Fracturing in coals with different fluids: an experimental comparison between water, liquid CO2, and supercritical CO2

The present work conducted laboratory experiments of fracturing in fat coals, anthracites, and mudstones. Three different fluids were selected as the fracturing fluids, including water, liquid CO2 (L-CO2), and supercritical CO2 (Sc-CO2). The resulting fracture morphologies and fracture apertures of the coal specimens were investigated using 3D morphological scanning, and the permeabilities of the samples were measured before and after fracturing. The experimental results showed that the breakdown pressures of Sc-CO2 fracturing were the lowest among the three fracturing fluids, and the average single fracture apertures of the ScCO2-induced cracks were the smallest amongst the three fracturing fluids. In addition, the number of cracks and the roughness coefficients induced by Sc-CO2 fracturing were larger than those caused by water and liquid CO2. The viscosity of the fracturing fluid and the capillary effect are key factors that affect the crack propagation path and fracture surface topography. The results suggest that Sc-CO2 has the largest diffusion length, and thus is capable of permeating the coal matrix through small pores and causing more extensive fractures. Additionally, the effective hydraulic apertures of coal specimens produced by Sc-CO2 fracturing were wider than those induced by water and liquid CO2. The experimental results indicate that Sc-CO2 fracturing has huge potential to enhance coalbed methane recovery.