Page 235 - A Practical Companion to Reservoir Stimulation

PRACTICAL COMPANION TO RESERVOIR STIMULATION

be measured with a rotational viscometer such as a Fann-35. is determined by monitoring the time for the vortex to close
Figures P-76 and P-77 use marsh funnel times to calculate after the crosslinker has been added. Some crosslinked fluids
viscosities. Whenever possible, a rotational viscometer should require temperature in addition to time to crosslink. For these
be used to check viscosities. fluids, the vortex closure must be tested in a cup that can be
heated with a thermocouple or in a water bath. Another test
P-8.1.3: Crosslinked Fluids checks the crosslink time in a falling ball viscometer. The
The crosslink time of a fracturing fluid must be adjusted to crosslinked fluid is added to the viscometer, and the time for
prevent shear degradation and excess friction. The ultimate the ball to fall is recorded. This type of viscometer can be
viscosity of a crosslinked fluid, made from organometallic made from a 50-cm’ syringe and a 1.9 specific gravity %-in.
crosslinkers, depends on the exposure of the fluid to shear. ball sealer. The fluid is considered crosslinked when the ball
Figure P-78 shows the effect of shear on an organometallic takes 7 skx to fall from one end to the other. The apparent
fluid. The viscosity of the fluid not exposed to shear is several viscosity of the fluid can be calculated using Fig. P-8 1. The
times higher than the fluid exposed to high shear during the floppy lip method of checking crosslink time should be
crosslink process. Borate-crosslinked fluids are not shear avoided if possible. This method does not adequately simu-
sensitive. They can recrosslink after being exposed to shear late the shear conditions a fluid experiences during pumping.
and therefore do not have to be delayed to ensure proper The viscosity of a fluid crosslinked under shear conditions is
rheological properties. very different from viscosity development under static con-
The ultimate viscosity degradation of an organometallic ditions. The super-high viscosity of a floppy gel never forms
fluid is a function of both shear rate and time exposed to during pumping.
shear. To prevent the shear degradation of these fluids, their Crosslinking requires the precise addition of a crosslinker
crosslink must be delayed. However, precise control of the solution to a gelled fluid. Varying the concentration of this
crosslink time is not required to ensure optimum fluid per- solution has significant effects on the stability and rheologi-
formance for most applications. A window can be established cal performance capabilities of the fracturing fluid. Some
to prevent the shear degradation while minimizing friction. crosslink solutions have a limited shelf life and may degrade
The crosslink process does not have to be extended past the when exposed to air or water. Therefore, large treatments
perforations. The shear at the perforations is extremely high may require mixing several crosslink solutions throughout
but lasts for such a short time that the fluid will not be the job. If several batches of crosslink solution are required
adversely affected. Hodge and Baranet have shown that for the treatment, each batch should be tested for crosslink
delaying the crosslink for as little as 1 min is adequate for performance prior to pumping.
preventing shear degradation (Fig. P-79). Each of these fluids
was subjected to 5 min of shear history at 1350 sec-’ (1 3 BPM P-8.1.4: Gelled Oil
down 2%-in. tubing). The amount of shear a fluid experiences The quality of a gelled oil depends not only on the proper
at different rates in different tubing sizes can be found on Fig. additive chemistry but also on the quality of the crude used as
P-80. the fluid base. Crudes that are high in either asphaltenes or
Delaying the crosslink time also lowers the friction pres- paraffins cannot be used to create a gelled fluid. For this
sure drop of a crosslinked fluid. Experience has shown that reason, gelled oils are often prepared from diesel or kerosene
delaying the crosslink time to a point where the fluid will be rather than crude. Even these refined products should be
two-thirds down the tubulars usually controls excess friction. tested in the laboratory to ensure they can be gelled. Other
This delay eliminates high friction from most of the tubing, chemicals added to these refined oils may interfere with
yet allows the fluid to maintain adequate viscosity for trans- viscosity development. Antigelling additives added to diesel
porting proppant as it enters the formation. Both organome- in cold climates often deteriorate the long-term performance
talk and borate crosslinkers can be delayed to minimize the of gelled oils. Care should also be taken to prevent contami-
amount of frictional pressure drop they create. nation of the oil with common polar species such as water,
Several methods of checking crosslink times can be used acid, bases or salts. These polar species often destroy the
on location. A long-time standard test requires checking the association between the aluminum ions and phosphate ester
time a fluid needs to develop a “floppy lip.” The fluid is groups. Water usually makes the gel structure more rigid at
considered crosslinked once it forms a rigid structure that surface temperatures but greatly reduces gel stability at el-
will hang from the lip of a cup rather than pouring out evated temperatures. Figure P-82 shows the effect of water
completely. A vortex closure test has become popular in on the surface viscosity of a typical gelled oil based on a
recent years. In this test, a gelled fluid is stirred in a blender at marsh funnel time. From this chart, it is evident that 10 gal to
a rate just high enough to create a vortex. The crosslink time 20 gal of water in a standard frac tank can affect the quality of

P-74