Study On Wellbore Pressure In Deep Water Shallow Hydrate Drilling

There were many risks in the process of natural gas hydrate drilling.For example,in the process of ocean hydrate drilling,the original hydrate will decompose and generate gas due to the change of equilibrium conditions.When the gas generation speed is too fast,it will cause the wellbore.The dangers of sudden decrease in pressure,reservoir wall collapse,overflow or blowout,this paper aims to study the decomposition characteristics of hydrated drill cuttings into the wellbore and analyze the effect of hydrate decomposition on wellbore pressure.Firstly,according to the law of conservation of energy and the principle of heat transfer,considering the influence of hydrate phase change heat,the prediction model of wellbore temperature field in hydrate drilling process was established,and the influence of hydrate decomposition on the transient temperature distribution of wellbore was compared and analyzed.The D-G prediction model of the chemical equilibrium method was selected from the four methods of graphic method,equilibrium constant method,empirical formula method and chemical level balance method as the hydrate phase equilibrium judgment method.The combination of hydrate decomposition inhibitors was selected under the effects of drilling fluid compatibility and rheology.According to the principle of mass transfer,the principle of mass conservation and the molecular structure of hydrate,on the basis of the decompressional decomposition velocity model,added with the surface area of hydrate decomposition reaction and the equivalent molar mass of methane hydrate,and the decomposition model of hydrate drill cuttings was established.The hydrate drill cuttings and the initial decomposition position and hydrate decomposition rate at different time points above the initial decomposition position were determined,and the effects of temperature,pressure and drill cutting diameter on the decomposition rate of the hydrate were simulated.According to the continuity equation and the momentum equation,considering the transient temperature distribution and the dynamic decomposition rate of hydrate,a multi-phase flow model of deepwater hydrate drilling wellbore was established to simulate different drilling rates,drilling fluid displacement,drilling fluid inlet temperature,the effects of decomposition inhibitors and wellhead pressure on hydrate decomposition rate,wellbore multiphase flow parameters,and bottomhole pressure.Then proposed a treatment method for incomplete decomposition of hydrate drill cuttings.After considering the influence of hydrate decomposition,the wellhead temperature was lower than before,so the influence of hydrate decomposition on the wellbore temperature cannot be ignored.A combination of a polymer inhibitor of 0.1 wt%PVP + 0.5 wt%lecithin was selected,and its effect can increase the decomposition temperature of methane hydrate by 3 ℃ and reduce the decomposition rate by about 50%.The initial decomposition position of the hydrate is located in the riser with a depth of 500-600 m,and there is no reformation of hydrate at the mud line.Smaller drilling rate,smaller drilling fluid displacement,lower drilling fluid inlet temperature,addition of hydrate decomposition inhibitor,and application of a certain wellhead pressure will slow down the decomposition rate of hydrates in the cuttings and prolong the decomposition time.Increasing the drilling fluid displacement,reducing the drilling rate,cooling the drilling fluid,adding decomposition inhibitors,and increasing the back pressure of the wellhead all reduced the gas holdup of the wellbore and slowed down the reducing of bottom pressure.What’s more,the addition of inhibitors and the application of wellhead back pressure will cause the hydrate uncomplete decomposition after reaching the wellhead.The research results and understanding of this paper not only provide a theoretical basis for the development of technical measures for safe drilling of hydrate reservoirs,but also provide new ideas for the safe treatment of hydrate drill cuttings.