Characteristics of P-wave and S-wave times and their relationship with Young’s modulus of coals with different degrees of deformation

Abstract

The degree of deformation of coal under the action of geological structure has important effects on the permeability and gas-bearing properties of a coal reservoir. Therefore, it is critical to understand how the characteristics of coals vary with degree of deformation in coalbed methane (CBM) exploration and exploitation. In this work, Young’s modulus and Poisson’s ratio of coal samples with various degrees of deformation, cored from the CBM wells in South Qinshui Basin, China, were tested with an electro-hydraulic servo-controlled rock mechanic testing system (RMT 150B). Statistical analysis was subsequently conducted with the corresponding P-wave times. The results demonstrate that the destructive effect of geological structure had significant influences on the Young’s modulus of coals. Poisson’s ratio had no significant correlation, but Young’s modulus decreased with increased deformation, and coals can thus be classified into one of three types based on the critical values of Young’s modulus. Further study found that P-wave time had a positive linear correlation with S-wave time in coal samples, and P-wave time is obviously shorter than S-wave time with the same coal. Both P-wave and S-wave times become longer with increased deformation. P-wave time is also linearly correlated with Young’s modulus. Based on well log data, values inferred from the statistical formulas among P-wave time, S-wave time, and Young’s modulus can provide a more accurate classification in terms of the degree of coal deformation.