Effects of swelling clay layers on fluid production from hydrate-bearing sediments induced by depressurization

Clay-rich sediments are common in natural gas hydrate reservoirs and pose significant technical challenges in past field production tests. Among these, Na-montmorillonite significantly affects methane hydrate (MH) formation and dissociation due to its swelling and water absorption properties. However, the role of swelling clay layers on fluid production by depressurization remains unclear, thus limiting CH₄ recovery efficiency. This study aims to elucidate the role of swelling clay on fluid production from natural gas hydrate deposits under depressurization. We synthesized MH-bearing sediments in the presence of sand, clay, and clay-sand alternating layers to examine fluid production and water-gas ratios under different bottom-hole pressures of 6.0 and 10.0 MPa. Results suggest that MH formation rates and final saturation are significantly lower in clay layers (14%) compared to sand layers (29%). During depressurization, MH in clay layers decomposed earlier due to the thermodynamic inhibitory effects of Na-montmorillonite. Swelling clay reduced water recovery (4.3%) and increased gas recovery (88.4%) in clay-sand layers at 6.0 MPa. Lowering pressure enhanced CH₄ recovery to 88.4% in clay, 71.8% in clay-sand, and 68.3% in sand layers. Depressurization created persistent low-temperature region in clay layers (ΔT = 2.0 ℃), potentially promoting MH reformation. Heat conduction was more dominant in sand layers than clay layers, indicating imbalance interlayer heat transfer characteristics. These findings offer critical insights for optimizing depressurization strategies in clay-rich hydrate reservoirs, enabling secure and efficient energy recovery from clay-rich NGH deposits.