Numerical simulation on development effect of tar-rich coal through in-situ conversion by convective heating in Huangling Mining Area

The effective development of tar-rich coal to extract oil is of great practical significance to alleviate the oil and gas tension in China, and the convective heating technology of underground in-situ mining is one of the main technologies for the effective development of tar-rich coal. Herein, the mining effect and energy return rate of the two heating media (high-temperature steam and nitrogen) under different injection pressures were studied by convective heating technology for underground in-situ conversion mining based on the tar-rich coal seam of Yan'an Formation in Huangling Mining Area. The results show that: the heating efficiency of steam injection mining is higher when the injected gas is at 400℃. Besides, the time required for steam injection mining to finish the extraction of stratum kerogen is less than that of high-temperature nitrogen injection under the same injection pressure, and the ratio of the two is 0.52‒0.68, with obvious time saving effect. Meanwhile, the ratio of total gas production by steam injection mining to that by nitrogen injection mining is 1.07‒1.11, while the ratio of total gas production is 0.82‒0.85, and the difference between the two values decreases with the increase of injection pressure. In addition, the ratio of energy return rate of steam injection mining and nitrogen injection mining is 1.754‒2.363. Specifically, the energy return rate of steam injection mining reaches a peak of 1.796 at 4.99 a at an injection pressure of 6 MPa, but no positive energy return is obtained by nitrogen injection mining. Regardless of the heating medium, increasing the injection pressure could shorten the heating reaction time, which is conducive to improving the energy return rate of steam injection. However, the effective permeability of formation fluids in nitrogen injection mining is high, which could facilitate the flow of oil and gas in the formation. Generally, steam injection mining has superiority in clean mining of tar-rich coal, and provides some data reference basis for the specific production process for clean mining of tar-rich coal.