Workovers
Click here for a .pdf file
which summarizes some common workover methods
[Below, Squirt describes some of the problems that can beset an oil
well. Described are the sources of these problems, and the solutions: from minor repairs
to major "workovers". Click on the following bookmarks to go directly to the
topic, or read through the entire article.]
An oil well is essentially a mechanical device and like any other mechanical device it
is subject to problems and failures which require repairs. Repairing an oil well sometimes
requires a workover.
Of all the problems that can occur during production, three stand out the most:
equipment failure, wellbore problems, and saltwater disposal. These problems can result in
a workover operation. Let's look at these common production foul-ups and learn more about
them.
EQUIPMENT FAILURE
Equipment failure is probably the most common type of production problem. For example,
a rod may break in a pumping well, requiring special equipment called a service rig to
be moved to the well to recover the rod from the hole and put the well back into
production. The service rig is mounted on a self-propelled chassis and has a special crew
of its own. Since wells drilled in recent years do not have a derrick, the unit will
include a mast and drawworks for removing equipment such as rods, tubing and the pump from
the wellbore. This equipment is directly descended from the first service rigs used in
this area - they were called pole rigs.
Another common production problem is subsurface pump failure, due in most cases to
physical wear of one or more of the pump's moving parts. When this occurs, a service rig
can quickly remove the pump, which is attached to the sucker rods, and make the necessary
repairs.
If tubing develops a leak or a break from corrosion, or mechanical stresses, the
service rig is again called to the well. The tubing is removed from the hole, the damaged
joint is replaced, and the tubing is returned to the wellbore.
If the well needs a quick jolt of clean fluid to get it working again a
truck mounted unit called a FLUSHBY is called. This unit pulls the pump up the well using
its derrick and drawworks. A load of clean oil is pumped down by the same unit which also
has a mounted pressure pump and tank. The pump is then reseated and the well placed on
pump.
WELLBORE PROBLEMS
Sanding, formation damage, paraffin accumulation, oil-water emulsions, and corrosion
are common wellbore problems.
Sanding
In wells which produce from loosely consolidated sandstone formations, a certain amount
of sand is usually produced with oil. Although some of this sand will be produced at the
surface, most of it will accumulate at the bottom of the hole. Continued accumulation of
the sand in the wellbore will eventually cut the oil-producing rate and may even halt
production altogether. When this problem, known as sanding, occurs, a service rig equipped
with a sand pump on a wire line is called to the scene. The sand pump is a special tool
which removes the sand from the wellbore.
If a well continues to present sanding problems, preventive action may be needed.
Various types of plastics can be used to consolidate or compact the sand. The chief
problem here is to obtain a plastic which will consolidate the sand yet permit oil to flow
through the result.
Formation Damage
This common problem occurs when something happens to the formation near the wellbore,
slowing oil production. For example, excessive buildup of water saturation in the vicinity
of the wellbore impedes oil flow. A mud block, an accumulation of drilling mud around the
wellbore producing zone, can also reduce the rate of oil flow, In a shaly producing
formation, the drilling mud used in a workover operation can cause clay swelling and
completely stop oil flow.
Wells with such formation damage may be treated with acids, mud cleanout agents,
wetting agents, and/or other special-purpose chemicals. These materials are pumped into
the formation and are eventually produced to the surface. These are highly specialized
operations, requiring special pump trucks and equipment, and they are usually performed by
oilwell service companies specializing in this type of work.
Oil-Water Emulsions
Emulsions of oil and water are a fourth common production problem. Under certain
conditions, oil and water may form an emulsion that will not separate at the surface
without special treatment. This is a problem because the process to break up the emulsion
is very expensive. Methods of breaking up such emulsions include heat treatment, chemical
treatment, and various combinations of chemical treatment. Since the chemical composition
of crude oil varies from one field to another, the nature of the chemicals used to break
up emulsions also varies.
Corrosion
Corrosion of equipment is one of the most costly problems plaguing the oil industry.
Salt water produced with oil is highly corrosive, and most crude oils contain varying
amounts of hydrogen sulfide, which is also quite corrosive. Anticorrosive measures include
the injection of a chemical corrosion inhibitor down the casing/tubing annulus; the use of
plastic-coated tubing; and the use of special corrosion-resistant alloys and cement-lined
pipe. Each of these methods has distinct advantages and disadvantages. Frequently the cost
of reducing the corrosion rate is so high that it cannot be justified, in which case no
anticorrosion measures of any kind are taken and the equipment is replaced at the end of
its useful life.
SALTWATER DISPOSAL
Disposing of the salt water produced with oil can be very expensive. Salt water cannot
be run into surface streams and pools since it is harmful to plant and animal life. The
most common method of saltwater disposal is through wells drilled especially for this
purpose.
Salt water must not be injected into a freshwater formation, and wherever injected it
must be handled with care to prevent the accumulation of excessive foreign materials which
might plug the formation. It is common practice to backflow saltwater injection wells from
time to time to remove some of the foreign materials that have accumulated on the
formation at the bottom of the hole. Acidizing the injection well also helps to clean the
formation.
WORKOVER OPERATIONS
Workover operations are major remedial operations sometimes required to maintain
maximum oil producing rates. If, for example, a well begins to produce an excessive amount
of salt water, a service rig is moved onto the well, and operations to reduce the
saltwater production are begun.
It may be first necessary to "kill" the well with some fluid, such as
drilling mud, salt water, oil, or possibly a special workover fluid, which has sufficient
hydrostatic pressure to counteract the formation pressure when the hole is filled with the
fluid. If the salt water is coming from the lower part of the reservoir, it is usual to
squeeze-cement the perforation with either a low-pressure or a high-pressure squeeze.
If the high-pressure squeeze-cementing technique is used, a special packer is run on
the bottom of the tubing to protect the casing and other equipment at the wellhead. If the
low-pressure or "bradenhead" squeeze-cementing method is used, then a packer is
not required, since the pressures applied will not exceed the working pressure of the
wellhead equipment and casing. After the cement has set, it may be necessary to drill out
the cement from inside the casing and reperforate the casing at the desired intervals,
since the cement will have sealed off all the old perforations.
If a well is producing with an excessive gas-oil ratio, it may be possible to reduce
the gas-oil ratio by the same squeeze-cementing and reperforating technique. Where there
is more than one producing interval in the wellbore and a lower zone has been depleted, a plugback
to a high zone is in order. The plugback can be accomplished with a cement plug in the
casing or with a bridge plug-a mechanical device which can be set in the casing to
effectively seal off all production below the point at which it is set.
The so-called permanent completion permits all workover operations to be conducted with
wireline equipment, eliminating the need for workover rigs. Permanent completion equipment
features special types of valves which can be opened and closed by wireline equipment. A
complete line of equipment has been designed for this type of workover operation, and even
cementing and reperforating can be satisfactorily accomplished with it.
