Research On Wellbore Pressure Prediction And Control Method For Multi-Gradient Drilling In Deep Water

The oil and gas resources are abundant in deepwater,and the prospect for exploration and development is very broad.However,due to the narrow pressure window formed by high pore pressure and low fracture pressure,it is difficult to control the wellbore pressure in deep water drilling.What’s more,it is easy to cause complex downhole conditions such as well kick,gas invasion or lost circulation.Compared with the existing managed pressure drilling(referred to as MPD)technology,the multi-gradient MPD drilling technology can well adapt to the narrow safe density window in deepwater formation and the prospect for application is broad.A downhole separator is used in the new technology to separate the hollow glass spheres(referred to as HGS)from the drilling fluid inside drill string and inject it into the annulus,multiple pressure gradients can be achieved in annulus.Although the multi-gradient MPD method is feasible in principle,the separation efficiency of the existing separators is low,and the basic theory of wellbore pressure prediction and control methods are not perfect.Therefore,the prediction and control of wellbore pressure is taken as the main line in this paper,and combines theoretical research with laboratory experiment to carry out the following research:I.Based on the filtration mechanism of porous media,a new type of downhole filter separator was developed with spherical filter plug and metal filter screen as key components.Numerical simulation research was carried out on flow field,separation efficiency,erosion wear rate and strength check.The indoor simulation experiment system of multi-gradient drilling was established,and the separation efficiency of the filter separator,the slip rate of the HGS and the wellbore pressure gradient were carried out.The test experiment has verified the feasibility of the multi-gradient drilling method and the separation efficiency of the separator was improved to 87%.II.Considering the influence of variable mass flow when the separated HGS entered the annulus,the slip of HGS in the drilling fluid,and the energy exchange between the wellbore and the formation on the physical parameters of the drilling fluid,a transient model of coupled wellbore temperature and pressure based on single-phase flow for multi-gradient drilling in deep water was established.The model was solved by finite difference method and the sensitivity was analyzed.The results showed that the wellbore pressure has an inflection point at the separator and the overall distribution was "broken line" in multi-gradient drilling.As the number of separators increases,the wellbore pressure decreases and the inflection point of the broken line increases,which can better adapt to the narrow safe density window of the deep water formation.An increase in wellbore temperature reduces annulus pressure,but the overall effect is limited.Variable mass flow of HGS at the position of separator reduces the annulus temperature at the separator and slightly increases the wellbore pressure.The multi-gradient parameters such as location or number of separators,HGS density or volume fraction and the other factors such as HGS slip distance,multi-pressure system location can significantly affect drilling fluid density and wellbore pressure distribution.III.Based on the non-equilibrium interface mass transfer theory,the effect of the mass transfer resistance caused by surface tension at the mass transfer boundary layer of the bubble,variable mass flow induced by multiple gradient parameters in annulus,the energy exchange between the wellbore and the formation on the gas-liquid interphase transfer rate were considered,the multiphase flow model for multi-gradient drilling based on the transient non-equilibrium interphase mass transfer in deep water was established.Then,the implicit finite difference method was used to solve the model,and the influence of different multi-gradient parameters on the non-equilibrium interphase mass transfer rate and wellbore multiphase flow behavior was studied.The results showed that compared with the equilibrium interphase mass transfer condition,the interphase mass transfer is more difficult and the mass transfer rate is lower under the non-equilibrium mass transfer condition.The multi-gradient parameters cause the change of variable mass flow in annulus and the gas-liquid interphase mass transfer rate,which has a significant impact on the flow law of multiphase flow in the wellbore.IV.Based on the multiphase flow model of transient non-equilibrium mass transfer for multi-gradient drilling in deep water,the numerical study of wellbore pressure control was carried out as the gas invasion occurred by adjusting different multi-gradient parameters.The multiphase flow behavior in wellbore was described.According to the principle of variation coefficient weighting method,a comprehensive evaluation index model of bottom-hole pressure increment,the peak value of overflow and the gas void fraction at the wellhead cross-section based on different control parameters was established.A optimization method of key parameter was gained under the control target of the maximum increment of bottom-hole pressure,the minimum peak of overflow flow or the minimum gas void fraction at wellhead cross-section.The results showed that compared with the constant bottom hole pressure MPD,at the initial stage of gas invasion,multi-gradient drilling can identify gas invasion earlier.At the control stage of gas invasion,the increment of bottom hole pressure is larger when the same bottom hole pressure reachs in the same time.The optimal control parameters is to reduce the HGS volume fraction,increase the HGS density,or lift the position of separator.the self-developed downhole filter separator and indoor simulation experiment system of multi-gradient drilling are taken as the hardware basis,the wellbore pressure prediction model is established as the theoretical premise,the effective control of wellbore pressure is regarded as the core purpose,and the laboratory experiment was used to verify the theory as the key means in this paper.The distribution law of wellbore pressure and control method for multi-gradient are studied,which has important theoretical and engineering significance for using this method to solve the drilling problem of narrow safe density window in deep water formations.