Equipment Selection
Equipment Selection
The basic equipment components required to perform a frac
treatment are high-pressure pump trucks, blender(s), and storage
equipment. Most frac treatments also employ a wide array of
auxiliary support equipment. For an offshore situation in which the
equipment must be temporarily installed on a flat barge, the
equipment should be skid-mounted rather than mounted on trucks, to
keep the center of gravity as low as possible.
Pump Trucks The pump trucks used for fracturing have
high-horsepower prime movers, each of which drives one or more
positive displacement, high-efficiency triplex pumps mounted on a
heavy-duty oilfield chassis. The fluid end of each pump is designed
to operate over a sizable pressure range, with the transmission
system giving a relatively constant horsepower performance. The
fluid end of the pumps can easily be changed to extend the
performance range of the pumps.
Some pumping equipment is operated from remote control panels to
facilitate overall treatment control and improve safety conditions.
The output performance of these units is typically in the 800-1500
horsepower range, with some units having two prime mover/pumps
installed on the same truck chassis. Because of the extreme
operating conditions encountered when pumping proppant/fluid
slurries at high pressures, at least one extra unit should be
available as a standby for most jobs. For some jobs with long
pumping times, as much as 100% excess horsepower should be kept in
reserve, ready for use.
Frac Boats A preferred solution for offshore operations is to
use a special frac boat with all the necessary equipment
permanently installed and plumbed for efficient operation. Several
frac boats are currently in use in various locations around the
world and have already effectively extended the safe and economic
application of fracture stimulation, including acid fracturing, to
the more difficult offshore environment. These boats typically
incorporate the latest technological advances in sophisticated
monitoring equipment, as well as the special automated blending
operations control that is essential to the use of continuous-mix
fluid systems.
Storage and Mixing units typically include multiple compartments
to allow a variety of proppants to be used on each treatment.
Because excellent fracture conductivity is the ultimate objective
of all frac treatments, correct handling and blending of the
proppants are among the most critical elements of on-site
operations. Fluid storage facilities have not changed considerably
in the last few years, the industry mainstay being a 500-barrel
frac tank, sometimes with an axle to facilitate movement to the
location. These tanks can be manifolded together to allow high-rate
fluid transfer with a minimal labor requirement. Recent
advancements in the associated monitoring equipment allow the fluid
level of each tank to be monitored continuously from a central
point as a cross-check of the volume of fluid pumped.
Most fracturing treatments use fluids that are specially mixed
on location prior to the actual pumping operation, and then stored
in large frac tanks until needed. This is impractical, however, for
exceptionally large treatments or for offshore locations having
limited storage facilities. The viscosity of frac fluids may start
to deteriorate slowly within a few minutes after being mixed.
Sometimes, due to unforeseen job delays, it is necessary to discard
considerable amounts of expensive fluids. The total optimal surface
storage time is highly dependent upon bacterial content and
temperature. During the summer, fluid properties may deteriorate
after one or two days unless a bactericide is added. A bactericide
extends the surface life of the fluid as well as limiting the
buildup of bacteria in the reservoir. If there is doubt, the fluid
properties should be re-tested before use.
It is not advisable to add gelling agents to previously gelled
fluids, because the partially viscous fluid is very inefficient at
hydrating the dry polymer gel particles. As a result, the partially
wetted polymer encapsulates other particles of completely dry
polymer, creating "fisheyes" of unhydrated polymer. These fisheyes
are highly undesirable, since some of them may be large enough to
permanently plug the formation and the propped fracture.
Many of the fluid systems used today, and others currently under
development, are designed to allow the continuous mixing of all the
necessary additives as the fluid is pumped downhole. Special mixing
systems are usually used to provide quality control. Alternatively,
certain other fluids are designed for use in a modified
continuous-mix mode, where additives can be safely added
continuously to the partially batch-mixed system.
Blender The most critical piece of equipment in fracture
stimulation is the blender. This unit transfers the frac fluid from
the storage tanks, blends the proportionate amount of proppant and
chemical additives with the fluid, and pressurizes the suction of
the high-pressure pumps with this slurry. Since all the fluid and
proppant must go through this single unit, its continuous operation
is essential to the success of every frac treatment. The modern
blender includes the following components:
a suction pump to transfer fluid
control devices to meter the simultaneous addition of several
liquid and solid additives
a complete manifold system to allow fluid selections to be
changed at will
a densiometer to determine the exact proppant concentration
a system to maintain a continuous discharge pressure at the
suction end of the triplex pumps
a monitoring system to ensure that everything is functioning
properly
