Cementing casing across highly depleted zones and weaker formations requires low-density cement systems capable of reducing the hydrostatic pressure of the fluid column during cement placement. Low-density or lightweight cement systems help achieve the specified top of cement by avoiding or minimizing lost circulation (losing cement to the formation). Halliburton offers cement solutions made light enough to circulate in such situations while retaining the ability to withstand downhole conditions.
Lightweight cement can be achieved in one of three ways: water extended, injection of gas (foamed cement), or by adding low-specific-gravity microspheres or other enhancing additives.
- Water extended: adding additional water to the cement slurry is a common means of reducing slurry density, yet it requires water-extending additives to help maintain slurry stability and overcome slow compressive strength development and low ultimate compressive strength.
- Foamed cement: Injecting gas (nitrogen, compressed air, or gas-generating solids) into the slurry provides the benefit of increased slurry compressibility, increase set-cement elasticity, and the flexibility to vary density during operations.
- Microspheres: While typically bulk blended, Halliburton also offers liquid-additive beaded systems, microspheres are available as
- solid, plastic beads of approximately 1.0 specific gravity;
- hollow-pozzlanic spheres of approximately 0.7 specific gravity;
- hollow glass bubbles of approximately 0.32 to 0.61 specific gravity.
Properly designed microsphere slurries can achieve the highest strength-to-weight ratio and lowest permeability of any lightweight cement design. Additionally, microsphere slurries have a reduced transition time from liquid to cement setting, helping minimize the gas migration. Some other mechanical enhancement additives such as fibres may also assist with reduction of slurry densities.