Research On Wellbore Mechanical Integrity During Carbon Dioxide Injection

With the development of the global industry,the consumption of petrochemical fuel continues to grow rapidly,and a large amount of CO2 is emitted into the atmosphere,resulting in a significant increase in the global greenhouse effect.At present,carbon dioxide capture,storage and utilization technologies are effective solutions to the greenhouse effect.However,during the CO2 injection process,the temperature of the injected fluid is low and the changes in thermal properties are complex,which poses a threat to the integrity of the wellbore,there is a risk of leakage in the injection wellbore.Therefore,the integrity of the injection wellbore is an important guarantee for the safety of CO2 storage and the evaluation of wellbore integrity is of great significance for the implementation of the project.This paper analyzes the integrity of CO2 injection wells from two aspects: risk assessment and wellbore mechanical integrity.Through the investigation of relevant literature,a database of risk factors affecting the integrity of the CO2 injection wellbore is established;a risk assessment model is established based on the FEP and TOPSIS methods to assess the possibility and severity of wellbore damage caused by various factors.In addition,this paper establishes the calculation model of the temperature and pressure of the wellbore during the injection of pure liquid CO2.The proposed model analyzes the effects of injection temperature,pressure,speed,and duration on the temperature and pressure distribution of the wellbore,and determines the temperature of each component of the wellbore,which provides boundary conditions for wellbore mechanical integrity evaluation.Finally,considering the temperature effect,bulging effect,piston effect and tubular string buckling,combined with the temperature and pressure distribution of the wellbore,this study provides a tri-axial stress calculation model of the wellbore structure(tubing,casing and cement ring).Based on the model,a comprehensive analysis for wellbore mechanical integrity during CO2 injection is available through the triaxial safety factors for the tubing and casing and the safety factors of cement for shear compressive failure,radial cracking and interfacial debonding.The results of this study indicate that the damage to the cement ring seal caused by the CO2 injection due to later operations is the most important factor that threatens wellbore integrity.The injection speed has the most significant influence on the temperature distribution of the wellbore,and the injection pressure has the greatest impact on the wellbore pressure distribution.Under practical circumstances,the tubing and casing have a high safety margin,and the debonding of the cement’s interfaces is the most dangerous failure mode.The research provides a comprehensive evaluation of the integrity of a CO2 injection well and the optimization of construction parameters.