Operational Considerations

Operational restrictions may impose limits on the hypothetically optimum

treatment. Operating conditions encountered offshore frequently impose

additional economical and unique logistical considerations on the fracturing

program design.

Surface Location

The size and shape of the surface location that is required for a fracturing

treatment is quite different from that needed to drill the well. Surface locations

used for drilling deep wells are generally large enough to accommodate all the

frac pumps and associated equipment. But the smaller sites that are

characteristic of shallow wells frequently impose severe space restrictions,

especially if the rig is still on location. Whenever practical, the rig should be

moved off the location to make more room during a frac treatment. Alternatively,

it should be shut down during the pumping operation for safety purposes.

Industry safety standards define the minimum distances to be maintained

between the wellhead and potential ignition sources. They specify that the

storage facilities for treating fluids should be located a safe distance from the

wellhead, and from potential ignition sources if the frac fluid is flammable. There

must be sufficient space available for spotting the blender, proppant storage

facilities, frac pumps, pumping manifold, and recording/command center, and still

enough room for personnel to move easily around the equipment. The equipment

that will be in operation during the treatment should be located upwind of the well

to further minimize fire hazards. The site should also have enough room for

logging equipment or other specialty equipment involved in the overall

completion program.

Sometimes it is necessary to enlarge the location prior to a fracturing treatment,

or to use space adjacent to the hard pad. The cost of any special preparations, as

well as other eventualities (such as pulling heavy equipment out of the mud

because the road or location was not adequately prepared), must be taken into

account when finalizing the treatment costs.

The injection rate for most fracturing treatments today ranges from 15 to 40

barrels per minute (bpm), with a maximum possible rate of about 150 bpm.

Higher-rate treatments are pumped down open casing, but most treatments are

directed down large-diameter tubing, or simultaneously through the casing and

tubing. The minimum pump rate for fracturing depends primarily on individual

formation characteristics and treating fluids. It is typically around five to eight

barrels per minute.

Because large-diameter tubing is frequently impractical for production purposes,

special "frac" strings are often used. When fracturing sensitive formations, this

practice should be delayed until after cleanup in order to return the fracturing

fluid as soon as possible and thereby minimize damage to the formation.

Treatments may be pumped through either the tubing or casing annulus, or

through both simultaneously, to minimize friction losses. When pumping through

both the tubing and the annulus, the rates having the same friction losses would

be combined to determine the total injection rate possible. If the treatment is

pumped down the annulus, the pressure limit on the casing must be considered.

When designing the production casing for a new well that has even a slight

chance of requiring fracture stimulation, the pressure rating should be high

enough to accommodate a typical frac treatment to be pumped via the annulus.

Tubing that has been used in rod pumping wells should never be used for

fracturing because of possible rod wear, which would reduce tubing strength. New

tubing should be cleaned with acid. Any old production tubing must be thoroughly

cleaned prior to use as a frac string because of the possible presence of scale or

paraffin deposits that could come loose and interfere with the frac. Tubing may be

conveniently cleaned in-place by scouring it with sand. A small volume of sand-

water slurry circulated down the tubing and then reverse-circulated at a high rate

has been used very successfully to clean tubing.

Timing

A successful fracture treatment is the result of detailed planning and thorough

pretreatment organization. The key element is good communication between all

involved participants, including the service company, rig supervisor, trucking

contractors, downhole tool service company, suppliers of frac tanks, logging

company, and the company representative.

The time required for each facet of the operation depends on the job’s size and

complexity, and on local conditions. For example, transporting frac water to a

remote location may require up to two weeks, while the same size job in an active

region with many water trucks available may take only a day or two.

The freshly cleaned fluid storage tanks are the first pieces of equipment to be set

on location. They should be spotted by the fracturing supervisor in a level area of

the location that meets the company's safety standards and allows the proper

setup for the proppant and pumping equipment. They should be filled with the

type of fluid recommended by the fracturing service company representative,

making sure there is enough for an extra stage of flush or for other unplanned

contingencies.

For treatments involving large amounts of proppants, the storage facilities will be

large portable tanks that are filled after being set on the location. These may take

several hours or days to load, depending on the job size and remoteness of the

well location. On small treatments, the proppant is usually transported to the

location in trucks along with the pumping equipment, and poured from these

trucks directly into the blender for mixing. The pumping equipment is normally

brought to the location on the morning of the job and hooked up in a few hours

while the frac fluid premix is underway. Exceptionally large treatments or critical

operations may require an extra day to set up all the equipment. In certain areas,

it is recommended that the frac fluid be heated to formation temperature to

reduce paraffin formation and other damaging conditions.

The actual pumping operation may require only a few minutes or a few hours, but

preparing for a frac job may take several weeks.

Safety and Quality Control

Safety is of paramount importance throughout the fracture treatment. All service

companies have stringent standards for pumping operations that must be

adhered to by all site personnel. These standards may differ depending on the

type of fluid being pumped.

A fracturing treatment is normally pumped at high surface pressures. A maximum

treating pressure that will protect the tubular goods (and/or prevent fracture

growth through defined barriers) should be established prior to the treatment.

Because of the high complexity of the materials and equipment used in hydraulic

fracturing, quality control has emerged as a major issue. The role of quality

control is to ensure that all the fluids meet the design specifications. On-line

quality control involves, but is not restricted to, pH and viscosity measurements.