Multi‐Scale Pore Structure of the Deep Coal Seam Gas Reservoir in the Daning‐Jixian Block of the Southeastern Ordos Basin

ABSTRACT The micro‐nano pore structure characteristics of coal are the basis for revealing the mechanism of gas enrichment and the law of diffusion and migration in coal seams. Based on the samples from the No. 8 deep coal seam of the Benxi Formation in the Daning‐Jixian area on the southeastern margin of the Ordos Basin, through field emission scanning electron microscopy analysis, high‐pressure mercury adsorption experiments, low‐temperature N 2 adsorption and CO 2 adsorption combined measurements, the full‐pore‐fracture structures of the deep coal seam were finely characterised. Moreover, the movable fluid space and the potential for accommodating free gas of the deep coal seam were analysed using low‐field nuclear magnetic resonance technology. The research displays that the pore types of the deep coal in the Daning‐Jixian area are dominated by gas pores, intergranular pores and residual pores of compressed and deformed plant cells. Together with exogenous fractures, endogenous fractures and clay mineral microfractures, they constitute a complete deep coal pore system. The pore structure of the coal sample exhibits significant cross‐scale effects and extremely strong heterogeneity, mainly composed of micropores with diameters less than 2 nm, followed by macropores with diameters ranging from 50 nm to 1 μm and fractures with diameters greater than 10 μm. The development of mesopores with diameters ranging from 2 to 50 nm and macropores with diameters greater than 1 μm is the poorest. The micropores have an extremely large specific surface area (SSA) and strong adsorption potential energy, making them the main storage space for adsorbed gas; both the macropores and the microfractures contain certain movable fluid spaces, with the movable porosity ranging from 0.23% to 0.75% (with an average of 0.54%), providing conditions for the storage of free gas in the reservoir. Fractal analysis indicates that the complexity of pore structure decreases with scale, showing the trend of micropores > mesopores > macropores/fractures. The results indicate that the relationship between gas and water saturation affected by pore‐fracture structure is a key issue that restricts the precise prediction of ‘high saturation − super saturation’ prospective gas zone in deep coal seams.