Analysis On Characteristics Of Transient Pressure Produced By Super Critical CO₂ And Influence On Annular Pressure

Managed pressure drilling?MPD?is used to control bottom pressure in a certain range so that risk of accidents which include well blowout,leakage,sticking can be reduced.The core principle of MPD is that bottom pressure consists of three parts,hydrostatic fluid column pressure and annular pressure loss and wellhead back pressure.This is a model without time variation,but the process of closing BOP and close or open valve is a dynamic process which will produce transient pressure and may have influence on bottom pressure of well in the form of pressure wave.BOP,a special components of well-control technology,the influence of transient pressure produced in its closing process has been a problem in choosing shut-in modes.Previous research analyzed the transient pressure on the basis of water-hammer theory but they do not give firm conclusion to choose hard shut-in or soft shut-in when overflow occurs.Super critical CO₂ is very sensitive on change of pressure,quick movement of opening throttle valve may will cause phase change,or even lead to partial gasification of CO₂,all of this will have an impact on wellbore pressure control of super critical CO₂.A mathematical model of inertial pressure is established on the basis of unsteady flow theory to describe the transient pressure in the process of dynamic well-control process.Model describe how parameters of drilling fluid which included drilling fluid velocity,drilling fluid density,drilling fluid viscosity and fluid motion control body parameters which included gate structure of ram and closing-speed of the ram influence transient pressure.According to theoretical analysis and CFD results,there are same change rules of consistency between inertial pressure and transient pressure,drilling fluid velocity and moving speed of spool or ram have great impact on transient pressure.The mathematical model can offer some theoretic guidance on the transient pressure which produced by throttle valve and BOP in the dynamic well-control process.In the analysis on transient pressure in the closing process of BOP,theoretical analysis and CFD results show that transient pressure produced at the ram hole is less than one out of tens of the calculated value of water hammer pressure,which illustrates that the transient pressure has a limited impact on wellbore pressure,therefore,quick operation can be taken to prevent more formation fluids coming into the well.Analysis also shows that even when severe gas influx,which lead to faster velocity of slug flow,transient pressure caused-by hard shut-in still have little impact on annular pressure and bottom hole pressure.Studies is shown that when super-critical CO₂ flow to well head through the annulus,the phase of CO₂ is nearly in gas state due to the reduction of pressure,density of gas state of CO₂is one sixth of super-critical CO₂,in this case,transient pressure produced by CO₂ in the valve adjusting process is small.Transient pressure in the wellbore propagates rapidly for the reason of low bulk modulus of gas state CO₂,so the influence of transient pressure on annular pressure can be ignored.