Research On Wellbore Mechanical Integrity Of HPHT Gas Well

Based on the research of wellbore mechanical integrity at home and abroad, this paper mainly adopts the theoretical analysis and numerical simulation method to study wellbore mechanical integrity of gas well string and cement sheath in detail under the high temperature and high pressure conditions.(1) Based on the steady pipe flow model and wellbore radial heat transfer model,this paper established a prediction model coupled pressure with temperature and made a calculation procedure.Through the calculation of wellbore temperature and pressure,providing basic parameters for wellbore mechanical integrity evaluation of oilpipe and cement sheath.(2) A model of gas well tubing string deformation calculation and strength check has been established and a calculation procedure has benn made. With the procedure, the strength of the tubing is checked and the deformation of the tubing during different cases,such as setting,well-shut,production,fructing, is calculated.The results can leae to guide the design and evaluation of tubing mechanical integrity.(3) Considering the effect of temperature,this paper has established a coupled casing-cement-formation elastic mechanics theory model and used this model to analyze the influence of casing pressure,formation confining pressure, the temperature effect on cement sheath stress, results show that:when formation pressure is certain,with the increasement of casing pressure or temperature,the sustained radial compressure stressure of cement shearth is enhanced,while the tangential tensile stress is also increased, and both kinds of stress along the radial direction reached the largest at the in-wall of the cement sheath; when casing pressure is certain, with the increasement of formation pressure, the cement sheath along the tangential derection gradually changes from tension to compression and the sustained radial compressure stressure of cement shearth is also increased.(4) A casing-cement-formation finite element model of thermal structure coupled has been established. Using Ansys Workbench to simulate the influence of wellbore temperature, formation temperature and rate of the cement sheath heat release on the radial thermal stress,the results show that:when formation temperature and cement sheath heat release rate at certain, the higher the wellbore temperature, the greater the maximum temperature and radial thermal stress in the cement sheath, and the maximum value is more close to the first interface;when ignoring the cement exothermic, the higher the range of temperature between formation and wellbore, the greater the thermal stress.(5) Based on the theoreticalcement sheath mechanics model to analyze the effect of cement thickness and the well head casing pressure on cement mechanical integrity, the results show that:when cement sheath elasticity modulus is certain, to enhance the mechanical integrity of cement sheath,the samller the elastic modulus of formation, the lager the needed thickeness of cment sheath;when casing pressure test,the test pressure value and the elastic modulus of cement sheath has some influence on the cement sheath mechanical integrity.(6) Considering the cement sheath height,this paper established a casing-cement-formation finite element model and simulated the effect of cement sheath height on its mechanical integrity.Results show that:free casing section ballooning effect along the axial tensile stress on cement sheath end face two interface the following local scope, when the local tensile stress exceeds to the gel strength of cement sheath can damage its mechanical integrity in high temperature and high pressure gas wells. By increasing the cement sheath height and the drilling fluid column height, decreasing wellhead casing pressure value can improve the cement sheath in cementing surface along the axial tensile state,thus increasing the mechanical integrity of cement sheath.(7) Based on the cement sheath mechanics finite element model above to simulate the uniform and non-uniforum crustal stress impact on its mechanical integrity, results show that: for the formation with different elastic modulus,through optimizating cement slurry systems and improving the elastic modulus of cement sheath can increase the mechanical integrity of cement sheath in HTHP gas wells.This paper research contribuate to improve the level of the wellbore integrity design and management of high temperature and high pressure, to prolong the life of gas well service, realize safe and efficient development in natural gas is of great significance.