Slant drilling is most commonly called directional drilling, though it is also called horizontal and deviated drilling. It is the process of drilling non-vertical wells or indirect paths to an oil reserve that cannot be reached by drilling directly under the drilling site. Although the technique is fairly old – the first patent was issued to Robert E. Lee in 1891- early applications were severely limited and it often took up to 2,000 feet for the well to complete the transition from vertical to horizontal. With modern technology, a horizontal well can be drilled in as little as 100 feet.
Types of Directional Wells
– Short-radius wells – radius of 20 to 45 feet and are considered the “sharpest-turning” of the three kinds of horizontal wells. Short-radius wells are easily drilled from an existing vertical well and are used to increase the productivity of an existing oil field by locating bypassed reserves of oil or natural gas.
– Medium-radius wells – radius of 300 to 700 feet with the horizontal portion of the well measuring up to 3,500 feet. These are used primarily when drilling a long distance from the drill site or when attempting to drill into several pockets of reserves at the same time.
– Long-radius wells – radius of 1,000 to 4,500 feet with a huge distance of horizontal well. These are primarily used to drill offshore from a single platform.
Benefits
Directional wells are used for many reasons:
1) They increase the exposed length of a well through the reservoir by drilling at an angle.
2) They allow access to a reservoir where vertical access is very difficult or impossible. An example of a difficult reserve to access would be one under a lake.
3) They allow more wellheads to be grouped at one location, which in turn requires less movement of the rig and less disturbance of the surface, making the production of wells cheaper, easier and reducing their overall environmental impact.
4) They can be used as a “relief well”. Most people who watch the news and are following the Gulf oil spill know that a relief well will relieve the pressure on the leaking well thereby reducing the amount of oil that is spilling into the Gulf.
5) They can extract more oil and gas from these reservoirs because they use long, lateral arms to extend into the flat reservoirs and drain larger areas.
6) They can locate bypassed reserves in existing oil fields. Vertical wells are often used to begin horizontal ones in a search for any overlooked reserves. According to US Department of Energy, “Since 1990, the vast majority of reserve additions in the United States-89 percent of oil reserve additions and 92 percent of gas reserve additions-have come from finding new reserves in old fields”(Environmental Benefits of Advanced Oil and Gas Exploration and Production Technology, 1999).
Disadvantages
Many of the traditional disadvantages of slant drilling have been overcome with advances in technology.
1) Slant drilling is costlier due to the need for more specialized tools and drill bits wearing out quicker. Additionally, there is the added cost of cameras and surveys to ensure the well continues on its intended path.
2) Traditionally, horizontal drilling has been slower due to the need to stop and take surveys. With the invention of better tools to measure inclination and azimuth as well as the use of cameras and computers, the time it takes to survey the well has greatly decreased.
3) Another disadvantage of horizontal wells was that prevention of sand influx into the well was less reliable and needed higher effort. Again, this disadvantage has decreased so that, with proper sand control, it is possible to carry it out reliably.
SOURCES:
Natural Gas Supply Association
Enermax
Oil and Gas Accountability Project
