Safety Evaluation Of Wellbore Integrity Of High Sulfur - Bearing Gas Wells

In recent years, with more and more sour reservoirs containing high H2S and CO2concentration being developed, well integrity has captured increasing people’s attention. In order to keep well integrity, factor which may lead to the failures of well integrity should be evaluated. So that efforts can be made to mitigate the various specific risks, and to make the risky under the reasonable and acceptable extent, which can result in the purpose of safe and efficient development of oil and gas wells.The corrosion environment of tubing and casing is badly tough because gas wells have high concentrations of H2S and CO2.In order to predict the life of casing and tubing over life cycle scientifically, this paper applies the self-designed high temperature and high pressure reactor to imitate the condition of the XX well conducting assessment of sulfide corrosion and sulfide stress cracking experiment for the final purpose of evaluating the safety of the casing pipe structure through predicting the corrosion rate and calculating the critical stress-intensity factor. The corrosion rate is tested under the conditions of different testing time,and the time effect has been analyzed.The formula of corrosion rate and testing time is built using the mathematical statistic method, which provides the basis for predicting the life of casing and tubing. This paper tests the critical stress intensity factor in the H2S environment based on Double Cantilever Beam (DCB) of NACE TMO177-2005.The effect of the critical stress intensity factor on internal pressure strength of casing has been analyzed with the mechanical model of stress corrosion crack propagation.Based on experiment of the electrochemical corrosion, the calculation model of casing strength has been built considering pitting corrosion, uniform corrosion and sulfide stress corrosion cracking. To address the inevitable sustained casing pressure, calculation model of maximum allowable annulus pressure values has been built considering electrochemical corrosion and environment sensitive fracture based on the fundamentals above, which provides theoretical basis for evaluating the security situation and managing the gas wells with SCP. Lastly, according to well structure, fluid in annulus and the situation of SCP, this paper analyzes the effect of corrosion rate on casing strength and determines the maximum allowable annulus pressure values, and carried out the safety evaluation.This paper carries out the test of electrochemical corrosion rate and builds prediction model of corrosion rate considering time effect, which is innovative. For the existing calculation model of maximum allowable annulus pressure values based on the original API strength of casing and tubing, the calculation model of casing strength has been built based on the time effect of corrosion rate, and calculation model of maximum allowable annulus pressure values has been built considering electrochemical corrosion and environment sensitive fracture.