One of the most fascinating tools in the oil exploration arsenal is directional drilling, sometimes called “slant drilling.” The concept of directional drilling is straightforward: drill a well or borehole that runs in an “unnatural” direction; usually something other than straight down.
Though the concept is simple, the execution is not; so some very high-tech solutions have evolved around the task. Over the years, the oil industry has advanced the techniques to where it is possible for a well to be guided to within just feet of a location two or three miles away, passing through solid rock all the way.
Why drill directional wells? There are several reasons to drill directionally, but most of the time it comes down to just one: you can’t place a drill rig on the surface above the target. For instance, the surface may be in a park or protected in some other way. A well that starts off to one side and curves in underneath the protected area may be the only way to reach valuable resources thousands of feet under the ground. The shoreline near Long Beach. California, is dotted with wells that start on land before curving west beneath the shoreline to tap a huge oil field, far below waters protected by environmental laws. In Illinois, one of the largest discoveries of the decade lies beneath Stephen A. Forbes State Recreation Area, reached by drilling a directional well from outside state lands.
Most wells are deviated (drilled directionally), however, because several wells must be drilled from the same location. This is very common offshore, where dozens of wells, all drilled from one production platform, cover large areas surrounding those gigantic structures. Since platforms cost hundreds of millions of dollars to build and maintain, it is more economical to drill directionally from a central location than to build a platform for every well.
On-shore, directional wells are often drilled from stable pads in hostile environments such as swamps or the permafrost of Alaska’s North Slope. Use of such pads may be mandated by environmental regulations, or may be for economic reasons similar to those for offshore wells.
How are directional wells drilled? The earliest deviated wells were drilled along the shores of lakes, aiming for locations under the water. At the time, the technology only allowed drilling wells on dry land, so some bright wildcatter simply tipped the entire derrick to the side. The result was a straight hole, but a hole with a slant – slant drilling was born!
Modern directional drilling is far more complex, of course – it has to be when deviated wells run for thousands of feet horizontally or bend and curve throughout the subsurface to intersect multiple reservoirs. The technology evolved first by planting wedges and ramps in the wellbore to force the entire drillstring to bend in the desired direction. As more complex paths were needed, however, oilfield service companies developed more versatile methods.
Current directional drilling uses “mud motors” to turn the drill bit when a bend is desired. The drill pipe, which runs from the surface to the bit, is held stationary while a hydraulic pump driven by the mud (drilling fluid) spins the bit. A precisely-bent section of drill pipe just above the bit forces it to move in the desired direction. Once the proper direction is achieved, the mud motor is turned off and the entire string of pipe rotates to drive the bit. Since the bent section is also turning, the bit continues to move in the same direction.
Hitting a small target tens of thousands of feet away from the surface location takes careful planning. It also requires instrumentation to allow precise positioning of the bit and motor, as well as constant communication of the bit’s current position and orientation to the surface. Modern accuracy is sufficient to place a drill bit within just a few feet of a designated point in space.
Offshore platforms are surrounded by dozens of directionally-drilled wells that fan out to cover a space up to ten kilometers (six miles) across. A map of all of the paths around the platform resembles spokes on a demented wheel or the web of a drunken spider. Such a map is called, for obvious reasons, a spider plot.
Schlumberger Oilfield Glossary
Halliburton
