Page 170 - A Practical Companion to Reservoir Stimulation

PRACTICAL CONSIDERATIONS FOR FRACTURE TREATMENT DESIGN

P.3.2: Rheological Properties and Viscosity Requirements viscosity, respectively, of 30 lb/lOOO gal borate-crosslinked
Experience has shown that the viscosity of fracturing fluids gel; Figs. P-8 through P-10 contain the same variables for 40
must nearly always be above 100 cp at 170 sec-' to success- lb/1000 gal fluids. The time dependency occurs because of
fully transport proppant throughout the fracture. When de- the delayed crosslink nature of these fluids. These are some
signing a fluid to meet this requirement, the reservoir static of the most common fracturing fluids and the ones recom-
temperature must be taken into account. As shown in Fig. mended for the majority of fracturing treatments.
5-5, the temperature of the fracturing fluid approaches this Figures P- 1 1 through P- 1 3 include the n ', K' and apparent
maximum temperature in the first 25 to 30% of the fracture. viscosity for 40 lb/lOOO gal zirconate-crosslinked fluid; Figs.
The viscosity of a fracturing fluid will decrease as a function P-14 through P-16 are for 50 lb/1000 gal fluid; and Figs. P-17
of both increasing temperature and increasing exposure time through P-19 are for 60 lb/1000 gal fluid. The latter has been
at this temperature. used in reservoirs with static temperatures up to 400°F.
Wells with temperatures above 300°F require fracturing Figures P-20 and P-21 contain the rheological properties
fluids with polymer loads of 50 to 60 lb/lOOO gal for the pad of three different quality foams (554, 654 and 754) with 30
fluid and for the early proppant stages. As treatment progress- Ib of polymer/l000 gal of liquid. Figure P-22 shows the
es, polymer loading may be reduced because this portion of apparent viscosity of these fluids and their substantially lower
the treatment will not be exposed to reservoir temperature for values when compared to the crosslinked liquid fluids. These
an extended period of time. The final stages can be as low as viscosities would have a highly detrimental effect on the
30 lb/lOOO gal and may incorporate a less damaging crosslinker created fracture width, and in moderate-permeability reser-
(i.e., borate instead of an organometallic material). Currently, voirs (k > 0.5 md), such a fracture would exhibit lower
major advances in continuous fluid mixing allow the engi- posttreatment production.
neering and tailoring of fracture fluids and additives to affect However, if foams are made with crosslinked polymer
optimum properties. fluids, they exhibit much higher viscosities. Figures P-23 through
The data that follow cover common fracturing fluids and P-25 contain the rheological properties and apparent viscosi-
include the power law rheological properties n' and K' and a ties of foams made with 40-lb crosslinked polymer at 150°F,
calculated apparent viscosity at 170 sec-'. The latter can be whereas Figs. P-26 through P-28 contain the same informa-
readily obtained using the method outlined in Table 5-1. tion at 200°F. In both cases, three quality foams are used:
Example calculations are shown in Chapter C of this volume. 25Q, 50Q and 654.
Figures P-5 through P-7 include the n', K' and apparent

P-11