Numerical Investigation of the Heat and Mass Transfer during the In Situ Pyrolysis Process of Oil-Rich Coal

A multi-physics numerical method coupling fluid flow, heat transfer, and a chemical reaction was used to determine the temperature distribution and the conversion rate of a coal seam during underground pyrolysis. The coal seam was fractured to enhance the heat and mass transfer. The influences of the pyrolysis pressure on the heat transfer, oil and gas production, and pyrolysis time were also analyzed. When the injection gauge pressure was increased to 14 MPa, the conversion rate on the 120th day was 98.8% and the promotion was not obvious any more at further higher pressures for the model without a fracture. For the model with a fracture, the pyrolysis was completed in only 90 days at the much lower pressure of 4 MPa, which is beneficial for both reducing the heating period and enabling the rapid harvesting of oil. Then, the fractured zone was designed and optimized by investigating different radii of the fractured zone at both the inlet and the outlet of the domain. The dead zones around the two corners at the right side of the computational domain near the outlet well were reduced effectively with an increase in the diameter of the fractured region. The heat and mass transfer were enhanced with a larger area of the fractured region at the outlet well for the reason that the flowing dead zones experienced a longer effective heating time.