Mechanisms of permeability alteration during water invasion in unconsolidated sandstone gas reservoirs

Water invasion in unconsolidated sandstone gas reservoirs affects sand and gas production. With changes in displacement pressure difference, sand production and gas permeability variation characteristics in different unconsolidated sandstones also change, primarily influenced by three factors: clay mineral content, cementation strength, and pore throat size. The classification of unconsolidated sandstone is determined by analyzing gas permeability variation and sand production during water flooding, identifying three types: type I—permeability increases after sand production, type II—permeability decreases after sand production, and type III—permeability decreases without sand production. Type I formations have low clay mineral content (<30 wt. %), high cementation strength, and large pore throats, which promote sand production. After sand production, fluid seepage conditions improve, resulting in increased gas permeability. Although Type II formations have high cementation strength and large pore throats, their high clay mineral content (>35 wt. %); and strong water–rock interaction with formation water leads to the rapid generation of sand particles, which block pore throats and reduce gas permeability. The clay mineral content of type III formations is low (<30 wt. %), but their cementation strength is weak, or their original pore throats are small. Weak cementation strength leads to stress-induced pore compaction after water invasion, reducing gas permeability. Additionally, small pore throats hinder sand particle migration, further decreasing gas permeability. Based on this study, unconsolidated sandstone formations can be further classified, and targeted sand prevention and control strategies are proposed to enhance the efficient development of water-rich unconsolidated sandstone gas reservoirs.