Effects of Supercritical CO2-Brine/shale Interaction on Fracturing Behavior

ABSTRACT As a caprock for CO2 geological sequestration, the mechanical properties of shale may change significantly from the long-term CO2-fluid interaction. To study the long-term effects of supercritical CO2-brine interaction on the mechanical properties of shale, the marine LMX shale was treated with supercritical CO2-brine at temperature of 110 °C and pressure of 30 MPa for 15, 30, and 60 days, respectively. The Brazilian tests combined with Acoustic Emission (AE) monitoring were conducted on shale before and after the treatments. The results show that: (1) Compared with the dry shale, the tensile strength of shale saturated with supercritical CO2-brine for 15, 30, and 60 days decreases by 40%, 48.6%, and 68.8%, respectively. The fracture morphology changes from a straightly single fracture to a fracture band consisting of two to four curved fractures; (2) The effects of supercritical CO2-brine interaction on the initiation and damage stresses are quantitatively analyzed. Taking the initiation and damage stresses of dry shale as the reference, the initiation stresses of shale saturated with supercritical CO2-brine for 15, 30, and 60 days decrease by 17.7%, 37.8%, and 48.4%; the damage stresses decrease by 35.5%, 58.3%, and 75.8%; (3) The AE characteristics of shale are closely related to fracture modes. The tensile fracturing of shale is sudden, manifested as an obvious step in the cumulative AE energy, while the shear fracturing of shale is gradual. With the supercritical CO2-brine treatment time, the fracture mode gradually changes from tensile-dominated to tensile-shear mixed failure mode, which can be inferred from that the slight step increases significantly and the ratio of tensile to shear events decreases from 10.7:1 to 0.8:1. The mechanical properties of shale weaken with the increase of supercritical CO2-brine treatment time. INTRODUCTION As one of major greenhouse gases, excessive CO2 exacerbates global climate change. CO2 geological sequestration is one of the important means to mitigate the greenhouse effect by injecting large amounts of CO2 into deep formation (Bachu et al., 2007; Wood, 2015; Jia et al., 2019; Nie et al., 2022). The deep shale formation has low porosity, low permeability, with beddings, which can effectively prevent and isolate the upward migration and leakage of CO2, which is suitable for the target formation of CO2 geological sequestration (Kalantari-Dahaghi, 2010; Lyu et al., 2021; Xie and Economides, 2009; Liu et al., 2013; Chen et al., 2015; Chen et al., 2023; Zhang et al., 2021). CO2-shale-fluid interaction will significantly affect the mechanical properties of shale under the condition of high temperature and high pressure for long-term CO2 geological sequestration (Zou et al., 2018; Al-Ameri et al., 2016; Choi and Song, 2012; Zhou et al., 2022).