China's First Offshore Ultra-HT/HP Venture in CO2 Rich Gas Formation - Success with Mechanical and Corrosion Resilient Cement

Abstract The South China Sea hosts significant natural gas reserves, but HT/HP production remains limited due to fragile reservoir formations and high CO2 concentration in production zones. In a recent cementing operation for China's first offshore HT/HP campaign within a CO2 rich gas formation, a resilient cement design was developed and deployed to mitigate well integrity risks and hazards in these complex downhole conditions. This paper details the design and testing of a modified HT/HP cement slurry and spacer system engineered to address challenges such as narrow operational window, corrosive CO2 exposure, and extreme temperature and pressure variation during the well construction and production phase. A specialized wide-temperature-range retarder was rigorously tested for sensitivity to temperature, density, and retarder loading before deployment. The bulk cement transportation process was closely monitored from warehouse storage, bulk plant, vessel, and offshore rig silos to mixing. Different field cement sampling methods were evaluated to investigate their effects on the slurry thickening time. Mechanical testing of the resilient cement confirmed excellent mechanical properties, ensuring its ability to withstand extreme temperature and pressure fluctuations between drilling and production. This contributed to well integrity by providing a reliable barrier between the casing and borehole. Long term laboratory testing and monitoring of compressive strength development under down hole conditions (200 °C/392°F with 40% CO2) showed no evidence of cement strength retrogression, demonstrating the cement's durability and resilience against corrosive formation fluid in HT/HP environments. The ability to maintain compressive strength under these conditions is critical for ensuring long-term zonal isolation and well integrity. The cement slurry exhibited a linear thickening time response to the selected retarder loading at bottom hole circulation temperature (BHCT) ranging from 145 °C (293°F) to 195 °C (383 °F), as well as to changes in slurry density and temperature. Maintaining predictable thickening time behavior is crucial to HT/HP cementing preventing "flash set" hazard. The resilient cementing system was successfully deployed in two liner and tie-back cementing jobs during China's first offshore ultra-HT/HP campaign. Both wells demonstrated good cement bond log (CBMT logging) and passed negative pressure tests up to −4,450 psi, meeting well integrity requirements. The successful application of this engineered cementing system establishes a proven, field validated solution for ensuring long term well integrity in HT/HP production wells exposed to corrosive CO2 environment. Its adaptability can be extended to CCS/CCUS applications as well as more complex, hard to reach deepwater projects, providing a durable and resilient barrier system for extreme downhole conditions.