Experimental Study of Coal Directional Sorption-Induced Strain Under Different Temperatures

Laboratory measurements that consider coal directional sorption-induced strain in response to different gases and temperatures are very scarce, though they are of critical importance for reservoir simulation and analysis. This work aims to quantify coal Langmuir isotherm curves and the directional sorption-induced strain during both adsorption and desorption processes. To achieve this, a series of sorption tests on a coal sample were conducted under free swelling/shrinkage (i.e., hydrostatic pressure) conditions using three gases in the order of He, N₂ and CO₂at three different temperatures. Three strain gauges were attached perpendicular to the face cleat, butt cleat, and the bedding directions of the sample. The coal sample was firstly vacuumed before being injected with a gas at a constant 1.0 MPa pressure. Once gas adsorption reaches equilibrium, the pressure then increases by 2.0 MPa and is kept constant until reaching new equilibrium. This process is repeated until the pressure reaches 9.0 MPa, from when the desorption process starts following the reverse order. Results from He injection show that coal demonstrates evident anisotropy in Young's modulus, with the ratio of E_Face: E_butt: E_Vertical being 0.81: 0.93: 1.00. The mean effective stress coefficient of the sample is 0.34, 0.39, and 0.40 under 35ºC, 40ºC, and 45ºC, respectively, which is significantly lower than the unity value conventionally adopted in reservoir analysis. For adsorbing N₂ and CO₂, a difference in the directional sorption-induced strain was observed, with a ratio of 0.87:0.87:1.00 for N₂ gas and 0.71:0.60:1.00 for CO₂, in the direction perpendicular to the face cleat, butt cleat, and bedding plane, respectively. Furthermore, it is found that the total volumetric strain and adsorption volume are linearly correlated regardless of the type of adsorbing gas, indicating that sorption-induced strain can be directly estimated from the Langmuir isothermal curve. The comprehensive data set from this work provides useful information for understanding coal anisotropic response to swelling/shrinkage under various temperatures and selecting appropriate input parameter values into reservoir simulation and analysis.