Tailored Engineering Cementing Solution to Maintain Well Integrity in Shallowest Liner for Underground Gas Storage Project - Case History from Southwest China

Abstract

Underground gas storage (UGS) wells have emerged as a strategic solution in China. Success of UGS projects largely depends upon maintaining long term well integrity. Cement slurries that are placed across a wellbore should exhibit superior cement bonding as evidenced through a cement bond log (CBL) and long-term integrity to sustain the cyclic stress change by the injection and production process. Such slurries should have improved mechanical properties and the job execution should follow all cementing best practices. The well architecture included a 9 5/8-in. surface casing and a 7-in. production liner. The 7-in. Liner was run inside an 8.5-in. open hole and extended to surface using a tie-back liner. This well architecture should have a superior quality of cement across the entire liner. Multiple Finite Element Analysis (FEA) runs were performed to determine an optimum Young's modulus and Poisson Ratio for the cement slurry. These rigorous tests can take weeks to complete. As the well was shallow, to cover a wide range of well profiles, three different slurries were tested prior to the job. The initial mud weight planned for the well was in the range of 1.25 g/cm3 to 1.4 g/cm3. Due to gas influx, the mud density in the section was increased to 1.90 g/cm3. However, losses were also encountered at this mud density. Hydraulic modelling was revised, and slurry rheology and pumping rates were optimized to ensure equivalent circulation density (ECD) control within the pore pressure and fracture gradient window. Displacement rates were optimized to facilitate good displacement efficiency for hole cleaning. The slurry design was tailored with special additives to provide a synergetic effect of improving mechanical properties and minimizing seepage losses. Multiple computational fluid dynamics (CFD) runs were performed to evaluate the cementing job quality and based on the simulations it was decided to increase the cement volume to minimize any impact of contamination. The cementing job was performed with no operational issues and cement returns were observed above the top of the liner. Two different cement evaluation logs - CBL and ultra-sonic log, were conducted and showed good cement quality in the open hole section, meeting the well objectives. With this successful implementation, the tailored engineered cementing solution was highly recognized. The design and execution methodology were highlighted as the guideline for further successful cementing operations in UGS projects. This study shows a fully comprehensive and scientific way to improve cementing quality for long-term well integrity for UGS projects.