Evaluation of Water-Sealed Gas Reserves and Identification of the Development Potential in an Unconsolidated Sandstone Reservoir with Edge Water: A Case Study of Layer Group II-2 in the Sebei-2 Gas Field

Abstract

Water invasion can isolate gas and reduce gas-phase mobility in unconsolidated sandstone reservoirs, complicating reserve evaluation and target selection. This study evaluates dynamic reserves and remaining gas potential in the II-2 layer group of the Sebei-2 Gas Field by combining classified well-level reserve estimates with numerical simulation. Wells were divided into weakly water-affected and water-producing groups using their water–gas ratios. The flowing material balance (FMB) method was applied to the former, whereas a material balance method that accounts for water sealing was applied to the latter. A history-matched gas–water model was then used to characterize the spatial distribution of remaining gas and water invasion patterns. The classified estimates yield dynamic reserves of 70.72 × 108 m3, including 33.91 × 108 m3 of remaining recoverable reserves. The difference between the FMB reference estimate and the integrated estimate is 49.18 × 108 m3. This method-dependent difference is interpreted as an indirect indicator of restricted gas mobilization under the current water invasion conditions. Channel invasion and uniform invasion types account for 96.1% of the difference. Remaining gas occurs mainly in structural highs and the central and eastern reservoir areas. Intervals 2-5-11 and 2-5-12 are identified as candidate intervals for further development evaluation.