Mr. Erik Milito - The House Committee on Natural Resources ...

30 API STANDARD 65-2
it will be with foamed cement prepared under atmospheric pressure in the laboratory. Foamed cement prepared
under atmospheric pressure cannot be tested in pressurized laboratory equipment. This is because the volume of
slurry prepared at atmospheric pressure will significantly decrease when pressurized in lab equipment. For example,
a foamed cement with 20 % by volume nitrogen prepared at atmospheric pressure will undergo an approximately 75
fold decrease in nitrogen volume when placed under 1000 psi of pressure.
Since the gas used in foamed cement is inert the rate of cement hydration does not change. This means that the
thickening time is not affected and thickening time tests should be performed in a standard HTHP consistometer on
the base slurry. <strong>The</strong> surfactants and stabilizers as well as any other additives should be added to the base slurry
because they will affect the thickening time.
<strong>The</strong> time required for foamed cement to begin to develop strength is also unaffected by the addition of an inert gas but
the magnitude of the strength will be lower. Compressive strength of foamed cement is normally performed by
crushing specimens of the cement after they have been cured in a sealed curing vessel submerged in a water bath at
atmospheric pressure. Studies with specialized equipment have shown that the compressive strength of foamed
cement generated and cured under pressure is generally higher than the compressive strength of samples generated
and cured under atmospheric pressure.
Even though the chemistry of the base slurry remains unchanged, many slurry properties such as fluid loss and
rheology will be altered due to the compressible nature of a multi-phase fluid. Compressible fluids like foamed cement
have a lower inherent rate of fluid loss than the base slurry from which they were prepared because as differential
pressure is applied across a porous media the gas bubbles will compress more readily than the water can be forced
from the base cement slurry. Specialized test equipment can be built to measure the fluid loss of foamed cement
slurries but generally the fluid loss of the base slurry is used as the design criteria.
<strong>The</strong> rheology of foamed cement is difficult to characterize but in general it can be considered to be higher than the
rheology of the base slurry. Measuring the rheology of foamed cement on a rotational viscometer will give erroneous
results. <strong>The</strong> rheology of the base slurry can be measured and correlations can be applied to estimate the rheology of
the foam.
<strong>The</strong> stability of foamed cement is one of the most important parameters to evaluate. If the foam is not stable, the
bubbles will begin to coalesce and migrate through the slurry. This will result in a decrease in the density of the
column as the bubbles rise. It will also cause the set cement to have an unacceptably high permeability and a very
low compressive strength. If the gas completely breaks out of the foamed slurry and migrates upwards it will likely
result in a loss of overbalance pressure. <strong>The</strong> loss of the gas from the foamed cement will also result in a loss of
volume and thus a lower TOC. <strong>The</strong> methods for evaluating the stability of both the foamed cement slurry and the set
foamed cement described in API RP 10B-4/ISO 10426-4 should be followed to evaluate stability. If there is a
possibility that the foamed cement could come into contact with other fluids in the well that could potentially
destabilize it, such as NAF, additional stability tests simulating the contact should be performed.
5.7.14 Cement Slurry Techniques for Controlling Annular Flow
A number of slurry design methods have been developed to control annular gas flow. <strong>The</strong>se methods differ
fundamentally in their mechanisms of control and it is up to the designer to determine that the selected method is
likely to perform well in the specific application. <strong>The</strong>se methods include compressible slurries, such as foamed
cement and cement with in-situ gas generating materials, latex of certain types, systems containing microsilica,
slurries with surfactants or polymer dispersions and static gel strength controlled slurries. Certain of these require
special laboratory testing techniques.
Some of these slurries and techniques are proprietary and service company design criteria should be considered for
their use. Simple mathematical expressions are sometimes used to gauge the potential for gas flow within a
cemented annulus. Care should be exercised when evaluating the potential for gas flow using this type of generalized
expression. Variations in wellbore diameter, the undefined hydrostatic contribution of by-passed drilling fluid,
uncertainty in the location of the top of cement and unknown degree of annular hydrostatic pressure reduction owing