Page 240 - A Practical Companion to Reservoir Stimulation

PRACTICAL CONSIDERATIONS FOR FRACTURE TREATMENT DESIGN

the gel. Weak acids and bases are used by design to break where V, and V, are the volumes of gas and liquid, respec-
gelled oils. It is therefore imperative that these species do not tively. The liquid ratio of the total foam is then 1 - Q.
contaminate the gelled oil. Proppant is added only to the liquid portion. Therefore, the
Care must be taken to add the phosphate ester gelling proppant concentration at the blender must take into account
agent and the aluminate crosslinker at the proper concentra- the dilution of the gas. The required concentration can be
tion and ratios. An overactivated gelled oil initially becomes calculated by:
highly viscous but quickly reaches a point where the gel
breaks out and viscosity is lost. Materials must be added in Blender Proppant Concentration =
the proper sequence for most gelled oils. The two materials Foam Proppant Concentration
can be added simultaneously during the mixing procedure, (P-9)
but directly mixing together the gelling agent and activator 1-Q
must be avoided. After the gel structure begins to form, The downhole volume of these fluids can be calculated by
viscosity measurements should be taken to ensure the fluid using standard gas calculations. The formation volume factor,
has the correct rheological properties. Very small amounts B (SCFhbl), is defined as
(less than 1 gal when mixing 500-bbl frac tanks) of either
activator or gelling agent can be added to create a fluid with Velum e Reservoir Chidition s
the proper viscosity. If the viscosity of the fluid is low, then B= Volume Stntidurd OKlditims (P-10)
additional activator is probably needed. If the viscosity is
high, then more phosphate gelling agent should be added. where
Often, these fluids slowly build viscosity for several hours (P-11)
after mixing. Slow viscosity development can lead to very PVRe.srrvoir = (ZnR r) Reservoir
high static viscosities that cause fluid transfer problems. To and
prevent this, the concentration of the aluminate activator may
be deliberately kept low. Small amounts of the activator can
then be added until the desired fluid properties are reached. Therefore,
Recent developments in continuous-mix technology have
made new gelled oil fluids practical. These fluids exhibit
lower friction pressures while providing adequate viscosities. (P-13)
As with any continuous mix procedure, all additive rates
must be closely monitored to ensure the desired fluid is being With standard conditions of 60°F [620"R], pressure of
created. 14.7 psia and a Z factor of 1; reservoir conditions of 180"F,
Breaker tests should be run on all gelled oil fluids before fracture pressure of 5000 psi and a Z factor of 1.2, the B is 7.7
they are pumped. Each different oil displays a characteristic x lo4 bbl/SCF, or 1299 SCFhbl. Z factors for N2 and COz
break. Using a standard breaker concentration based only on can be found in Figs. P-83 and P-84, respectively. A quick
temperature and gel loading must be avoided. estimation of the volume factor can be obtained from Figs. P-
85 and P-86.
P-8.1.5: Foamed Fluids These two gases are also soluble in the base fluid. The
Using nitrogen or carbon dioxide as part of the fracturing amount of gas lost to solution should be accounted for.
fluid requires extra effort to ensure proper execution. The Figures P-87 and P-88 show the varying solubilities of Nz and
compressibility and solubility of these fluids must be ac- C02, respectively. The total downhole volume of gas needed
counted for based on downhole conditions. An error in the for the quality calculation is the sum of the compressibility
compressibility calculations can significantly change both volume factor and the solubility.
the rheological properties and the overall volume of the It is critical to know the accurate fracture gradient before
fracturing fluid. attempting a treatment with either C02 or N2. All the com-
Foams are usually characterized by quality. The quality of pressibility calculations are based on this value. These gases
a foam, Q, is defined as the ratio of gas volume to the total occupy a different amount of space for any given pressure.
liquid and gas volume: The volume of these gases must be calculated at fracturing
pressures. Changing the amount of gas pumped based on
Q=- "8 surface pressures during a treatment will almost always lead
v, + v,' to an error in the downhole foam quality.

P-79