Abstract: In the Shenfu block located on the eastern margin of the Ordos Basin, the deep coal seam is intimately interlayered with the tight sandstone layer. During the development of deep coalbed methane, a notable phenomenon has been observed whereby tight sandstone gas from the roof migrates into the coal seam. To date, studies on the impact of roof tight sandstone gas migration on deep coalbed methane production remain scarce. Taking the No.8+9 coal seam and its roof and floor in the Shenfu block as the study object, a numerical simulation of vertical well drainage was conducted for deep coalbed methane under different sandstone coverage ratios (i.e., the proportion of roof sandstone covering the coal seam within the well control range). This study investigated the roles of reservoir pressure conduction, gas desorption/diffusion, and dynamic changes in permeability in governing the influence of roof tight sandstone gas migration on the gas production performance of deep coalbed methane wells and explored its underlying mechanism. The results show that during the development of deep coalbed methane, the migration of roof-tight sandstone gas into the coal seam contributes to the gas output of deep coalbed methane wells. With the increase of roof sandstone coverage, the contribution of tight sandstone gas to cumulative gas production correspondingly increases by 11.43%, 23.54%, and 29.35% at sandstone coverage ratios of 40%, 60%, and 80%, respectively, In comparison to cases with a mudstone roof. Outside the control range of hydraulic fracturing, gas and water production from the sandstone layer occurs more rapidly. This enlarges both the magnitude and spatial extent of the pressure drop within the sandstone layer. As a result, pressure relief is indirectly facilitated in the coal seam, enhancing the rates of gas desorption and diffusion. Additionally, it strengthens the shrinkage effect of the coal matrix and promotes the recovery of permeability. The improvement in permeability further augments the reservoir pressure conduction and gas desorption/diffusion, thereby improving the gas production performance of coalbed methane wells.

Key words: Shenfu block, deep coalbed methane, tight sandstone gas, reservoir pressure, permeability