Pore-Water Occurrence Characteristics in High-Rank Coal Based on NMR and Its Water-Blocking on Coalbed Methane Production

Significant productivity differences exist among high-rank coalbed methane (CBM) wells in the southern Qinshui Basin, and the water-blocking effect is a key microscopic factor limiting CBM desorption and efficient production. To reveal the pore-water occurrence law, water-blocking effect, and the control on CBM production in high-rank coal, the southern Fanzhuang Block (Block A) and eastern Qinnan Block (Block C) were selected, and the No.3 coal samples were collected from Sihe Coal Mine (SH, anthracite) and Gucheng Coal Mine (GC, lean coal). Low-field nuclear magnetic resonance (NMR) pore-fluid test and high-pressure methane isothermal adsorption experiment were conducted. Combined with long-term production data from CBM wells, the differences in pore-water occurrence and water-phase distribution in coal pores were systematically analyzed, and the coupling response mechanism of pore-water occurrence, water-blocking effect, and CBM desorption-production was clarified. SH coal is dominated by water in transition pores (10-100 nm) with a proportion of 55.97%, while micropores (<10 nm) account for 40.87%. In contrast, GC coal has a micropore water proportion of 81.72% and a transition pore proportion of 14.51%. After centrifugation at 1.61 MPa, the adsorbed water proportion of SH coal is 73.89%, significantly lower than 97.94% of GC coal, indicating that Block C has well-developed micropores, a high volume of adsorbed water, and strong capillary binding forces, making the risk of damage from water-blocking effect significantly higher than that in Block A. Under reservoir pressure, the average methane desorption rate of Block A reaches 5.27 m³·t^-1·MPa^-1, far exceeding 1.45 m³·t^-1·MPa^-1 of Block C, demonstrating stronger desorption driving force and lower fluid migration resistance. Production data confirm that Block C exhibits a longer single-phase water drainage period, greater difficulty in pressure reduction and desorption, limited pressure-drop funnel expansion during the two-phase flow stage, and slow productivity improvement. Both the cumulative gas production and the peak daily gas production capacity in Block A are significantly higher than those in Block C. This paper analyzes the effect of the microscopic pore-water occurrence on macroscopic CBM production performance of high-rank coal. It is clarified that enriched adsorbed water in micropores induces high-intensity water blocking and restricts efficient gas-water migration, which is the core cause of block productivity differentiation. The results provide theoretical support for productivity evaluation, water-blocking damage prevention, and production parameter optimization of high-rank coal reservoirs.