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

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.

Thermal Evolution Characteristics of the Pore Structure in Coal and Its Dominant Factor Conversion

“…CBM mainly exists in pores of coal in the form of micropore filling and monolayer adsorption. In the process of CBM drainage, the free methane in the fractures first flows into the borehole and a methane pressure difference forms between the fracture and the matrix; then, the methane in the pore diffuses into the fractures and finally seeps into the borehole. , Free-methane capacity in the fracture is small, and thus CBM migration can be regarded as a cascade process of desorption, diffusion, and seepage of adsorbed methane. , Heat treatment for coal reservoir will increase the kinetic energy of methane molecules, which reduces methane adsorption ability of coal and improve the methane migration ability, thereby enhancing the CBM recovery. Additionally, pores act as the methane diffusion channels; their evolution characteristic contributes to master the methane migration law in the actual coal reservoir with heat injection.…”

Water Injection Into Coal Seams for Outburst Prevention: The Coupling Effect of Gas Displacement and Desorption Inhibition

“…Coal mines often use coal seam water injection, hydraulic punching, hydraulic cutting, hydraulic fracturing, hydraulic extrusion, and its integration with blasting and many other hydraulic-based measures to improve coal seam permeability and gas extraction efficiency and so as to eliminate the outburst risk. After water invasion, a large number of water molecules inhibit the diffusion and seepage of gas in coal seams and reduce the desorption rate and desorption capacity of gas . Studies have shown that methane molecules adsorbed on the surface of coal are replaced by water molecules in the heat release process, which releases energy and requires energy for adsorption, indicating that the adsorption capacity of coal for water molecules is stronger than that for methane molecules .…”

“…After water invasion, a large number of water molecules inhibit the diffusion and seepage of gas in coal seams and reduce the desorption rate and desorption capacity of gas. 9 Studies have shown that methane molecules adsorbed on the surface of coal are replaced by water molecules in the heat release process, which releases energy and requires energy for adsorption, indicating that the adsorption capacity of coal for water molecules is stronger than that for methane molecules. 10 There is a critical injection pressure for coal injection in the coal seam.…”

Influence of Water Injection Pressure on Methane Gas Displacement by Coal Seam Water Injection

“…The changes in the coal physical and mechanical properties reduce the stress concentration of the coal body in front of the mining face [10,11]. The water entering the coal seam reduces the gas desorption rate and desorption capacity by blocking the gas diffusion and seepage channels [12,13]. Many scholars consider coal seam water injection as an effective measure to prevent coal and gas outbursts, and this measure has been widely used in many countries [14][15][16][17].…”