Effect of Thermal Cycling on Cement Sheath Integrity: Realistic Experimental Tests and Simulation of Resulting Leakages
- Torbjørn Vrålstad (SINTEF Petroleum Research/DrillWell) | Ragnhild Skorpa (SINTEF Petroleum Research/DrillWell) | Nils Opedal (SINTEF Petroleum Research/DrillWell) | Jesus De Andrade (Norwegian University of Science and Technology (NTNU)/DrillWell)
- Document ID
- Society of Petroleum Engineers
- SPE Thermal Well Integrity and Design Symposium, 23-25 November, Banff, Alberta, Canada
- Publication Date
- Document Type
- Conference Paper
- 2015. Society of Petroleum Engineers
- 5.5.2 Core Analysis, 5.4.6 Thermal Methods
- Leakages through cement, Cement integrity, Microannuli, Surface casing vent flow, Thermal cycling
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The cement sheath is one of the most important well barrier elements in the well, both during production and after abandonment. However, normal production operations which involve temperature variations in the well, such as steam injection, stimulations and shut-down periods, may damage the integrity of the cement sheath. Temperature increase and decrease, i.e. thermal cycling, cause the casing to expand and contract, which creates debonding and cracking of the cement sheath and thereby loss of zonal isolation.
This paper presents novel results from an experimental study of cement sheath integrity during thermal cycling. The temperature was cycled repeatedly from 5 °C to 125 °C in a controlled manner from inside the casing, and Portland cement with silica additive was tested with both sandstone and shale as surrounding rock. Debonding and cracking of cement were quantified and visualized by X-ray computed tomography (CT), and it was found that cracking and debonding occurred for the sandstone sample, whereas the shale sample remained almost unaffected. There were some initial defects in the cement sheath in the sandstone sample, and these small and scattered defects grew together during thermal cycling into a continuous leak path; i.e. resulting in a loss of zonal isolation.
The digitalized 3D geometry of this leak path was imported into Computational Fluid Dynamics (CFD) software, thereby enabling a unique visualization of fluid flow through an actual leak path in degraded cement and an estimation of leak rates for different pressure differences. It is seen that microannuli are not homogeneous or uniform, and that fluid flow through microannuli and cracks is complex and not easily predictable.
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Garnier, A., Saint-Marc, J., Bois, A.-P., Kermanac'h, Y., A Singular Methodology to Design cement Sheath Integrity Exposed to Steam Stimulation. 2008. Paper SPE/PS/CHOA 117709 presented at the SPE International Thermal Operations and Heavy Oil Symposium held in Calgary, Alberta, Canada, 20-23 October 2008