Sandia LabNews

Hot electronics could boost geothermal industry, earth sciences

It’s a hot technology. Not pre-2000 hot, but hot hot.

Sandia researchers Joe Henfling and Randy Normann of Geothermal Research Dept. 6211 have developed high-temperature electronics that function reliably in temperatures up to at least 250° C and probably up to 300° C. (That’s about 480°F to 570°F.) Nobody else, says Randy, has developed electronics that work at that temperature for any extended period of time.

"This is stuff that’s never been done, it’s unique," says Randy. "We’re even getting the attention of the scientists outside of geothermal in the earth sciences community."

Conventional modern electronics begin to fail at much lower temperatures, say in the 150°C range.

Randy and Joe developed the electronics for geothermal energy research, but the technology clearly has major relevance for researchers in volcanology and seismology. It has potential application in the oil and deep natural gas drilling and exploration business. And there are more than a few high-temperature industrial processes that could be enhanced and made more efficient if better electronic-based monitoring tools were available.

The geothermal industry needed a tool — an electronics instrument package — that could be sent down a geothermal well to collect data without heat shielding. The current state of high-temp probe technology left more than a little to be desired, bearing a closer resemblance in some ways to 19th century plumbing than to 21st century electronics.

Randy explains the problem: "One of the probes that’s commonly used to study volcanoes and earthquakes uses tubing to bring fluid from the bottom of the well to the surface. You’ve gotta bring the fluid to the surface to measure it, to analyze it, because it’s too hot for your instruments down there. Obviously, this isn’t very precise, because the fluid cools as it rises through the tube. You can compensate for that, but you can never be as precise as you want to be. Nobody has ever depended on deep-well electronics [in high-temperature research], because electronics drift, they fail. At that temperature, they’re just not reliable." That is to say, conventional "bulk" silicon microelectronics aren’t reliable.

Again, Randy explains why: "Silicon is by nature a semiconductor; for electronics applications, you dope silicon [treat it with other elements] to make it more of a conductor or less of a conductor. The problem is that high temperatures tend to excite the atoms, resulting in leakage currents in the bulk silicon. The silicon moves from being a semiconductor to being a conductor. And once that happens the electronic device no longer functions. A technology that sidesteps silicon’s temperature-related limitations is silicon-on-insulator, or SOI."

Reducing leakage currents

"Silicon-on-insulator builds up the transistors on glass, which is a nonconductor," Randy says. "And, since you’re building your transistors on nonconducting material, they’re isolated and these leakage currents don’t destroy the device. SOI reduces leakage currents by a factor of 100 at these higher temperatures."

These aren’t theoretical projections. Randy and Joe have already built and demonstrated a tool, a complete telemetry data acquisition system, based on the SOI technology. It’s been field-tested in a geothermal well at 240°C, measuring real-time pressure and temperature. Researchers who’ve tried the tool can’t wait to get their hands on one of their very own.

While the SOI technology is familiar, the application-specific integrated circuit, or ASIC, is the work of Randy and Joe. The proprietary ASIC is, in fact "the glue that holds the tool together," Randy says.

"We’ve applied for a copyright on the ASIC, and we’re going to license that to Honeywell, which will sell to the high-temperature industry. By combining commercially available SOI components and our ASIC, we’ve developed a universal instrumentation system for any number of high-temperature applications."

Randy and Joe are working with a number of industry partners on this work, funded by DOE’s Geo-thermal/Wind Energy Program Office. Randy says several industry partners are developing specialized components and sensors to work with the Sandia ASIC.

"For example," Randy says, "one of our partners is developing a tilt sensor — that’s a combination MEMS [microelectromechnical system] and SOI device."

Such an instrument could tell researchers a lot about movement deep within the Earth, Randy notes, an important data point in understanding earthquakes and volcanoes.

"MEMS has the capability of operating at very high temperatures; in Europe, they’ve been testing silicon-based MEMS devices up to 500°C [980°F]. It appears to be really quite viable to merge high-temperature electronics with a new family of specialized MEMS devices."

With MEMS devices like tilt sensors mated to magnetic sensors, Randy says, "we can do not just logging — gathering information — but guided drilling at depths greater than 20,000 feet. So we’re building a steering tool to sense direction and movement." Such a drill would enable researchers — and geothermal explorers — the ability to follow the unpredictable branching of a geothermal upwelling.

Other potential applications:

  • An instrument to measure wellbore bending. (Pipes are bent when earth faults shift.)
  • A tool for gauging pipe wall degradation. (Pipe can get eaten away in the caustic environment of a hot well.)
  • A device to measure and control the flow of fluids and determine if the flow is oil or water. That could be very useful to beef up the efficiency of multi-completion (several wells feeding into one) oil fields, Randy says.

"While you’re drilling, if you see your pressure fall off, that’s because fluid is running into the formation," he says. "Well, that is a potential production place for geothermal energy, because that means fluid could come back out of the hot rock to drive a geothermal power plant. In other words, they were able to see production zones immediately using these electronics. That was pretty cool. We wrote up a report to DOE saying, ‘All we did was loan them a tool and look at all this free research we’re getting.’ "

"We’ve just seen the tip of the iceberg, but the whole iceberg is beginning to appear. The fact that right now it’s just the two of us [Joe and Randy] complicates things for us because we can’t do all the things we’d like to do."