Experimental Study on Shale Mechanical Properties Interacted with Supercritical Carbon Dioxide and Brine

Abstract

ABSTRACT: Supercritical carbon dioxide (SC-CO2) can be utilized to displace methane in shale gas reservoirs due to its low viscosity, high diffusivity, and high density. However, injection of SC-CO2 can cause a series of physical and chemical interactions with the pore brine and shale rock, leading to irreversible changes in the mechanical properties of the shale. To investigate changes in the mechanical properties of the shale that interacted with SC-CO2 and brine, Longmaxi shale samples were soaked in pure SC-CO2 and SC-CO2 + brine for seven days at 50 ° C and 15MPa. Uniaxial and triaxial compression tests were performed on the soaked samples, and acoustic emission was monitored during the loading process. The results indicated that the Longmaxi shale exhibited a more pronounced reduction in strength under the SC-CO2+brine condition compared to the pure SC-CO2 condition. In the loading process, the acoustic emission signals exhibit a distinct staged characteristic, and the signals are generated primarily during the unstable crack expansion stage. The observed increase in the fractal dimension of the signals at the same circumferential pressure indicates that the shale damage is multifaceted, resulting in a reduction in the stability of the shale. This study may help to elucidate the alterations in the mechanical properties of shale as a consequence of its interaction with SC-CO2. Furthermore, the findings may provide a foundation for the recovery of shale gas using CO2 displacement. 1. INTRODUCTION With the rapid growth of the global economy and the increasing level of industrialization, the supply of traditional fossil energy has gradually failed to meet the energy needs of today's society (Zou et al., 2016). Shale gas, as one of the most important unconventional oil and gas resources, has attracted a lot of attention because of its abundant reserves, cleanliness, low cost, and long supply cycle (Zhou and Zhang, 2020). Shale reservoirs are typically characterized by low porosity, low permeability, and high clay content (Jin et al., 2012), so hydraulic fracturing is commonly used to enhance shale gas production (Daneshy, 2010). However, it can result in significant water loss, damage to the physical properties of the reservoir, and contamination of surface water with back-draining fluids (Estrada and Bhamidimarri, 2016; Yang et al., 2019; DiStefano et al., 2019). Supercritical carbon dioxide (SC-CO2) is often used to enhance shale gas recovery due to its low viscosity, low surface tension, and high extraction capacity in its supercritical state (Adwani, 2007; Wang et al., 2011; Yang et al., 2008). This method has several advantages, including no damage to the reservoir, high efficiency of jet breakthrough, and the ability to lower the viscosity of crude oil and displace shale gas (Lu et al., 2021; Zhao et al., 2021).