Optical Sensors Aid Oil Drilling

by Alfred Poor, Contributing Editor

Alfred PoorIf you’re not familiar with the modern oil industry, you might think that drilling for oil still resembles the scene in the movie “Giant”, where James Dean’s character hits a classic Texas gusher. The truth is that it’s much harder than that to get oil out of the ground these days. Most of the oil exists in a solid form, trapped in rock crevices or layers of sand. And oil drilling companies are using light to help get it out.

The task of extracting the oil is complex, but in simple terms, you have to steer the drilling rig to where you think the oil is. This typically means creating branches off the center well below an oil rig. You inject superheated steam into some of these wells, which melts the crude oil and causes it to rise up where it can be pumped out through another well. The engineers need to know the pressure and temperature of the wells at various critical points, so they need to rely on remote sensors.

The problem is that the average oil well is not a particularly hospitable environment for sensing devices. Components are subjected to pressures of 10,000 psi and temperatures of 300° Celsius or more. The crude oil itself is a soup of various hydrocarbons that can attack all sorts of materials.

Fiber optic temp sensorOne solution is the use of electronic sensors. These often require semiconductors, and need to be lowered into the well with wires that are used to conduct the signals back to the surface over long distances, sometimes a mile or more. The sensors are expensive and delicate, and easily damaged or lost.

Enter the optical sensor solution. Using passive devices, systems are now able to remotely monitor temperature, pressure, flow, and other metrics. They can even take seismic readings along sections of the well, which can help engineers locate oil deposits.

The clever aspect of this approach is that all the sensitive electronics stay up on the surface. The optical sensors are lowered into the well using optical fiber. It is possible to multiplex more than one sensor on a single fiber, or you can use multiple fibers.

The surface equipment includes a laser, and the light is transmitted through the fiber, down the hole to the sensor. The sensor then alters the light in predictable ways, depending on its surrounding conditions. For example, diffraction gratings are used to measure temperature and pressure. The characteristics of the gratings are altered by the heat or pressure, which alters the laser beam. The altered beam is then reflected back up the same fiber, where it is received and analyzed by the electronics at the surface.

Optical flow meters can measure oil, gas, or water flow rates, using sonic sensors to listen to the flow of gas or liquids through the pipe. It is a non-contact sensor, so that there is no risk of jamming on solids that may be pumped up, and it’s not exposed to the pipe’s contents.

Some measurements don’t require any sensors at all, aside from the optic fiber itself. The laser beam gets scattered as it travels through the fiber. The characteristics of this backscattering vary with temperature. By using a laser pulse, it is possible to determine where the backscattering light came from along the fiber, by timing the lag between sending the pulse and when the return is sensed. This simple solution can monitor the entire length of the well with no additional sensors.

Because the data is transmitted on a beam of light generated at the surface, there is no risk of electromagnetic interference, making the sensor systems more accurate and reliable. The end result is a sensor system that can be left in place permanently, monitoring well production in real time to increase production efficiency and output.

Maybe you don’t work with oil wells. (If you do, you probably already know all about this.) The point is this; give a kid a hammer and the whole world looks like a nail. We make assumptions when we go about solving a problem. For example, need to move data over a distance? Often, the answer is to just use an electrical cable. That’s no longer the only option, however. Light also can be used to transmit data using fiber optics, whether it’s sound in a home theater system or monitoring the vital signs of an oil well. And it doesn’t have some of the problems that electrical circuits have, such as signal loss or interference from electromagnetic sources.