Page 166 - A Practical Companion to Reservoir Stimulation

PRACTICAL COMPANION TO RESERVOIR STIMULATION

P-3
nent fracture damage. The temperature limitation is that
Fracturing Fluid Selection below which borate-crosslinked fluids can be used. This is an
important limit in all fracturing treatments because (as will be
While historically there have been several hundred fracturing shown later) these fluids have demonstrated several favorable
fluids used by the industry, three considerations must govern properties.
their selection. In order of importance these are: Figure P-3 allows the division for below -225°F gas wells
1. Ability to create a fracture with large conductivity (i.e., to offer the flexibility of using linear gels or foams, but with
transport high proppant-slurry concentrations). the strong recommendation to always consider borate-
2. Result in as little polymer-induced proppant-pack crosslinked fluids first.
damage as possible. If the, gas well temperature is more than 225"F, then
3. Require lower pumping and treatment pressure capac- organometallic-crosslinked polymer fluids are indicated.
ity by reducing the friction pressure drop. While these are far more damaging than borate-crosslinked
fluids, they are necessary at these high temperatures. At
All these considerations are affected by the poly'mer load ultrahigh temperatures (>300"F), titanate or zirconate-
that controls both the fluid viscosity and the resulting friction crosslinked hydroxypropyl guar (HPG) must be used. If the
pressure drop. Thus, it is important that the amount of poly- reservoir is underpressured, these fluids can be energized
mer is engineered appropriately so that it is adequate but not with the addition of nitrogen or carbon dioxide.
excessive. In an oil well, the water sensitivity of the reservoir rock
There are other less important considerations in fluid has traditionally prompted a division between water-base
selection. While these should be taken into account, they fluids and oil-base fluids. However, this consideration is
should not govern the fluid selection to the detriment of the often the cause of inappropriate fluid selection and less-than-
previously mentioned important concerns. These include optimum fracturing treatments. Essentially, if the reservoir is
minimization of fracture face damage, which would be the mildly or moderately water sensitive, the selection process
result of unavoidable leakoff and compatibility problems outlined for a gas well should be followed.
between the fracturing fluid and reservoir fluids and rock. In While the use of oil-base fluids in oil wells is often
addition, there has been much concern in the industry about suggested, these fracturing fluids deserve certain additional
posttreatment cleanup. This concern led to the use of ener- considerations. The cost of pumping is much greater than for
gized and foamed fluids. Although they have a decided edge water-base pumping fluids because of the cost of the oil itself.
on cleanup, these fluids become impractical when high slurry Excessive hydraulic horsepower may be needed to place
concentrations are necessary because proppant is added ex- fracture treatments using oil-base fluids because of their
clusively to the liquid portion of the foam. Thus, superhigh inordinately high friction pressure losses.
proppant concentrations may be needed to exceed the proppant Finally, safety considerations must be addressed because
handling capabilities of today's pumping equipment. Foams of the flammability of the base fluid. Any problem leading to
may then be more appropriate in very tight formations where fluid leakage has the potential for posing an extreme fire
the fracture conductivity is less important. hazard.
The perceived advantage of oil-base fluids is that the
P-3.1: Fracturing Fluid Selection Guide reservoir is exposed only to a fluid that is related to the
Figure P-3 represents a compendium of current and evolving reservoir fluids. However, several studies have shown that
industry practices. While this selection guide should be con- fluid leakoff and relative permeability-induced damage are
sidered as general recommendations, the actual use (or even not usually severe problems. Figure P-4 shows that a dam-
appropriate use) of fluids may be lopsided toward certain aged zone of 5 in. (much deeper than most fluids will reach
types of fluids. Furthermore, this chart should always be used during leakoff) has a minimal effect on production as long as
with a degree of caution, bearing in mind the considerations the fracture itself has adequate conductivity. With this in
outlined in the previous subsection. Thus, fracturing fluid mind, water-base fluids can be used in most all reservoirs
must be engineered with the particular reservoir in mind and without creating significant damage. Figure P-4 suggests that
with consideration for the desired performance of the fractur- a tenfold decrease in the reservoir permeability (or even a
ing treatment. hundredfold decrease) has very limited impact on the produc-
The first and obvious division is whether the well is oil or tivity index ratio between the fractured and the unfractured
gas. In the case of a gas well (the left branch in Fig. P-3), the well. Thus, fracture face damage should never be a criterion
reservoir temperature provides the first decision. If the tem- for the fracturing fluid selection to the detriment of fracture
perature is less than 225"F, then all fracturing fluids can be conductivity. This issue is discussed extensively in Chapter
used with the obvious considerations of diminishing perma- 11 of Reservoir Stimulation.

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