Influence of Supercritical CO 2 Fracturing Fluid on the Permeability of Shale Reservoir and Mechanism Analysis

Reduction of the reservoir permeability during supercritical CO fracturing caused significant reservoir damage, which directly affects the crude oil recovery rate. This investigation explored a novel thickener for supercritical CO fracturing fluid, this CO thickener not only effectively improves the viscosity and rheological properties of CO fracturing fluid but also contributes to reduce reservoir damage and improve permeability. The research results indicated that the synthesized CO thickener (3 wt %) can increase the apparent viscosity of supercritical CO 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% CO fracturing fluid. However, 3 wt % of commercial CO thickener only increases the apparent viscosity of supercritical CO 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 CO thickener shows excellent characteristics of reducing reservoir permeability and is also conducive to protecting shale reservoirs. Furthermore, supercritical CO 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 CO reduce the adsorption in shale crevices, but a large amount of commercial thickeners can adsorb on the surface of shale.