Directional drilling
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Directional drilling (sometimes known as slant drilling outside the oil industry) is the science of drilling non-vertical wells. Directional drilling can be broken down into three main groups; Oilfield Directional Drilling, Utility Installation Directional Drilling (commonly known as H.D.D./Horizontal Directional Drilling) and in-seam directional drilling (Coal-Bed methane).
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[edit] History
A number of prerequisites were necessary before this suite of technologies could become productive. Probably the first requirement was the realisation that oil wells (or water wells, but since their depths are normally trivial, the development was essentially done in the oil industry) are not necessarily vertical. This realisation was quite slow, and didn't really grasp the attention of the oil industry until the late 1920s when there were several cases of lawsuits alleging that a well drilled from a rig on one person's property had actually crossed the boundary and was penetrating a reservoir on an adjacent property. Initially proxy evidence such as changes in production from pre-existing wells was accepted, but such cases fuelled the development of small diameter tools capable of surveying wells as (or during) their drilling.
Measuring the inclination of a wellbore (its deviation from the vertical) is comparatively simple—one needs a pendulum of some sort. But measuring the azimuth (direction with respect to the geographic grid in which the wellbore is running from the vertical) was much more difficult. In certain circumstances magnetic fields could be used, but were open to the influence of the metalwork used to line wellbores, as well as the metalwork used in drilling equipment itself. The big step forward was in the modification of small gyroscopic compasses by the Sperry company, who were making similar compasses for aeronautical navigation. Sperry did this work under contract to Sun Oil (who were involved in a lawsuit as described above), and a spin-off company was formed under the name "Sperry Sun", which brand continues to this day, absorbed into Halliburton, the second-largest oil services company.
Prior experience with rotary drilling had established a number of principles for the configuration of drilling equipment down hole ("Bottom Hole Assembly" or "BHA") that would be prone to "drilling crooked hole" (initial accidential deviations would be increased away from the vertical). Counter-experience had also given these early directional drillers ("DD's", on many whiteboards on many rigs around the world to this day) principles of BHA design and drilling practice which would help bring a crooked hole back towards the vertical.
Combined, these survey tools and BHA designs made directional drilling possible, but it was perceived to be decidedly arcane. Some DDs allegedly took a perverse delight in making it sound more arcane than it actually was - using Ouija boards to perform calculations instead of slide rules for example. Actually the Ouija board performs simple trigonometric functions quickly and in a somewhat graphic format. [citation needed]
The next major advance was in the 1970s, when downhole drilling motors became commonplace. These allowed the bit to be rotated on the bottom of the hole, while most of the drill pipe was held stationary (power to the motor is supplied by the hydraulic effect of the drilling fluid pumped down the inside of the drill pipe). Including a piece of bent pipe (a "bent sub") between the stationary drill pipe and the top of the motor allowed the direction of the wellbore to be changed without needing to pull all the drill pipe out and place another whipstock. Coupled with the development of MWD (mud pulse telemetry or EM telemetry, which allows tools down hole to send data back to the surface without disturbing routine drilling operations), directional drilling got much easier. Certain profiles could not be drilled without the drill string in rotation at all times.
The most recent major advance in art of directional drilling has been the development of a range of Rotary Steerable tools from various companies which allow 3 dimensional control of the bit without shutting down the drill string rotation. These tools (PowerDrive from Schlumberger, AutoTrak from Baker Hughes and GeoPilot from Sperry Drilling Services/Halliburton) have almost automated the process of drilling highly deviated holes in the ground. But they are not cheap, so more traditional directional drilling will continue for the foreseeable future.
Until very recently the drive towards lowering the high cost of these devices has been led from outwith the "Big Three" oilfield service companies by individual entrepreneurs and inventors working effectively alone. However, with the advent of a recent acquisition by Halliburton, this is gradually changing and the drive to introduce a viable low-cost Rotary Steerable System is on.
[edit] Benefits
Directional wells are drilled for a number of purposes:
- Increasing the exposed section length through the reservoir by drilling through the reservoir at an angle
- Drilling into the reservoir where vertical access is difficult or not possible. For instance an oilfield under a town, under a lake, or underneath a very difficult to drill formation
- Allowing more wellheads to be grouped together on one surface location can allow fewer rig moves, less surface area disturbance, and make it easier and cheaper to complete and produce the wells. For instance on an oil platform or jacket offshore, where up to about 40 wells can be grouped together. The wells will fan out from the platform into the reservoir deep below. This concept is also being applied to land wells, allowing multiple subsurface locations to be reached using only one leveled-out pad, reducing the environmental impact.
- Drilling "relief wells" to relieve the pressure of a well which is producing without restraint (blown out). In this scenario, another well could be drilled starting at a safe distance away from the blow out, but intersecting the troubled wellbore beneath the surface. Then, heavy fluid (kill fluid) could be pumped in the new relief wellbore to suppress the high pressures in the original wellbore causing the blowout.
Most directional drillers are given a well path to follow that is predetermined by engineers and geologists before the actual drilling commences. When the directional driller starts the drilling process, he relies on a MWD (Measurement While Drilling) Engineer to provide survey data (inclination and azimuth (horizontal direction) of the well bore every 90 ft or so. This data indicaties to the directional driller whether the well is following the planned well path. Corrections are regularly made.
With modern technology great feats can be achieved. Whereas 20 years ago wells drilled at 60 degrees through the reservoir were achieved, horizontal drilling is now quite normal. However, drilling out far from the surface location is still something that requires careful planning and design; the current record holders manage wells of over 10 km (6 miles) away from the surface location at a depth of only 1600–2600 m (5,200–8,500 ft). These are all wells drilled from a land location to underneath the sea (Wytch Farm (BP), south coast of England, ARA (Total), south coast of Argentina (TFE) Dieksand (RWE), north coast of Germany, and most recently Chayvo (ExxonMobil), east coast of Sakhalin Island, Russia.
In 1990 Iraq accused Kuwait of stealing Iraq's oil through slant drilling.
[edit] Popular culture
Directional drilling makes an appearance in the Simpsons episode “Who Shot Mr. Burns Part One” in which the elementary school strikes oil which is then stolen by Montgomery Burns’s nearby Slant Drilling Company.
[edit] Web
http://info.sbo.at/_movies/dd_320_de.wmv
