William Albert Challener

MOEMS pressure sensors for geothermal well monitoring

The technology for enhanced geothermal systems (EGS), in which fractures connecting deep underground wells are deliberately formed through high pressure stimulation for energy generation, is projected to enormously expand the available reserves of geothermal energy in the U.S. EGS could provide up to 100,000 MWe within the U.S. by the next 50 years. Pressure measurements, in particular, are important for determining the state of the fluid, i.e., liquid or steam, the fluid flow, and the effectiveness of the well stimulation. However, it has been especially difficult to accurately measure pressure at temperatures above ~200°C at a distance of 10 km below ground. MEMS technology has been employed for many years for extremely accurate pressure measurements through electrical readout of a MEMS fabricated resonator. By combining optical readout and drive at the end of a fiber optical cable with a MEMS resonator, it is possible to employ these highly accurate sensors within the harsh environment of a geothermal well. Sensor prototypes based on two beam and four beam resonator designs have been designed, fabricated and characterized for pressure response and accuracy. Resonant frequencies of the sensors vary between ~15 kHz and 90 kHz depending on sensor design, and laboratory measurements yielded sensitivities of frequency variation with external pressure of 0.9-2.2 Hz/psi. An opto-electronic feedback loop was designed and implemented for the field test. The sensors were packaged and deployed as part of a cable that was deployed at a geothermal well over the course of 2½ weeks. Error of the sensor versus the reference gage was 1.2% over the duration of the test. There is a high likelihood that this error is a result of hydrogen darkening of the fiber that is reducing the temperature of the resonator and, if corrected, could reduce the error to less than 0.01%.

Hollow-core fiber sensing technique for pipeline leak detection

Recently there has been increased interest on the part of federal and state regulators to detect and quantify emissions of methane, an important greenhouse gas, from various parts of the oil and gas infrastructure including well pads and pipelines. Pressure and/or flow anomalies are typically used to detect leaks along natural gas pipelines, but are generally very insensitive and subject to false alarms. We have developed a system to detect and localize methane leaks along gas pipelines that is an order of magnitude more sensitive by combining tunable diode laser spectroscopy (TDLAS) with conventional sensor tube technology. This technique can potentially localize leaks along pipelines up to 100 km lengths with an accuracy of ±50 m or less. A sensor tube buried along the pipeline with a gas-permeable membrane collects leaking gas during a soak period. The leak plume within the tube is then carried to the nearest sensor node along the tube in a purge cycle. The time-to-detection is used to determine leak location. Multiple sensor nodes are situated along the pipeline to minimize the time to detection, and each node is composed of a short segment of hollow core fiber (HCF) into which leaking gas is transported quickly through a small pressure differential. The HCF sensing node is spliced to standard telecom solid core fiber which transports the laser light for spectroscopy to a remote interrogator. The interrogator is multiplexed across the sensor nodes to minimize equipment cost and complexity.

Optical self-excitation and detection for inertial MEMS Sensors

We report on the development of an optical self-excitation and detection method to allow for high stability operation of inertial MEMS sensors. A single constant amplitude laser beam is used to both drive and optically interrogate the resonant frequency of a MEMS resonator. A parametric study of x,y,z position of the laser spot location, laser power, and laser wavelength with respect to frequency stability of the MEMS sensor is presented. A frequency bias instability of 3 ppb at 1 s without any calibration and a frequency white noise of 2.4 ppb/√Hz for a MEMS resonator with resonant frequency of 68.28 kHz and quality factor of 4,600 have been measured.

Fugitive methane leak detection using mid-infrared hollow-core photonic crystal fiber containing ultrafast laser drilled side-holes

The increase in domestic natural gas production has brought attention to the environmental impacts of persistent gas leakages. The desire to identify fugitive gas emission, specifically for methane, presents new sensing challenges within the production and distribution supply chain. A spectroscopic gas sensing solution would ideally combine a long optical path length for high sensitivity and distributed detection over large areas. Specialty micro-structured fiber with a hollow core can exhibit a relatively low attenuation at mid-infrared wavelengths where methane has strong absorption lines. Methane diffusion into the hollow core is enabled by machining side-holes along the fiber length through ultrafast laser drilling methods. The complete system provides hundreds of meters of optical path for routing along well pads and pipelines while being interrogated by a single laser and detector. This work will present transmission and methane detection capabilities of mid-infrared photonic crystal fibers. Side-hole drilling techniques for methane diffusion will be highlighted as a means to convert hollow-core fibers into applicable gas sensors.

Multipoint temperature and pressure sensing system for monitoring CO 2 sequestration wells

We describe a novel interrogation scheme that enables multipoint measurement of temperature and pressure in harsh environment CO2 sequestration wells. Measured MEMS sensor responses are compared with finite element modeling to suggest accuracies better than ±0.1% for downhole pressure measurements.