Dissociation Of Natural Gas Hydrate In Deepwater Drilling And Its Effect On Drilling Safety

During the deepwater drilling process,when drilling to shallow gas hydrate formation,the high temperature drilling fluid circulation will lead to the dissociation of natural gas hydrate near wellbore region.Which may cause gas ejection or affecting borehole and stratigraphic stability,thereby endangering the safety of drilling.Using self-built hydrate dissociation experiment system,this paper simulates the formation of different saturation of methane hydrate in the porous medium,then simulates the methane hydrate dissociation characteristics of different drilling fluid temperature and methane hydrate saturation.At the same time,combined with the experiment data and relevant theory,this paper establishes the hydrate dissociation kinetics model,heat transfer model and gas ejection model.Using these models,the gas production characteristics of methane hydrate dissociation and its influence on drilling safety are analyzed.The experimental results show that the starting temperature difference is low.That is,under the condition of constant pressure,the circulating temperature of the drilling fluid is slightly higher than that of the phase equilibrium temperature.When the dissociation temperature is reached,the gas production rate increases rapidly in a short time,and then reduces to low gas production state.It is revealed that the dissociation of near wellbore hydrate is mainly controlled by the heat transfer process,and the overheat and hydrate saturation are the main factors affecting the gas production and gas production rate.By using the models established above,analyses the influence of drilling fluid circulating temperature,methane hydrate saturation and drilling speed under typical deepwater drilling condition.Put forward the corresponding control measures to ensure the safety of drilling.