Slant drilling, also known as directional or horizontal drilling, allows oil reservoirs to be accessed at an angle other than vertical by manoeuvring the drill bit in the desired direction. This in turn frees directional drilling from the traditional need to place the oil rig on top of where the oil is. Directional drilling now accounts for between 5% and 8% of all active land wells in the United States.
Not so long ago, nearly all wells were drilled vertically. The earliest reference to a horizontal drilling technology is the 1891 patent issued to Robert E. Lee Jr, son of the famous general, for drilling a horizontal drainhole to a vertical well. The first hints that wells themselves could be dug in any other direction began with a rash of lawsuits in the late 1920s over compromised oil reservoirs due to cross-property drilling. Slant drilling first officially started in 1939, but was necessarily limited in its scope at the time due to technological limits. Compared to modern methods, most drilling was hit and miss. Even if a field was known to exist, lateral extensions to the drill string could not go very far without disrupting the whole rig.
Modern slant wells are of a different cut altogether. Whereas previous directional wells needed up to a kilometre to coax a reluctant drill string to change direction using inserted wedges, modern directional wells can bend up to 90 degrees in just a few metres. These directional wells are designated according to their radius: short-radius wells (7-20 metres) are used primarily to optimise productivity in an existing field, medium-radius wells (100-250 metres) are commonly used to drill at a distance from the reservoir, while long-radius (300-1500 metres) are usually reserved for offshore drilling.
What makes this type of drilling possible is modern telemetry tools which allow precise directional data and guidance while the drill string is rotating. Mud motors, also used in vertical drilling, allow only the drill bit to rotate while the rest of the drill pipe remains stationary. With the addition of a piece of bent pipe, the direction of the wellbore can be easily changed at will. Modern location sensors, cameras, and even gamma ray spectrometers can be located directly behind the drilling bit, giving up-to-the-second feedback on the current drill bit environment (providing they don’t get clogged or otherwise damaged). These kinds of sensors eliminate the constant need to stop and take surveys, one of the major reasons horizontal drilling takes longer than conventional vertical drilling.
With modern directional drilling techniques, there is no longer a need to move the rig to take advantage of each separate lenticular reservoir: which in turn significantly increases productivity and reduces cost. The ability to drill in several directions from a single production platform (a ‘spider plot’) is especially valuable in offshore drilling. In some formations, using directional drilling in soft earth may even negate the need to use drilling mud. On the other hand, with inadequate sand control, sand is more likely to leak into a horizontal well.
Long, thin reservoirs and fracture reservoirs, both natural and man-made, cannot be efficiently tapped with a conventional vertical setup. Without directional drilling, oil fields under cities, ecologically sensitive areas, permafrost, and even protected shoreline could never be tapped. Directional drilling protects natural areas by shifting the surface risk to well outside those areas. In some cases, directional drilling may be legislated as part of the agreement allowing tapping of a particular reservoir.
A final important use of slant drilling is to drill a relief well where another damaged well needs to be shut down, such as the plan to finally bring the 2010 Deepwater Horizon drilling rig disaster completely under control. Although this new well is drilled into the same reservoir as the damaged well, the slant avoids the high oil pressure at the original well. Once the connection is established, first seawater and then mineral mud is pumped into the reservoir to displace the oil. If these have successfully displaced the oil and gone up the broken well, the last step is to pump concrete into the well and allow it to set.