Background Given China's significant demand shortfall of oil and gas, numerous constraints on oil and gas supply, an arduous task of green and low-carbon energy transition of coals, tar-rich coals, a type of coal resource integrating the properties of coal, oil, and gas, have great potential for increasing the oil and gas supply of the country.
Advances Despite the successful engineering test conducted in Yulin, Shaanxi Province, in-situ pyrolysis of tar-rich coals remains in its exploratory stage. This technology holds great strategic value in two aspects: (1) It enables reduced demand shortfall of oil and gas in China to enhance the country's ability to independently guarantee oil and gas supply. (2) It allows for reforms of coal mining technologies to promote the green, low-carbon coal energy transition and development of the coal industry. The in-situ pyrolysis of tar-rich coals is primarily achieved using boreholes and mines, aiming to extract oil and gas from coals in a sustainable and efficient manner. This technology principally faces challenges in siting, heating technology, and efficient heat and mass transfer.
Prospects The key to the research and development of in-situ pyrolysis involves the suitability of geological conditions, the matching of heating technology, the effectiveness of heat and mass transfer, and the safety and stability of pyrolysis, which are detailed as follows: (1) It is necessary to elucidate the geological foundation suitable for the in-situ pyrolysis of tar-rich coals and reveal the constraints on the dynamic sealing stability of surrounding rocks within the thermal radiation range from the perspective of the resource conditions of tar-rich coals, stratigraphic sealing performance, hydrogeological conditions, and structural conditions. The purpose is to provide a geological basis for the siting and engineering design of in-situ pyrolysis. (2) It is necessary to thoroughly understand the evolutionary behavior of the thermophysical properties of tar-rich coals under temperature and stress constraints, demonstrate the suitability of in-situ heating technology based on geological and engineering conditions, design efficient heating processes tailored to the poor thermal conductivity of coal seams, and achieve an economical energy supply for heating via mutual complementation of multiple energy sources such as wind, light, and electricity. (3) Major factors restricting the transport and production of oil and gas from in-situ pyrolysis include in-situ stress, large-scale coals, and high tar viscosity. Correspondingly, potential methods for enhancing the heat and mass transfer of coal seams and improving the producibility of oil and gas through pyrolysis encompass coal seam fracturing, the optimization of heat-carrying media combined with temperature and pressure control, and tar viscosity reduction for lightweight. (4) The sustained and stable operation of in-situ pyrolysis relies on the whole-process monitoring and dynamic early warning, requiring support from comprehensive monitoring means, precise inversion of geological information, modeling of multiphase and multifield environments, and prediction and early warning of thresholds for abrupt changes. To advance the development of in-situ pyrolysis of tar-rich coals, the key is to further explore vital technologies for sustained and efficient pyrolysis of tar-rich coals tailored to geological conditions in order to resolve the contradiction between coal resource exploitation and geological environments.
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