Production capacity prediction of U-shaped horizontal wells for in-situ pyrolysis of tar-rich coal: Experimental and numerical simulation study

Under the goal of “dual carbon”, “taking hydrogen and retaining carbon” has become an inevitable choice for the clean utilization of coal, and underground coal pyrolysis provides a new idea for increasing domestic tar and gas production. Scientific prediction of underground tar-rich coal pyrolysis capacity is critical for project economics and energy return assessment, but fewer studies have been conducted on the underground coal pyrolysis in mid to deep layers, and pyrolysis experiments on block coal considering real confining pressure conditions have not yet been reported. To predict underground tar-rich coal pyrolysis capacity and identify the laws of oil and gas production, this paper proposed a horizontal well development method suitable for underground coal pyrolysis in mid to deep layers based on the characteristics of underground coal pyrolysis. Block coal overburden pyrolysis experiments were conducted on two sets of main coal seams in the Santanghu Basin. The overlying pyrolysis yield evaluation model of tar-rich coal in Badaowan Formation based on Boltsmann function and the productivity prediction method of U-shaped horizontal well were established. The variation law of tar and coal gas production capacity and energy return rate were discussed. The results show that: ① Underground coal pyrolysis has the advantages of high resource abundance, clean and low-carbon gas products, and low geological risk. U-shaped, L-shaped and multi-branch horizontal Wells can be produced intermittently or continuously. ② The coal of Badaowan Formation has a higher tar and coal gas yield than that of Xishanyao Formation. Tar production reaches its peak at 400−500 ℃, confining pressure also has a negative effect on reducing the mass transfer capacity of pyrolysis products and a positive effect on improving heat transfer efficiency. When the temperature is lower than 400 ℃, the negative effect dominates, and the positive effect gradually dominates as the temperature rises. The coal gas production increases rapidly at 300−400 ℃, slows down at 400−600 ℃, and rapidly decreases after 600 ℃. H₂ and CO in coal gas increase monotonously with increasing temperature. CH₄ and CO₂ increase first and then decrease with temperature, the effect of confining pressure on the two sets of coal is different, and the application of confining pressure is more beneficial to coal gas production of Badaowan Formation coal. ③ Tar-rich coal pyrolysis product yield shows an S-shaped variation pattern of “slow increase - rapid increase - tends to stabilize” with increasing temperature, the Boltsmann function can better fit the law of tar-rich coal pyrolysis product yield. The linear and triangular well patterns form rectangular and circular temperature fields, respectively. To reach the effective pyrolysis temperature (350 ℃) required for the coal seam between wells, the well spacing of horizontal wells is 4.5 and 5.5 meters respectively, which shows that the heating effect of triangular well pattern is better than that of linear well pattern. ④ Tar and coal gas productivity is divided into three stages: low production in the early stage, rapid production in the middle stage, and stable production in the late stage. According to 1 w/(m·℃) coal thermal conductivity, the production capacity of tar, methane and hydrogen in a single U-type well for 5 years is expected to reach 1.56×10⁴ t/a、260.21×10⁴ m³/a and 201.83×10⁴ m³/a, and the energy return rate can reach 2.09. The mass ratio of pyrolysis water and tar in pyrolysis products is the largest, and the production capacity is the first to stabilize. The mass proportion of CH₄ and CO₂ in coal gas is the highest, CO₂ increases first and then decreases with production years, while CO and H₂ increase monotonically with production years. The production capacity contribution in the later stage of heating is mainly from CO and H₂ in the coal gas. ⑤ In the case of equal calorific value, underground coal pyrolysis reduces carbon emissions by 97% compared with surface coal combustion, ideally, and the CO₂ produced can be absorbed and buried through the semi-coke layer to achieve carbon neutrality. Overall, underground coal pyrolysis has the resource base, technical feasibility and good economic prospect for scale development, which is a new technology for clean and low-carbon development of fossil energy with great development potential.