Variations in the pore structure and fluid mobility under anionic surfactant assisted matrix acidification of coal based on nuclear magnetic resonance T₁-T₂ spectra

Matrix acidification is a potential method for increasing the permeability of coalbed methane reservoirs. However, current research on the acid corrosion process is still unclear, and acid solutions can cause water blocking damage to reservoirs. Therefore, this article obtained a T-1-T-2 joint spectrum through nuclear magnetic resonance (NMR) test and conduct contact angle (CA) test to analyze the pore structure and fluid mobility evolution of bituminous and anthracite coal under sole acidification and Sodium secondary alkyl sulfate (SAS) surfactant assisted acidification. The results show that the acidification process exhibits two distinct stages. Stage 1: acidification mainly expand the pore space of the free fluid, while stage 2: acidification significantly increases both the bound fluid and free fluid space. After sole acidising, the porosity of bituminous and anthracite coal increased by 4.44% and 31.31%, while increased by 18.28% and 38.35% under SAS assisted acidification, respectively. The adding of SAS resulted in better connectivity of coal samples, with pore throat (>0.1 mu m) of bituminous and anthracite increasing by 56.02% and 351%, respectively. Under SAS assisted acidification, the maximum decrease of T-2cutoff value of bituminous and anthracite coal is 62.79% and 20.28% respectively. Surfactants can effectively alleviate the negative impact of acidification on coal surface wettability. The results can provide references for field application of matrix acidification in coalbed methane reservoir.