Sensitivity analysis of reservoir risk in marine gas hydrate drilling

Abstract It is easy to change the original temperature state of marine gas hydrate reservoir by drilling, which leads to uncontrollable decomposition of gas hydrate and release of large amount of gas. The decomposition gas will further escape and expand, and the reservoir will break and collapse due its weak cementation characteristic, which will easily lead to a series of other potential risks. Therefore, in this study, based on the drilling process of marine gas hydrate, we establish the theoretical model and numerical calculation method of wellbore temperature field, analyze the influence on wellbore temperature of drilling fluid displacement, density, viscosity and injection temperature, and seawater depth. Then the sensitivity laws of reservoir risk in marine gas hydrate drilling are obtained. The results show that with the increase of drilling fluid displacement, density, viscosity and injection temperature, the temperature in lower well section and bottom hole will increase, making the increasing amplitude of temperature in hydrate reservoir larger and the scope of influence on hydrate reservoir stability bigger. Moreover, drilling is more likely to raise the temperature of reservoirs in shallow seawater depth, posing greater risks. Thus, engineering measures to avoid risks caused by rising reservoir temperature in marine gas hydrate reservoir drilling are presented. This study is of great significance to ensure the safety of marine gas hydrate reservoir drilling.