Abstract:

The eastern margin of the Ordos Basin is rich in tight gas resources. However, the reservoirs are characterized by low pressure coefficients, poor physical properties, a high concentration of sensitive minerals such as clay minerals, and complex pore structures. These features result in a high degree of potential reservoir damage, which constrains stable and high gas production. To clarify the sensitivity characteristics of tight gas reservoirs in the study area, the study used tight sandstones from the first member of the Permian Shanxi Formation (Shan-1 member) and the eighth member of the Permian Shihezi Formation (He-8 member) as the research objects. Cast thin-section analysis, scanning electron microscopy (SEM), X-ray diffraction, and core fluid displacement tests were employed to investigate reservoir sensitivity. Based on the results, targeted strategies for reservoir protection were proposed. Experimental results indicated that the reservoir rocks were fine to medium grained lithic sandstones, with an average clay mineral content of 21.59%, primarily consisting of mixed-layer illite-smectite. The main pore types were residual intergranular pores and secondary dissolution pores. Nanopores were well-developed, with poor connectivity between micro- and nanopores. In cores from the He-8 member, the median porosity and permeability were 6.43% and 0.149 × 10^-3 μm², respectively. In cores from the Shan-1 member, the median porosity and permeability were 6.46% and 0.387 × 10^-3 μm², respectively. The pH values of the formation water ranged from 5.47 to 6.83, with an average total salinity of 118 077.21 mg/L. The reservoirs in the study area were low-temperature, low-pressure tight gas reservoirs, exhibiting weak velocity sensitivity, weak to moderately weak water sensitivity, weak to moderately weak salt sensitivity, moderately weak to moderately strong acid sensitivity, weak to moderately strong alkali sensitivity, and moderately strong to strong stress sensitivity. The average critical flow velocity was 0.3 mL/min, the critical salinity was 60 000 mg/L, and the average critical pH value was 7.79. During development, attention should be paid to the low pressure, low temperature, and high nanopore proportion of the reservoirs. Acid sensitivity, alkali sensitivity, and stress sensitivity should be prioritized, and improvements and optimizations should be made in drilling, completion, fracturing, and production practices. The research findings provide significant guidance for the efficient development of low-pressure tight sandstone gas resources.

Key words: Ordos Basin, tight sandstone gas, fluid sensitivity, stress sensitivity, reservoir protection