Investigation and prediction of thermal cracking and permeability enhancement in ultra-low permeability rocks by in-situ thermal stimulation

Abstract

Thermal cracking in rocks can cause a significant increase in permeability, which is of great significance for enhancing the oil recovery of ultra-low permeability reservoirs. To evaluate the potential for thermally-induced permeability enhancement in ultra-low permeability rocks, in-situ thermal stimulation (ITS) and pulse-decay gas permeability (PDP) real-time measurement of Chang 8 tight sandstone and Longmaxi shale cores were performed in a specially designed joint test system of the high-temperature pseudo-triaxial core holder and PDP real-time tester at different temperatures and simulated in-situ stresses. Stereo light microscope (SLM) observation and computerized tomography (CT) real-time scanning were applied to qualitatively and quantitatively characterize the dynamic variation in the thermal crack during the ITS, respectively. The experimental results show that above the threshold temperature of about 500 °C, thermal cracks were rapidly initiating, propagating, and forming a multi-scale fracture network in situ, thus leading to an improvement in permeability up to 11.23 and 29.82 times, respectively. By combining lithology and thermal analysis with reservoir physical property measurements in a multi-scale approach, the huge difference in thermal expansion coefficient (DTEC) between minerals caused by the quartz phase transition from α to β (QPT) at 573 °C is proved to be the most crucial mechanism for thermal cracking and permeability enhancement. To describe the intrinsic relationship between thermal crack and permeability with temperature-pressure during the ITS, a mathematical model based on thermoelasticity, fracture mechanics, and percolation theory was proposed. The QPT from α to β was further verified as the crucial mechanism by coupling the DTEC as a function of temperature into the mathematical model and obtaining better fitting results for experimentally measured permeability. Through experimental investigation and mathematical modeling, ITS is demonstrated as a less promising reservoir stimulation strategy for the economic and efficient development of tight oil and shale gas reservoirs.