Multi-physics field simulation study on in-situ pyrolysis characteristics of fractured tar-rich coal seam

In-situ pyrolysis of tar-rich coal refers to the process of extracting tar and gas through underground in-situ modification and heating of coal seams. This technology offers advantages such as a small land footprint, a wide range of resource extraction depths, and no surface subsidence. It represents a new clean and low-carbon utilization technology for coal resources. In this study, thermogravimetric experiment was firstly conducted to obtain the kinetic parameters of coal sample. Subsequently, the pyrolysis effects with and without hydraulic fractures were compared. And then, the in-situ pyrolysis characteristics of a 10 m × 10 m square coal seam under multi-physics field coupling with different hydraulic fracture parameters were investigated by numerical simulation. The effects of varying hydraulic fracture numbers and spacings on the flow heat transfer, structural evolution and product generation performance were analyzed. The results reveal that hydraulic fracturing can notably achieve higher permeability of coal seam, enhance the flow diversion capacity, produce more tar in the same period of time. When the coal seam contains three hydraulic fractures, it undergoes complete transformation earlier than that with one or five hydraulic fractures. The results suggests that more hydraulic fractures do not necessarily produce better outcomes, which is precisely conducive to balancing benefits and costs. All conditions stimulated with hydraulic fractures produce more than 8000 kg of tar within one year. At the beginning of pyrolysis, the smaller hydraulic fracture spacing leads to a relatively rapid increase in every indicator. With the progress of pyrolysis, the larger the hydraulic fracture spacing, the more these performance indicators improve, indicating that a larger fracture spacing facilitates rapid pyrolysis, fluid transportation, heat exchange and products output. The research results can provide a reference for the underground pyrolysis scheme of coal seams with fractures and offer guidance for the efficient and sustainable utilization of tar-rich coal.