In Oman, certain fields contain heavy oil and recovery of this oil is done through steam injection, which leads to rapid heat-up of the cemented annulus to very high temperatures. Throughout the lifecycle of steam injection wells, stresses in the cement sheath induced by rapid temperature cycling, results in mechanical damage and ultimate failure of the cement sheath. Such failure leads to loss of steam down-hole and an increased amount of steam is required to extract the oil. This translates into higher energy costs for steam production. In extreme cases steam can be seen breaking through to the surface.

In Oman heavy oil reserves are found in naturally fractured limestone formations prone to severe losses while drilling. To ensure proper cement placement, systems with densities below 1,400 kg/m3 are required. In the past certain wells were cemented using foam cements with density close to that of water. Data collected from earlier steam injection drilling campaigns by PDO suggests that maintaining cement integrity is a key challenge. The issue is related to initially not being able to place the cement properly due to losses and subsequent degradation of set cement as it does not withstand the stresses created during steam injection process.

A recently developed specialized cement system was used to successfully cement one such well. The system was placed successfully using fibers based pill ahead of the slurry to cure the losses. Stresses created on the cement sheath during steam injection were simulated, mechanical and thermal properties of the cement system were optimized to prevent failure, and evaluation was performed for wellbore integrity. Excellent mechanical and thermal properties for a 1,400 kg/m3 slurry system showed no breakthrough of steam when exposed to multiple temperature cycles of up to 300 degC. Multiple wells in Oman have been cemented using this technology.

The current paper looks at various aspects of design, execution and evaluation of such cement systems.

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