To develop offshore fields as economically as possible, numerous directional wells radiate out from a single platform to drain a large area of reservoir (F94). For directional drilling special weighted drill collars are used with a 'bent sub' to deflect the drill bit at a certain angle in the required direction (F93). Wells which deviate at more than 65 degrees from the vertical and reach out horizontally more than twice their vertical depth are known as extended reach wells. In order for the driller to guide the deviated well to a specific target zone in the reservoir a monitoring-while-drilling (MWD) 'directional sub' is run above the bit to relay information back to the surface on the bit location and inclination. This information can be transmitted to the surface using a mud-pulse telemetry system or recorded in the directional sub and recovered when the bit is changed.
As the angle of deviation from the vertical increases, the friction of the rotating drillstring becomes excessive. Also, as drilling becomes slower the risk of sticking the drilling assembly against swelling shales rapidly increases. Environmental restrictions limit the use of friction-reducing oil-based muds in many areas, so that oil-contaminated cuttings from wells need to be shipped back for onshore disposal. The alternate is water-based which needs additives to reduce its frictional effects, and to inhibit its chemical reactivity with the clays drilled.
Deviated wells which exceed 80 degrees from the vertical are known as horizontal wells (F92) and the horizontal section of the well is maintained in the reservoirs to give the highest production rate possible. Horizontal wells are used when the reservoir permeability is low, or the reservoir interval is very thin or the oil and gas is being produced from vertical fractures in the rock. The flow from a horizontal well may be over 5 times the flow from a normal vertical well. The higher flow rates more than offset the higher cost of drilling a horizontal well.
More than one horizontal section can be drilled in one well as a multilateral well (F96). This technique is used to reduce drilling costs and to maximise the number of wells that can be drilled from small platforms.