Effects of maturity on the oxidative pyrolysis characteristics and heat balance in autothermic in-situ conversion of low-medium maturity organic-rich shales

Global low-medium maturity organic-rich shale (OS) has significant reserves and is expected to be exploited using the autothermic pyrolysis in-situ conversion process (ATS). The maturity of organic matter in OS significantly affects its pyrolysis characteristics. This study prepared low-medium maturity OS with varying maturity levels via a hydrothermal method. We systematically investigated the oxidative pyrolysis behavior, dynamic characteristics, and combustion performance of the samples using thermogravimetry-Fourier transform infrared spectroscopy (TG-FT-IR). Results showed that higher maturity OS exhibited lower initial temperatures (Ti), reduced activation energies (Ea), and improved combustion indices due to increased free bitumen and light hydrocarbons, facilitating semi-coke low-temperature oxidative initiation and efficient combustion. Specifically, Ti in low-temperature oxidation pyrolysis (LOP) and high-temperature oxidation pyrolysis (HOP) decreased by approximately 26-27 degrees C, while Ea decreased by 5.9-61.8 %. The ignition index (C) in LOP and combustion stability index (Rw) in HOP increased by 9.9 % and 26 %, respectively. Additionally, the heat balance relationship of OS in the ATS process is discussed, confirming the feasibility of ATS exploiting. This study emphasizes the importance of retaining light hydrocarbons and bitumen in medium maturity OS reservoirs to enhance oxidative pyrolysis, providing insights for the efficient in-situ application of the ATS method.