Development and application of a supercritical CO₂ pulsed fracturing-seepage coupling test system

Objective To effectively enhance the extraction effects of coalbed methane (CBM), this study proposed a novel approach for fracturing coals (rocks) using pulsation of supercritical carbon dioxide (CO₂) by combining pulsed fracturing with supercritical CO₂ fracturing. Methods A true triaxial test system coupling supercritical CO₂ pulsed fracturing with seepage was independently developed, and its primary structure, characteristics, and functions were elucidated. Then, the supercritical CO₂ pulsed fracturing - seepage/acoustic emission monitoring tests under true triaxial conditions were conducted in the laboratory. The test system, controlling the triaxial stress by combining an independent servo system with a central digital system and achieving pulsed pressurization using a dual pump with constant speed and pressure, features high precision, true triaxiality, and resistance to high temperature, pressure, and stress and covers the whole process. Results and Conclusions The results indicate that the true triaxial test system that couples supercritical CO₂ pulsed fracturing with seepage can achieve effective pulsed fracturing. Using supercritical CO₂ pulsed fracturing, the permeability of coals can increase by two to nine times, manifesting a pronounced exponential growth. During the acoustic emission - supercritical CO₂ pulsed fracturing, new fracture channels were formed in coals. These channels penetrated directly from the center of the fractured pores to the coal surface, with supercritical CO₂ being observed ejecting directly from coals. Therefore, supercritical CO₂ pulsed fracturing plays a certain role in propagating and connecting fractures, effectively improving CBM extraction. The findings of this study provide certain test support for reinforcing the permeability enhancement technology of deep, low-permeability coal seams and offer practical guidance for field CO₂ fracturing engineering.