventing CO2 from escaping to the upper layers and ensuring the safety of carbon sequestration. After CO2 injection into the reservoir, it affects the mechanical properties of the caprock and alters it sealing ability. Therefore, it is necessary to conduct relevant research to explore the evolution characteristics of the mechanical properties and sealing ability of the caprock during CO2 injection. This paper has conducted experiments on the evolution of the mechanical property of caprock before and after CO2 exposure. Furthermore, it delves into the research on the breakthrough pressure of CO2 in the caprock, aiming to determine the CO2 sealing characteristics of caprock. The results indicate that CO2 injection causes a slight change in the mechanical properties of the caprock. Compressive and tensile strength show a decreasing trend, and the elastic modulus experiences a decrease, which mainly caused by partial dissolution of mineral and alterations in rock pore structure observed by SEM. The breakthrough pressure of CO2 in target caprock range from 2.08 to 10.27 MPa. With increasing permeability, the CO2 breakthrough pressure significantly reduces, while CO2 breakthrough pressure averagely decreases by 9.7% under SC-CO2 exposure. The above research showed that CO2 injection will not only cause changes in the mechanical properties and a decrease in breakthrough pressure for caprock. This study can provide theoretical guidance for carbon sequestration in depleted oil reservoirs and the sealing feasibility analysis of the caprock against CO2.

1. INTRODUCTON
Carbon capture and storage (CCS) is widely recognized as one of the main pathways for reducing carbon emissions, playing a crucial role in supporting China's achievement of its "dual carbon" goals. The mature development and scaled application of CO2 storage technology are of strategic importance [1-4]. CO2 storage primarily involves geological structures such as depleted/exhausted oil and gas reservoirs, deep saline aquifers, and unmineable coal seams [5-7]. Depleted/exhausted oil and gas reservoirs, due to their well-developed geological structures and infrastructure, make CO2 storage more straightforward and economically viable. Additionally, injecting CO2 into depleted/exhausted oil and gas reservoirs not only helps maintain the integrity of the reservoir's pressure system but also holds potential for enhancing the displacement of residual oil. Therefore, utilizing depleted/exhausted oil and gas reservoirs for carbon storage is not only significant in terms of economic prospects but also presents positive development [8, 9].