Investigation of the Temperature Dependence of the Chemical–Mechanical Properties of the Wufeng–Longmaxi Shale

Abstract

Shale rocks have been widely investigated to evaluate the productivity of oil/gas. The high temperature generated by the explosive fracturing to stimulate the gas reservoir has a significant impact on the chemical−mechanical properties of shale rocks. Pioneering works have been carried out at temperatures below 500 °C, but little has been done to quantify the correlation between the chemical and mechanical properties of shale at temperatures above 500 °C. Therefore, an experimental study on the effect of temperature on the chemical−mechanical properties of shale rocks is presented in this paper. The temperatures used in our experiments are between 0 and 800 °C. Results indicate that there exist strong chemical reactions leading to a big reduction in the sample's weight and mechanical strength for a temperature over 500 °C. Thermogravimetric analysis data demonstrates that the weight of shale powders has little change below 400 °C and largely decreases after 600 °C. It shows that the chemical reaction rate corresponding to shale compositions varies with temperature. X-ray diffraction and Fourier transform infrared are integrated to quantify the occurrence of contained reactions including the decomposition of kerogen, carbonates, and quartz transition. This can provide a temperature range for all possible reactions. Changes in the compositional information of shale samples have been proven to significantly influence the mechanical properties. A 25% decrease in dynamic Young's modulus emerges as the temperature approaches 700 °C. As the brittle minerals, for instance, carbonates, decrease with temperature, a brittle-ductile transition happens in shale. This work provides very meaningful results different from that at low temperatures to help people better understand the effects of high temperatures in many fields, such as explosive fracturing and radioactive waste disposal.