Non-linear seepage characteristics and influential factors of water injection in gassy seams

Abstract

High-pressure water injection, as an approach for the prevention and control of gas- or dust-induced disasters, has been adopted in coal exploitation globally. To further decode the complicated seepage process of water injection in gassy seams, we introduced an experimental approach to uncover the dynamic characteristics of water injection and gas scavenging. We also built a mathematical model describing non-linear seepage of water injection in gassy seams based on experimental data. We then discussed how overburden pressure and coal seam gas pressure affect the starting pressure gradient of water injection. We determined that a hydraulic gradient within coal seams is the major driving source for water injection and gas scavenging. The entire process goes through (1) the initiation of water injection, (2) water-gas coupled seepage and gas scavenging, (3) the saturation of water in fractures and pores and (4) the end of injection. During the priming stage, there exists a starting pressure gradient, which is non-linear, for water flow within a coal seam. The starting pressure gradient soars as overburden pressure increases, which are attributed to the drop in coal seam permeability. Specifically, the starting pressure gradient forms a power exponential function relationship with coal seam permeability, and we thus established a functional relationship between starting pressure gradient and overburden pressure of different embedded depths. Gas pressure can exert an influence on the starting pressure gradient mainly in the following ways: (1) The coal seam absorbs gas, leading to the decline in the coal seam permeability, which allows the starting pressure gradient for water injection to increase; or (2) A high gas pressure is held responsible for a low level of water saturation in pores and fractures, which subsequently leads to the decline of relative water permeability in coal seams. We hope that our research can be of great theoretical support for the improvement of anti-dust techniques and high-pressure water injection approaches for gas drainage.