Gyroscopes guide directional drilling
Wells have been made for thousands of years. Most early wells were dug to reach water. In Gydry regions, water was found deeper. Digging deep wells was difficult; drills were developed to make this process easier.
Early drills were a metal rod, sharpened on the end. They were connected to a rope or cable, which lifted the drill and then released it to impact the bottom of the hole. A pipe, called a “bail,” with a valve at the bottom had to be put down the hole periodically to capture the crushed material and lift it out of the hole.
In the early 1800s, petroleum became important. It had appeared in water wells (often to the driller’s dismay). It often was found at much greater depths than water. The lifted drill and bail system was too slow and led to the development of rotary drills. These had gearlike sharp teeth on a member fastened to a heavy rotating pipe, which was driven by a powerful motor and gear system at the well head. Fluid could be pumped down the pipe to flush the crushed dirt and rock up and out of the well.
With petroleum discoveries thousands of feet deep, keeping the well straight became a problem, and methods of steering the drill bit were devised.
“Shoes” that pushed against the side of the hole at the same point in each rotation were employed, then ports were developed that released the flushing fluid at the appropriate point of rotation to force the drill sideways.
It was apparent that if holes could be guided straight down, they also could be guided in curved paths. This would allow the well to follow along oil-bearing strata, making the well more productive.
The location and orientation of the strata were often known from previous drillings.
A way of locating the guided drill was obviously needed so that the driller could know what corrections to make to the drill direction.
An applicable method was borrowed from the military. It is the same technology used to guide missiles and torpedoes, called “inertial navigation.” A group of gyroscopes and accelerometers mounted in the drill bit assembly provides the necessary data.
These devices are marvels of precision and miniaturization. They operate by measuring the accelerations that the assembly experiences. If these accelerations are integrated over time, the distance and direction that the assembly has traveled can be determined.
The method requires no radio communication or wires connected to the surface.
The gyroscopes are based on solid-state lasers. Two circular light paths, with the light traveling in opposite directions, experience different travel times when they are rotated (angular acceleration).
This time difference is a tiny fraction of a second. It must be measured with extremely high accuracy (like a millionth of a millionth of a second).
The gyroscope must be small, consume little power, and operate over a wide temperature range (well temperatures can easily be hundreds of degrees Fahrenheit).
The accelerometers use a weight suspended on springs, which senses acceleration by the amount the weight moves as the system is accelerated. Three of these, oriented at right angles to each other, provide data in all directions.
Like the gyroscopes, these must be small, use little power and withstand a wide range of temperature.
An amazing technology, called MEMs (Micro Electronic Mechanism) is used to fabricate these devices. It is an offshoot of the technology used to make microcircuits.
Drawings of the structures are optically reduced many times and exposed on a specially coated wafer of silicon. The unwanted parts are chemically etched away to create the desired configuration. This makes it possible to produce rugged, accurate navigation systems in a size of a few inches.
They can be made very sensitive — motions smaller than the diameter of an atom can be detected.
It is possible to drill wells several miles long with the location controlled to 10 feet or less. Productive strata can be followed for long distances, making a single well more useful than several wells used to be.
As an example, drilling from the suburbs of Paris (yes, France) under the center of the city is being seriously considered.
Phillip Jessen holds a degree in electronic engineering. He has designed a wide range of systems, including radars, nuclear instrumentation and space electro-optics.