Permeability Enhancement via Acetic Acid (CH3COOH) Acidizing in Coals Containing Fracture-Filling Calcite

Calcite is one of the most common fracture-filling minerals in low-permeability coal seams and can be dissolved easily by acid. Although much attention has been paid recently to coal seam acidizing, only few studies have focused on the permeability evolution during the injection of organic acid, which has a very different reaction rate and dissolution structure morphology from those of conventional strong acids (e.g., HCl and HF). In this study, 0.25-1.0 mol/L acetic acid (CH3COOH) was injected into the coal plug samples from the Fuxin Basin in China, whose fractures were completely infilled by calcite. The evolution of plug permeability during CH3COOH flooding was monitored. Before and after flooding experiments, the coal plugs were characterized using micro-computed tomography imaging, scanning electron microscopy, and nuclear magnetic resonance. The results showed that fracture-filling calcite exhibits high reactivity after exposure to CH3COOH at the temperatures of 20, 30, and 40 degrees C, and the macro-/microscale highly conductive flow channels between the inlet and the outlet of coal plugs can be formed in several hours. Thus, a significant permeability improvement from less than 0.02 mD to over 27 mD was observed during CH3COOH flooding. These increases ranging from 131 to 2720 times of the initial permeability are mainly due to the presence of fracture-filling calcite with high connectivity and the absence of fine migration, clay swelling, and secondary Fe-rich precipitation in the CH3COOH environment. The results also suggested that the minimum volume of CH3COOH required for a highly conductive flow channel breakthrough can be less than 1.0 PV. This investigation demonstrated the potential of deep matrix-acidizing treatment using CH3COOH for improving the permeability and enhanced coalbed methane recovery in coal seams containing fracture-filling calcite.

Automated summary

This study investigates the use of acetic acid (CH3COOH) for improving permeability in coal seams with fracture-filling calcite, demonstrating significant increases in permeability due to the high reactivity of calcite in the presence of CH3COOH. The results show the formation of highly conductive flow channels and the potential for deep matrix acidizing treatment using CH3COOH for enhanced coalbed methane recovery.