Experimental Investigation of Anisotropic Thermal Deformation of Oil Shale Under High Temperature and Triaxial Stress Based on Mineral and Micro-fracture Characteristics

The anisotropic deformation of oil shale at different temperatures should be given particular attention considering that thermal deformation affects borehole stability, oil and gas permeation channel formation and surface subsidence during the pyrolysis of oil shale. The deformation of oil shale parallel and perpendicular to the bedding directions under different triaxial stresses was studied from room temperature to 600 °C. Evolutions in mineral compositions and thermal fracturing were studied by X-ray diffractometry and computer tomography, respectively. The main results are as follows. (1) Under 2.5 and 5 MPa axial stresses, the thermal deformation perpendicular to the bedding direction shows compressive deformation with increasing temperature, and the strain rate increases sharply at 200 °C, 350 °C and 400 °C. When the axial stress is 10 MPa, the deformation rate increases sharply when the temperature increases to 100 °C, and then, the compression deformation increases almost linearly. (2) Under 2.5 and 5 MPa axial stresses, the thermal deformation parallel to the bedding tends to be expansion–compression–expansion–recompression with increasing temperature. When the axial stress increases to 10 MPa, the overall deformation of the material will be compression deformation, and no expansion deformation will occur with the increase in temperature. (3) The deformation characteristics under the coupled action of temperature and stress are less related to mineral changes but more related to fracture evolution characteristics. (4) Thermal stress has a great influence on the thermal deformation of oil shale perpendicular to the bedding directions but little effect on the thermal deformation parallel to the bedding directions.