Effects of thermal damage on natural gas transport capacity of tight sandstone after electrical heating

As an environment-friendly method, downhole electric heating can cause thermal damage to the rock around the well, so as to improve the gas transport capacity of tight sandstone gas reservoir (TGR), but its mechanisms are not well established. In the present study, a comprehensive analysis of the thermal damage characteristics of tight sandstones has been conducted, employing a suite of microscopic investigative techniques such as differential thermal gravimetry, X-ray diffraction, and scanning electron microscopy. The experimental results were related to permeability test results and then combined with constant-pressure heating acoustic rock tests. Subsequently, the heating and cooling processes along with the effects of confining pressure on the damage evolution and gas transport capacity were analysed. According to the difference in thermal damage mechanism, three temperature ranges were identified: 100–300 °C, 300–1000 °C, and 1000–1200 °C. In the first stage, free water escaped, clay minerals expanded, and a small number of weak cementation cracked, resulting in a 26.18% decrease (200 °C) and a 56.95% increase (300 °C) in permeability. In the second stage, the clay minerals were stripped of their bound water; with an increase in temperature, heat-induced cracks were initiated, propagated, and aggregated to form a network. The permeability increased exponentially by 938-fold. Finally, in the third stage, with the initiation of feldspar melting, the mineral surface was smooth; furthermore, permeability increased rapidly by 164.08%, after which the pores were blocked rapidly. Acoustic tests under constant-pressure heating conditions showed that thermal damage continued to evolve during the cooling phase because of the temperature variable stress. External pressures could only partially limit the evolution of thermal damage. The findings of this study illustrate that downhole electric heating (300–1000 °C) can improve natural gas transport capacity by eliminating water phase trap damage and creating a multi-scale crack network in a near-wellbore area. In addition, the nonlinear evolution of permeability in tight sandstones is different from that in other rocks, and the S-wave attenuation coefficient can well characterise this phenomenon. Thus, a mathematical model of nonlinear permeability evolution based on the principle of dual percolation is derived. Therefore, the results of this research provide a reference for the temperature selection and acoustic monitoring of the TGR downhole heating process.