Introduction to Drilling and Well Completions (Books / Manuals)
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Importance of Oil and Gas Wells (Books / Manuals)
Introduction to Drilling and Well Completions The Petroleum Well Construction Book was published by John Wiley and Sons. Content provided has been authored or co-authored by Halliburton employees to be used for educational purposes. Michael J. Economides, Texas A&M University Shari Dunn-Norman, University of Missouri-Rolla Larry T. Watters, Halliburton Energy Services
Importance of Oil and Gas Wells Few industries and certainly no other materials have played such a profound role in modern world history and economic development as petroleum.
Yet deliberate access to geologic formations bearing petroleum through drilled wells is relatively recent. The "Drake well," drilled in the United States by Colonel Edwin L. Drake in 1859, is considered by many to be the first commercial well drilled and completed. It heralded the creation of an industry whose history is replete with international adventure, color, frequent intrigue, and extraordinary characters. Many believe that the majority of twentieth century social and political events, including two World Wars, a Cold War, and many regional conflicts are intimately connected to petroleum.
Until the late 1950s, much petroleum activity was originated and based in the U.S.
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Importance of Oil and Gas Wells (Books / Manuals)
Amyx, et al. (1960) reported that through 1956, the cumulative world crude-oil production was 95 billion bbl, of which 55 billion had been produced in the U.S.
Beginning tenuously in the early 1900s, speeding up in the period between the World Wars, and accelerating in the 1960s, petroleum exploration and production became a widely international activity. In the late 1990s, the U.S. is still the world's largest oil consumer both in terms of shear volume (18.2 million bbl/d) and, overwhelmingly, per capita (28 bbl/person/year compared to 1 bbl/person/year in China). The U.S. is also the largest petroleum importer (9.5 million bbl/d, representing over 50% of consumption); worldwide production is about 62 million bbl/d. The bulk of petroleum reserves is clearly outside the industrialized world of North America and Western Europe (combined 57 billion bbl vs. 1.1 trillion bbl worldwide). The majority of petroleum is found in the Middle East, where 600 billion bbl are produced, 260 billion of which are from Saudi Arabia alone.
Drilling activity is reflected by the geographical shifting of petroleum operations. The numbers of drilling rigs are now roughly equally distributed between North America and the remainder of the world, although this statistic is somewhat misleading. Wells drilled in mature petroleum environments, such as the continental U.S., are far less expensive, and drilling prices rely on mass utilization but, of course, production rewards are lackluster. In the U.S. and Canada, approximately 34,000 wells were drilled during 1995 and 1996, representing almost 60% of all wells drilled worldwide (about 58,000). Yet the United States and Canada, combined, account for only 13% of the world's petroleum production.
On the contrary, offshore drilling from either platforms or drill ships, drilling in http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-01.htm (2 of 4)05/12/2007 12:52:41 a.m.
Importance of Oil and Gas Wells (Books / Manuals)
remote locations, or drilling in industrially and developmentally deficient countries is far more expensive and involved.
Maturity in petroleum production is characterized by a marked decrease in both the total production rate and the petroleum rate per well, in addition to an increase in the water-oil ratio. "Stripper wells," representing the vast majority of wells in the USA, imply a production of less than 20 bbl/d of petroleum and a total production rate where water constitutes more than 90%.
Darcy's law, the most fundamentally basic petroleum engineering relationship, suggests that the production rate is proportional to the pressure driving force (drawdown) and the reservoir permeability:
(1-1) This law can readily explain current worldwide petroleum activities and the petroleum industry's shifting focus. Mature petroleum provinces are characterized by depletion in the reservoir pressure or by the necessity to exploit less attractive geologic structures with lower permeability, k.
The permeability in Equation 1-1 is effective; that is, it is the product of the absolute permeability and the relative permeability of a flowing fluid competing with other fluids for the same flow paths. The relative permeability is a function of saturation. Thus, water influx from an underlying aquifer not only results in an increase in water production (which is a nuisance in itself) but an associated decrease in petroleum relative permeability and the petroleum portion of the total http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-01.htm (3 of 4)05/12/2007 12:52:41 a.m.
Importance of Oil and Gas Wells (Books / Manuals)
production rate.
Although such problems do not burden newer reservoirs to the same extent, it must be emphasized that all petroleum reservoirs will follow essentially the same fate.
The two extreme fields of operation, mature reservoirs on land in developed nations and newer discoveries either offshore or in developing countries, result in very different well construction costs.
These costs range from a few hundred thousand dollars to several million dollars. (Or tens of millions if ancillary costs such as the extraordinary testing or the building of an artificial island in the Arctic are considered.)
The total annual worldwide expenditure for petroleum well construction is estimated at over $100 billion. To give a relative measure for this figure (and to avoid a misunderstanding from a reader) only a handful of nations have national budgets of larger magnitude. This book will provide a comprehensive and integrated treatment of today's technology for the substantial and profoundly international industrial activity of constructing oil and gas wells.
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Petroleum Formations (Books / Manuals)
Petroleum Formations Because the development of well construction technology has had a rather fragmented past and many practitioners are not trained as either reservoir or production engineers, it is worthwhile here to provide an elementary description of the targets that the drilling of a well is supposed to reach.
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Petroleum Fluids
The Geology of Petroleum Accumulation
Petroleum Fluids Petroleum is a mixture of hydrocarbons consisting of about 11 to 13% (by weight) hydrogen and 84 to 87% carbon. Chemically, "crude" petroleum may include several hundred compounds, encompassing practically all open-chain and cyclic hydrocarbons of single, double, and triple bonds.
A description of these mixtures by composition was abandoned early in industry history with the exception of very generic divisions that denote important distinguishing content (such as, paraffinic or asphaltenic crudes). Instead, bulk physical properties such as density and viscosity have been used to describe crude behavior.
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Petroleum Formations (Books / Manuals)
Specifically, the phase and thermodynamic behavior has been reduced to the simplifying division of crude petroleum into (liquid) oil and (natural) gas. While such a description is apparent and relatively easy to comprehend given a temperature and pressure, crude petroleum content is generally referred to as volumes at some standard conditions (for example, 60°F and atmospheric pressure). With the definition of pressure and temperature, a volume unit also clearly denotes mass.
Oil, then, consists of higher-order hydrocarbons such as C6+ with much smaller and decreasing quantities of lower-order hydrocarbons, while gas consists of lower order hydrocarbons—primarily methane, and some ethane—with much smaller amounts of higher-order hydrocarbons.
An important variable is the bubblepoint pressure, which, for a given temperature, denotes the onset of free-gas appearance. At lower pressures, oil and gas coexist.
Petroleum found at conditions above the bubblepoint pressure is all liquid and is referred to as undersaturated. Below the bubble point, the petroleum is referred to as two-phased or saturated. At considerably lower pressure and below the dew point pressure, hydrocarbons are all in the gaseous state.
In all natural petroleum accumulations, water is always present either as interstitial, cohabiting with the hydrocarbons, or underlying, in the form of (at times very large) aquifers.
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Petroleum Formations (Books / Manuals)
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The Geology of Petroleum Accumulation (Books / Manuals)
The Geology of Petroleum Accumulation Petroleum is found chiefly in sedimentary basins, and although fanciful theories of inorganic origin have surfaced in the past, it is almost universally accepted that petroleum has its organic origin in a source rock.
Decay of organic remains under pressure and temperature and under conditions preventing oxidation and evaporation has been the most likely process in the formation of petroleum. Associated saline water suggests environments near ancient seas and, thus, a plausible and often repeated scenario is one of ancient rivers carrying organic matter along with sediments and depositing successive layers, eventually buried by substantial overburden.
The formation of petroleum was followed by accumulation. The gravity contrast between hydrocarbons and water, along with capillary effects, would force oil and gas to migrate upward through rock pores. Connected pores provide permeability, and the ratio of pore volume to the bulk volume, the porosity, is one of the most important variables characterizing a petroleum reservoir.
The natural tendency of hydrocarbons to migrate upward would continue to the surface unless a trapping mechanism intercedes. This is precisely what happened.
At depths as shallow as a few tens of feet to over 30,000 ft, natural traps, which are special geological formations, allowed the accumulation of the migrating
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The Geology of Petroleum Accumulation (Books / Manuals)
hydrocarbons. Common to all cases is an overlain impermeable layer forming a caprock.
Figure 1-1 (after Wilhelm, 1945) sketches some of the most common petroleum traps. Convex traps, either by simple folding (Figure 1-1A) or because of differences in reservoir thickness (Figure 1-1B) and overlain by an impermeable layer are the easiest to intersect with drilling.
Figure 1-1 Petroleum trapping mechanisms (eco1f01.jpg)
A permeability trap (Figure 1-1C) and a pinchout trap (Figure 1-1D) denote that laterally and upward the permeable rock vanishes. Such traps may have been http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-03.htm (2 of 7)05/12/2007 12:53:29 a.m.
The Geology of Petroleum Accumulation (Books / Manuals)
created by the rotation of layers, and they can sometimes be associated with continental rift and subsequent drift.
An interesting trapping mechanism is provided by the movement of faults (Figure 11E). Upward or downward motion of the layers on one side of the fault may bring an impermeable layer against a permeable one, and this interface can form a very effective trap. At times, the "sister" formation of a structure can be found several hundred feet above or below, and it may also contain attractive quantities of petroleum.
Finally, piercement traps (Figure 1-1F), formed by the intrusion of a material of different lithological composition, may form an effective seal to a petroleum trap.
While a trap may contain a petroleum reservoir (defined as a structure in hydraulic communication), oil may coexist with overlain gas, gas may be the only hydrocarbon, and in all cases, water is likely to underlie the hydrocarbons.
An oil field (or a gas field) may contain many reservoirs distributed either laterally or in layers, often separated by nonhydrocarbon formations that may be considerably thicker than the reservoirs themselves. Furthermore, the contained hydrocarbons, reflecting geological eras that may be separated by millions of years, may have considerably differing makeups. Coupled with different lithological properties and reservoir pressures, more often that not, petroleum production from multilayered formations may preclude the commingling of produced fluids for a variety of operational reasons (including the danger of fluid crossflow through the well from higher to lower pressure zones). http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-03.htm (3 of 7)05/12/2007 12:53:29 a.m.
The Geology of Petroleum Accumulation (Books / Manuals)
A reservoir itself may be separated into different geological flow units, reflecting the varying concentration of heterogeneities, anisotropies, and reservoir quality, such as thickness, porosity, and lithological content.
The era of finding petroleum reservoirs through surface indicators (such as outcrops), conjecture, and intelligent guesses has been replaced by the introduction of seismic measurements, which have had one of the most profound influences on modern petroleum exploration and, in recent years, on petroleum production.
Artificially created seismic events (air bubbles offshore, large vibrators on land) send seismic waves downward. Reflected and refracted through formations, these vibrations are detected back on the surface. Processing of the signals results in the construction of seismic response images that can be two-dimensional (2D), threedimensional (3D), or even four-dimensional (4D), if taken at different time intervals.
Seismic measurements are then processed and can be represented by a 3D visualization (Figure 1-2). For such an image to be constructed, massive amounts of data are collected and processed through very powerful computers that use sophisticated algorithms.
Figure 1-2 displays a typical, processed seismic 3D volume of amplitude vs. time. Much more data is collected than what is displayed in Figure 1-2. Seismic attributes such as reflection, strength, phase frequency, and others may be correlated with several reservoir properties such as porosity, net pay, fluid saturation, and lithological content. http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-03.htm (4 of 7)05/12/2007 12:53:29 a.m.
The Geology of Petroleum Accumulation (Books / Manuals)
Figure 1-2 3D seismic volume
Modern formation characterization is the integration of many measurements that allow for a more appropriate reservoir description and improved reservoir exploitation strategies. Formation characterization involves the combination of various modeling approaches, including geological descriptions and pore volumes, and it is often combined with production history matching.
With powerful visualization and interpretation technologies, as shown in Figure 1-3, geoscientists and engineers can examine a seismic or geological data volume and identify and isolate significant features in ways not possible before. This new means of geological visualization is the basis of modern formation characterization; it is
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The Geology of Petroleum Accumulation (Books / Manuals)
rapidly forcing the abandonment of the traditional, yet simplistic reservoir approximations of parallelepiped boxes or cylinders. While single-well drainages can be tolerably considered through the use of simple approximations, reservoir-wide estimates of hydrocarbons-in-place can now be far more realistic and inclusive of heterogeneities.
Figure 1-3 3D seismic visualization and interpretation showing significant features
The expression for oil (or gas) in place is provided in Equation 1-2.
(1-2) This expression may now make use of seismic measurements that can provide A
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The Geology of Petroleum Accumulation (Books / Manuals)
(area) and h (thickness).
Obviously, better formation description can allow for targeted drilling. The fraction of dry holes is likely to be reduced and optimum reservoir exploitation can be envisioned, especially with the emergence of horizontal and multilateral/ multibranched wells. Along with seismic images, these wells constitute the two most important technologies of the last decade, if not the entire post-World War petroleum era.
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Drilling Fundamentals (Books / Manuals)
Drilling Fundamentals The basic principles and technology of drilling an oil or gas well are established and are described in texts (Bourgoyne et al., 1991; Mitchell, 1993; Gatlin, 1960). The following is a brief overview of drilling fundamentals.
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Equipment
Drilling Fluids
Vertical, Deviated, And Horizontal Wells
Equipment Drilling a petroleum well is a complex process that requires large, heavy-duty equipment. A conventional drilling rig consists first of a structure that can support several hundred tons. A "million-pound" rig is routinely supposed to support 10,000 ft and, in some cases, as much as 30,000 ft of drillpipe and additional equipment.
A drill bit (Figure 1-4) is attached to the bottom of the drillpipe by one or more drill collars. The entire assembly ends at the floor of a drilling rig and is connected to a rotary table. This table, along with a special joint called the kelly, provides rotational motion to the drilling assembly.
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Drilling Fundamentals (Books / Manuals)
Figure 1-4 Rotary drilling rig with the important components
While rotary drilling has dominated the petroleum industry in the last 50 years, cable-tool drilling preceded it and was the mainstay of early drilling. In some rare cases, it is still used today. For cable-tool drilling, the drilling assembly is suspended from a wire rope. The assembly is then reciprocated, striking blows to the formation, which becomes fragmented. The drilling assembly is retrieved, and cuttings are brought to the surface with a lowered bailer.
Rotary drilling can continue uninterrupted unless a worn-out drill bit must be http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-04.htm (2 of 3)05/12/2007 12:53:45 a.m.
Drilling Fundamentals (Books / Manuals)
replaced. Manufacturers have conducted extensive research to improve the durability of drill bits so that the number of trips (pulling the drilling assembly out and then running it in the hole) can be reduced, which results in reduced drilling time.
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Drilling Fluids (Books / Manuals)
Drilling Fluids A critical component of drilling is the drilling fluid, which is also widely referred to in the industry as drilling mud. One of the main roles of drilling fluid is to lift the drilling-rock cuttings to the surface and to lubricate the bit in its grinding, rotary action against the rock.
The drilling fluid has other important functions. The weight of the drilling fluid (the fluid density) and the resulting hydrostatic pressure at the drilling point are supposed to impart a positive pressure into the formation. Otherwise, formation fluids under pressure may cause a kick, which is an involuntary influx of fluids into the well. Under extreme circumstances, a kick may cause a catastrophic blowout.
To provide drilling fluids with the appropriate density for the pressure ranges that will likely be encountered, drilling operators must select the appropriate weighting agent. Drilling fluid weights have ranged from about 8.5 lb/gal (almost neat water) to as much as 15 lb/gal for highly overpressured and deep reservoirs.
Although bentonite clay has been widely used as the main constituent in waterbased drilling fluids, other drilling-fluid formulations have been used. General families include oil-based and gas-liquid-based fluids. These fluids are supposed to reduce the formation damage caused by water-based fluids and their contained solids when they penetrate the porous medium. One mechanism of controlling formation damage is the formation of a filter cake, which coats the walls of the well, http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-05.htm (1 of 2)05/12/2007 12:53:56 a.m.
Drilling Fluids (Books / Manuals)
thus reducing fluid leakoff.
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Vertical, Deviated, And Horizontal Wells (Books / Manuals)
Vertical, Deviated, And Horizontal Wells Through the mid-1980s, vertical wells were drilled almost exclusively. Earlier, deviated wells were introduced, which allowed for the use of surface drilling sites that could be a considerable distance from the targeted formation. This type of well became particularly useful both offshore, where drilling from platforms is necessary, and in the Arctic and other environmentally sensitive areas, where drilling pads can be used.
Although Soviet engineers had drilled several horizontal wells in the 1950s, such activity was limited until the early 1980s, when two western companies, Agip and Elf, reported some impressive results with horizontal wells in an offshore Adriatic oil field. Not only was oil production from the horizontal well several times greater than that of vertical wells in the same field, but the water-oil ratio, a considerable problem with vertical wells, was significantly reduced with the horizontal wells.
This success literally ushered a new era in the petroleum industry, and although horizontal wells today account for perhaps 10% of all wells drilled, their share is steadily increasing. More important, their share in new hydrocarbons produced is disproportionately favorable. Estimates suggest that by the year 2000, perhaps 50% of all new hydrocarbons will come from horizontal and multilateral wells.
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Vertical, Deviated, And Horizontal Wells (Books / Manuals)
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Long-radius wells may turn the angle at a rate of 2 to 8°/100 ft; thus, they require a vertical entry point about 1500 ft away from the desired reservoir target. For these wells, conventional drilling assemblies can be used, and conventional well sizes can be constructed. The horizontal lengths of such wells can be considerable; records have been established at over 10,000 ft, but typical horizontal lengths range between 3000 and 4000 ft. Medium-radius wells require approximately 300 ft to complete a turn from vertical to horizontal. Medium-radius wells use directional control equipment similar to that used in long-radius wells, but drilling practices for such wells are somewhat different. Short-radius wells can go from vertical to horizontal in 50 ft or less. Specialized, articulated drilling assemblies are needed, and typical well diameters are generally smaller than those for conventional wells. For these wells, coiled tubing drilling is often used. Ultrashort-radius drilling technology is available, which allows a well to run from vertical to horizontal within a few feet.
A good driller, aided by modern measurement-while-drilling (MWD) equipment and an appropriate reservoir description, can maintain a well trajectory within ± 2 ft from the target. Therefore, if the well is intended to be perfectly horizontal or slightly dipping to reflect reservoir dipping, the departure from the well trajectory can be controlled and minimized.
Once a well is drilled, it must be completed. Section 1-4 provides an overview of well completions.
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Well Completions (Books / Manuals)
Well Completions The purpose of drilling oil and gas wells is to produce hydrocarbons from, or to inject fluids into, hydrocarbon-bearing formations beneath the earth's surface. The borehole, described previously, provides a conduit for the flow of fluids either to or from the surface. Certain equipment must be placed in the wellbore, and various other items and procedures must also be used to sustain or control the fluid flow. This equipment and any procedures or items necessary to install it are collectively referred to as a well completion.
In the early twentieth century, oil and gas wells were commonly completed with only a single string of casing. The casing was a large diameter (e.g. 7-in.) string of steel pipe, consisting of threaded sections. Initially, casing was set with drilling fluid only.
A casing string in a well extends from the surface to some setting depth. If the top of a casing string is set at a depth below the surface, it is referred to as a liner. Liners are commonly found in wells completed during the early part of the twentieth century.
Cementing technology evolved in the 1920s, and by the 1930s, most casing strings were set with some cement. Cementing a well is an essential step in almost all well completions, irrespective of whether a perfect bond is achieved between the
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Well Completions (Books / Manuals)
reservoir and the casing. Currently, most wells are cemented at least some distance above the target reservoir.
In early completions, casing was either set at the top of the producing zone as an openhole completion (Figure 1-5) or set through the producing reservoir.
Figure 1-5 Openhole completions
Openhole completions minimize expenses and allow for flexible treatment options if the well is deepened later, but such completions limit the control of well fluids. Phillips and Whitt (1986) show that openhole completions can also reduce sand and http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-07.htm (2 of 14)05/12/2007 12:54:18 a.m.
Well Completions (Books / Manuals)
water production. Although many wells completed in this manner are still operating today, this method of completion has been superseded by cased completions (Figure 1-6).
Figure 1-6 Cased-hole completions with perforations
In a cased completion, casing is set through the producing reservoir and cemented in place. Fluid flow is established by the creation of holes or perforations that extend beyond the casing and cement sheath, thereby connecting and opening the reservoir to the wellbore (Figure 1-6). Wells that are cased through the producing http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-07.htm (3 of 14)05/12/2007 12:54:18 a.m.
Well Completions (Books / Manuals)
reservoir provide greater control of reservoir fluids because some or all of the perforations can be cemented off or downhole devices can be used to shut off bottom perforations. However, openhole wireline logs must be run before the casing is set so that the exact perforation interval is known.
Cased-hole completions are more susceptible to formation damage than openhole completions. Formation damage refers to a loss in reservoir productivity, normally associated with fluid invasion, fines migration, precipitates, or the formation of emulsions in the reservoir. Loss of productivity is expressed as a skin factor, s, in Darcy's equation as follows:
(1-3) A positive skin value indicates that a well is damaged.
Formation damage can be removed or bypassed through the use of stimulation techniques. In instances where the formation damage extends only a few feet from the wellbore, the well may be acidized to dissolve or remove the damage. Matrix acidizing is used to restore initial productivity. Hydraulic fracturing is a stimulation technique that creates a fracture that is intended to extend beyond the damage area. Significant advances in well stimulation have been made since the early part of this century, when openhole completions were stimulated by jars of nitroglycerin that were placed and detonated downhole.
As noted previously, reservoir pressure will decline as hydrocarbons are produced. Many of the wells completed in the early part of the twentieth century were http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-07.htm (4 of 14)05/12/2007 12:54:18 a.m.
Well Completions (Books / Manuals)
produced through the casing, and the reservoirs had sufficient pressure for the hydrocarbons to flow to the surface. With declining reservoir pressures and producing volumes, production through smaller-diameter tubing became necessary, since the velocity through the casing could not sufficiently sustain natural flow. Figure 1-7 shows a simple cased-hole completion with tubing.
Figure 1-7 Cased completion with tubing
These early completion techniques proved adequate in relatively shallow wells. However, as deeper, multiple, and higher-pressure reservoirs were encountered, it http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-07.htm (5 of 14)05/12/2007 12:54:18 a.m.
Well Completions (Books / Manuals)
was recognized that the completions imposed limitations on well servicing and control and designs would require improvement to meet increasing requirements for wellbore re-entry and workover operations.
A wide range of downhole equipment has been designed and manufactured to meet the needs of more complex well completions. In situations where multiple reservoirs cannot be commingled, the zones are separated with a production packer. Packers are devices that are run on, or in conjunction with, a string of tubing. The packer has a rubber element that is extruded by compression to form a seal between the tubing and the casing (Figure 1-8). Packers are used for a variety of reasons in well completions.
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Well Completions (Books / Manuals)
Figure 1-8 Production packer with detail of annular seal
Another component that has become an integral part of well completions is the sliding sleeve. The sliding sleeve provides annular access between the tubing and the casing. It is used to produce a reservoir isolated between two production packers and for circulating a well above the uppermost packer. The sleeve is opened or closed through the use of wireline servicing methods. Many other functions can be performed with wireline devices set in landing nipples.
The evolution of offshore drilling in the 1930s (ETA, 1976) and the production of http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-07.htm (7 of 14)05/12/2007 12:54:18 a.m.
Well Completions (Books / Manuals)
wells from offshore platforms in the 1940s (Graf, 1981) demanded methods of well shut-in for safety and environmental concerns. Storm chokes were pressurecontrolled devices set inside the tubing string. These devices were intended to shut in wellflow during storms or a major platform catastrophe. At today's offshore locations, these direct-control devices have been supplanted by surface-controlled subsurface safety valves (SCSSVs).
Equipment such as packers, sleeves, landing nipples, and safety valves provide various functions for well control. These devices are only a few examples of an extensive range of equipment that enables engineers to control fluid flow selectively and to stimulate producing reservoirs.
Many reservoirs contain sediments that are so poorly consolidated that sand will be produced along with the reservoir fluids unless the production rate is severely restricted. Sand production may erode the tubing or surface valves and flowlines. In addition, sand could accumulate in downhole equipment and create problems in wireline servicing. Gravel-packing was devised as a means of eliminating sand production without greatly restricting production rates. In a gravel-pack completion (Figure 1-9), sand with a grain size larger than the average formation sand grain is placed between the formation and a screen or slotted liner (Economides et al., 1994). More recently, high-permeability fracturing (fracpacking) has been proven as a technique for sand control.
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Well Completions (Books / Manuals)
Figure 1-9 Gravel-pack completions
Engineers designing well completions must consider that the wells will eventually be unable to flow naturally to the surface. The loss of natural flow occurs because the reservoir pressure declines with production and reservoirs produce increasing amounts of water with time, which increases the density of the flowing fluid. Various techniques of artificially lifting fluids from the wellbore have been developed. Artificial lift techniques include sucker rod pumping, electrical submersible pumps, gas lift, and other types of hydraulic lift. Each method of artificial lift requires unique downhole and surface equipment that must be considered during the design of the http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-07.htm (9 of 14)05/12/2007 12:54:18 a.m.
Well Completions (Books / Manuals)
well completion.
Well stimulation techniques introduced in the early part of the twentieth century have been improved through a more complete understanding of the processes involved. Acidizing models have been developed to describe the use of various types of acids in a range of lithologies. Hydraulic fracturing has experienced even more dramatic improvements since the introduction of crosslinked polymer fluids, highstrength proppants, and analytical techniques, such as the net pressure plot. Such techniques have enabled engineers to substantially improve the flow from both lowpermeability and high-permeability reservoirs.
Another notable advance in well completion design is the evolution of coiled tubing for servicing and completing wells. Coiled tubing servicing involves the deployment of a continuous string of small-diameter tubing into the wellbore. This coiled tubing is run concentric to existing tubulars, is used for the required service, and is then removed without damaging the existing completion. Coiled tubing servicing is of increasing importance in highly deviated and horizontal wells, since wireline servicing poses problems at angles greater than 50o.
Completion methods such as gravel-packing and stimulation, a variety of downhole equipment, and enhancements to servicing methods have enabled engineers to design more complex well completions, which offer greater fluid flow control, stimulation alternatives, and operational flexibility. An extensive range of downhole designs has been implemented to meet a number of producing requirements. Example designs include dual completions, slimhole and monobore completions
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Well Completions (Books / Manuals)
(Ross et al., 1992; Robison, 1994), completions for high-pressure, hightemperature (HPHT) reservoirs (Schulz et al., 1988), subsea completions (Cooke, 1992) whose wellheads are located on the seafloor, and waterflood or CO2 injection applications (Stone et al., 1989). Two examples, a dual completion (Figure 1-10) and a subsea completion with gravel-packing and artificial lift (Figure 1-11), illustrate the wide range of well completion designs available today.
Figure 1-10 depicts a dual completion. Dual completions are used when multiple reservoirs will be produced. Two tubing strings and at least two production packers are included. The packers may separate two or more producing reservoirs. A sliding sleeve can be included between or above packers so that one or more reservoirs can be selectively produced at any time. Other downhole equipment, such as landing nipples, safety valves, or side-pocket mandrels (for gas lift) may be included in a dual completion. Sanku et al. (1990) shows the use of a dual completion with gas lift in the Sockeye Field, offshore California (Figure 1-10A). Farid et al. (1989) shows the application of a dual completion for gas injection in a three-layered reservoir in Abu Dhabi (Figure 1-10B).
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Well Completions (Books / Manuals)
Figure 1-10 Dual completions
Figure 1-11 depicts a single-string subsea completion. This completion has been run in the Balmoral Field in the North Sea (Shepherd, 1987). Initial test data indicated that the Balmoral wells would produce significant amounts of sand, and it was decided to gravel-pack the wells to control sand production. The gravel screen is set across the producing zone and a packer is set above the gravel pack. A tubing expansion joint, run above the packer, allows the tubing to expand or contract with changes in downhole pressure or temperature. A sleeve is run above the expansion joint to circulate the well, and a number of gas-lift mandrels, equipped with dummy http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-07.htm (12 of 14)05/12/2007 12:54:18 a.m.
Well Completions (Books / Manuals)
valves, are included in the tubing above the sleeve. The gas-lift mandrels were included in the Balmoral design to provide for future gas lift, since reservoir models predicted a rapid onset of water production and the need for artificial lift. This type of forward planning is crucial in subsea wells, where the cost of mobilizing offshore rigs is substantial.
The evolution and growing application of horizontal drilling techniques has provided additional challenges in well completion design.
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Well Completions (Books / Manuals)
Figure 1-11 Balmoral subsea completion (Shepherd, 1987)
At present, most horizontal wells are either completed with an openhole horizontal section, with a slotted liner laid in the openhole section (Cooper and Troncoso, 1988; Lessi and Spreux, 1988), or with a gravel-pack screen (McLarty et al., 1993). To date, the use of casing, production packers, sleeves, and other downhole devices has been limited because they cannot provide a mechanical/hydraulic seal at the junction between the vertical wellbore and the horizontal hole. Completion technology in this area is evolving rapidly, and such capabilities will likely be available in the near future, enabling the use of downhole devices and techniques that will provide greater control of fluid flow and stimulation in horizontal and multilateral wells.
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Organization of this Book (Books / Manuals)
Organization of this Book The following chapters were written by experts from various Halliburton companies, operating companies, and academia. This book is divided into two major categories: drilling and well completions . A brief overview of each chapter is provided in the following paragraphs.
Drilling Directional Drilling describes technologies for drilling-trajectory monitoring and control, drilling assemblies, and requirements for the complicated well systems of today.
Horizontal, Multilateral, and Multibranch Wells in Petroleum Production Engineeting describes the wide variety of well configurations possible with drilling systems today. Horizontal, multilateral, and multibranch wells are explained, and their applications for a variety of reservoir management problems are outlined.
Measurement-while-drilling (MWD) and logging-while-drilling (LWD) equipment and techniques are discussed in Measurement-While-Drilling (MWD), Logging-While Drilling (LWD), and Geosteering. The formation characterization that these methods allow and the ability to drill complex well systems intelligently are the two most important new technologies in the petroleum industry.
Rock mechanics principles have several applications during drilling, well completion, http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-08.htm (1 of 4)05/12/2007 12:54:29 a.m.
Organization of this Book (Books / Manuals)
and subsequent production. Rock Mechanics in Wellbore Construction lists and explains most rock mechanics-related problems associated with well construction.
Well Completions Primary Cementing, Formation-Fluid Migration After Cementing, Cement-Sheath Evaluation, and Remedial Cementing describe, respectively, primary cementing, gasmigration problems, cement sheath evaluation, and in the case of detected problems, remedial cementing.
All cased-hole completions must be perforated, and these perforations can affect the well's capacity to produce reservoir fluids. Perforating describes perforating technology, which has evolved considerably in recent years in terms of diameter, density (shots per foot), tunnel length of the perforation, and orientation.
A wide variety of completion hardware is available for controlling fluid flow or providing operational flexibility in a well. Completion Hardware describes attributes of basic downhole equipment, such as packers, sliding side-doors, landing nipples, side-pocket mandrels, and SCSSVs.
Well performance is dominated by two main components: reservoir deliverability, described traditionally by the well inflow performance relationship (IPR), and flow in the production string. The combination of these two components is well deliverability. Inflow Performance/Tubing Performance provides a comprehensive description of these concepts for oil, gas, and two-phase reservoirs fitted with vertical, deviated, or horizontal wells.
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Organization of this Book (Books / Manuals)
Artificial-Lift Competions describes fundamental methods of artificial lift. Artificial lift must be included in completions in which the reservoirs no longer have sufficient pressure to sustain natural flow. Artificial lift can also be used to enhance production in flowing wells by augmenting the natural reservoir pressure. Challenges of applying artificial lift systems in horizontal and multilateral wells are also discussed.
Stimulation can also improve well productivity. Stimulation includes both acidizing and fracturing a reservoir. Stimulation improves a well's ability to flow by providing a more direct flowpath, which results in more production per unit of drawdown. The main concepts of stimulation are discussed in Well Stimulation.
Sand production is a significant problem in many wells, particularly wells producing from incompetent reservoirs or reservoirs that have experienced pressure depletion. Sand production is a serious concern because it affects the completion and surface facilities and may also deconsolidate the formation with a potentially catastrophic collapse.
Two general approaches to solving sand production problems are available: sand exclusion techniques, which require the use of filters, and sand-production control techniques, which affect the sand production mechanisms within the pore structures. The prominent sand exclusion technique is gravel-packing augmented by screens, as described in Sand Stabilization and Exclusion. Sand-production control has been demonstrated by high-permeability fracturing or fracpack processes (HighPermeability Fracturing).
Water production and its control is the other great, persistent problem in reservoir http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-08.htm (3 of 4)05/12/2007 12:54:29 a.m.
Organization of this Book (Books / Manuals)
management. Water Control describes the mechanisms of water production and a holistic approach to water-production management.
Finally, proper engineering suggests that well construction must be based on the entire life of the well. Thus, planning completions for such well management is essential. Designing Well Completions for the Life of the Field provides such an approach and combines several of the concepts previously presented in this book.
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Importance of Oil and Gas Wells (Books / Manuals)
References Amyx, J.W., Bass Jr., D.M., and Whiting, R.L.: Petroleum Reservoir Engineering, McGraw-Hill Book Company, New York (1960) 1-20.
Bourgoyne Jr., A.T., Millheim, K.K., Chenevert, M.E., and Young Jr., F.S.: Applied Drilling Engineering, SPE, Richardson, TX (1991).
Cooke, J.C. and Cain, R.E.: "Development of Conventionally Uneconomic Reserves Using Subsea Completion Technology: Garden Banks Block 224, Gulf of Mexico," paper OTC 7004, 1992.
Cooper, R.E. and Troncoso, J.C.: "Overview of Horizontal Well Completion Technology," paper SPE 17582, 1988.
Economides, M.J., Hill, D.A., and Ehlig-Economides, C.A.: Petroleum Production Systems, Prentice Hall, Englewood Cliffs, NJ (1994) 119.
ETA Offshore Seminars: The Technology of Offshore Drilling, Completion and Production, PennWell Books, Tulsa, OK (1976) 3-32.
Farid, E.A., Al-Khaffaji, N.H.J., and Ryan, M.A.: "Drilling and Completion of Dual Gas Injection Wells in Multilayered Reservoirs," paper SPE 17982, 1989.
Gatlin, C.: Petroleum Engineering—Drilling and Well Completions, Prentice-Hall,
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Importance of Oil and Gas Wells (Books / Manuals)
Inc., Englewood Cliffs, NJ (1960).
Graf, W.J.: Introduction to Offshore Structures—Design, Fabrication, Installation, Gulf Publishing Company, Houston (1981) 4-19.
Lessi, J. and Spreux, A.: "Completion of Horizontal Drainholes," paper SPE 17572, 1988.
McLarty, J.M., Dobson, J.W., and Dick, M.A.: "Overview of Offshore Horizontal Drilling/Completion Projects in the Unconsolidated Sandstones in the Gulf of Mexico," paper OTC 7352, 1993.
Mitchell, B.: Advanced Oilwell Drilling Engineering Handbook, Mitchell Engineering, Houston (1993).
Phillips, F.L. and Whitt, S.R.: "Success of Openhole Completions in the Northeast Buttery Field, Southern Oklahoma," SPEPE (May 1986) 110-119.
Robison, C.E.: "Monobore Completion for Slimhole Wells," paper OTC 7551, 1994.
Ross, B.R., Faure, A.M., Kitsios, E.E., Oosterling, P., and Zettle, R.S.: "Innovative Slim-Hole Completion," paper SPE 24981, 1992.
Sanku, V., Weber, L.S., and Masoner, L.O.: "Development of Sockeye Field in Offshore California," paper SPE 20047, 1990.
Schulz, R.R., Stehle, D.E., and Murali, J.: "Completion of a Deep, Hot, Corrosive East Texas Gas Well," SPEPE (May 1988) 153-157. http://www.halliburton.com/customer/common/PWC_Book/Chapter_01/Chapter01/pwc01-09.htm (2 of 3)05/12/2007 12:54:39 a.m.
Importance of Oil and Gas Wells (Books / Manuals)
Shepherd, C.E.: "Subsea Completions in the Balmoral Field," paper OTC 5433, 1987.
Stone, P.C., Steinberg, B.G., and Goodson, J.E: "Completion Design for Waterfloods and CO2 Flood," SPEPE (Nov. 1989) 365-370.
Wilhelm, O.: "Classification of Petroleum Reservoirs," Bull. of Am. Assoc. Petr. Geol. (1945) 29.
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