The Study Of Natural Gas Hydrate Formation In Deepwater Well Drilling

The deepwater petroleum exploration and exploitation is doomed to be a vital measure to ensure the national energy source security with the rapid development of economy and the increasing demand for energy in China.In deepwater oil and gas exploitation,the temperature near mud line is low under the influence of water depth and ocean temperature.Natural gas hydrate may appear in wellhead,BOP,or choke line.,which will bring great risks to deepwater well drilling.Therefore,studying on the theory of natural gas hydrate formation and prediction of formation conditions are of great theoretical and practical significances for the overall process of deepwater drilling.This paper established a model of Natural gas hydrate phase equilibrium according to the extensive research of the prediction method of hydrate formation.Based on the principle of conservation of mass,principle of conservation of momentum,principle of conservation of energy and theory of heat transfer,a wellhole temperature and pressure prediction model was established.According to the heat transfer characteristics of different heat transfer units and the phase change of gas,the model could be divided into four working conditions such as no kick circulation,kick circulation,shutting and well control.Combining with the auxiliary equation and definite condition,the model was solved by finite-difference method and verified through the field data.Then,the gas hydrate formation under the conditions of no kick circulation,kick circulation,shutting and well control was predicted.The influences of flow rate,shut-in time,cycling time,hydrate inhibitor and heat insulation layer of riser were discussed.The changes of air phase volume fraction,bottom hole pressure,pit gain,and choke pressure were simulated to study the influences of phase change to wellbore multiphase flow.Afterwards,a water hammer pressure model for shut-in was established to compute the water hammer velocity and water hammer pressure.In order to study the effects of pressure fluctuation,the molecular dynamics method was used to analysis the hydrate formation under the conditions of stable Finally,a software of nature gas hydrate formation prediction and analysis of wellhole pressure in gas kick and well contorl for deepwater drilling was written.The simulation result showed that there was a little gas hydrate formed in gas kick condition,and the phase change almost had no effect to fluid flow.But in shutting condition,there was some gas hydrate formed because the low temperature and high pressure at mud line.Meanwhile,the pressure surge also improved the nucleating rate and growing rete of hydrate.In well control condition,a little gas hydrate may be formed in choke line.The hydrate near mud line may dissociated as the increased temperature which affected the gas phase volume fraction and choke pressure in choke line.Hence,some safeguard measures such as increasing flow rate,reducing the shut-in time,using hydrate inhibitor or thermal insulation material to reduce the hydrate formed.The study result in this paper could be used in natural gas hydrate prediction and analyzed the multiphase flow patterns.It can provide the theoretical support to hydrate prevention and well control measures in deepwater well drilling.