Title : Mechanical damage behavior and constitutive model of cement sheath under ultra-high temperature thermal cycling
Abstract:
The cement sheath faces extreme service conditions in ultra-high temperature and ultra-high pressure wells, which easily leads to the failure of the cement sheath and safety accidents such as annulus pressure and wellhead movement. Based on the wellbore structure design and conditions in the LD-10 gas field, the evaluation and testing method for the damage behavior of cement sheath under ultra-high temperature thermal cycling (UTTC) is proposed. The mechanical properties test of cement sheath under thermal cycling (25??150??25?, 25??200??25?, 25??250??25?) is conducted by combining the full-scale experiment device of “9-5/8" production casing-cement sheath-13-3/8"technical casing”. The evolution law of mechanical properties of cement sheath after thermal cycling is obtained and the damage behavior and mechanism of cement sheath is clarified. The results show that the temperature and thermal cycling significantly negatively affect the mechanical properties of cement sheath and the degree increases with increasing number of cycles and temperature. The damage to the cement sheath is caused by five factors, which could be summarized as the changes in mechanical properties of the cement sheath caused by temperature itself and fatigue damage caused by thermal cycling on the cement sheath. Based on the test results, a uniaxial compression damage constitutive model under thermal cycling with consideration of the pore compaction stage and elastic deformation stage and a triaxial compression damage constitutive model with consideration of the elastic deformation stage and plastic deformation stage are established, by which experimental results validate these models. The research results could provide a reference and basis for optimizing and controlling cement sheath integrity in deep wells.
Audience Takeaway Notes:
- A method for evaluating the mechanical damage behavior of full-scale cement sheath under thermal cycling was proposed.
- This could benefit to clarify the damage mechanism of cement sheath under severe alternating temperatures.
- This could contribute to broadening the cognition of the boundary for cement sheath damage theory and enrich the wellbore integrity theory, and more beneficial to investigate the control and prediction method of wellbore integrity.
- This could contribute to establishing the indicator system for the evaluation of cement sheath integrity under severe alternating temperatures and propose the recommended practices for maintaining the long-term integrity of cement sheath in ultra-deep wells.
- The research results can provide a new method and concept for the optimization design of wellbore integrity and cement sheath integrity in deep wells and ultra-deep wells.
