Evaluation of liquid CO2 phase change fracturing effect on coal using fractal theory

Abstract

As an alternative and safe method for coal reservoir stimulation, liquid CO2 phase change fracturing (LCPCF) technology has large potential for enhancing coalbed methane drainage. In order to gain clearer insights into the changing laws of pore structure and gas permeability for samples before and after LCPCF treatment, various methods including mercury intrusion porosimetry (MIP) test, gas emission index experiments, gas permeability analysis were employed in this study. Fractal theory was also adopted to evaluate the fracturing effect. Based on the results, compared with the raw coal, the average pore diameter of the treated coal is greatly enlarged and the total pore volumes are increased from 15.75% to 41.31%. The fractal dimension (D) varies in the range of 2.6414 to 2.8701 and 2.2899 to 2.4971 for the raw coals and treated samples, respectively. The reduced D values of treated samples indicate that LCPCF treatment enables the coal pore structure to become more regular and homogeneous. Both the gas content and gas emission index exhibit an inverse relationship with fractal dimension, but coal permeability demonstrates an exponential increasing trend with the decline of fractal dimension. New pore generation, pore transformation and mineral dissolution account for this enhancement of gas permeability. The change of fractal dimension not only reflects the pore alteration, but also represents the methane seepage capacity in coal. Fractal theory bridges the gap between pore structure and methane flow, and it is expected to be widely used in evaluating fracturing effect of coal seams.