Experimental evaluation method for permeability changes of organic-rich shales by high-temperature thermal stimulation

Abstract

High-temperature thermal stimulation (HTS) can effectively remove the damage caused by water phase trapping during the drilling and completion of tight oil and hydraulic fracturing of gas reservoirs. Since HTS induces new rock fracture, the permeability of a reservoir will be increased to a certain extent. However, there are no standardized methodologies for pretreatment, experimental procedures, or evaluation indicators for HTS rock samples. Taking the Longmaxi Formation shale from the Sichuan Basin as the research object, an experimental process for evaluating the impact of HTS on permeability was established. The temperature increased at a rate of 5 °C/min, considering the conditions of dry and water-bearing rock samples. Three variables are included in the evaluation index for HTS of shale reservoirs: threshold temperature, raise multiples of permeability in 4 MPa confining pressure, and in-situ conditions. The results show that the threshold temperature of the dry shale is 650–700 °C, while the water-bearing shale has two threshold temperatures. Furthermore, the low temperature is 100–150 °C, while the high temperature is 450–500 °C. The permeability increase ratio of drying shale samples is 1.5–10.0 after HTS under in-situ effective stress, while that of water-bearing shale samples reach 20–50. This research has shown that using the HTS method to reduce water phase trapping damage and drastically improves well production are being conducted in a shale gas reservoir with horizontal wells and staged hydraulic fracturing. Our recommended experimental methods and evaluation indicators are effective in establishing the HTS potential of a shale reservoir.