Wellbore Flow Analysis During Marine Gas Hydrate Drilling

While the gas hydrate drilling, because gas hydrate has phase transition in wellbore or formation, the traditional annular multi-phase flow theory can not meet the engineering requirements, which need to undertake more study on this problem.Combined of the characteristics of gas hydrate drilling and Managed pressure drilling technology, we proposed Managed pressure drilling technology can be suitable for the gas hydrate drilling, and probed into the attentions and solutions while drilling.In view of the reservoir characteristics and the drilling process of the natural gas hydrate, established the phase equilibrium equations of the gas hydrate in the wellbore and in porous media. In study of the formation and decomposition kinetics of the gas in both of the wellbore and the porous media, and finally get the equations to calculate the formation and decomposition kinetics.Simulate the gas production rate by controlling the rate of gas injection, study the law that gas production rate and the annular pressure impact on the annular multi-phase flow pattern transition through the experiment: under the experimental condition, there are four kinds of flow patterns: bubble flow, cap-bubble flow, cap-churn flow and churn flow, the slug flow does not exist. While the decomposed gas went into the wellbore, it increased the annular multi-flow's fluctuation, promoting the flow pattern transformation occurring. Plot the flow pattern maps under different pressures based on the experiment (up to 0.6Mpa).Compared the boundary lines among the four flow patterns under different pressures, we can see with the pressure increase, the gas velocity when the flow pattern changed need to increase. So the pressure represses the flow pattern transition.Based on the experiment and the theory, considering the hydrate phase transition, we established the multi-phase flow calculation model for gas hydrate drilling annulus, and developed the program to solve the equations. By numerical calculation, we analyzed the gas volume fraction in the wellbore, pit gain, bottom-hole pressure, annulus pressure considering or non-considering gas hydrate transition, and studied factors to the parameters and points for attention in practice, which provide theoretical basis for gas hydrate drilling.