Click here for a .pdf file which summarizes some common workover methods
[Below, Crudey 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 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.
Pole Rig: on Dina # 2 Summer 1937?
The first service rigs in the Lloydminster area
were known as "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.
Sanding, formation damage, paraffin accumulation, oil-water emulsions, and corrosion are common wellbore problems.
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.
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.
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 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.
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 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.
So you see how amazing it is that humans would go to so much work just to get to me, Crudey, the Heavy Oil drop.