Experimental Study On Prediction Of Cement Sheath Failure

The integrity of cement sheath is an important guarantee of wellbore integrity in the whole life cycle.The integrity failure of cement sheath will not only cause serious economic loss and environmental problems,but also threaten the safety of personnel.Generally speaking,the research on the integrity of cement sheath focuses on unconventional oil and gas wells and production measures,such as HTHP production,water injection and gas injection,acid fracturing to enhance oil recovery.It can be seen from the previous research that the factors affecting the integrity of the wellbore include chemistry,machinery,construction technology,etc.As the time goes by in the production process,the change of temperature and pressure in the wellbore greatly affects the mechanical integrity of the casing and cement sheath,which causes additional stress to the downhole string and cement sheath.The most important effects include the oil and gas leakage via the cementing interface,the impact on the mechanical properties of the cement and the casing,etc.In order to better understand the influencing factors of cement sheath integrity and how to protect the wellbore integrity under complex formation conditions,studies was committed in sealing failure between cement sheath and casing.Based on the theory of elastic-plastic mechanics and thermodynamics,this paper focuses on the laboratory experiment of bond strength of cement sheath interface,the prediction model of wellhead growth under the condition of constraint,and the prediction model of micro annular gap.The research content mainly includes:1.Study the failure principle of the longitudinal and radial cementation strength of the interface between the cement sheath and the casing;carry out the laboratory experimental research on the interface cementation of the two directions.By studying the influence of the cement slurry curing time,temperature,density and the surface roughness of the casing on the cementation strength of the interface between the cement sheath and the casing,obtain the influence rule of different factors on the cementation strength of the interface between the cement sheath and the casing.2.According to the different yield state of the cement sheath during the loading unloading process of the pressure in the casing,combined with the mechanical model of the casing cement sheath formation combination,the prediction model of the micro annulus size at the interface between the casing and the cement sheath after debonding is established,and the influence of different construction parameters on the calculation results is simulated.3.The mechanism of wellhead growth is explained,the calculation model of wellbore temperature field distribution is established,the comprehensive influence of temperature,cement sheath cementation and other factors on wellhead growth is studied,and the prediction model of wellhead lifting height is established.The field data is used to check the calculation of temperature field distribution and the calculation of wellhead lifting height,and the influence law of each influencing factor on the wellhead lifting height is analyzed.