Sandia LabNews

Inexpensive disposable fiber optics can relay real-time information about well-drilling process

A new technique developed by Sandia researchers using an inexpensive disposable fiber optics telemetry system to relay real-time information about the drilling process is capturing the attention of the oil and gas industry.

"We have come up with a unique system using throwaway fiber optics that relays information about what is going on at the end of the drill string as it is happening," says David Holcomb (6117), Labs researcher who devised the technique. "Information is instantaneously sent to the surface about temperatures, pressure, chemistry, and rock formation — all obtained without stopping the drilling operation."

David conceived the idea of a disposable cable about eight years ago at an oil and gas industry meeting. People there indicated the need for immediate access to information about the drilling process and the formations being drilled. They wanted to obtain data without halting the drilling operation and have it be transmitted to the surface immediately at a rate high enough to support video or televiewer systems.

"Traditionally to gather this type of information, drilling would have to be stopped so that instrumentation could be lowered into the drill hole," David says. "Ceasing the drilling process is extremely expensive — as much as $100,000 to $200,000 a day for offshore drilling. A better way was needed."

Familiar with disposable fiber developed for the non-line-of-sight missile guidance systems in the 1980s, he realized the technology might be applied to well drilling.

The use of fiber optics telemetry to transmit information from the down-hole end of the drill string while drilling is in process has been of interest to the oil and gas well drilling industry for some time. However, it was considered expensive. A bulky armor was required to protect the delicate optical fiber, and deploying the cable interfered with drilling. David’s technique, instead, uses unarmored fiber, protected only by a thin, clear protective plastic coating, similar to that found in missile guidance systems which can deploy miles of fiber from a small spool at missile speeds.

"The key difference from other attempts to insert fiber optic cable in drill pipes is to consider the cable a throwaway item, to be used once and then ground up and flushed out in the drilling mud," David says. "If the cable only has to survive for a few hours and need not be retrieved, it is feasible to use ‘unarmored’ fiber, which is cheap and can be wound into packages small enough to be inserted into the drill pipe without interfering with operations."

The extreme lightness and compactness of the unarmored optical fiber make it easy to manipulate, compared to the massiveness of a conventional reusable cable. The fiber flows through the drill string with the mud to reach the bottom of the hole, where it instantaneously sends back information to drill operators.

Actual oil and gas drill holes are about eight to ten inches, or larger, in diameter. The pipe (drill string) running down the middle of the hole through which the mud flows is about five inches. To deploy and control the optical fiber in the rapidly moving mud in the drill string, researchers developed a deployment tool. The optical fiber is wound up like a spool of thread inside the tool, which controls the payout of the fiber into the drill string. The entire deployment tool weighs about 75 pounds; the actual fiber optics being placed into the drill string weigh only about one pound.

David said he and other researchers successfully showed that the disposable fiber optics telemetry system works in tests last September at the GRI/CatoosaSM Test Facility, Inc., a subsidiary of the Gas Technology Institute (GTI), located in Catoosa, Okla.

"We got the fiber down and the information back all successfully," he says. "While in field tests the optical fiber was dropped only 3,000 feet, we see no limitations. The cable could easily reach 10,000 to 20,000 feet."

Researchers had two primary concerns going into the test. They were worried that the abrasive nature of the mud flowing through the drill string would chew up the fiber. They also were concerned that the drag of the mud flow down the drill hole would break the fiber.

Both concerns turned out to be inconsequential.

"The abrasiveness was not a problem," David says. "The fiber doesn’t fight. It has a light plastic coat that gives. It doesn’t resist abrasive particles."

The second concern, drag, also proved not to be a problem. Laboratory drag measurements over-predicted the drag by a factor of two or three. The researchers aren’t quite sure why the inconsistencies between the lab tests and the field tests exist. One possibility might be that the fiber moves to the wall of the drill string, out of the way of the mudflow. The only way to know for sure is to send a camera down with the fiber optics.

Researchers also determined the new system could transmit data at about one megabit per second, five orders of magnitude faster than commercially available MWD (measure while drilling) transmission system, without interfering with the drilling process.

David says that before he could get support to research this new technique of obtaining information in a drill hole, he had to get past the "giggle" factor.

"Everyone thought of the drilling environment as a rough environment that would rapidly chew up the optical fiber," he says. "What we learned is that once the fiber flows with the mud, it’s a pretty benign environment."

Early funding came from Sandia’s Laboratory Directed Research and Development (LDRD) program. GTI has funded more recent research.

Currently GTI is seeking a partnership from the oil and gas industry to put the new inexpensive disposable fiber optics telemetry system into production.

"There’s been a lot of interest," David says. "This could change the future of oil and gas drilling around the world."