Design for Well Longevity: Optimizing Cement Barrier Quality in UGS Wells
Authors: Gao Fei , Wang Rui , Su Hongsheng , Zhong Shouming , Guo Yabin , Taoutaou Salim 
Published: April 2019

A design approach was developed to obtain an optimum annulus cement barrier during well construction of a 29-well underground gas storage (UGS) project to maximize the longevity of the UGS wells in the years that followed in production state. The effectiveness of this approach was verified by multi-year post-job surface and subsurface field data from all the wells from the time the wells were put into production in this project.

The UGS wells are adjacent to a residential area, making reliable well integrity of great significance. To facilitate this, the cementing design for an entire well was prepared to obtain good and durable zonal isolation. The design focused on long-term well integrity and longevity rather than short-term effectiveness. First, the cementing technique was reviewed so that best practice for a high-quality cementing operation was applied in this project. Second, an advanced third-interface pulse-echo cement evaluation logging tool was adopted for better understanding of the cementing job results and to use the indications for subsequent job improvement. The tool measures the state of annulus material and casing standoff, which considerably impacts the cementing quality. Third, a high-performance flexible/expandable (HPFE) cement system with engineered mechanical properties was introduced to the project. With a specialized cement stress simulation software, the mechanical properties of the cement were optimized to deal with the downhole varying pressures and temperatures the cement would see during completion and production. Temperature change was predicted to fluctuate between 69 and 81°C and pressure between 17 MPa and 34 MPa. From the 5 years of production of the UGS wells, downhole temperature and pressure were recorded. Pressure values were within the predicted range. The temperature was higher than expected, but the designed set cement was robust enough to deal with the higher temperature.

In all, 29 UGS wells were cemented from 2011 to 2016 in this campaign. Due to the continuous effort of improving the cementing quality through the iterations of optimizing job program, the cement bond log result was better in the later 4 years of the project compared to the first 2 years. In June 2013, some of the wells were put into production and gas injection initiated. In the following 5 years, the whole storage block was gradually put into production up to its full capacity. We conducted a post-job numerical cement integrity study based on the acquired field data from the last 5 years with the measured set-cement mechanical properties. The result indicates the cement barrier can remain intact under the varying downhole conditions. This is evidenced by the post-job production behavior, which was being monitored during the gas injection and withdrawal cycles and no sustained casing pressure (SCP) problem was ever reported during the process.

The design for the cementing program focused on the well barrier quality from the beginning with the goal of maximizing well longevity by means of a durable well integrity. The design iterations based on a job-by-job design-execute-evaluate cycle helps improve the cement job quality throughout the whole project. The combination of best practice implementation, reliable cement bond evaluation tool, and the engineered HPFE cement system realized a robust well integrity for the wells. This lasting well integrity is evidenced by continuous post-job downhole and surface data acquisition. The acquisition also fine-tuned the model for predicting downhole dynamics for future UGS wells in the same block, facilitating a more realistic start for the engineered HPFE cement system design.