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

This article focuses on the mechanisms and influencing factors of sand production and gas permeability alteration during water invasion in unconsolidated sandstone gas reservoirs, based on experimental studies of core samples from the Tainan gas field in China's Qaidam Basin. It identifies three types of permeability changes after water flooding: Type I, where gas permeability increases following sand production due to low clay content, strong cementation, and large pore throats; Type II, where permeability decreases after sand production because high clay content and water–rock interactions cause pore blockage; and Type III, where permeability decreases without sand production due to weak cementation or small pore throats leading to pore compaction. The study highlights clay mineral content, cementation strength (quantified by Young's modulus, compressive and shear strength), and pore throat size as key factors controlling sand production and permeability changes, and proposes targeted strategies such as selecting reservoirs with favorable properties, controlling production pressure differences, and mitigating water–rock interactions to optimize gas recovery and minimize sand-related damage.