Dynamic behavior of deep high-temperature layered shale under medium strain rates

Abstract During deep shale gas extraction or deep shale oil recovery involving in-situ upgrading of low-maturity oil through electric heating, the process is challenged by high-temperature and high-stress environments. Against this backdrop, impact fracturing technologies such as in-situ deflagration or carbon dioxide blasting are considered promising extraction approaches under medium strain rates (10 2 s − 1 ). This paper reports the results of split Hopkinson pressure bar (SHPB) experiments under medium strain rates on dry layered shale samples with five bedding orientations (0°, 30°, 45°, 60°, and 90°) after thermal treatment at simulated reservoir temperatures (25–400 °C). The effect of reservoir temperature on dynamic strength of shale samples is both bedding orientation- and strain rate-dependent, where the greatest temperature sensitivity is seen at the bedding orientation of 60°. With increasing reservoir temperature, the strength anisotropy magnitude of shale samples gradually increases within 300 °C and then minorly decreases. Rate dependence of dynamic strength is affected by both reservoir temperature and bedding orientation of shale samples. Fragmentation degree and energy absorption rate of shale samples as functions of reservoir temperature can be classified into same categories according to bedding orientation, indicating no apparent rate dependence. With increasing reservoir temperature, the fragmentation degree increases whereas the corresponding energy absorption rate decreases. This temperature-induced trend is further enhanced at higher strain rates, particularly for shale samples with bedding orientations of 45° and 60°. The mechanisms governing the observed anisotropic response of dynamic strength are analyzed through physical property measurements and theoretical interpretation. Finally, possible challenges and suggestions of impact fracturing in deep high-temperature shale reservoir under medium strain rate are proposed.