Numerical Simulation Of Cementing Seal Integrity During Electric Heating Of Oil Shale

With the continuous development of unconventional oil and gas resources,development technologies such as electric heating of well,in-situ thermal injection and steam stimulation have been applied to field engineering practices,resulting in increasingly complex well conditions during the development process.Especially,the cement sheath has been exposed to high-temperature and high-pressure environments for a long time,which has brought a significant test to the sealing integrity of the cement sheath.Seal failure of the cement sheath can directly lead to a series of problems such as wellhead gas channeling or annular pressure.It poses a serious threat to the safety of wellhead production,so it is of great significance to study the formation mechanism of cementing sealing integrity under high temperature and pressure.In this paper,based on the macro-mechanics parameter of cement sheath and oil shale,the microscopic parameters are calibrated systematically and accurately through mechanical tests such as compressive and tensile tests.The numerical model of oil shale and cement sheath that conforms to indoor mechanical characteristics is established,and the combined model is subjected to thermal and pressure coupled numerical simulation research.The thermal crack evolution of the combination under the conditions of heating mode,layering angle of oil shale,confining pressure,temperature and prefabricated cracks is mainly studied.The results show that:(1)After the electric heating reaches the set temperature,the longer the continuous heating time is,the larger the heating range of the combination,and the stronger the thermal cracking effect.Compared to the oil shale matrix,the thermal cracks are more likely to expand and develop in the layering plane,and tensile cracks are primarily developed in the cement sheath and matrix,while both tensile and shear cracks are developed in the oil shale layering plane.(2)The inclination angle of the oil shale layering has an inhibitory effect on the development of thermally induced cracks in the matrix.The greater the inclination angle of the layering,the easier the thermal stress is released at the layering plane,the more difficult it is to develop thermally induced cracks in the matrix,in general,tensile cracks dominate.(3)The greater the buried depth of the combination,the greater the confining pressure,the more compacted the particulate material,and the lower the porosity,resulting in the greater strength of the specimen.However,in the study of heating the specimen to different temperatures under the same confining pressure,it was found that the temperature at which cracking begins of the combination is around 150 ℃,and When the temperature reaches the range of 400℃ to 500℃,a large number of thermally induced cracks are generated.The higher the temperature,the lower the compressive strength and elastic modulus of the combination,and the greater the porosity.(4)In the study of prefabricated cracks in combination,it was found that thermal stress is concentrated at the tip of the prefabricated crack during the heating process,and the thermal crack extends along both ends of the prefabricated crack.The addition of multiple microcracks can be seen as an enhancement of the thermal effect under a single microcrack.The addition of steel fiber into the cement sheath can effectively improve its sealing performance.The research results are helpful to understand the fracture behavior and mechanism of cement sheath and formation rock in oil shale electric heating project,and provide a theoretical basis for oil shale in-situ thermal recovery.