Experimental Study on CO2 Fracturing of Continental Shale with Dense Laminas

ABSTRACT: Using CO2 to increase shale oil production is a practical requirement to ensure energy security and reduce carbon emissions. Therefore, studying the fracture propagation mechanics of CO2 fracturing is of great significance. This article applies a triaxial fracturing simulation system combined with a CO2 control system to perform supercritical CO2 (SC-CO2) fracturing, CO2 aqueous solution (CO2 aqs) fracturing, slick water fracturing, and gel fracturing on continental shale. The results indicate that CO2 fracturing has the effect of reducing breakdown pressure, and the breakdown pressure of supercritical CO2 fracturing and CO2 aqs pre-immersion fracturing is lower than that of slick water fracturing. The artificial fractures induced by low-viscosity slick water fracturing in shale with dense laminas are easily inhibited by the laminas, and it is difficult to propagate in the direction of fracture height. The ultra-low viscosity and strong diffusivity of SC-CO2 lead to multiple laminas and micro-fractures being activated, and the main fracture is not obvious. The main fracture width of CO2 aqs pre-immersion fracturing is relatively large, the fracture propagation path is tortuous, and multiple laminas are activated. The complexity of the fracture induced by CO2 aqs solution is 75% higher than that induced by slick water. The effect of CO2-water-rock interaction on fracture propagation cannot be ignored. In addition, there is a significant positive correlation between the complexity coefficient of fractures and acoustic emission shear activity ratio. 1. INTRODUCTION The success of the United States in the efficient development of marine shale oil has sparked a wave of exploration and development of unconventional oil and gas resources worldwide (Zou et al. 2020). Under the tectonic background of the foreland basin, the Jurassic in the Sichuan Basin of China is located in a sedimentary environment of deep to semi-deep lakes, forming a large and stable set of dark shale interbedded with carbonate or siltstone formations, laying an important material foundation for the formation and enrichment of shale oil (He et al. 2022). However, the strong heterogeneity, complex lithology, and dense laminas of continental shale pose challenges for fracturing development (Guo et. 2023). To improve the effect of reservoir stimulation, CO2 fracturing technology was explored and applied in the oilfield (Li et al. 2020). CO2 as a fracturing fluid can effectively reduce formation damage and increase formation pressure, and help reduce greenhouse gas emissions (Li et al. 2022). Therefore, studying the fracture propagation mechanics of CO2 fracturing is of great significance.