Coupling Effect Of Temperature And Pressure On Deepwater Casing And Subsea Wellhead

Oil & gas well under deepwater is different from onshore well structure.Deepwater casing and subsea wellhead are subjected to coupling effects of the thermal stress and annular pressure caused by temperature variation during production process,which increases the failure risk of oil & gas wells under deepwater.Thus,it is of great value to analyze the coupling effects of temperature and pressure on deepwater casing and subsea wellhead.Is is of great importance to reveal the wellbore temperature distribution,casing pressure loads and subsea wellhead stability for well structure design,production plan and safety control.For the well structure under deepwater,considering the transient heat transfer of the oil tubing and the annulus liquid,the calculation model of the temperature field is established in sections,which can calculate the thermal expansion effect of the annulus liquid and the thermal stress of the casing.Results show that the wellbore temperature tend to stabilize with the increase of production rate and time.The insulated tubing can increase the temperature of oil tubing fluid while reducing the temperature of outer fluid.The deepwater multi-layer annular pressure buildup calculation model is established based on the state equation and thermal elasticity theory,which considered the coupling effect of thermal property of annular fluid and annular volume change.Results indicate that the annular pressure increases with the increase of production rate and time,but reduces from the inside to the outside space.Since the thermal properties of the annular liquid change nonlinearly with temperature,when the average temperature rise of the annular liquid is the same,the higher the temperature of the annular liquid,the greater the annular pressure;the application of the thermal insulation tubing to lower the temperature of the annular liquid or the application of the annular pressure relief tool are effective methods to reduce the annular pressure of a deepwater wellbores.Aiming at the seal failure problem of cement ring,considering the influence of annular volume variation with annular pressure,a multi-layer annular pressure calculation model caused by cement ring seal failure is established.Numerical simulation results show that the annular pressure of the multi-layer annular increases due to gas channeling among which the B cement ring seal failure causes the highest annular pressure while adjacent annular is relatively low;the annular volume change affects the stable speed but does not affect the maximum annular pressure;the application of the annular pressure calculation model can quantitatively determine the comprehensive permeability of the cement ring,and the comprehensive permeability of the cement ring calculated by considering the volume change of the annular is low.The above calculation model was verified by the measured data,and the calculation results were in good agreement with the field data.Based on the prediction results of annular pressure and temperature,the calculation model,which illustrates the influence of the thermal property of casings,production rate,production time on wellhead growth height and resultant axial force is established considering the coupling effect of thermal pressure,bulging force and annular pressure.This simplified calculation model is applied to analyze a wellhead growth accident and the results matches well with field data.