Experimental Investigation of Crack Propagation in High Carbonate Shale

In recent times, significant advances have been made in the exploration and exploitation of Permian shale gas reservoirs within the Sichuan Basin, China, resulting in commendable gas production rates. However, challenges have emerged during field tests of Permian shale gas fracturing, particularly in relation to high fracturing pressures and early-stage fracture initiation complexities. Mineralogical assessments have revealed a pronounced elevation in carbonate content within Permian shale formations in comparison to their Ordovician-Silurian counterparts. Presently, there exists a conspicuous dearth of scholarly inquiry into the crack propagation mechanisms operative within high-carbonate shale matrices. This study endeavors to bridge this knowledge gap by undertaking a comprehensive investigation utilizing shale samples exhibiting varying carbonate concentrations through true triaxial fracturing physical model experiments. By meticulously analyzing fracture morphological characteristics and pressure profiles, we are able to discern the nuanced attributes of fracture initiation and expansion across diverse in-situ stress regimes and operational parameters, including viscosity, pumping rates, and acid pretreatment. Our findings serve to elucidate the multifaceted factors governing fracture expansion within high carbonate content shale formations, thereby enhancing the holistic understanding of shale gas extraction dynamics. 1. INTRODUCTION The Sichuan Basin and its surrounding regions host a variety of organic-rich shale formations, including those from the Cambrian, Ordovician-Silurian, and Permian periods. These regions have emerged as pivotal locales for shale gas extraction in China (Bao et al., 2023). Notably, the Permian strata within the basin serve as crucial hydrocarbon source rocks for the Puguang and Yuanba gas fields. Recent endeavors have witnessed a notable breakthrough with the attainment of high-yield shale gas flows, marking a significant milestone in the exploration and exploitation of deep-seated Permian shale gas resources. However, unlike their counterparts in shallower strata, Permian shale formations exhibit a substantial carbonate mineral content, averaging 33.9%. Consequently, the tensile strength and fracture toughness of the rock are significantly elevated, posing considerable challenges to the initiation and propagation of shale fractures. Currently, operational strategies primarily are derived from fracturing construction parameters that have been tailored for medium and shallow shale reservoirs. However, a transformational technology specifically addressing the unique attributes of high-carbonate shale reservoirs remains elusive.