Editing Directional drilling (section)

==Benefits==
Wells are drilled directionally for several 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 difficult-to-drill formation.
*Allowing more [[wellhead]]s 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, 40 or more wells can be grouped together. The wells will fan out from the platform into the reservoir(s) below. This concept is being applied to land wells, allowing multiple subsurface locations to be reached from one pad, reducing costs.
*Drilling along the underside of a reservoir-constraining fault allows multiple productive sands to be completed at the highest stratigraphic points.
*Drilling a "[[relief well]]" to relieve the pressure of a well producing without restraint (a "[[Blowout (well drilling)|blowout]]"). In this scenario, another well could be drilled starting at a safe distance away from the blowout, but intersecting the troubled wellbore. Then, heavy fluid (kill fluid) is pumped into the relief wellbore to suppress the high pressure in the original wellbore causing the blowout.

Most directional drillers are given a blue 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 instrument to provide survey data (inclination and azimuth) of the well bore.<ref>{{Cite web|url=http://chinookconsulting.ca/News/Directional-drilling-glossary.html|title=Glossary of geo-steering terms|date=26 August 2010 |access-date=5 September 2010}}</ref> These pictures are typically taken at intervals between {{convert|10|and|150|m|ft|abbr=off|sp=us}}, with {{convert|30|m|ft|abbr=off|sp=us}} common during active changes of angle or direction, and distances of {{convert|60|–|100|m|ft|abbr=off|sp=us}} 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, a [[measurement while drilling]] (MWD) tool will be added to the [[drill string]] to provide continuously updated measurements that may be used for (near) real-time adjustments.

This data indicates 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 dropping the camera instrument in a sealed tubular housing inside 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 depth intervals (approximately every {{convert|30|m|ft|abbr=off|sp=us}} 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").

Drilling to targets far laterally from the surface location requires careful planning and design. The current record holders manage wells over {{convert|10|km|mi|abbr=on}} away from the surface location at a true vertical depth (TVD) of only {{convert|1,600|-|2,600|m|ft|abbr=on}}.<ref>{{cite web|url=http://www.gulf-times.com/site/topics/article.asp?cu_no=2&item_no=219715&version=1&template_id=48&parent_id=28 |title=Maersk drills longest well at Al Shadeen |publisher=The [[Gulf Times]] |date=21 May 2008 |access-date=5 March 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120214025536/http://www.gulf-times.com/site/topics/article.asp?cu_no=2&item_no=219715&version=1&template_id=48&parent_id=28 |archive-date=14 February 2012}}</ref>

This form of drilling can also reduce the environmental cost and scarring of the landscape. Previously, long lengths of landscape had to be removed from the surface. This is no longer required with directional drilling.