Temperature effect on the mechanical behavior of shale: Implication for shale gas production

Abstract

• The variation in mechanical strength parameters of shale at elevated temperatures. • Tensile strength, mode-I fracture toughness and point load strength investigation. • Mechanical strength attributes were supported with XRD and SEM analysis. • Micro-crack and thermal behaviour of minerals induced thermal behaviour of shale. Deformational shale attributes such as fracture toughness, tensile strength, and axial point load strength (PLS) index under elevated temperatures are crucial for the petroleum and gas industries. Here, an attempt has been made to investigate the thermal response of shale from room temperature to 400°C. Three-point bending (TPB) test using semi-circular bending (SCB) specimen, Brazilian tensile test, and the point load strength index test were conducted to evaluate mode-I fracture toughness, tensile strength, and PLS index, respectively, with varying temperatures. Additionally, the X-ray Diffraction (XRD) analysis was performed to determine the bulk rock composition and the resulting phase changes with increasing temperature. Furthermore, Scanning Electron Microscope (SEM) was used to study the induced microcracks in the sample due to thermal treatment. It was found that the mode-I fracture toughness increased from 25°C up to 100°C and, after that, a slight decline up to 400°C. The fracture toughness of the shale got increases from 0.256 MPa·m 1/2 to 0.454 MPa·m 1/2 in a transition from ambient room condition to 400°C. The same trend was obtained for tensile strength and PLS index. With the same temperature range, the tensile strength of the shale changed from 7.94 MPa to 9.75 MPa. The SEM images of the thermally treated samples demonstrate the formation of noticeable microcracks at 400°C.