Permeability enhancement and porosity change of coal by liquid carbon dioxide phase change fracturing

Liquid carbon dioxide phase change fracturing (LCPCF) technology can effectively increase the coal permeability. In order to evaluate the influence of LCPCF on pore structure and permeability characteristics of coal, mercury intrusion porosimetry (MIP) analyses and permeability tests were used in this study. The experimental results show that LCPCF has less effect on transition pores, but it indeed has significant influence on pores (> 100 nm) and fracture structure of coal, further affecting the gas permeability characteristics within coal. This influence is heavily dependent on the distance from the fracturing borehole. When 1.26 L of liquid CO₂ (weighing 1.25 kg–1.40 kg) was used to conduct physical blasting, according to the change trends of pore/fracture and gas permeability within coal, the influence degree of LCPCF on coal can be divided into three stages at the distance of 0.2–1.0 m, 1.0–6.0 m and > 6.0 m, respectively. For the first stage, the influence of LCPCF is strengthened, and mesopores within coal are reduced and shift to the larger pores under the effect of high-energy gas and shock wave, leading to the increase in the number of macropores and microfractures, which in turn improves the gas permeability of coal to a large degree. At the second stage, due to the energy attenuation of shock wave and high-pressure CO₂ gas, the fracturing effect of LCPCF is reduced with the distance increasing from 1.0 m to 6.0 m, and the increase of coal permeability is rapidly diminished. At the third stage, both the change rates of pore structure and permeability characteristics of coal tend to be stable over 6.0 m away from the fracturing borehole, indicating that the influence scope of LCPCF is approximately 6.0 m for a single fracturing borehole.