Experimental study on characteristics of Jiamin damage in high-volatile bituminous coal reservoirs

In recent years, the high volatile bituminous coal reservoir in Xinjiang has become the focus area for Coalbed Methane (CBM) development. The Jamin effect in the reservoir can be caused by well repair and pumping suspension, which in turn reduces the single-well productivity of CBM wells. This paper takes the high volatile bituminous coal reservoir in Xinjiang as the research object, studies the characteristics of Jamin damage formation during the drainage process in high volatile bituminous coal reservoirs through physical simulation experiments, and uses low-field nuclear magnetic resonance technology to analyze the characteristics of the damage process and the pore-fracture space occupied by gas phase after Jamin damage. The results show that: ① The permeability of the water phase in the parallel bedding coal sample (0.872×10−3～1.066×10−3 μm2) in the single-phase flow stage is one order of magnitude higher than that of the vertical bedding coal sample (0.064×10−3～0.112×10−3 μm2). The water phase permeability damage rate after Jiamin in parallel bedding and vertical bedding coal samples at steady-state reached 40.60% and 50.00%, respectively. ② After Jiamin damage, part of the water in coal core becomes non-mobile, and the volume of this part increases over time. This increase in volume leads to a decrease in permeability during the post-Jiamin waterflooding process. ③ The physical simulation suggests that when a CBM well temporarily halted for maintenance, the accumulated gas within the reservoir cannot be fully displaced by the formation water during subsequent re-opening and waterflooding. The gas tends to cluster in the medium and large pores and fractures, occupying the pore space for fluid flow. This leads to a reduction in water permeability and inhibits the effective removal of water upon re-opening of the CBM well, resulting in reduced drainage efficiency and pressure reduction. ④ It is recommended to reduce Jamin damage in the vicinity of the well and release the productivity of low-yield CBM wells by strengthening gas well management, extending well repair and pump inspection cycles, maintaining stable drainage, developing Jamin-suppressing antifoaming agents, and implementing pulse excitation flushing methods.