The Influence of Fracturing Fluid on CBM Desorption-Diffusion-Seepage Capacity of Coal

Abstract

Abstract The production process of coalbed methane (CBM) includes desorption, diffusion and seepage, and the production is controlled by these three processes together. The permeability of coal reservoir is generally low. To increase the coalbed methane production, well stimulation needs to be applied, in which the contact between fracturing fluid and coal seam is inevitable. Developed micro-fractures, large specific surface area and high capillary pressure of coal generate serious damage easily caused by fracturing fluid, which can affect the production of coalbed methane. Therefore, the multi-scale mass transfer ability damage evaluation method considering CBM desorption-diffusion-seepage capacity was put forward based on the coal reservoir microscopic pore structure and coalbed methane migration output mechanism. Taking the NO.9 coal from Ningwu basin and fracturing fluid in site as the research object, comprehensive damage evaluation experiment of fracturing fluid on CBM desorption-diffusion-seepage were conducted in order to optimizing the fracturing fluid in construction site. Combining the methods of infrared spectrum, wettability, scanning electron microscope and coal mechanical experiments, the impact mechanism of fracturing fluid on CBM desorption, diffusion and seepage were analyzed. Results show that both the coalbed methane desorption rate and the diffusion coefficient of samples treated by fracturing fluid decrease compared with that of treated by formation water; compared with formation water, fracturing fluid can decrease the mechanical strength of coal and strengthen its stress sensitivity, microcracks close further under the action of geostress which will make the flow channel more narrow, eventually the seepage capacity decreases. Through the results of infrared spectrum, wettability and scanning electron microscope we can get the conclusion that fracturing fluid can change the surface structure, wettability and pore connectivity of coal, and affect the migration ability of coalbed methane. The field test of optimized fracturing fluid were conducted, the results are consistent with that of indoor experiment. This study can provide theoretical guidance for fracturing fluid optimization in the operation of coalbed methane wells fracturing.