Risk Analysis Of Wellbore Integrity Of HPHTHS Gas Wells

Gas wells with high temperature,high pressure and high sulfur(HTHPHS)were festured with exploitation technology limition,harsh exploitation environment,massive potential risk factors and severe impact of wellbore failure.However a higher requirement for wellbore safety is puted with natural gas gap increasing in China.Based on reducing failure risk of wellbore,this paper identified and evaluated wellbore failure factors,researched the casing reliability and annulus pressure risk,and proposed management and controlling measures.Firstly,this paper investigated the fault types of HTHPHS wells,including the failure of Christmas tree,Casing strings,Tubing strings,Cement rings and Downhole seals,further analyzed the causes of each failure type and accidents prevention measures,and evoluted the process of wellbore failure,obtained it's key causes applying Bayesian network.Secondly,based on the theory of limit equations and the principle of reliability,this paper got the variable and parameters affecting the strings reliability,established the theoretical analysis model and the finite element simulation model for the reliability of wellbore strings,and obtained the affection of different factors on well reliability.Taking the casing at 6 km as an example,the safety factor and reliability index of the casing string at different confidence levels are calculated.Conclusion is wellbore casing at 6 km can meet the safety design requirements of HTHPHS wells.Thirdly,this paper analyzed the damage caused by the annulus pressure to the wellbore integrity of HTHPHS well and the means of prevention,determined the weak point of the A/B/C annulus.According to the annulus pressure recommended of API RP-90,combined with actual damage situation,established the determination method and calculation model of the maximum allowable pressure of annulus,and the monitoring methods,diagnostic methods and treatment measures are established for the annulus pressure.Finally,this paper,with reference to HTHPHS well integrity management specifications,systematically analyzed and developed management measures of wellbore integrity in construction phase,production phase and disposal phase of HTHPHS wells,such as detailed management for well barriers,annulus pressure,and risk assessment.