Targeted Modification Effects of HCl Acidification on Methane Adsorption in Coal

Acidification is a commonly used chemical modification method to ameliorate the coal seam permeability. Current research on acidified coal mainly focuses on mineral composition, pore structure, and permeability. However, as one of the key stages of coalbed methane (CBM) development, there is still insufficient research on adsorption in acidified coal. The study selected HCl as the modification reagent and explored the pore and adsorption evolution characteristics through pore-combined testing technology and isothermal adsorption experiments. The results demonstrate that HCl acidification has a pore-enlarged effect on the macropore, with an increase in pore volume and a decrease in pore area. Mesopore displays pore-enlarged and transformed effect, with a decrease in pore volume and area. But HCl acidification has a weak effect on the micropore structure with chemical properties. The changes in the pore structure influence the Langmuir constant. Specifically, the decrease in macropore and mesopore area reduces the gas adsorption sites and weakens the adsorption capacity. The Langmuir constants generally show a decrease in Langmuir volume V L and an increase in Langmuir pressure P L. The study clarified the targeted control effect of HCl acidification on reducing coal adsorption and enhancing desorption efficiency and revealed the mechanism of CBM increasing production from the perspective of adsorption and desorption, which provides theoretical guidance and an evaluation basis for the HCl application in reservoir modification and CBM increasing production.