Identification of the relatively low permeability area in coal and gas outburst seams by seismic wave tomography technique: Field application and validation

Abstract

Identifying relatively low permeability areas (RLPA) is significant for site selection and parameter design for implementing enhanced gas mining measures (e.g. hydraulic fracturing, hydraulic perforation, etc.) in coal and gas outburst seams. The P-wave velocity of coal is sensitive to the variation of porosity. This paper researched the P-wave velocity change of coal with respect to confining pressure and gas pressure, the mathematical relationship model between P-wave velocity with porosity and permeability, and the relative permeability identification index. The research found that the P-wave velocity response to gas pressure varies non-monotonically. When the gas pressure is <2 MPa, the confining pressure is the main control factor leading to P-wave velocity variation. The mathematical model of P-wave velocity versus porosity and permeability was derived based on the linear relationship between wave velocity and porosity in dense rocks and the Seidle permeability model for porous media. Then the wave velocity anomaly coefficient (An) is used as the identification index for the relative permeability of the coal seam. That is, when the anomaly coefficient of coal seam wave velocity An < -8%, the coal seam is a relatively high permeability area (RHPA); when −8% <An < 8%, the coal seam is relatively normal permeability area (RNPA); when An > 8%, the coal seam is (RLPA). Based on the identification indexes studied, two RHPA (H1 and H2) and two RLPA (L1 and L2) were identified in the field engineering application. Comparing the drainage parameters in different relative permeability areas, the gas drainage volume fraction and flow in the RLPA are 56% and 90% lower than RHPA, respectively, validating the accuracy of identifying RLPA in the coal seam.