Stability Study of Tight Sandstone Reservoirs During CO2 Dynamic Storage Process
Authors: He Yuting , Liu Yuetian , Wang Jingru , Li Jingpeng , Zhang Bo , Chai Rukuan , Fan Pingtian 
Published: August 2024

ABSTRACT:
The tight sandstone reservoirs present a viable option for geological CO2 storage. The stability and safety of the reservoir rock are fundamental for long-term CO2 storage. However, the injection may lead to interactions between H2O, CO2, and rock, potentially impacting the mineral composition and physical properties of the reservoir. This ultimately leads to a decrease in the mechanical properties of the rock and disrupts the stability of the reservoir in high-pressure environments. In this study, rock mechanics experiments, nuclear magnetic resonance (NMR) experiments, and scanning electron microscopy (SEM) experiments were carried out to investigate the changes in mechanical properties, pore size, and microstructure of cores with different mineral compositions treated with aqueous carbonate solution under different confining pressures, in order to analyze the effects of the dynamic CO2 storage process on reservoir stability. The results show that the presence of CO2 under confining pressure leads to a decrease in rock strength. Carbonic acid solution can effectively dissolve feldspars and clay minerals in the rock composition, causing changes in pore structure and leading to a decrease in rock strength. The study aims to provide insights and recommendations for developing oil and gas extraction and carbon sequestration.

1. INTRODUCTION
Carbon dioxide geological storage technology is one of the effective ways to mitigate the global warming trend, and its basic principle is to inject captured carbon dioxide into deep underground rock pores for long-term isolation and fixation (Chen et al., 2023; Farquhar et al., 2015). Among the many potential storage media, tight sandstone reservoirs have received great attention from scientists and industry for their complex pore structure and high storage capacity (Bachu et al., 1994; Michael et al., 2009). However, CO2 injection affects the porosity, permeability, and mechanical properties of reservoirs, which may cause reservoir damage and reduced storage efficiency (Yu et al., 2012). For example, CO2 reacts chemically with minerals in the rock, which may lead to dissolution or precipitation of minerals, and thus change the physical structure of the rock (Fischer et al., 2010). The physical structure of the rock may be altered. Therefore, the mechanical response of rocks during CO2 injection must be thoroughly studied to ensure the safety and efficiency of the storage process. In recent years, in the field of CO2 geological storage research, scholars have carried out a large number of experimental and simulation studies, aiming to explore the mechanism of CO2 interaction with reservoir rocks and trying to predict and optimize the reservoir performance (Vafaie et al., 2023). The CO2 has different dissolution capacities for different minerals, leading to changes in the microstructure and mechanical properties of mineral surfaces, which indirectly leads to the decline of rock mechanical properties, thus affecting the production of unconventional oil and gas and reservoir stability (Vogt et al., 2014; Zhang et al., 2020; Zhang et al., 2022). Among various minerals, carbonate minerals (e.g., calcite and dolomite) have the highest solubility and the fastest reaction rate, while silicate minerals such as feldspar and quartz have lower solubility and slower reaction rate (Memon et al., 2022; Ranjith et al., 2019; Xing et al., 2019). Under the combined effect of CO2, the compressive strength and modulus of elasticity of the rock are significantly reduced (Huang et al., 2020; Rathnaweera et al., 2015; Zhou et al., 2024). The geological sequestration of carbon dioxide is a dynamic process, with the continuous injection of fluids, the formation pressure gradually increases. In fractured and low-permeability formations, the physical properties of rocks are affected by the increase in pore pressure, and their physical properties change significantly, which affects reservoir capacity (Dávila et al., 2017; Pavan & Govindarajan, 2023; Vialle & Vanorio, 2011). Zhao et al. carried out CO2 in brine solution saturated dense sandstone reservoir core samples for oil repulsion and explained the increase in porosity and permeability due to the increase in pore pressure (Zhao et al., 2021). Liteanu et al. investigated the effect of supercritical CO2 on rock mechanics, where carbonic acid solution led to stress corrosion fracturing and strength reduction of rocks (Liteanu et al., 2013).