Page 195 - A Practical Companion to Reservoir Stimulation

PRACTICAL COMPANION TO RESERVOIR STIMULATION

Controlling the timing of the breaking process is critical to fluid relative to its dimensionless position along the fracture,
the success of the treatment. Once a breaker is added to a based on fluid efficiency. Time- and temperature-dependent
fracturing fluid, the degradation process immediately begins. viscosity values can then be applied to develop a breaker
Careful planning must go into the design of breaker sched- schedule that maximizes the amount of breaker used while
ules. If too much breaker is added early in the treatment, the never allowing the fracturing fluid to degrade below the
viscosity required for fluid loss control and proppant transport critical viscosity of 100 cp 170 sec-1. Table P- 10 gives a hy-
may be prematurely lost, resulting in a screenout. If the pothetical treatment schedule using a 40-lb borate-cross-
breaker schedule is not ambitious enough, the molecular linked fracturing fluid. The tapered breaker schedule was
chains may not degrade sufficiently, causing the treatment to calculated based on a fluid surface temperature of 80°F and a
clean up slowly. Even worse, the fluid may not completely bottomhole temperature of 150°F. The pad fluid will be
degrade without sufficient breaker quantities, limiting well exposed to bottomhole temperature for more than 1 hr so only
production because of proppant-conductivity damage. This is a minimal amount of breaker can be added. The last proppant
particularly true at high polymer concentrations. As the poly- stage is exposed to bottomhole temperatures for less than 10
mer concentration increases, higher breaker concentrations min, allowing the breaker concentration to be very aggressive
must be maintained to sufficiently degrade the polymer and for this one stage.
prevent proppant-permeability damage. It is now recognized Breakers for oil-base gels operate much differently than
that the in-situ polymer concentration in the fracture may their water-base counterparts. Most gelled oil breakers are
increase more than tenfold (concentrations greater than 500 slowly dissolving bases, such as lime or bicarbonate, and
lb/lOOO gal) because of fluid loss. The breaker level in this they are intended to reverse the crosslinking reaction. These
concentrated fracturing fluid decreases as the polymer con- systems work quite well at temperatures above 150°F, but
centration increases. The breaker is dissolved into the water large quantities of breaker are required at lower temperatures.
portion of the slurry and is lost in the fluid leaking off as the The low-temperature break mechanism is not precise and is
fracturing fluid dehydrates. This can result in a damage to the often expensive because of the quantity of breaker needed.
proppant pack, which may exceed 90%. Amine systems can also be used in gelled oil systems to aid in
Tapered breaker schedules allow much greater quantities the break process at low temperatures.
of breaker to be added to the fluid while minimizing the risk Ideally, a breaker would not become active until after the
of excessive degradation. To design the breaker schedule, the treatment has finished and the formation has closed on the
time of exposure to bottomhole temperature for each stage of proppant. The ideal breaker would then completely degrade
the fracture treatment must be determined. From this, a the polymer in a very short period of time, and the fracturing
maximum quantity of breaker can be calculated without fluid could be produced back. Recent advances in encapsula-
risking premature loss of viscosity. This can best be demon- tion technology have allowed the development of breakers
strated through the following example: Table P-9 gives vis- that perform in a similar manner. Even though the current
cosity data for a borate fracturing fluid with breaker included. techniques are not perfected, they do allow significantly
The table shows how time, temperature and breaker concen- higher concentrations of oxidative breaker to be placed into
tration contribute to the viscosity degradation of a fluid. The fracturing fluids without sacrificing viscosity (see Fig. P-44).
temperature of the fracturing fluid along the fracture will These breakers also have the advantage of remaining with the
vary significantly as a result of cool-down effects. Figure polymer inside the fracture. Once encapsulated, the breaker
P-43 can be used to estimate the temperature of a fracturing is large enough that it will not be lost during fluid leakoff.

P-36