Effect of Fracturing Fluid on Coalbed-Methane Desorption, Diffusion, and Seepage in the Ningwu Basin of China

Abstract

Summary The production process of coalbed methane (CBM) includes and is controlled by desorption, diffusion, and seepage processes acting conjointly. A coal reservoir commonly has a low permeability. Well stimulation is applied to increase the CBM production, during which contact between the fracturing fluid and the coal seam is unavoidable. Developed microfractures, large specific surface area, and high capillary pressure of coal generate serious damage easily caused by fracturing fluid, which can affect the production of CBM. Therefore, we presented a multiscale mass-transfer damage-evaluation method dependent on the coal-reservoir microscopic-pore structure and CBM migration-output mechanism that includes the capacity of CBM desorption/diffusion/seepage. The No. 9 coal in the Ningwu Basin of China and the associated fracturing fluid at the site are the object of our research. A comprehensive evaluation experiment on the damage caused by fracturing fluids on CBM desorption/diffusion/seepage was conducted to optimize the fracturing fluid in construction sites. Combining the methods of mechanical testing, scanning electron microscopy (SEM), low-temperature N2 adsorption, wettability, solution ratio, and infrared spectrum, we investigated the effect of fracturing fluids on CBM desorption/diffusion/seepage. Results show that the CBM desorption rate and the diffusion coefficient of samples treated by the fracturing fluid decrease compared with those treated by formation water. Compared with formation water, the fracturing fluid can decrease the mechanical strength of coal and strengthen its sensitivity to stress. Microcracks will seal more under the action of geostress, producing a narrower flow channel and causing the seepage capacity to eventually decrease. The results of our analysis suggest that the fracturing fluid can change the pore connectivity, wettability, and surface structure of coal, as well as affect the migration capacity of CBM. This study provides a theoretical guidance for fracturing-fluid optimization during the operation of CBM-well fracturing.