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.