Foamed Conventional Lightweight Cement Slurry for Ultra-Low Density and Low ECDs Solves Loss-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. Introduction Cementing CBM wells usually requires a cementing system that reduces the risk of lost circulation, while providing excellent annular displacement efficiency with the aim of achieving 100% annular fill. When set, the cement should give complete zonal isolation for the life of the well (stimulation, production, and selective workover) while providing lateral pipe support to combat compaction-induced failures. Foamed cement has historically fulfilled these requirements.[1] Table 1 summarizes required cement properties. Foam cement offers many advantages, including the following:Low hydrostatic pressure. Circulation losses while drilling and completing in CBM fields are common. A reduction of cement density from 1920 to 1440 kg/m[3] (16.0 to 12.0 lb/gal) can reduce the hydrostatic pressure in a typical WCSB well by ~5.5 MPa, or 400 psi.Dynamic control of losses. The thixotropic and expansive nature of foam, together with the structural features of the bubble cells, helps reduce losses to vuggy or fractured formations and helps reduce fluid loss to permeable formations.Strength-to-density ratio. Foamed-cement slurries can yield higher strengths than low-density slurries extended by adding water only.Mechanical properties of set cement. The high ductility and bubble structure of foamed cements has been shown to be beneficial when the cemented annulus is subjected to thermal and mechanical loading. These features of foamed cements can enable internal deformation without cracking.[2–4]Hole cleaning. The high apparent viscosity of foamed fluids can enable them to exceed the shear stress required to mobilize highly gelled muds and to exhibit superior solids-carrying capability.[5–7]Improved mud displacement, expansion properties, and fluid loss.[8] Foamed-cement slurries are considered to have superior mud-removal properties and the capability of filling lost-circulation voids. The two phases of foamed cement lower the overall slurry fluid loss, which is known to be one of the primary properties for controlling gas and fluid influx into the setting cement.[9–13]