1 Aug, 2017
Introduction to Directional Drilling
This post contains material from DHD(Directional, Horizontal, and Multilateral Drilling) and BDT(Basic Drilling Technology).
At one time, it was assumed all oil wells were essentially vertical or the bottom of the hole was directly under the drilling rig. The petroleum industry did not become fully aware of deviated well problems until the development of the Seminole, Oklahoma field. The wells in this field were drilled very close together and as a result wells were drilling into one another, and ones which were already producing. Deviations as high as 46º from vertical were measured in the Seminole wells. The average deviation from vertical was approximately 13°. Directional drilling began emerging in the late 1920's when curvey instruments were developed that could measure both inclination and azimuth.
The first controlled directional well was drilled in California in 1930 to tap offshore oil reserves. Unfortunately, operators were drilling across lease lines in order to drain oil owned by another individual, resulting in legal problems. In the 1930's, wells were directionally drilled to tap oil reserves that would otherwise be inaccessible. In one case, directional drilling was employed to produce oil from under a cemetery. Oil was produced from under the ocean by placing the rig on the shore and directionally drilling into the offshore oil deposits.
Little attention was paid to directional drilling until a relief well was drilled to kill a blowout near Conroe, Texas. The relief well was drilled near the surface location of the blowout. Directional drilling techniques were used to intersect the producing formation near the blowout, and the blowout was killed by pumping fluid down the relief well and into the blowout well. Since then, directional drilling has been widely accepted. Today, the on-going research and development of new tools and techniques are making directional drilling more accurate and economical.
Figure 1 Examples of directional drilling situations.
Directional drilling is now common from platform and offshore locations. The expense of placing production equipment off shore, or in the arctic, requires that wells be produced into a common area. The well heads can be located on one platform instead of several if wells are drilled directionally from one location. The drainage area for a platform may be extended with extended reach drilling (ERD) so that fewer wells and platforms may be needed. This will also minimize the environmental impact of the operations and maximize the economics of hydrocarbon production.
Platforms such as the one shown below may be taller than the tallest buildings in the world. Most of the structure will be below sea level and not visible. These platforms may be larger than three football fields. The structural loading is crucial to the cost of these tall structures. Each ton of material on the decks will add significantly to the costs, so space is at a premium. However, the cost of a structure this big makes it imperative that as many wells as possible be drilled from this one location. Directional drilling has permitted development of many subsea production intervals in an economic manner even though these platforms are expensive. The investment for these platforms, before a single drop of oil or cubic foot of gas is produced, is frequently in the hundreds of millions or billions of US dollars. Every effort is made to decrease drilling costs for these production platforms.
Figure 2 Example of a platform drilling operation.
Horizontal drilling is an important application of directional drilling and is used to increase the productivity of various formations (Figure 3). One of the first applications for horizontal drilling was in vertically fractured reservoirs. In fractured reservoirs, a significant quantity of the production comes from fractures. Unless a vertical well encounters a fracture system, production rates will be low. A horizontal well has a much greater chance of encountering a prolific fracture system. Horizontal wells are a very common way to produce formations. The Austin Chalk in Texas is a classic example of using horizontal drilling techniques to produce a fractured reservoir.
Figure 3 Horizontal drilling.
Horizontal drilling is used to produce in thin oil zones with water or gas coning problems. The horizontal well is optimally placed in the oil leg of the reservoir. The oil can then be produced at high rates with much less pressure drawdown because of the amount of formation exposed to the wellbore.
Additionally, horizontal wells are used to increase productivity from low permeability reservoirs by increasing the amount of formation exposed to the wellbore. Numerous hydraulic fractures can be placed along a single wellbore to increase production and reduce the number of vertical wells required to drain the reservoir.
Horizontal wells can also be used to maximize production from reservoirs which are not being efficiently drained by vertical wells. These wells usually have permeability streaks in combination with natural fractures. The horizontal well can connect the portions of the reservoir that are productive.
Horizontal drilling was a major innovation in the industry. It revolutionized shale drilling and led to the technique of fracking. Some believe that fracking "obscured the far more important role played by horizontal drilling in enabling oil and gas to be produced from previously inaccessible rock formations, revolutionizing energy output and even international relations". Today approximately two-thirds of all wells are horizontal. 
Directional drilling can be used to drill multilateral wells as well. Multilaterals are additional wells drilled from a parent wellbore as illustrated in Figure 4. Multilaterals can be as simple as an open hole sidetrack or it can be more complicated with a junction that is cased and has pressure isolation and reentry capabilities. Multilaterals are used where production can be incrementally increased with less capital costs. Multilaterals can be used offshore where the number of slots are limited. It is also used to place additional horizontal wells in a reservoir.
Figure 4 Multilateral wells drilled from a platform.
Sidetracking is one of the primary uses for directional drilling. Sidetracking is an operation which deflects the borehole by starting a new hole at any point above the bottom of the old hole as in Figure 5. The primary reason for sidetracking is to bypass a fish which has been lost in the hole; however, there are several other reasons for sidetracking. A sidetrack can be performed so the bottom of the hole can intersect a producing formation at a more favorable position such as up dip above the oil-water contact. A well can be sidetracked to alleviate problems associated with water or gas coning. A sidetrack can be performed in an old well to move the location of the bottom of the hole from a depleted portion of the reservoir to a portion that is productive, such as, across a fault or permeability barrier. Sidetracking an exploration well can lead to a better geologic understanding of an area (Figure 6) especially where the geology is complicated. Sidetracking and directional drilling can be more economical than multiple exploration wells if the upper portion of the well is expensive to drill.
In horizontal wells, it is a common practice to sidetrack existing vertical wells. A whip stock is set inside the casing and the well sidetracked. Then the formation is drilled horizontally to increase productivity. Multiple sidetracks can be drilled from the same well, which are termed multilaterals.
Most often, a sidetrack is accomplished by setting a cement plug in the hole and dressing off the plug to a depth at which the sidetrack will commence. The sidetrack can be either "blind" or "oriented". In a blind sidetrack, the direction of the sidetrack is not specified and is not considered a directional well. In either case, a deflecting tool is used to drill out the old hole and start a new hole.
Figure 5 Sidetracking a stuck bottomhole assembly.
Figure 6 Multiple sidetracks.
STRAIGHT HOLE DRILLING
In some areas of the world, deviation from vertical is caused by the natural formation tendencies. Packed hole assemblies are employed to keep the dogleg severity within reason. Pendulum assemblies are used to keep the inclination as low as possible though with limited success at lower inclinations. If the inclination is already too great to hit a previously specified target, pendulum assemblies, and sometimes downhole motors are used to bring the hole back within range of the target. It should be noted here that sometimes targets are unduly restricted. Controlling the inclination of a well costs significantly more than letting it deviate and keeping the dogleg severity within reason. If there are no restrictions on bottomhole location, the well should be allowed to deviate.
Figure 7 Straight hole drilling.
Controlled directional drilling is used when drilling multiple wells from an artificial structure such as offshore platforms, drilling pads, or man-made islands (Figure 8). The economics of building one offshore platform for each well would be prohibitive in most cases. However, since wells can be directionally drilled, forty or more wells can be drilled from a single platform. Without controlled directional drilling, most offshore drilling would not be economical. Some fields are developed using drilling pads where multiple wells are drilled from one location due to economic or environmental pressures. Where the environment is concerned, roads and production facilities may not be allowed for each surface location with a vertical well. As oil companies become more environmentally conscious, it may be politically advantageous to develop fields from drilling pads in sensitive areas. In areas of shallow water depth, multiple wells can be drilled from artificial islands. Subsea wells are drilled from a template on the ocean floor. In all cases, location construction expenses and rig move expenses are reduced. Also, due to the proximity of the wells, production costs are lower. However, for most land wells, it is usually more economical to drill vertical wells rather than drill directional wells from a pad.
Figure 8 Multiple wells from an artificial structure.
SEALED SAND ZONES DRILLING
There are special cases when multiple sands are drilled with a single wellbore. This occurs when steeply dipping sand zones are sealed by an unconformity, fault, or salt dome overhang. Several vertical wells would be required to produce each sand, which are separated by a permeability barrier. However, all the sand zones can be penetrated with one directionally drilled well thereby greatly reducing the cost of production (Figure 9).
Figure 9 Drilling multiple sands from a single wellbore.
REACHING INACCESSIBLE DEPOSITS
There are times when oil deposits lie under inaccessible locations such as towns, rivers, shorelines, mountains, or even production facilities (Figure 10). When a location cannot be constructed directly above the producing formation, the wellbore can be horizontally displaced by directional drilling. This allows production of an otherwise inaccessible hydrocarbon deposit.
Figure 10 Inaccessible location.
Directional drilling is also applicable in fault drilling (Figure 11). It is sometimes difficult to drill a vertical well in a steeply dipping, inclined fault plane. Often, the bit will deflect when passing through the fault plane, and sometimes the bit will follow the fault plane. To avoid the problem, the well can be drilled on the upthrown or downthrown side of the fault and deflected into the producing formation. The bit will cross the fault at enough of an angle where the direction of the bit cannot change to follow the fault.
Figure 11 Fault drilling.
SALT DOME DRILLING
Many oil fields are associated with the intrusion of salt domes. Directional drilling has been used to tap some of the oil which has been trapped by the intrusion of the salt. Instead of drilling through the salt overhangs, the wells can be directionally drilled adjacent to the salt dome and into the underlying traps as shown in Figure 12. However, since the development of salt saturated and oil based muds, the amount of directional drilling has decreased. It is difficult to drill long intervals of salt with fresh water muds. Directionally drilling around the salt, alleviates a lot of the problems associated with drilling salt.
Figure 12 Salt Dome drilling.
RELIEF WELL DRILLING
A highly-specialized application for directional drilling is the relief well. If a well blows out and is no longer accessible from the surface, then a relief well is drilled to intersect the uncontrolled well near the bottom (Figure 13). Water or mud is then pumped through the relief well and into the uncontrolled well. Since it is sometimes required that the relief well intersect the uncontrolled well, the directional drilling must be extremely precise and requires special tools. Survey data is not accurate enough to intersect a wellbore at depth; rather, proximity logging is required when drilling relief wells.
Figure 13 Relief well drilling.
EXTENDED REACH DRILLING
Another application of directional drilling is what is commonly termed as extended reach drilling. As illustrated in Figure 14, extended reach drilling is where wells have high inclinations and large horizontal displacements for the true vertical depth drilled. It is used to develop reservoirs with fewer platforms or smaller sections of a reservoir where an additional platform cannot be economically justified. Extended reach drilling will become more popular as the costs of platforms in deeper water and severe environments become more expensive.
Figure 14 Extended reach drilling.
Advances in technology have allowed operators to drill extended reach wells with very high HD/TVD ratios (the ratio of the horizontal displacement to true vertical depth). Wells have been drilled with HD/TVD ratios in excess of 6/1 as illustrated in Figure 15. In these wells the horizontal departure was more than six times the true vertical depth with the total measured depth exceeding 32,800 feet (10,000 m).
Figure 15 Extended reach wells drilled by BP.
One of the most important benefits of directional drilling is the total well cost per equivalent barrel of oil is greatly reduced. If a field extends over a large geographic area, several platforms might be required to produce the hydrocarbons. Eliminating just one platform will have a several million dollar impact on development costs. Hydrocarbons from the fringe areas of the reservoir or in remote fault blocks may also be economically produced by directionally drilling from an existing platform. These additional reserves will extend the life of existing platforms or pad locations. Frequently, in the arctic or in swamp country, drilling locations are so expensive that rigs are positioned on pads. In California, for example, many wells south of Los Angeles are drilled from land westward under the sea. Some platforms are positioned in relatively shallow water and directionally drill under deeper water. On land, directional wells are so inexpensive now that wells are drilled under forests or trees to avoid compensating the land owner for cutting down trees.
Directional drilling does require a higher level of technology than is normally associated with straight hole drilling. Equipment must be available to determine direction and angle of the hole. Usually this means that some telemetry devices must be installed in the BHA. One problem with directional wells is a frequent change in direction – either vertically or horizontally. Too many changes in the vertical direction can result in the well bore penetrating the top or bottom shale, penetrating a gas cap, or penetrating a water leg. There is also a problem in severe cases of pulling the BHA back through the well bore. A directional driller is added to the rig crew to supervise interpretation of data received from the BHA and make adjustments to keep the well on track and prevent too many doglegs in the well. The rig crew and drillers need additional training to drill without unexpected problems. The wells are much more difficult to plan and require expertise to properly engineer the program and well path. With expensive equipment in the hole, stuck pipe and other problems are much more expensive. The equipment rental for telemetry and directional driller expenses increase the daily rig costs. Down time or time not spent drilling must be minimized to achieve economic benefits from directional drilling.
To learn more about directional drilling see our Directional, Horizontal, and Multilateral Drilling course.
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 Kemp, John. (14 July, 2014). The real shale revolution: Kemp. Reuters. Retrieved from http://www.reuters.com/article/us-shale-usa-kemp-idUSKBN0FJ26220140714
 SPE International. (27 July, 2016). Extended Reach Wells. SPE International. Retrieved from http://petrowiki.org/Extended_reach_wells