Effects of Injection Pressure and Duration on Alternate High-Pressure Water-Gas Sequestration of Coalbed Methane

Abstract

To solve the problems of high ground stress, low permeability and low coalbed methane recovery, this paper proposes an optimization technique for improving coal seam permeability, i.e., water-gas alternating displacement enhances coalbed methane recovery (WGA-ECBM). The seepage field of gas-water two-phase flow is derived and a multiphase fluid-solid coupling model between physical parameters is established. This paper studies the effects of three displacement methods (water injection, air injection and water-air alternating injection) on methane recovery. The influence of injection pressure and injection duration on the WGA-ECBM technique is also studied. The VCH4 (methane volume fraction) of the three methods decrease by 17.84%, 12.72% and 44.62% in the borehole, respectively. The influence ranges (diameter) are 24.68 m, 149.73 m and 135.21 m, respectively. Pore pressures and displacement area also increase as injection pressure increases in a 5 MPa gradient. And VCH4 in the borehole decrease by 35.57%, 43.82%, 46.78% and 51.72% from 10 MPa to 25 MPa, respectively. With the increase of injection duration, the influence range expands significantly but the pore pressure distribution changes little. And VCH4 decreases slightly in the displaced area and the displacement velocity gradually slows down. This technique promotes multiphase flow migration in coal seam, and increases the displacement range and methane recovery efficiency.