Pore structure evolution of anthracite modified by CO2 foam fracturing fluid under different temperature and pressure conditions

Abstract

CO₂ foam fracturing technology has significant potential for enhancing coalbed methane (CBM) recovery. Intrusion of the CO₂ foam fracturing fluid into coal seams alters coal pore and fracture structures, thereby influencing CBM recovery. However, coal pore structural evolution modified by CO₂ foam fracturing fluid under varying reservoir temperatures and injection pressures remains unclear. To address this, a self-constructed high-temperature and high-pressure reactor was used to modify anthracite under different temperature (30–60℃) and pressure (3–6 MPa) conditions for 15 h. The pore structure and mineral composition of the modified anthracite were subsequently analyzed using low-temperature nitrogen adsorption, low-field nuclear magnetic resonance, environmental scanning electron microscopy, and X-ray diffraction. The results showed that temperature and CO₂ pressure significantly affected the modified coal-pore structure. Specifically, as the temperature decreased or the CO₂ pressure increased, the average pore diameter, total and effective porosities, NMR permeability, proportion of free fluid, and adsorption pore fractal dimension D₂ showed increasing trends, whereas the Brunauer–Emmett–Teller (BET)-specific surface area, and fractal dimensions D₁, and D_S decreased. Moreover, the combined effects of carbonic acid and sodium dodecyl sulfate in the CO₂ foam promoted the dissolution of clay and carbonate minerals, resulting in an increase in the number of surface fractures, pore enlargement, and the transformation of isolated pores into interconnected fractures. Pore structure evolution is primarily driven by mineral, and organic matter dissolution and fracturing, and CO₂ adsorption-induced matrix swelling. After performing CO₂ foam fracturing in deep high-temperature coal seams, implementing higher shut-in pressures can further enhance CBM recovery. These findings provide a fundamental theoretical basis for engineering applications of CO₂ foam fracturing technology to enhance CBM recovery.