Measuring the inclination of a wellbore (its deviation from the vertical) is comparatively simple—all that is needed is 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 more difficult. In certain circumstances magnetic fields could be used, but could be influenced by metalwork used inside wellbores, as well as the metalwork used in drilling equipment. The next advance was in the modification of small gyroscopic compasses by the Sperry Corporation, who were making similar compasses for aeronautical navigation. Sperry did this under contract to Sun Oil (who were involved in a lawsuit as described above), and a spin-off company "Sperry Sun" was formed, which brand continues to this day, absorbed into Halliburton.
Prior experience with rotary drilling had established several principles for the configuration of drilling equipment down hole ("Bottom Hole Assembly" or "BHA") that would be prone to "drilling crooked hole" (initial accidental deviations from the vertical would be increased). Counter-experience had also given early directional drillers ("DD's") principles of BHA design and drilling practice which would help bring a crooked hole nearer the vertical.
Combined, these survey tools and BHA designs made directional drilling possible, but it was perceived as arcane. The next major advance was in the 1970s, when downhole drilling motors (aka mud motors, driven by the hydraulic power of drilling mud circulated down the drill string) became common. These allowed the bit to be rotated on the bottom of the hole, while most of the drill pipe was held stationary. 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 (using mud pulse telemetry or EM telemetry, which allows tools down hole to send directional data back to the surface without disturbing drilling operations), directional drilling became easier. Certain profiles could not be drilled without the drill string rotating at all times.
The most recent major advance in directional drilling has been the development of a range of Rotary Steerable tools which allow three dimensional control of the bit without stopping the drill string rotation. These tools [Revolution] from Weatherford Drilling Services,Well-Guide from Gyrodata, PowerDrive from Schlumberger, AutoTrak from Baker Hughes, PathFinder, GeoPilot & EZ-Pilot from Sperry Drilling Services/Halliburton) have almost automated the process of drilling highly deviated holes in the ground. They are costly, so more traditional directional drilling will continue for the foreseeable future.
Until recently the drive toward reducing the high cost of these devices has been led from outside the "Big Three" oilfield service companies by entrepreneurs and inventors working essentially alone. With a recent acquisition by Halliburton, this is gradually changing and the drive to introduce a viable low-cost Rotary Steerable System is on.
Most directional drillers are given a well path to follow that is predetermined by engineers and geologists before the drilling commences. When the directional driller starts the drilling process, periodic surveys are taken with a downhole camera instrument ("single shot camera") to provide survey data (inclination and azimuth) of the well bore.
These pictures are typically taken at intervals between 30-500 feet, with 90 feet common during active changes of angle or direction, and distances of 200-300 feet being typical while "drilling ahead" (not making active changes to angle and direction)
During critical angle and direction changes, especially while using a downhole motor, an MWD (Measurement While Drilling) tool will be added to the drill string to provide continuously updated measurements that may be used for (near) real-time adjustments.
These data indicate if the well is following the planned path and whether the orientation of the drilling assembly is causing the well to deviate as planned. Corrections are regularly made by techniques as simple as adjusting rotation speed or the drill string weight (weight on bottom) and stiffness, as well as more complicated and time consuming methods, such as introducing a downhole motor.
Such pictures, or surveys, are plotted and maintained as an engineering and legal record describing the path of the well bore. The survey pictures taken while drilling are typically confirmed by a later survey in full of the borehole, typically using a "multi-shot camera" device.
The multi-shot camera advances the film at time intervals so that by sealing the camera instrument into a tubular housing and dropping the assembly into the drilling string (down to just above the drilling bit), and then withdrawing the drill string at time intervals, the well may be fully surveyed at regular intervals (approximately every 90 feet being common, the typical length of 2 or 3 joints of drill pipe, known as a stand, since most drilling rigs "stand back" the pipe withdrawn from the hole at such increments, known as "stands".)
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 normal.
Drilling far from the surface location still requires careful planning and design. The current record holders manage wells 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 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, Chayvo (ExxonMobil), east coast of Sakhalin Island, Russia, and most recently Al Shaheen (Maersk Oil Qatar AS), Offshore Qatar.
What remains is a difference in operating costs: for wells with an inclination of less than 40 degrees, tools to carry out adjustments or repair work can be lowered by gravity on cable into the hole. For higher inclinations, more expensive equipment has to be mobilized to push tools down the hole.
Another disadvantage of wells with a high inclination was that prevention of sand influx into the well was less reliable and needed higher effort. Again, this disadvantage has diminished such that, provided sand control is adequate planned, it is possible to carry it out reliable.
Between 1985 and 1993, NCEL (now the NFESC) of Pt Hueneme, California developed horizontal drilling technologies. These technologies are capable of reaching 10,000 to 15,000 feet and may reach 25,000 feet (approximately the distance from Kuwait to Iran, across the coast area of Iraq) when used under favorable conditions.
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