Abstract: Accurate characterization of microscopic pore structure is the key to evaluating reservoir resources and productivity. Taking the low-tight permeability sandstone reservoir of the Huagang Formation in the HY Gas Field of the Xihu Depression as an example, pore structure parameters are characterized on a full-scale basis by combining low-pressure nitrogen adsorption, high-pressure mercury injection, and nuclear magnetic resonance technology. Combined with the steady-state gas-water relative permeability and movable fluid saturation experiments, the reservoir seepage mechanism and control factors are comprehensively evaluated. The results show that the proportion of dissolved pores in reservoir samples increases, and the pore size decreases gradually from type I to type IV reservoirs in the study area. The proportions of pore throats with radius greater than 1 μm are 50.7%, 41.3%, 25.2% and 5.7%, respectively. With the deterioration of pore structure quality, the gas-water relative permeability curve in the study area shifts to the right, and the irreducible water saturation ranges from 15% to 55%. The effective permeability of the gas phase ranges from 0.01 to 9.00 mD, and the water saturation in the range of gas-water co-seepage is 6%~65%. When the flowback pressure gradient is 9 MPa/m, the average movable fluid saturation of the reservoir exceeds 50%, and the pore throat connectivity is good. The water film occurs in the pore throat radius μm. The reservoir seepage capacity is worse where a larger proportion of the water film. The research results have significant guidance for formulating effective development countermeasures and predicting gas well productivity in this area.

Key words: Huagang Formation, low-tight permeability sandstone, pore structure, seepage mechanism, movable fluid