For years, the industry has recognized the benefits hollow glass spheres (HGS) provide to cement slurries--near-normal strength development at drastically reduced densities. When drilling through weak formations, drilling fluid can be lost. In one of the subject wells discussed in this work, more than 1,200 bbl of mud were lost. When cementing across these intervals, unless the density of the cement is drastically reduced, the cement slurry can also be lost to the formation. If cement is lost to the formation, the top of cement (TOC) will be reduced. If the TOC is substantially reduced, remedial cement jobs might be required, costing precious time and extra money. Previous HGS materials have been available in a variety of densities and pressure ratings. Typically, as the strength increases, so does the cost and the density. Additionally, as the density of the HGS material increases, more of that product is required to yield the same reduction in slurry density. With the new HGS material introduced in the case histories presented, the product is both lighter and stronger, improving both slurry quality and economics. With the reduced specific gravity of this new HGS material, the slurry was designed with the required downhole properties at a density low enough to provide complete circulation and yield the required TOC. Multistage cementing (MSC) tools are briefly discussed and can provide an alternative option, but the overall job cost is typically higher, the jobs take longer, and additional operational issues are possible. Real well case histories detailing the design and advantages of the new material are presented, as well as comparisons of the new slurries' effectiveness in an area where previous attempts to circulate cement have failed. BACKGROUND API Class A, G, or H cements, when mixed with their specified testing-water concentration, weigh 15.6 to 16.4 lbm/gal. Wells drilled through depleted intervals or in areas with low fracture gradients cannot handle full hydrostatic cement columns. To successfully circulate complete columns of cement slurry, under these conditions, the slurry density should be reduced. There are three primary methods for reducing cement slurry density:
adding extra water,
adding nitrogen, and
adding lightweight microspheres.
All three of these methods are widely used in the industry and have their own individual sets of advantages and disadvantages. Optimizing a lightweight cement slurry requires the designer to consider the positive and negative aspects of these various systems while considering the well's needs and requirements, as well as the operator's short- and long-term objectives
