Study On Mechanical Integrity Of Deep Well Cement Sheath

The mechanical integrity of cement sheath is an important part of wellbore integrity of oil and gas wells,which is closely related to the safe production of oil fields.With the continuous development of deep and ultra-deep wells,the service environment of downhole cement sheath is becoming more and more harsh.Therefore,it is very important for optimizing cement slurry system,recommending reasonable construction parameters,improving annulus sealing capacity of deep well cement rings and maintaining wellbore integrity of oil and gas wells to establish mechanical integrity model of cement rings suitable for deep wells,design indoor interface cementing quality testing devices,and explore the mechanical properties and interface cementing quality of cement pastes in different cement slurry systems.In this paper,the mechanical properties of cement paste with different cement slurry systems,the bonding quality of cement sheath interface and the mechanical model of casingcement sheath-formation combination are studied.The mechanical model of casing-cement sheath-formation combination considering temperature and pressure variation is established,and the corresponding calculation program is compiled.The influence of borehole pressure and mechanical properties of cement paste on the sealing ability of cement annulus is studied.According to API SPEC 10 "Standards for Cement Materials and Tests for Oil Wells",three kinds of cement slurry systems A,B and C were allocated.The mechanical parameters of three kinds of cement paste systems were tested,including tensile strength,compressive strength,elastic modulus and Poisson’s ratio.The effects of three cement slurry systems,mud cake thickness,different formation lithology(sandstone,mudstone)and different temperature cycles on the interface integrity of cement ring were studied by using self-developed cement ring interface cementing quality testing device.The results show that the longer the solidification time of the three kinds of cement slurries,the better the cementing strength and the lower the risk of cement sheath failure;the lower the elastic modulus of cement paste,the better the mechanical properties and the better the logging interpretation results.Based on the formation parameters and the basic parameters of cement sheath of an example well in the field,combined with theoretical model and mechanical experiment results,the mechanical properties of cement paste of three kinds of cement slurry systems in downhole service are studied to guide the optimization of cement slurry system in the field.The integrity of cement sheath under pressure test and shut-in conditions is discussed.The results show that tensile failure is the most prone failure form of cement sheath.The failure of cement sheath is mainly caused by the excessive construction pressure at wellhead and the attenuation of tensile strength of cement sheath at high temperature.