Foamed Conventional Lightweight Cement Slurry for Ultralow Density and Low ECDs Solves Lost-Circulation Problem Across Coal Formations: A Case History

Abstract

Abstract Cementing gas wells in coalbed methane (CBM) formations can present a cement-circulation challenge because the coal formations tend to have low fracture gradients and break down under the hydrostatic pressure of the cement column. The use of conventional lightweight cement slurries and/or foam cement significantly increases the prospect of maintaining circulation throughout cement placement. However, wells with exceedingly low fracture gradients can suffer either partial or complete loss of circulation during cement placement even with lightweight slurries. Both conventional lightweight cement slurries and foam cements have restrictions. Conventional lightweight cement systems use low-density materials (i.e. hollow spheres, gilsonite) and high water-to-cement ratios as a means of reducing the equivalent circulating density (ECD). The lightweight materials and high water requirements detract from the compressive strength of the cement, thereby limiting ultra-low densities while achieving adequate compressive strength. For foamed cement, the gas-to-slurry ratio should stay within a specific range to help ensure optimal cement properties; otherwise, the set cement could become permeable. This requirement has led to the hybridization of two systems to create an effective cementing solution. By combining the two systems, the best attributes of both can be captured. Starting with a lightweight cement system provides a low base-slurry density that allows the gas to be added to lower the density further while maintaining the compressive strength and foam stability. This paper discusses an operator's challenge and the use of the above solution to cement wells in the Western Canada Sedimentary Basin (WCSB) across CBM formations.