Underground gas storage (UGS) wells have become the national strategy for natural gas storage and supply throughout China. PetroChina is building UGS facilities throughout the country, with planned construction extending from 2011 to 2015. The Chongqing gas storage well project in Central China is one of the constructed facilities. Because the life expectancy of gas storage wells is at least 50 years, long-term maintenance of well integrity is one of the major challenges faced during the construction and operations phases of UGS wells. The wells must bear the injection pressures, temperature changes, and frequent cycling of injection during production. With conventional cement, the pressure and temperature change during the injection and production phases can result in cement traction, compression, or microannulus failure. This leads to gas migration through the damaged cement sheath, which means the well will fail its mission as a UGS well.

To maintain well integrity, a novel engineered flexible and expanding cement system has been introduced for UGS cementing in the Chongqing project, whereas past service companies have failed to deliver the fit-for-purpose solution. The optimized mechanical properties of this novel cement allow the cement to remain intact under extreme pressure and temperature changes.

To prove the ability of the selected flexible and expanding cement to withstand the pressure change, logging was conducted at various times during the construction and after the casing pressure test cycle. The logs show excellent results. 

Since its implementation, the flexible and expanding cementing system has become the standard solution for China’s UGS wells, in which long-term well integrity and sustained casing pressure are required. This paper discusses the challenges for cementing UGS wells, which case histories describe the successful implementation of the FEC solutions.