Oil and gas well completion

The operations that prepare a well bore for producing oil or gas from the reservoir. The goal of these operations is to optimize the flow of the reservoir fluids into the well bore, up through the producing string, and into the surface collection system. See also: Oil and gas field exploitation; Oil and gas well drilling

Casing and cement The well bore is lined (cased) with steel pipe, and the annulus between well bore and casing is filled with cement. Properly designed and cemented casing prevents collapse of the well bore and protects fresh-water aquifers above the oil and gas reservoirs from becoming contaminated with oil and gas and the oil reservoir brine. Similarly, the oil and gas reservoir is prevented from becoming invaded by extraneous water from aquifers that were penetrated above or below the productive reservoir. See also: Aquifer The casing string is made up of joints of steel pipe which are screwed together to form a continuous string as the casing is extended into the well bore. The common length of an individual joint is 30 ft (9 m). Such factors as the depth of the well, the pressure, temperature, and corrosivity of the fluids to be produced and those in the reservoirs that are to be cased off (behind pipe) are taken into account in specifying the diameter, wall thickness, strength, and chemical composition of the steel pipe for a particular casing job. In deep wells, one or more intermediate strings of casing are set (Fig. 1) in order to cement off either high-pressure intervals which cannot be controlled by the weight of the drilling fluid, or low-pressure intervals into which large volumes of drilling mud may flow and result in lost circulation, preventing further controlled drilling. When drilling into a high-pressure formation, casing is frequently set on top of it in order to facilitate well control operations if a blowout appears to be imminent. Fig. 1  Casing detail; casing strings in an oil well. ' In order to achieve its objectives, the casing must be securely sealed to the well bore itself with cement, although special formulations may be required for specific wells. For example, high-temperature formations or producing formations which will be extensively fractured will require cement that will not set too rapidly at high temperature or will not crack too badly as a result of the pressure shock of hydraulic fracturing, respectively. The cement is pumped down the casing and then on up into the annulus to a predetermined height. Cement returns (to the surface) are not universally required. The cement is mixed, pumped, and metered with highly specialized mobile equipment which is supplied by an appropriate service company. In order not to end up with the casing filled with cement, a specially designed plug is inserted after the required amount of cement has been pumped in and displaced with water until the plug hits the bottom of the casing string. The plug and some minor amount of cement will have to be drilled out after the cement has set.

Well bore–reservoir connection The nature of the reservoir, evaluated from a core analysis, cuttings, or logs, or from experience with like productive formations, determines the type of completion to be used: barefoot, casing set through and then perforated, or a shop perforated or slotted liner. In a barefoot completion, the casing is set just above the producing formation, and the latter is drilled out and produced with no pipe set across it (Fig. 2). Such a completion can be used for hard rock formations which are not friable and will not slough, and when there are no opportunities for producing from another, lower reservoir. Fig. 2  Diagram of barefoot completion. ' Set-through and perforated completions are also employed for relatively well-consolidated formations from which the potential for sand production is small. However, the perforated completion is used when a long producing interval must be prevented from collapse, when multiple intervals are to be completed in the one borehole, or when intervening water sands within the oil-producing interval are to be shut off and the oil-saturated intervals selectively perforated. Perforations are made with bullets or shaped charges (jet perforation). The bullets are fired from a gun with multiple barrels, spaced at desired intervals, which is lowered into the hole on a wire line. An electric impulse detonates the bullets. The holes created by bullets are frequently lined with fused metal and mineral debris and as a result may offer some resistance to fluid influx. See also: Well logging The charges used in jet perforating are similar to the shaped charges used in bazookas. The shaped charges are run into the hole on a glass gun which disintegrates. A shop-fabricated liner is used for friable formations from which some of the formation sand may flow into the well bore. The passage of such sand into the well bore may cause scoring of the seats and valves in the pump and its consequent failure to be able to lift produced fluid; or it may result in accumulation of a sand plug in the lower joints of casing through which the flow of fluids would be impeded, or in erosion of surface valves and piping. The holes in the liner are designed to screen out any produced sand, and such liners are gravel-packed. A slurry of gravel is circulated (washed-in) down behind the liner, prior to setting the liner hanger (Fig. 3). The distribution of particle diameters in the gravel pack is chosen so that the pack is an effective screen for the reservoir sand. A prepacked gravel liner may also be used, but since a gap is left between the well bore and the liner, which may fill up with the fine silt that is carried with the produced fluids, it is generally preferable to use a washed-in gravel pack. Fig. 3  Liner-type completion; preperforated liner.

Production A string of steel tubing is lowered into the casing string and serves as the conduit for the produced fluids. The tubing may be hung from the well-head or supported by a packer set above the producing zone. The packer is used when it is desirable to isolate the casing string from the produced fluids because of the latter's pressure, temperature, or corrosivity, or when such isolation may improve production characteristics. Artificial lift The tops of wells from which fluids flow as a result of the indigenous reservoir energy are equipped with a manifold known as the Christmas tree (Fig. 4). However, only some reservoirs have sufficient pressure and sufficient gas in solution (which is released at the lower pressure existing in the well bore and therefore lowers the effective density of the fluid in the tubing) to permit natural flow to the surface. The reservoir fluids from other reservoirs and, after pressure depletion, even from those which initially flowed must be brought to the surface by one of several methods of artificial lift. Fig. 4  Typical layout of a Christmas tree manifold. The most common method is the use of a rod pump which is set near the bottom of the hole and operated by reciprocating sucker rods which are in turn attached to the walking beam on the surface (Fig. 5). The walking beam is driven by a motor, and by the use of suitable cams and cranks the beam's seesaw movement raises and lowers the sucker rod string. The cycle and stroke length of the sucker rods are adjustable. Tubing pumps attached to the bottom of the tubing string have a relatively high capacity, but the entire string must be pulled to repair a damaged pump. Insert pumps are set within the tubing. Because of their restricted diameter, they have a limited capacity for lifting reservoir fluids, but they have the advantage that they can be pulled and replaced with a wire line without pulling the entire tubing string. For deeper wells, hydraulic motors can be used for which the actuating fluid (crude oil) is pumped down the tubing and returns with the produced fluid up through the annulus. Wells which produce a large amount of fluid (both water and oil) can economically use a submerged centrifugal pump driven by an electric motor for which an electric cable is run down the annulus. See also: Centrifugal pump Fig. 5  Schematic diagram of most commonly used downhole pumps. For deep wells which produce a significant amount of gas, gas lift can be employed in which some of the produced gas is compressed and returned to the casing-tubing annulus. A series of pressure-actuated valves inserted in the tubing string permits the gas to enter the string at various levels to lower the effective density of the fluids in the tubing and propel the fluids to the surface (Fig. 6). A plunger lift system to assist with unloading liquids can be easily installed inside tubing without the need to pull the tubing. Such systems are used to produce high-gas-oil-ratio (GOR) wells, water-producing gas wells, or very-low-bottom-hole-pressure oil wells (used with gas lift). Fig. 6  Schematic representation of operation of a gas lift string. (a) Oil level above first valve. (b) First valve open and gas entering tubing; oil level in casing/tubing annulus moving downward. (c) Oil level has moved downward, and each valve has closed as gas has entered the next lowest valve. Multiple completion In some geological provinces, several successive but separated intervals are productive of oil and gas. In some instances, the production from all the intervals may be commingled in a single well bore. However, if the properties of the reservoirs or their fluids are different, then commingling may be unacceptable because of the potential for cross flow between the individual reservoirs. Multiple completions in which the producing zones are separated by the use of packers and individual tubing string are then used (Fig. 7). Fig. 7  Schematic diagram of a multiple completion.

Water problems Excessive water production increases the cost of oil production since energy must be expended in lifting the water to the surface. Water production may also jeopardize the production of oil and gas by saturating the oil-productive interval with water. Such damage is more likely to occur in low-pressure formations or formations which contain water-sensitive clays that swell in an excess of water. Water-exclusion methods Water exclusion may be effected by the application of cements of various types. If it is determined that water is entering from the lower portion of a producing sand in a relatively shallow, low-pressure well, a cement plug may be placed in the bottom of the hole so that it will cover the oil-water interface of the reservoir. This technique is called laying in a plug and may be accomplished by placing the cement with a dump-bottom bailer on a wire line or by pumping cement down the drill pipe or tubing. For deeper, higher-pressure, or more troublesome wells, a squeeze method is used. Squeeze cementing is the process of applying hydraulic pressure to force a cement into an exposed formation or through openings in the casing or liner. It is also used for repairing casing leaks; isolating producing zones prior to perforating for production; remedial or secondary cementing to correct a defective condition, such as channeling or insufficient cement on a primary cement job; sealing off a low-pressure formation that causes lost circulation of drilling fluids; and abandoning depleted producing zones to prevent migration of formation effluent and to reduce possibilities of contaminating other zones or wells. The squeeze tool is a packer-type device designed to isolate the point of entry between or below packing elements. The tool is run into the hole on drill pipe or tubing, and the cement is squeezed out between or below these confining elements into the problem area. The well is then recompleted. It may be necessary to drill the cement out of the hole and reperforate, depending upon the outcome of the job performed in the squeeze process. Water-exclusion plug back Simple water shutoff jobs in shallow, deep, or high-pressure wells may also be performed in multizone wells in which the lower producing interval is depleted or the remaining recoverable reserves do not justify recompletion. Here, water may be excluded by placing a packer-type plug above the interval, then producing formations that are already open or perforating additional intervals that may be present higher up the hole.

Production stimulation Production may be impaired from a well bore as a result of drilling-mud invasion or of accumulation of clays and fine silts carried by the producing fluids to the borehole, or the lithology of the formation itself may have a naturally low permeability to reservoir fluids. Since the permeability to fluids of the formation within the first few feet of the well bore has an exponential effect on limiting the influx of fluid, the productivity of a well can frequently be increased manyfold by increasing the permeability of this element of the reservoir or removing the skin just at the face of the producing interval. This is accomplished by acidization and fracturing, and in some instances by the use of surfactants, solvents, and explosives. Specialized service companies conduct the work by using their own specially designed equipment. Acidizing Inhibited hydrochloric acid contains a chemical additive (an inhibitor) which prevents the acid from attacking steel. In this way the acid can be used for dissolving carbonates, oxides, and other compounds without fear of it attacking the well's steel tubulars. This formulation is used to dissolve limestone and dolomitic matrices and thus enlarge the flow channels in production-impaired reservoirs. Hydrochloric acid is also used to shrink and disperse sheaths of drilling mud on the well bore and to dissolve calcareous cements, which results in larger channels through which fluids can flow to the well bore. Hydrofluoric acid (released by injecting a mixture of hydrochloric acid and a soluble fluoride salt) is sometimes used in sandstone reservoirs to dissolve and disperse drilling mud that has invaded the reservoir. Fracturing Formation fracturing is a hydraulic process aimed at the parting of the formation. Vertical fractures most frequently occur. Horizontal fracturing occurs only in relatively shallow formations, in formations where the major tectonic stress is horizontal, or in relatively plastic formations. The fracturing fluid is injected into the well, and the pressure is raised to maintain a given flow rate until formation breakdown occurs. Injection is continued with a slurry of a selected grade of sand or gravel or particles of other material (such as sintered bauxite or ceramic beads). These particles prop the fracture open after the hydraulic pressure is released. Crude oil, acid, and a variety of gelled liquids are used as fracturing fluids. The propping material guarantees that there will be a high-permeability path into the well bore, and the nature of fluid flow in the vicinity of the well bore is changed to being predominantly linear rather than radial with an associated decrease in pressure drop (or higher flow rate at the same pressure drop). Other stimulation techniques Explosives were the first means used to stimulate oil and gas production. However, this technique has largely been supplanted by more effective and safe fracturing and acidizing technology. Solvents are used when the substances believed to be inhibiting production are asphaltenes, waxes, and emulsions stabilized by such organic materials. Surfactants are frequently used with the solvents to aid in the dispersion of the sediments. Surfactants or alcohols (for example, methanol) are also used alone when the cause of impairment is believed to be a high saturation of water that has accumulated in the reservoir near the well bore.

Sand consolidation Sand exclusion techniques using liners and gravel packs are not perfect, and therefore technology has been developed that attempts to consolidate friable formations. The consolidating medium must be capable of cementing the grains together without significantly reducing the permeability of the reservoir to fluid flow. Epoxy and phenolic resins have been developed for such purposes; some techniques use thermally deposited nickel metal and precipitated aluminum oxides. However, liners with properly designed gravel packs continue to be the most economical and useful technique for sand control.

Coiled tubing Many of the well completion or workover techniques can be implemented with a coiled tubing unit that can greatly reduce costs. Instead of moving in a completion rig to lower or pull tubing, a coiled tubing unit may be moved next to the wellhead. With such a unit, instead of having to connect and disconnect stands of tubing, a continuous length of tubing may be uncoiled or coiled into the borehole by using a large spool. In this way, numerous operations may be performed on a well such as acidizing, setting and retrieving bridge plugs or packers, cementing, cleaning out the hole, and even light-duty or slim-hole drilling. A wide variety of remedial operations may be performed. See also: Petroleum reservoir engineering Todd M. Doscher R. E. Wyman

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