Research On Optimization Design Of Wellbore Structure Dased On CML Drilling Technology

The narrow density window between pore pressure and fracture pressure is one of the most important problems faced by deep-water drilling.Controlled Mud Level technology is a new type of controlled pressure technology developed on the basis of dual gradient drilling.The purpose is to solve the problem of drilling in narrow density window.When drilling with CML technology,the equivalent drilling fluid density of the wellbore pressure is a curve,which is more in line with the formation pore pressure and fracture pressure,which can effectively increase the running depth of a single layer of casing,reduce casing running levels,and save drilling time and reduce drilling costs.This paper introduces the basic principles and key equipment of CML technology and analyzes its technical characteristics and applications in detail.By establishing the physical model of the CML drilling process and optimizing the drilling fluid rheological model,a mathematical model of the wellbore pressure distribution during the CML drilling process was established.Considering the temperature and circulation friction,the effects of drilling fluid density,free height of riser and drilling fluid displacement on wellbore pressure were studied..The research results show that the drilling fluid density and the free height of the riser have the greatest influence on the wellbore pressure.The inclination of the equivalent density curve corresponding to the wellbore pressure increases with the increase of the free riser height and the drilling fluid density.A wellbore structure design method suitable for CML drilling technology was created.Using this method for casing design can minimize casing usage.Finally,based on the pore pressure and fracture pressure profiles,the conventional method and CML technology were used to design the casing respectively.The design results show that,compared with the conventional technical,the use of CML technology reduces the running of two layers of casing,reduces the total casing length by 5859 m,and reduces the casing consumption by33.74%.