Effects of key geological factors in the long-term transport of CH4 and the CH4-hydrate formation behavior with formation dip

Abstract

Methane hydrates (MH) are widely distributed below seafloor at marine locations. Most MH bearing geological deposits have dip angles because of the complicated geological structure and diagenesis. In this study, the impact of formation dip on CH4 migration and the associated MH formation was systematically investigated numerically. A numerical model was set up based on the geological information in the Shenhu area of Baiyun Sag in the northern South China Sea (SCS) to simulate the long-term transport of CH4 and the associated MH formation with formation dip. We further conducted a sensitivity analysis and a statistical analysis on three key geological parameters (i.e., dip angle, CH4 leakage rate, and water salinity). The results indicate that the dip angle (θ) of the reservoir promotes the lateral migration of CH4. The total accumulated amount of MH decreases 2.45% with θ increasing from 0° to 10°. Sensitivity analysis suggested that the formation of MH predominantly depends on the CH4 leakage rate (RCH4) in inclined reservoirs. The lateral migration distance of CH4 increases 13.1% but the total accumulated amount of MH decreases 8.8% with increasing water salinity (Xi) from 3.0 wt% to 3.5 wt%. We further analyzed the effect of heterogeneity, which shows that the inclined reservoir with alternating sand and clay layers favors MH formation more than a uniform reservoir with an 8.8% increase in the total MH. Understanding the effect of formation dip on the transport of CH4 and MH formation provide a useful reference in the identification of marine MH resources and the design of safe and effective exploration strategies.