Study On Wellbore Pressure Distribution And Control Method Of Variable Gradient MPD In Deep Water

Deepwater area has been an important succession part of oil and gas exploration and development in China for its abundant resources.However,the narrow safe density window issue and the difficulty in controlling the wellbore pressure of deepwater formation can lead to downhole complexities such as well kick,loss circulation and coexistence during well drilling,which brings great challenges to ensure safe and efficient drilling operation in deepwater area.The variable-gradient managed pressure drilling(MPD)method is a new technology proposed in recent years to address the above problems in well drilling,which can adapt to the narrow safe density window of deepwater formation well and has broad application prospects.This new technology can form multiple pressure gradients in the annulus by using a cyclone separator to separate the Hollow Glass Spheres(HGS)in the drill string.For this reason,it can lead to large error when using the traditional wellbore pressure calculation model and control method in variable-gradient MPD because both the fluid flow law and pressure control method in wellbore are very different from the traditional MPD technology.In addition,although the variable-gradient MPD method is feasible in principle,its feasibility has not been verified by laboratory experiments or field tests.Therefore,taking the wellbore pressure of variable-gradient MPD as the main line,the following research has been carried out:?.Based on the principle of segmented control of annulus density in deepwater drilling,we established a set of indoor physical simulation experimental system for variable-gradient MPD.We carried out experimental research on the slip rate of hollow glass spheres,separation efficiency of cyclone separator and variation of wellbore pressure separately.And explored the rules of influences of drilling fluid performance parameters,hollow glass spheres physical parameters,and engineering parameters on hollow glass spheres slip rate,cyclone separator separation efficiency,and variation of wellbore pressure.The experimental results show that the hollow glass spheres have low slip rate relative to the drilling fluid when using a small diameter so the mixed fluid of the drilling fluid and the hollow glass spheres in the wellbore can be regarded as a single-phase flow;the cyclone separator can separate the hollow glass spheres in the drill string effectively;variable-gradient MPD based on the cyclone separator method can control multiple pressure gradients in the wellbore effectively.?.Based on the mass,momentum and energy conservation equations of drilling fluid and hollow glass spheres,and considering heat and mass transfer during the process that hollow glass spheres are being separated and injected into the annulus from the drill string,we established a transient temperature-pressure coupling model under single-phase flow conditions,which is solved by using the finite volume method and the dynamic stratification method.Using this comprehensive model,we analyzed the rules of variation of the wellbore thermal behavior and wellbore pressure during the normal cycle of variable-gradient MPD.The results show that the hollow glass sphere reduces the rate of heat transfer between the wellbore fluid and the surrounding environment;the wellbore pressure of variable-gradient MPD is distributed as a "broken line" shape on the image as a whole,so it can better fit the narrow safety density window of deep water formation;the combination of the concentration of hollow spheres,the position and separation efficiency of the cyclone separator enables very flexible adjustment of pressure distribution throughout the wellbore.?.Coupling the interface mass transfer theory of slip-rising bubbles into the drift-flux model,and considering the annulus variable mass flow,the system heat transfer between the multiphase flow in the wellbore and the surrounding environment caused by the variable gradient parameters,we established a new model of all-transient non-isothermal gas-liquid two-phase flow under the condition of gas kick in variable-gradient MPD and solved by using the finite volume method.This model was used to analyze the thermodynamic behavior,the rate of interphase mass transfer,and flow rules of gas-liquid two-phase in the wellbore during the process of gas kick in variable-gradient MPD.The results show that the annular temperature near the bottom-hole increases gradually during the process of gas kick,so it can be used as a characteristic of response to detect gas overflow;the fluids flow state,heat transfer and inter-bubble interaction have significant effects on gas-liquid mass transfer rate;variable gradient parameters,gas-liquid mass transfer,and system heat transfer also have great influences on the flow rules of gas-liquid two-phase in the wellbore.?.Based on the wellbore pressure prediction model under normal circulation conditions,we established a key parameter optimization model of variable-gradient MPD during the process of dynamic drilling.In the optimization model,the maximum drilling depth after each adjustment of the variable gradient parameters for a single drill bit trip and the maximum drilling depth under the same well structure were set as the optimized subjects,the concentration of hollow spheres,drilling fluid density,position of cyclone separator and wellhead back pressure were set as the optimizing parameters,and safe density window as well as the drill bit life expectancy were the constraints.The optimization model was solved by particle swarm optimization algorithm.The results show that variable gradient parameters need to be adjusted multiple times when drilling to the maximum well depth under the same well structure;the drill bit life expectancy has great influence on the times of trips;variable gradient parameters and the design of bit should be coordinated to avoid waste of rising and drilling;variable-gradient MPD can simplify the wellbore construction effectively.?.By applying the dynamic well control method of conventional MPD to variable-gradient MPD,we established a dynamic control method of wellbore pressure in variable-gradient MPD under the condition of gas kick,and the variation of hydraulic parameters in the process of dynamic well control was simulated by using gas-liquid two-phase flow model.The results show that the variable mass flow caused by the transfer of hollow glass spheres and the abrupt change of wellbore diameter on the mudline can lead to more complex changes of wellhead back pressure during gas kick treatment.Then,based on the unscented Kalman filter theory and the simplified drift-flux model,an inversion model of formation parameters during gas kick was established.The calculation results show that using the overflow rate and wellhead back pressure during the process of gas kick can accurately invert formation pressure and production index,and accurate gas kick parameters can be obtained during inversion of formation parameters.This provides a reliable basis for determining the wellhead back pressure accurately during dynamic well control.This paper is based on the premise of the experimental verification of variable-gradient MPD,the basis of the prediction of wellbore pressure under single-phase and multi-phase flow conditions,and the goals of controlling the wellbore pressure under normal and gas kick conditions,studies the pressure distribution and control method of variable-gradient MPD.The study results have important theoretical and engineering significance for using this method to solve the drilling problems of narrow safe density window in deep water formation.