Foamed Conventional Lightweight Cement Slurry for Ultra ow Density and Low ECDsSolves Los -Circulation Problem Across Coal Formations: A Case History

摘要

Cementing gas wells in coalbed methane (CBM) formationsrncan present a cement-circulation challenge because the coalrnformations tend to have low fracture gradients and break downrnunder the hydrostatic pressure of the cement column. The usernof conventional lightweight cement slurries and/or foamrncement significantly increases the prospect of maintainingrncirculation throughout cement placement. However, wellsrnwith exceedingly low fracture gradients can suffer eitherrnpartial or complete loss of circulation during cementrnplacement even with lightweight slurries.rnBoth conventional lightweight cement slurries and foamrncements have restrictions. Conventional lightweight cementrnsystems use low-density materials (I.e. hollow spheres,rngilsonite) and high water-to-cement ratios as a means ofrnreducing the equivalent circulating density (ECD). Thernlightweight materials and high water requirements detractrnfrom the compressive strength of the cement, thereby limitingrnultra-low densities while achieving adequate compressivernstrength. For foamed cement, the gas-to-slurry ratio shouldrnstay within a specific range to help ensure optimal cementrnproperties; otherwise, the set cement could become permeable.rnThis requirement has led to the hybridization of two systemsrnto create an effective cementing solution. By combining therntwo systems, the best attributes of both can be captured.rnStarting with a lightweight cement system provides a lowrnbase-slurry density that allows the gas to be added to lower therndensity further while maintaining the compressive strengthrnand foam stability.rnThis paper discusses an operator's challenge and the use ofrnthe above solution to cement wells in the Western CanadarnSedimentary Basin (WCSB) across CBM formations.