Influence of Supercritical CO₂ Fracturing Fluid on the Permeability of Shale Reservoir and Mechanism Analysis

Reduction of the reservoir permeability during supercritical CO₂ fracturing caused significant reservoir damage, which directly affects the crude oil recovery rate. This investigation explored a novel thickener for supercritical CO₂ fracturing fluid, this CO₂ thickener not only effectively improves the viscosity and rheological properties of CO₂ fracturing fluid but also contributes to reduce reservoir damage and improve permeability. The research results indicated that the synthesized CO₂ thickener (3 wt %) can increase the apparent viscosity of supercritical CO₂ fracturing fluid to 7 mPa·s, and a 9% matrix permeability damage rate and a 0.5 mD permeability decrease value are shown in a 3% CO₂ fracturing fluid. However, 3 wt % of commercial CO₂ thickener only increases the apparent viscosity of supercritical CO₂ fracturing fluid to 3 mPa·s, while the reservoir damage rate increases to 13%. Two thickeners exhibit completely different damage capabilities to the reservoir, and the synthesized CO₂ thickener shows excellent characteristics of reducing reservoir permeability and is also conducive to protecting shale reservoirs. Furthermore, supercritical CO₂ fracturing fluid containing synthetic thickeners has better temperature and shear resistance compared with commercial thickeners. This may be because the synthesized thickener and the micro grid formed by supercritical CO₂ reduce the adsorption in shale crevices, but a large amount of commercial thickeners can adsorb on the surface of shale.