Experimental study and numerical modeling of methane hydrate dissociation and gas invasion during drilling through hydrate bearing formations

Gas hydrate has been concerned as a potential shallow hazard during deepwater drilling. In this study, hydrate dissociation and gas flow into wellbore induced by circulation of high temperature drilling fluid when drilling through hydrate bearing formations have been investigated. A specially designed experimental setup based on sandpack model was used, which can simulate the process of methane hydrate dissociation and gas production in wellbore with circulation of drilling fluid. The experimental results show that the rates of hydrate dissociation and gas production are greatly influenced by the temperature of drilling fluid, hydrate saturation and pressure. A mathematical model was derived to simulate the process of hydrate dissociation and gas invasion into wellbore within a few hours when hydrate zones being penetrated during drilling. The effects of various parameters on gas invasion rate have been evaluated, including the inlet temperature and circulation rate of drilling fluid, the rate of penetration, the wellbore size, and the circulation condition with or without drilling risers. The results show that small to moderate gas invasion can occur when drilling through hydrate zones mainly depending on the inlet temperature of drilling fluid and hydrate saturation in near wellbore formation, which can be manageable when low-temperature drilling fluid is used and with a low circulation rate. Optimizing penetration rate, reducing wellbore size and drilling without risers are also beneficial to decreasing the gas invasion from hydrate zones into the wellbore.