Study On Casing Strength And Safety Reliability Of UTUPUD Gas Wells

With the deepening of oil and gas exploration,the development of oil and gas field present four major directions,which are ‘U-deep,Low-permeation,Ocean and Unconventionality’.This leads to more and more tertiary recovery technology applied to production.And result in casing damage more serious under the increasingly rigorous demand of casing service conditions.Because of the difference of failure mode and mechanism under different working conditions,it has a striking difference in pipe quality control index and tubing test evaluation method.Therefore,casing failure control and integrity analysis needed to combine with the actual working condition.Then,combined with the casing damage mechanism,a quantitative safety evaluation model was established.That can help people clearly know again casing damage risk,and provides the technical basis for the oilfield production safety.Start with the casing damage failure modes and mechanism,combined with the UDUPUT(ultra deep well,ultra deep well,ultra-high pressure)casing damage,the casing damage classification and quantitative processing were completed.The detailed information at the casing damage position and size,etc were obtained,through the well log data.Then a calculation model of residual strength of damage casing was established,by which we can get the residual strength and strength reduction factor of damaged casing.Combined with the engineering field and indoor acoustic logging data and large-scale triaxial test,complete the logging information interpretation,get the well location dispose of in-situ stress distribution,and complete block in-situ stress based on multi-objective constraint inversion,then get the well’s ground stress load information by calculated.Based on the existed string strength and load statistical data,through the partial coefficient of reliable calculation method,establishes string reliability design and evaluation calculation model and complete pipe string design and quantitative evaluation.Specific research work and conclusions are as follows:(1)Studied the single well layered situ stress calculation based on the logging information and the situ stress inversion problem based on multi-objective constraint.The dynamic and static parameters of rock mechanics corresponding relation curve were obtained by contrast the interpretation results of filed logging information to test data,realized block in-situ stress calculation on the basis of logging information.To constraint condition of strata situ stress inversion analysis,complete the situ stress inversion research under the multi-objective constraints.The result shows that: the elastic modulus and poisson’s ratio increased linearly with the increase of confining pressure.There is a linear relationship between the dynamic and static of rock rock mechanics parameters.Through comparing with the situ stress inversion results and the results of logging information interpretation of some key well in block,Found that the error of inversion results and the measured between 1.71% and 6.62%,that indicate the situ stress inversion method based on multi-objective constraint is closer to the actual engineering.(2)Considering the wellbore pressure and the non-uniform coefficient of situ stress,study the stress field variation near wellbore area by theoretical derivation using elastic-plastic mechanics.The results show that situ stress non-uniform coefficient has a decisive influence on stress field near the wellbore.The circumferential stress,radial stress,shear stress and Mises stress all has the exponential function relation with increasing distance from the wellbore.And the influence was within 5 meters from the wellbore.(3)Discussed two damage patterns of corrosion and wear on the influence of casing residual strength.Established a simplified model by field investigation,and obtained the residual strength of damaged casing using the analytic calculation and finite element simulation method,and get the oil and gas well tubing string curves of residual strength,as well as the change rule.(4)Get the elastic modulus and yield strength of the pipe material under different temperature with triaxial test.And then the temperature stress distribution formula of casing-cement-formation was deduced.Considering the plastic numerical model of cement ring formation,simulated the system of casing-cement-formation temperature under different stress non-uniform coefficient and working conditions,and established the deformation and stress calculation model and numerical model for the tubing under various construction conditions,then compared simulated values and analytical value,with the error within a reasonable range,so the theoretical model is relatively accurate and meet the using conditions.(5)A reliability design of casing strength based on the partial coefficient method was put forward,which combined domestic and foreign construction ISO 2394,EN 1990,GB50153 and other unified standard for reliability design.This method divide the casing service process into two stages,drilling completion stage and normal production stage,on the basis of the differences of casing load combinations and the degradation of casing strength before and after cementing consolidation.This paper establish the limit state equations of the casing in the above two stages respectively,calculates the partial coefficient of casing load and intensity respectively by using reliability theory,and takes into account the influence of variation coefficient of load and strength on casing design values.It achieves the reliability quantitative expression according the casing service process,and so it can predict the safe reliability during the period of casing service process effectively.