New Liquid Microsphere System Simplifies Lightweight and Ultra-Lightweight (2006)

As reservoir targets are increasingly chosen below difficult drilling formations, the need for lightweight cement of higher quality is also increasing. While conventional water-extended cement slurries are often favored because of their low cost, slurry properties or high minimum densities may prevent them from being used in the more challenging wellbores. To obtain slurries of premium quality and lower density, using foam or microspheres is recommended. While foam has certain advantageous mechanical properties, it is not applicable in all well conditions. Because of these reasons, the popularity of microsphere cementing has continued to increase every year over the last decade. In spite of the growing market, several complications can exist in regard to actually using microspheres to cement oil and gas wells. Microspheres should be homogeneously distributed throughout the bulk material while it is being delivered to the mixing head, otherwise viscosity or density-control issues may arise. Industrystandard cement mixing equipment is designed to mix based on density, but the real concern when mixing a cement slurry (although not monitored) is delivering the slurry at the correct cement:water ratio. During conventional cementing, a heavy bulk material is mixed with a light fluid. When this heavy bulk material is mixed with a light liquid, physics assures that mixing at the correct density also means the slurry is being mixed at the proper cement:water ratio. With the addition of lightweight microspheres to the bulk material, the overall specific gravity of the bulk material is reduced. In some cases where the microspheres are added at high concentrations, the bulk material can actually become as light or even lighter than the mix fluid. As the concentration of the microspheres increases (or the density of the bulk decreases), it becomes hard to mix the resulting slurry at the wrong density. On the surface one might think, not being able to mix at the wrong density a good thing, but it is actually problematic. Remember, with density-based mixing equipment, we are relying on density to control the cement:water ratio. If all cement:water ratios yield a similar density, controlling the ratio by controlling density is no longer possible. Volumetric-based mixing systems that function well when the bulk density—mix fluid density is small have been introduced to the industry, but are not in widespread use. Some amount of bulk cement is lost every time it is transferred from one pressure vessel to the next. When expensive microspheres are included in those losses, the cost of the job rises. Also, increased fines introduced by the microspheres could cause dusting concerns. The liquid microsphere system discussed in this paper can help eliminate or minimize the foregoing issues. By minimizing or eliminating the standard microsphere cementing issues, lightweight cement can become easier to deliver successfully. Ultra-lightweight cementing also can be accomplished at most any time and place without requiring (1) specially designed or modified cementing equipment or (2) application of job volumes small enough to batch-mix. Background General When mixed with the recommended concentration of water, cement density ranges from 14.8 to 16.4 lb/gal depending upon the specific surface area or grind of the dry cement powder. When the goal of the cement job is to circulate cement above zones with a low fracture gradient, the cement density is reduced to help prevent losses of bulk cement fluid into the formation. Industry technology allows density reduction through three different mechanisms: (1) addition of extra water, (2) foaming the liquefied cement slurry, and (3) bulk blending microspheres into the dry cement powder. 1