New Evaluation Method Of Casing Collapse Strength For HPHT Well Considering The Annulus Temperature And Pressure

As large amount of early investment and high technical risk,the design of casing strength proposes high demand of life cycle.The heat carried by oil transfer from the wellbore to earth heating the annulus and building the annulus pressure.It pose a threat to life cycle design of casing strength.So,this dissertation discussed the relationship between heat carried by oil and annulus temperature and pressure.The casing collapse strength under annulus pressure and bending was analyzed.Based on this,the recommendation of HPHT casing collapse strength calculation method was outlined referencing to API 5C3.The follow was the main research contents and method.Firstly,the matrix of wellbore temperature field considering the disturbance of fluid thermal equilibrium and coupling effect of temperature and pressure is obtained by using the theory of multi-cylinder heat transfer discipline,principle of conservation of energy.Secondly,the computing method of the fluid pressure and volume in micro-annulus is proposed considering the temperature effect.The two-way coupling model between the annular pressure and fluid volume is given by combining equations with the mass-conservation,energy-conservation and heat stability of fluid.A code is developed to conclude the variation of annular pressure-volume and estimate the trend of annular temperature and pressure under different producing time and rate of production.Thirdly,based on the calculation results of annulus pressure,the recommendation of HPHT casing collapse strength calculation method was outlined referencing to API 5C3 and API-RP 90.Fourthly,taking care of the strain energy of flatten casing cross section under bending,the total strain energy formula of casing cross section was established.The minimum potential energy principle was apply to analyze the relationship between flattening of casing cross section and curvature.Considering the influence flattening of casing cross section to collapse,stress equivalence principle was apply to conclude the triaxial stress and extreme pressure.Finally,according to above analyses of annulus temperature,pressure and casing collapse strength,the recommendation calculation method of annulus temperature,pressureand casing collapse strength considering the annulus temperature and pressure was outlined referencing to existing calculation process of HPHT casing collapse strength.The study indicate that there are thermal shock pressure in annulus of free casing as high temperature oil flowing pass.The relationship between annulus temperature and pressure is almost linear under coupling effect.The diameter-to-thickness ratio has great influence to flattening of casing cross section comparing to curvature.Just for the P110 5 1/2"×7.72 casing,the flattening coursed by 0.01/m curvature decreases the collapse resistance concluded by API 5C3 about 7.5%.