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

24 API STANDARD 65-2
<strong>The</strong> information entered into the computer simulation should be as accurate as possible. This information should
include drilling fluid, spacer, and slurry rheologies, anticipated pump rates, temperature, caliper log information (if
available), survey data (if available), well architecture, fracture and pore pressures and hardware configuration.
5.6.5.9 Foamed Cement Modeling
Engineering software should be used in the design and placement of foamed cement. Foamed cement design
software is normally incorporated into the cementing service providers’ ECD engineering programs. <strong>The</strong>re are two
foamed cement placement methods that are commonly used: (1) constant nitrogen injection rate and (2) constant
foam density. Regardless of which method is selected, variances in hole size across the foamed cement column may
change the density and the downhole nitrogen volume (foam quality) from that which was designed. This density
variance is more pronounced when using the constant density method.
<strong>The</strong> constant nitrogen injection rate method calls for a single nitrogen injection rate (in volume of nitrogen per volume
of base cement) to be added to the base cement at surface. This produces a variable foamed cement density
downhole owing to the effects of temperature and pressure. <strong>The</strong> target foamed cement density used for the design
will normally be found at the midpoint of the foamed cement column in the annulus.
When using the constant nitrogen injection rate technique there are two points to be considered.
1) When the foamed cement is placed, the leading edge of the foamed cement, will have a density lower than the
target density used for the design. This is due to the lower hydrostatic pressure and lower temperature found at
the top of the foamed cement column compared to the pressure and temperature found at the mid-point of the
foamed cement column (which was used to calculate the average nitrogen injection rate at surface). When
using the constant nitrogen injection rate method the foamed cement density at the top of the foamed cement
column should not cause a loss of overbalance pressure.
2) When the leading edge of the foamed cement exits the casing/liner shoe and enters the annulus it will contain a
volume of nitrogen designed for a location higher in the annulus (which has a lower hydrostatic pressure and a
lower temperature). As such, the density of the leading edge volume of foamed cement, will be greater than the
density reduction of that same volume of foamed cement once in place. This produces a higher effective
foamed cement density as it exits the casing/liner shoe.
A second placement technique, constant foam density, calls for the nitrogen injection rate at surface to be varied as a
function of the temperature and pressure conditions found at expected placement point of the foamed cement in the
annulus. This produces a pseudo-constant foamed cement density once the cement is in place. This technique is
performed either by constantly ramping or incrementally stepping up the nitrogen injection rate at surface.
When using the constant density technique there are two points to be considered.
1) <strong>The</strong> density of the foamed cement column should be examined to ensure that it does not cause a loss of
overbalance pressure.
2) <strong>The</strong> density of the leading edge of the foamed cement when exiting the casing/liner shoe should be examined
to ensure that ECD does not exceed the fracture gradient.
An accurate cementing temperature profile for the column of foamed cement is necessary to calculate the volume of
nitrogen gas injected at surface to produce a foamed cement of desired in-situ density. Temperature simulators
should be used to characterize the circulating temperature profile of the well for use in the nitrogen injection rate
calculation.
A foamed cement will generally exhibit a higher viscosity than the base fluid from which it was generated. <strong>The</strong> higher
the nitrogen content of the foamed cement (foam quality), the greater the viscosity increase of the foamed cement
compared to the base cement from which it was generated. ECD models should account for this increase in foamed