Foamed Cement vs. Conventional Cement for Zonal Isolation-Case Histories

Abstract

Abstract Data from six deep gas wells in Wyoming indicate that foamed cement outperforms conventional cement for zonal isolation. Two of the six wells were cemented across the production zones with conventional non-nitrified cements. These wells experienced outer-zone communication after the stimulation treatments. In the other four wells, the foamed-cement sheath provided zonal isolation even after the sheath was perforated and stimulation treatments were performed. The case histories described in this paper provide an opportunity to compare the performance of foamed cement to conventional cement on similar wells within a particular geographic area. Large-scale laboratory testing has shown that foamed-cement is ductile and can deform as casing is pressurized, but will not crack like non-nitrified cement. These test results were confirmed by the results obtained in the six case-history wells. In the first two wells that were cemented, the operator used the conventional high-strength, non-nitrified cement across the formations. Tracer tags on the stimulation treatment showed that zonal isolation was not achieved and that the stimulation treatment communicated between the high- and low-pressure zones. The operator elected to use foamed cement for the last four wells to help obtain zonal isolation. In these wells, the stimulation treatment remained in the zone, little fracture growth occurred outside the target formation, and communication did not occur between the high- and low-pressure zones. This paper provides details about the slurry design and job execution on the six case-history wells, and also presents the postjob analysis that verifies the conclusions. Foamed Cement Properties and Applications Introduction Foamed cement's tensile strength, ductility, and displacement properties have made it especially useful in several zonal-isolation scenarios. Although the initial cost of conventional cement can be less than that of foamed cement, the improved zonal-isolation capabilities of foamed cement often provide substantial cost savings over the life of the well. The following section discusses both the advantages of using foamed cement for zonal isolation and its economic value compared to flexible cement. Mud Displacement/Fluid Influx and Migration Control During pumping operations, foamed cement can develop higher dynamic-flow shear stress than conventional cements, increasing its mud-displacement capabilities. A system that consists of cement slurry injected with nitrogen gas can be optimized for individual well conditions. Slurry density, which is determined by gas content or quality (the porosity of the set cement), depends on the pump rate of the base slurry, foamer and stabilizer injection rates, and nitrogen rate. Computer programs help optimize slurry design and predict job-placement pressures. The gas used to foam the system continues to expand while the cement volume decreases,1,2 allowing slurry pressure to remain almost constant during the system's transition period. Consequently, the system effectively controls gas migration and formation-fluid influx, which limits migration channels in the set cement sheath. Ductility The brittleness or lack of ductility of conventional cements has been identified as one of the primary failure mechanisms of primary cement jobs.3 Flexible cement systems have been used to successfully combat and prevent cement-sheath cracking. However, flexible cement systems, which incorporate vulcanized rubber, are very costly and require extreme conditions for justification of their technology. Introduction Foamed cement's tensile strength, ductility, and displacement properties have made it especially useful in several zonal-isolation scenarios. Although the initial cost of conventional cement can be less than that of foamed cement, the improved zonal-isolation capabilities of foamed cement often provide substantial cost savings over the life of the well. The following section discusses both the advantages of using foamed cement for zonal isolation and its economic value compared to flexible cement. Mud Displacement/Fluid Influx and Migration Control During pumping operations, foamed cement can develop higher dynamic-flow shear stress than conventional cements, increasing its mud-displacement capabilities. A system that consists of cement slurry injected with nitrogen gas can be optimized for individual well conditions. Slurry density, which is determined by gas content or quality (the porosity of the set cement), depends on the pump rate of the base slurry, foamer and stabilizer injection rates, and nitrogen rate. Computer programs help optimize slurry design and predict job-placement pressures. The gas used to foam the system continues to expand while the cement volume decreases,1,2 allowing slurry pressure to remain almost constant during the system's transition period. Consequently, the system effectively controls gas migration and formation-fluid influx, which limits migration channels in the set cement sheath. Ductility The brittleness or lack of ductility of conventional cements has been identified as one of the primary failure mechanisms of primary cement jobs.3 Flexible cement systems have been used to successfully combat and prevent cement-sheath cracking. However, flexible cement systems, which incorporate vulcanized rubber, are very costly and require extreme conditions for justification of their technology.