Oil and Gas : Well Completion
SETTING PRODUCTION CASING
If the operator decides to set production casing, a supplier brings it to the well. For the final time, the casing and cementing crews run and cement a string of casing in the well. In the case of our model well, the crew could run 5 -inch(127millimetre) casing in the 7 7/8-inch (200-millimetre) hole. Keep in mind that the operator may elect to set a liner string. As you recall, a liner string is the same as a casing string except that a liner does not run all the way to the surface. Instead, the casing crew hangs it inside a previously run casing or liner.
Usually, the casing and cementing crews set and cement the production casing or liner through the pay zone. The drilling crew drills the hole so that it goes all the way through the producing horizon and stops a short distance below. Then the casing crew runs the production string almost to the bottom of the hole. (It leaves a little room beneath the guide shoe to allow cement to flow out of the casing.) The production casing or liner and the cement actually seal off the producing zone. At this point, the drilling rig and crews are finished with their job: they have drilled, cased, and cemented the well to the depth specified by the drilling contract. Their only remaining job is to disassemble the rig (rig down) and move it to the next drilling location.
PERFORATING The operator is not through, however. Because the production string and the cement seal the producing zone, the operator has to provide a way for oil and gas to get from the formation and into the well. Usually, the operator hires the services of a completion rig, which is a relatively small portable rig whose crews perform the final operations required to bring the well into production (fig. 179). One important task is to perforate the well. A special gun shoots several relatively small holes in the casing. It makes them in the side of the casing opposite the producing zone. These holes, or perforations, pierce the casing or liner and the cement around the casing or liner. The perforations go through the casing and the cement and a short distance into the producing formation. Formation fluids, which include oil and gas, flow through these perforations and into the well.
The most common perforating gun uses shaped charges, similar to those used in armor-piercing shells. Several high speed, high-pressure jets of gas penetrate the steel casing, the cement, and the formation next to the cement. A perforating specialist installs the charges in the special gun and lowers it usually on wireline, rather than drill pipe-into the well to the desired depth. The depth can be determined by running a collar locator log, which identifies the depth of each casing collar.
By comparing the log with the overall number and length of the casing joints, the operator can accurately determine the depth. Once at the desired depth, the perforating specialist fires the gun to set off the charges (fig. 181). After the gun makes the perforations, the perforating specialist retrieves it.
RUNNING TUBING AND INSTALLING THE CHRISTMAS TREE
After the well is perforated, oil and gas can flow into the casing or liner. Usually, however, the operator does not produce the well by allowing hydrocarbons to flow up the casing or liner. Instead, the completion rig crew places small-diameter pipe called "tubing" inside the cased well. In fact, the operator sometimes runs tubing into the well before perforating it. In such cases, the perforating gun is lowered through the tubing to the required depth.
Tubing that meets API specifications has an outside diameter that ranges from 1.050 inches (26.7 millimeters) to 4 1/2 inches (114.3 millimeters). Seven sizes between the two extremes are also available. As it does with casing, the crew commonly uses couplings to join tubing, although an integral joint tubing is available that allows the crew to make up joints without using couplings.
Manufacturers also supply coiled tubing. Coiled tubing is a continuous length-it does not have joints-of flexible steel pipe that comes rolled on a large reel. Operators have completed wells over 20,000 feet (6,000 metres) deep with coiled tubing. Special equipment placed at the top of the well allows crew members to insert, or inject, the tubing into the well as they unwind it from the reel. The main advantage of coiled tubing is that crew members do not have to connect several single joints of tubing when installing the string. Consequently, coiled tubing takes considerably less time to run.
Whether using jointed or coiled tubing, the operator usually produces a well through a tubing string rather than through the casing for several reasons. For one thing, the crew does not cement a tubing string in the well. Accordingly, when a joint of tubing fails, as it almost inevitably will over the life of a well, the operator can easily replace the failed joint or joints or, in the case of coiled tubing, remove and repair or replace the failed area. Since casing is cemented, it is very difficult to replace.
For another thing, tubing allows the operator to control the well's production by placing special tools and devices in or on the tubing string. These devices allow the operator to produce the well efficiently. In some cases, the operator can produce the well only by using a tubing string. Casing does not provide a place to install any tools or devices that may be required for production. In addition, the operator installs safety valves in the tubing string. These valves automatically stop the flow of fluids from the well if damage occurs at the surface.
Finally, tubing protects the casing from the corrosive and erosive effects of produced fluids. Over the life of a well, reservoir fluids tend to corrode metals with which they are in contact. By producing fluids through the tubing, which the operator can easily replace, the casing, which is not so easy to repair or replace, is preserved.
Crew members usually run tubing into the well with a sealing device called a "packer." They install the packer on the tubing string and place it at a depth slightly above the casing perforations. The end of the tubing is left open or is perforated and extends to a point opposite the perforations in the casing. The packer expands and grips the wall of the production casing or liner. When expanded, the packer seals the annular space between the tubing and the casing above the perforations. The produced fluids flow through the perforations and into the tubing string. The packer prevents them from entering the annular space, where they could eventually corrode the casing.
After the crew runs the tubing string, the operator has a crew install a collection of fittings and valves called a Christmas tree (fig- 183) on top of the well. Tubing hangs from the tree so the well's production flows from the tubing and into the tree. Valves on the Christmas tree allow the operator to control the amount of production or to shut in the well completely to stop it from producing. They also allow the operator to direct the flow of production through various surface lines as required. In addition, a special safety valve on the tree automatically shuts in the well if the tree is damaged. This automatic shut-in valve prevents reservoir fluids from flowing onto the surface if damage occurs. Usually, once the crew installs the Christmas tree, the well is complete.
Hydrocarbons sometimes exist in a formation but cannot flow readily into the well because the formation has very low permeability. If the formation reacts favorably to acid, acidizing may improve flow. An acidizing service company can pump anywhere from 50 to thousands of gallons (or litres) of acid down the well's tubing. The acid, to which the acidizing company adds a chemical to prevent it from corroding the tubing, enters the perforations and contacts the formation. Continued pumping forces the acid into the formation, where it etches channels. These channels provide a way for the formation hydrocarbons to enter the well through the perforations.
Another treatment that may improve flow is fracturing. A fracturing service company pumps a specially blended liquid down the well's tubing and into the perforations. The pumps develop a great deal of pressure at very high rates of flow. Continued pumping causes the formation to split, or fracture, much as a steel wedge causes a log to split. The fracturing crew adds a finely graded sand or similar material(a proppant) to the fracturing fluid. The proppant enters the fracture in the formation. When the fracturing crew stops the pumps, the pressure dissipates. With the pressure gone, the fracture tries to close. The proppants, however, hold the fracture open. This propped-open fracture provides a passage for hydrocarbons to flow into the well.
From The Primer of Oilwell Drilling, 6th edition Copyright © 2001 Petroleum Extension Service (PETEX®) of The University of Texas at Austin. All rights reserved