where

γN = specific gravity of nitrogen (compared with air),

p = pressure (pounds per square inch absolute),

z = gas deviation factor (dimensionless), and

T = temperature (degrees Rankine = 460 + degrees Fahrenheit).

So, for every in situ gallon of slurry there will be 0.174 gallon of nitrogen mixed with 0.826 gallon of base 16.74-ppg cement slurry. Thus, the in situ foam quality is 17.4 percent. Note that the Chevron tests used a 13 percent quality foam, which corresponds to the weight fraction of nitrogen necessary to create a 14.5 ppg density foam at atmospheric conditions. Therefore, more nitrogen is required to create the same density foam at the much higher pressure and temperature of the bottom of the Macondo well.

At the mixer at the surface, the slurry is blended and pumped at about 600 psig. The volume of nitrogen introduced to 0.826 gallons of base cement is the in situ volume increased through the real gas law.

image

This is added to 0.826 gallon of base cement. Thus, for every 1 gallon of base cement, 1.94 gallons of N2 at 600 psig is required. This is a 66 percent quality foam.

The density of the foam slurry at the mixer will be as follows:

image

The previous equations and results can be combined to obtain an equation for the density of the slurry at any depth with a corresponding pressure, temperature, and gas deviation factor.

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