Cementing in Thermal Wells: A Three-Pronged Approach to Well Integrity

Abstract The primary objective of cementing a casing in a well is to provide zonal isolation. Thermal wells, however, pose different challenges in achieving this objective compared to conventional oil and gas wells. They require a cementing design comprising a robust cement system to withstand the thermal stresses during the life of the well, and proper placement techniques to ensure long-term well integrity. Thermal wells are usually not cemented in true "thermal" conditions. However, they experience high temperatures during the production phase. Therefore, although the cement slurry is not designed for placement at high temperatures, the set cement must be able to withstand high thermal stresses. Proper placement of cement in the annular space between the borehole wall and the casing is critical. Any non-cementitious fluid pockets would expand due to heat during the production phase, causing loss of well integrity and potentially a casing collapse in the worst case. Severe lost circulation is often encountered in thermal wells. While several lost circulation material pills and cement plugs might be needed to cure the losses, using a fit-for-purpose solution significantly reduces lost time. Thermal wells require the set cement to have a high coefficient of thermal expansion to minimize its expansion upon heating, a low Young’s modulus (high flexibility), and a high strength-to-flexibility ratio to maintain its integrity. In addition, the system must maintain these properties at high temperatures and have the ability to be designed at low densities if required, to avoid lost circulation in cases of low formation strength. Apart from designing the spacer fluid (viscosity and volume) for proper mud removal, spacer technology employing mechanical scrubbing of the casing and borehole walls is often used to achieve higher cleanliness. To combat lost circulation, an improved thixotropic lightweight cement system used in conjunction with high-performance lost circulation materials is employed to handle the severe losses often encountered in thermal wells. Presented in this paper are lab evaluation results and global case studies of cementing fluid systems that have been successfully used for thermal wells, both for heavy oil and geothermal applications. Also presented are operational best practices and a compilation of lessons learned from the authors' experience with cementing in thermal wells globally. Different operators use various methods to construct thermal wells. This paper presents an analysis of different approaches from a cementing perspective to manage the unique challenges posed by thermal wells. This includes the design of the cement system and its placement, along with operational best practices to achieve the ultimate goal of long-term well integrity.

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