The construction of underground gas storage (UGS) in a large-scale low-permeability lithologic gas reservoir presents an immense engineering challenge. Under the context of UGS, research on structural characteristics and storage capacity at the microscopic scale is insufficient, making it difficult to provide effective support for the engineering scheme. In this study, the microscopic storage spaces of a typical lithologic gas reservoir (i.e., YL block in the Ordos Basin) are comprehensively analyzed through experimental techniques (represented by computed tomography scanning), digital core analysis, and fractal analysis. Furthermore, the feasibility of UGS construction is examined. The results demonstrate that the large-scale low-permeability lithologic gas reservoir exhibits significant zonal heterogeneity in its microscopic structural characteristics at both morphological and statistical levels. Specifically, the microscopic storage spaces of the core zone within the YL block are notably higher than those in the transition and periphery zones, characterized by larger aperture, less tortuous, higher aggregation and connectivity. Consequently, the core zone provides adequate storage capacity and injection-extraction capability for large-scale underground storage of natural gas. In contrast, the transition and periphery zones exhibit inferior microstructural, storage, and flow properties, which are not suitable for rapid injection and production. However, these zones show a fairly strong lateral sealing capability, which can be utilized as a monitoring area to evaluate UGS integrity. These findings indicate that the reservoir's microstructural features meet the essential requirements of storage capacity, injection-extraction capability, and lateral sealing property for UGS construction. Based on this understanding, a series of zone-differentiated UGS engineering suggestions are proposed, including zonal function specification, well type selection, well deployment scheme, and management of old wells. These findings can provide valuable insights for the assessment and implementation of UGS projects from such gas reservoirs.

Due to the existence of fracturing fluid and formation water in shale gas reservoirs, the coexistence of gas and water in nanopores is prevalent. The pore water in the reservoir, on the one hand, affects gas flow behavior and permeability. On the other hand, it blocks pore throats and occupies adsorption sites on the pore surface, consequently reducing the gas adsorption capacity. The occurrence of pore water in shale reservoirs holds significant importance for shale gas resources exploration and development. In this paper, the shale from the Longmaxi Formation, Sichuan Basin was selected as the research target. The content and micro-distribution behavior of pore water were evaluated through centrifugation-nuclear magnetic resonance experiment and theoretical model. The results demonstrated that the content of free water would be underestimated by the experiment, with 2.55%-6.80% lower than that calculated by theoretical model. Moreover, due to the limitations of nuclear magnetic resonance experiment, the adsorbed water in mesopores and macropores might be mistakenly identified as that in smaller pores. As a result, the theoretical model is more applicable for characterizing the micro-distribution behavior of pore water than the origin nuclear magnetic resonance data.