Characterizing Anisotropic Swelling Strains of Coal Using Combined Rosette Strain Gauge and CT-Scans

Matrix shrinkage/swelling induced by gas sorption can be a major geomechanical driver for coalbed methane reservoir stress depletion and gas transport enhancement under in situ reservoir conditions. The pressure-dependent three-dimensional (3-D) coal matrix shrinkage/swelling is of obvious interest for investigating the mechanical failures and gas transport behaviors of coal. An integrated sorption and matrix shrinkage system was employed for simultaneously measuring the gas sorption capacity and 3-D anisotropic swelling/shrinkage strains. The sorbing methane gas and non-sorbing helium gas were used as the flooding fluids on two coals. The high heterogeneity and anisotropy features in micro- to macro- scale of coals were confirmed using FESEM-EDS imaging and elemental analysis. To quantify the 3-D anisotropic characteristics in directional swelling strains, rosette strain gauges were employed. The 3-D principal strains for the two coals were computed through combining the proposed strain transformation model and the visualized cleat coordinate system established via the reconstructed 3-D coal structure using the X-ray CT images. For helium injection, coals were in compression and the maximum compression strain in the newly established cleat system can be up to ~  – 0.11% and ~  – 0.10% for samples S1 and S2 at pressure of 1664.35 psi, respectively. The maximum principal strain in dilation for coals S1 and S2 due to methane injection can be up to ~ 0.21% and ~ 0.149%, respectively, at pressure of 2279.37 psi. The pressure- or methane content- dependent anisotropic degree based on the actual three-dimensional principal strains in the cleat coordinate system was calculated and discussed. The results will provide a comprehensive modeling framework to evaluate sorption-induced swelling effects under in situ complex boundary conditions.