Experimental study on erosion mechanism and pore structure evolution of bituminous and anthracite coal under matrix acidification and its significance to coalbed methane recovery

Acid fracturing is a potential method to resolve the mineralization restriction around wellbore and enhance the permeability in coalbed methane extraction. The previous study mainly focused on exploring the effect on permeability enhancement for oil reservoir. However, the performance of acid fracturing fluid in coal reservoir is still unclear. In this paper, the surface morphology and internal pore structure of coal were characterized by erosion rate test, scanning electron microscope-energy dispersive spectrum (SEM-EDS) and low-temperature nitrogen adsorption test (LT-N2GA), and the relationship between erosion rate and pore structure index of bituminous and anthracite coal under matrix acidification was obtained. The results show that the bituminous coal presented higher acid sensitivity than anthracite from no matter aspects of erosion rate, surface morphology and internal pore structure variation. The increment of erosion rate and pore structure index is increase with the coal ranks under matrix acidification, which can provide reference for the selection of acid pad fluids for acid fracturing in different coal seams.

Automated summary

Acid fracturing is a potential method to enhance permeability in coalbed methane extraction, but its performance in coal reservoir is still unclear. This study characterized the surface morphology and internal pore structure of coal under matrix acidification and found that bituminous coal showed higher acid sensitivity and greater erosion rate and pore structure changes compared to anthracite. These results provide reference for selecting acid pad fluids for acid fracturing in different coal seams.