High-pressure water and gas alternating sequestration technology for low permeability coal seams with high adsorption capacity

Abstract

Traditional ECBM techniques, which cannot extract methane adsorbed in coal pores effectively, is still difficult to greatly improve coalbed methane recovery rate until now. Waterflooding or CO2-ECBM can displace methane adsorbed in nanopores but increases the risk of coal and gas outbursts. We proposed a novel coalbed methane recovery technique, e.g. high-pressure water and gas alternating sequestration technique (H–P-WGAS), which can displace adsorbed methane in coal pores without introducing other gases with high adsorption capacity. Additionally, the injected H–P gas can eliminate the water lock and be easily extracted during methane drainage. Numerical simulation and field test were conducted to verify effectiveness of the H–P-WGAS technique. The numerical results show that the H–P-WGAS technique can enhance methane migration in coal seam by changing pore pressure distribution and increase influence radius to 70 m at 20 MPa in 48 h. For water and air co-injection, methane content can decrease by up to 37.6%, which is much more effective comparing with that under single water injection or gas injection. Meanwhile, field tests show that the reduction of methane content by percentage lies between 25.14% and 80.62% with an average of 53.17%. Meanwhile, the influence radius of field test reached ∼48 m. The average methane drainage flow rate of single borehole increased by 2.65 times. Average methane drainage concentration increased from 31.3% to 47.6%. The methane drainage flow rate and concentration of the whole coal mine increased by 50.2% and 32.4%, respectively. The above results indicate that the proposed H–P-WGAS has best effectiveness comparing with single water injection or air injection.