Research On Mechanical Integrity Of Cement Sheath In Heavy Oil Thermal Recovery Wells

The cement sheath is subjected to combined effects of thermal load and mechanical load during the heat injection of heavy oil thermal recovery wells,which is prone to various stress failure such as loss of structural sustain and regional isolation.It is of great significance to conduct research of the cement sheath integrity of thermal recovery wells.An analytical method for the stress field of casing-cement sheath-formation combination system based on the basic theory of elastic mechanics considering initial non-uniform geostress conditions,and the results is verified by numerical methods after appropriate approximation.It has been found that the increase in the temperature inside the casing causes a certain increase in the radial stress and the circumferential stress inside the cement sheath,and decreases as the temperature decreases.The triaxial core experiment is conducted to study the stress-strain relationship of different loading paths of cement,strength and failure law of cement under varied confining pressure are determined,and A constitutive relationship are selected to describe compaction property of cement.The mechanical characteristics of cement after cyclic temperature load to determine the influence of temperature load on the mechanical properties.It is found that the cement stone undergoes brittle failure under uniaxial conditions,and exhibits certain strain softening characteristics under low confining pressure conditions,and strain hardening characteristics under high confining pressure conditions,that is,cement stone has high confining pressure and shearing Two kinds of plastic phenomena:expansion and low confining pressure and shear compaction;DPCAP model can simultaneously describe the shear failure and compaction plasticity of cement stone;After multi-rounds heat injection,the cohesion parameter of shear failure yield surface decrease while the friction angle increase,and the effect on the compacted yield surface is not significant.A step-by-step simulation method has been established by considering well-building process,which can calculate the stress field development of cement sheath in different stages of subsequent development measures including drilling,casing setting,cementing until cement solidification.Based on the field oil well data,the regularity of the cement sheath integrity is studied in the following directions:mechanical properties of the cement,mechanical properties of the formation and heat injection parameters.It is found that cement sheath undergo different failure types under different formation conditions,and different mechanical parameters of cement,formation conditions and heat injection parameters will affect the different failure types of cement.The properties of cement and formation properties must be strictly controlled,and heat injection parameters should be optimized.