Experimental Study on Tensile Damage Localization Evolution of Shale After CO2-Brine Treatment

ABSTRACT CO2-brine corrodes the shale structure and damages mechanical properties in the Carbon Capture and Sequestration (CCS), potentially leading to the instability of reservoirs. In this study, Brazilian tests are conducted to analyze the tensile strain localization of brine and CO2-brine treated (30MPa, 110°C, 30 days) shale specimens by DIC (Digital Image Correlation), and tensile failure processes are compared. Moreover, the relationship between the tensile strain localization and the microscopic damage has been established. Results show that: (1) The initiation stress of strain localization decreases, and the heterogeneity of the strain field increases at peak stress for homogenous specimens. The initiation stress of strain localization for specimens loaded vertically to bedding planes is reduced, specimens loaded horizontally to bedding planes have two initiation points. (2) The role of the bedding on tensile strain localization is enhanced. The tensile strain is localized more significantly on the bedding after initiation, resulting in the fracture initiation along bedding planes; multiple parallel fractures are generated for specimens loaded horizontally to bedding planes. (3) The microscopic damage of laminated and homogenous shale is different in origin. Oriented micro-fractures are induced at the bedding of laminated shale, while slight-apertured, short, and disorderly distributed micro-fractures are developed in the homogenous shale. It is fundamental reason that the variation of tensile strain localization of CO2-brine treated laminated shale is more significant than that of CO2-brine treated homogenous shale. INTRODUCTION In the Carbon Capture and Sequestration (CCS) engineering of shale reservoirs, CO2 and fluid produce complex physicochemical interactions on reservoir rock minerals (Zhou & Zhang, 2016), new pore and fracture structures are formed on the micro-scale of rocks, and macro-mechanical properties (elastic modules, strength, fracture toughness, etc.) are degraded (Lyu et al., 2020; Bhuiyan et al., 2020; Tang et al., 2020). Due to continuous fluid migration in the rock seepage channel in the process of CCS, the pore pressure field and effective stress field have a long-term heterogeneous evolution. Such development may lead to the expansion of the original rock fractures and the initiation of new fractures. The long-term accumulation of damage may lead to the formation of macro fractures, resulting in formation instability and CO2 escape, and impacts CCS engineering safety (Han et al., 2020).