Study On Wellbore Integrity Evaluation Of CO₂ Flooding Injection

In recent years,CO₂flooding technology in Xinjiang Oilfield has played a significant role in improving recovery and production rate of low permeability and ultra-low permeability reservoirs.However,CO₂flooding technology is also faced with many problems.For injection wells,the problem of wellbore failure caused by the harsh downhole environment caused by CO₂is becoming increasingly serious,which seriously endangers production safety.At present,scholars at home and abroad have little research on the integrity evaluation of CO₂flooding injection wells.It is of practical significance to carry out the research on the integrity evaluation of CO₂flooding injection well bore for establishing the integrity management method of CO₂flooding injection well in China,timely improving the identification of adverse factors,prolonging the life cycle of CO₂flooding injection well,reducing the damage risk of CO₂flooding injection well,and protecting the interests of oilfield companies,stone workers and ecological environment safety.For this reason,the following research work has been carried out:（1）The evaluation index system of wellbore integrity for CO₂flooding injection is established.There are 7 first-class indexes,14 second-class indexes and 25 third-class indexes.The judgment methods of each index are introduced to provide scientific basis for integrity management of field workers.（2）The coupling calculation model of CO₂temperature and CO₂flow pressure considering the unsteady heat transfer characteristics of formation and cement sheath is derived and established.The pressure and temperature distribution of CO₂along wellbore is determined by VB programming.The software is applied to 80513 well of CO₂injection test well in Xinjiang Oilfield.The predicted bottom hole pressure is39.19mpa and the temperature is 57.54℃,which is in good agreement with the measured data of 40.43mpa and 58.67℃,with errors of 3.1%and 1.9%respectively.The accuracy of the temperature and pressure calculation model is verified.The software can be used to calculate the temperature and pressure of CO₂in the wellbore,which provides basis for the determination of corrosion environment and safety check of tubing.（3）The corrosion resistance of accessories for CO₂flooding injection well was tested by the high temperature and high pressure dynamic corrosion evaluation experiment in the laboratory.The results show that in the simulated corrosion environment,304L and 316L are not corroded,and the corrosion rate of other materials is 3Cr13>35Cr Mo>42Cr Mo,and the three materials,304L,316L and 3Cr13,met the conditions of CO₂corrosion resistance.3Cr13 is suitable for the main structure of underground accessories,and 304L can be considered for the parts of non structural main body,non wear-resistant parts and frequently contacting with corrosive medium.（4）Based on the analysis of various types of axial forces and stresses on the tubing during CO₂injection,the calculation model of axial forces and stresses on the tubing is established.According to the requirements of CO₂injection conditions on site,the axial forces and stress of 80513 well tubing under different injection conditions are calculated.The results show that the strength of tubing meets the requirements of CO₂injection conditions on site without corrosion,and the most dangerous injection condition is 40℃,40MPa,100t/d.Under this condition,wellcat software is used to check the safety of 80513 well with different tubing wall thickness.The results show that when the wall thickness is 4.34mm,the tubing just meets the safety requirements;when the wall thickness is less than 4.34mm,the tubing strength limit can not meet the working requirements,and the tubing fails;when the wall thickness is greater than 4.34mm,the tubing meets the safety requirements.（5）The integrity evaluation method is determined.According to the process of evaluating the integrity of CO₂injection well bore,an application software is developed by VB language,which can use AHP to calculate the index weight of all levels and risk matrix method to determine the risk level of the integrity of CO₂injection well bore,and evaluates the well 80513 of CO₂injection test well in Xinjiang Oilfield,realizing a quantitative analysis of the CO₂flooding injection wellbore integrity.The results show that the 80513 well wellbore risk value is 9.6876 and the risk level of wellbore integrity is low risk.