Study On Wellbore Flow Mechanics And Pressure Control In Deep-water Controlled Mud Level Dual Gradient Drilling

With the increasing demand of oil and gas resources in the world,deep water area has become an important succession part of global oil and gas resources.However,there are many problems and challenges in deep water drilling,such as narrow density window and difficult pressure control.Controlled mud Level(CML)dual gradient drilling can effectively solve the problem of pressure control in deep water drilling.This drilling system can accurately control the wellbore pressure by controlling the mud height level in the riser,which combines the advantages of managed pressure drilling and dual gradient drilling and effectively solves the pressure control problems in deep water drilling.For this new drilling technology,it has been widely applied abroad,however for domestic research it is just in its infancy.Effective wellbore pressure control is the key to the safety and speed of drilling,and the wellbore flow rules are the foundations for the prediction and control of wellbore pressure.Therefore,it is necessary to investigate into the wellbore flow law and well control technology of CML dual gradient drilling to ensure the smooth operation of the deepwater CML dual gradient drilling,thus providing reliable technical support for the effective development of rich oil and gas resources in South China Sea.Because of the influence of temperature and pressure field on the density and rheological properties of the mud in the wellbore,a wellbore heat transfer and flow coupling model considering the coupling of the mud density,rheological properties and temperature-pressure field of CML dual gradient drilling is established in this paper,combined with CML dual gradient drilling technology.And based on this model,the distribution law of temperature field and pressure field in the wellbore of CML dual gradient drilling well is analyzed.In view of the problem of the continuous changing of the mud level in the riser during the process of CML dual gradient drilling,a transient flow model of multiphase fluid between reservoir and wellbore is established and the boundary conditions of coupling flow between drilling fluid and formation fluid are established based on the gas-liquid two phase drift flow model.AUSMV format is adopted to solve the equation.In view of the deficiency of easier dissipation and not considering temperature effect,the first order AUSMV format is combined with the classic monotone MUSCL method,two-step Runge-Kutta methods and the optimized Vanleer slope limiter to obtain the second order accuracy of the AUSMV format,which improves the multiphase discontinuity capturing ability and enhances the calculation precision of this format.Furthermore,a mathematical treatment method of boundary conditions is given.Thus,a method of transient calculation of wellbore pressure is formed,which is suitable for the continuous changing of mud level in CML dual gradient drilling.For the problem of the occurrence of a U-tube effect after stopping the pump in the case of CML dual gradient drilling without installing a drill or the failure of drill pipe valve,a mathematical model of the U-tube effect of CML dual gradient drilling is established to simulate the variation law of the mud flow and the bottom hole pressure after stopping the pump,based on the Reynolds Transport Theorem of a deforming control volume.And the accuracy of the model is verified by comparing with the experimental data in the literature.Aimed at the problems of the incapability of timely monitoring of kick,a method of timely judging the kick in the process of U-tube effect is provided to ensure CML dual gradient drilling operation safety.In this paper,the method of kick detecting in CML dual gradient drilling is given and the applicability of conventional well control method in CML dual gradient drilling is discussed,based on the conventional well control theory and method.Against the shortages of conventional well control method,a well control method is presented for CML dual gradient drilling combined with the special process characteristics and equipments of CML dual gradient drilling system,which can provide a theoretical basis for well control operation in CML dual gradient drilling.According to the research of this paper,the flow law of multiphase fluid in CML dual gradient drilling is accurately described and a well control method is established,which is suitable for CML dual gradient drilling.The research results provide a theoretical basis for the CML dual gradient drilling,and it is of practical engineering significance to accurately control the wellbore pressure in the CML drilling and achieve the goals of safe and efficient drilling in the narrow safety density window in the deep water.