CEMENT SLURRY FORMULATION

CEMENT SLURRY FORMULATION

(a) Compressible Cements  Compressible cements maintain the cement pore pressure above that of the formation gas
pressure. Two general cement types are available:

•Foam cements

•Gas generating cements.

Foam cements do not work well at high pressure and are therefore limited to use at shallow  depths.

Gas generating cements maintain cement pore pressure by chemically producing gas  downhole (usually hydrogen). To avoid these gas bubbles coalescing and creating channels,  the slurries must be carefully designed.

(b) Expansive Cements

Expansive cements may be effective in eliminating small gaps between the cement and the  casing or formation, but are unlikely to be effective in eliminating large channels. There are two main techniques for expanding Portland cements:

•Gas generation
•Crystal growth.

Gas generating expansive cements work in the same way as gas generating compressible  cements, but with less gas-generating material added. Crystal growth expansive cements depend on the growth of added minerals within the set
cement. Formulating these cements must be carefully controlled. Uncontrolled mineral expansion  can disrupt and fracture set cement.

THIXOTROPIC AND HIGH GEL STRENGTH CEMENTS

Thixotropic and high gel strength cements resist gas percolation. However, if the gas bubbles  are smaller than the cement pore space gas migration can occur without slurry deformation.  In such a case gel strength is not a controlling factor. The inherent fluid loss of thixotropic  slurries tends to be high and must be controlled.

RIGHT-ANGLE SET CEMENTS

Right-angle set (RAS) cement slurries do not gel progressively, they set rapidly. The cement  transmits full hydrostatic pressure through the cement column up to when the cement begins  to set, and develops a low permeability matrix rapidly enough to prevent gas invasion. RAS  slurries can be difficult to design for temperatures below 250 deg F. In RAS slurries, the heat  generated from the rapid hydration that occurs is produced over a short period - possibly  leading to the formation of micro-annuli
.
IMPERMEABLE CEMENTS

Reducing the cement matrix permeability prevents gas migration through the cement  column. Several methods are available to do this:

•Viscosify the interstitial water of the cement slurry
•Prevent the interstitial water by using bridging agents and polymers
•Use latex additives that coalesce in the pore spaces when they contact gas.

SURFACTANTS

Under the right circumstances surfactants added to the cement slurry entrain  invading gas  and create a stable foam. This foam resists gas migration through the slurry