An experimental study on the effects of acidization on coal permeability: Implications for the enhancement of coalbed methane production

Acidization can enhance gas/oil production by increasing permeability. The coal permeability at one effective stress (ES) before/after acidization was compared in many studies, but the ES changes greatly during coalbed methane (CBM) production. Moreover, the coal permeability enhancement after acidization varies from -30% to 20,000%, and it is unclear which parameters contributed to those variances. In this study, the pore structure/ mineral composition/permeability at different ES of coal before/after acidization are determined. The results show that the key parameter affecting the acidization performance is the total carbonate mineral content, which is almost zero after acidization. The permeability at atmospheric pressure (k0) increases after acidization, with an average increase of 780.97%, but a high increase in the compression coefficient (cf) leads to a lower permeability than that before acidization at high ES in some samples, indicating a negative effect of acidization. This study finds that coals with a porosity of 5-9%, ash yield of 8-18%, carbonates of 10-40%, clays of 30-70%, and more filled-in-fracture minerals are more suitable for acidization. This study also establishes empirical equations for predicting permeability after acidization, with average predicted k0/cf errors of 17.94%/7.19%, providing foundations for preliminarily evaluating the potential of CBM production enhancement by acidization.