In an article published in Sensors, researchers have proposed a novel optical methane flux sensor to quantify unsteady methane venting from primary oil and gas sector sources, including well casing venting points and storage tanks.
The methane flux sensor quantified the mass flow of methane through a venting line by combining the Doppler shift measurements and volume fraction using the wavelength modulation emission spectroscopy approach with 2ƒ harmonic detection.
A systematic component-by-component investigation of the possible sources of thermally induced measurement drift was conducted for designing the new methane flux sensor.
An innovative signal processing method was utilized to examine test data, allowing the quantification of wavelength modulation emission spectroscopy measurement drift associated with specific components. The results were effectively applied to make the methane flux sensor drift-resistant.
In addition to being safe and user-friendly, the new methane flux sensor is also suitable for unstable methane venting measurements in hazardous areas, particularly the oil and gas sector.
Reducing Methane Emissions to Decelerate Global Warming
When released from fossil fuels, methane has a global warming potential many times greater than carbon dioxide (CO2). Methane has a much shorter efficient lifetime in the atmosphere than CO2, making it a more potent greenhouse gas for increasing global temperature.
The oil and gas sector is the most significant contributor to methane emissions and a primary target of global methane mitigation efforts. However, field analysis using various techniques shows that methane releases from the oil and gas sector may consistently be higher than those reported in inventories. Production storage tank venting points and well-casing gas venting points in the oil and gas sector are some of the significant sources of these unreported emissions.
The lack of appropriate direct measurement methods has contributed to the challenge of precisely quantifying methane emissions from the oil and gas sector. Some of the particular obstacles in methane quantification include unsteady venting rates where both mass flow rate and methane volume fraction can vary independently, the requirement of an extensive dynamic mass flow range, together with the precondition of near-zero backpressure, and the strict requirements for equipment to satisfy the safety standards for using in risky locations.
This research constructed and tested an upgraded methane flux sensor using the findings of a comprehensive examination of the sources of measurement drift. The upgraded methane flux sensor directly assessed the impact of individual components on the alterations in the wavelength modulation emission spectroscopy signals that induce measurement drift.
Regardless of the application and target gas, the methane flux sensor would help drive the design of other wavelength modulation emission spectroscopy sensors. The updated sensor is ideal for field deployments to quantify unsteady methane venting from the oil and gas sector point sources in explosive gas environments within hazardous locations.
Constructing the New Methane Flux Sensor
The methane flux sensor utilized a distributed feedback diode laser adjusted to a center frequency of 6026.23 cm-1 (1659.41 nm) to detect methane’s R 1 F1(1) absorption line. It was controlled by the Stanford Research Systems laser diode controller. A 30 Hz triangle waveform that swept the laser's frequency by 0.756 cm-1 was superimposed on a 10 kHz wavelength modulation frequency with a depth of 0.1754 cm-1.
The 2ƒ response was maximized by utilizing a wavelength modulation depth with a wavelength modulation index of m of 2.26. Here, m was the ratio of the wavelength modulation depth to the half width at half maximum of the absorption line.
The background wavelength modulation emission spectroscopy signals are unrelated to the absorbing gas. They are typically caused by laser speckle, varying component transmissivity and etalons, laser output measurement drift, and the thermal effects of detectors. Despite the success of the wavelength modulation emission spectroscopy technique in numerous applications, these wavelength modulation emission spectroscopy background signals may, however, limit performance.
The emission spectroscopy background signals can generate non-zero offsets in the measured mass flow velocity and species volume fraction. Moreover, the time- variations in these signals can also manifest as measurement drift.
A simplified approach was presented for quantifying changes in wavelength modulation emission spectroscopy background signals that influenced overall measurement drift and offsets. The approach involved recording a reference background signal at the beginning of the procedure and tracking any variations to this signal when an external disturbance was applied.
Notably, this method required the signal from one laser beam to be monitored without needing any light-absorbing gas. A more comprehensive range of components can be explored to develop an optimum system design owing to the much-simplified experimental setup that also accelerated testing.
The primary goal was to minimize the effect of background signal measurement drift on the recorded mass flow velocity. The effect of background signal drift on measuring the methane volume fraction was also evaluated.
However, the magnitude of the recorded methane volume fraction's drift was too low to be observed beyond the instrument's 0.5% precision. Therefore, the measurement drift was inferred by assessing the changes in the background wavelength modulation emission spectroscopy signals.
Upgraded Methane Flux Sensor and the Future of Global Methane Mitigation Efforts
The newly developed methane flux sensor was capable of steady, accurate, and time-resolved (1 Hz) quantification of methane fluxes. It could also be securely deployed in Class I, Zone 0/1 hazardous situations due to explosive gases.
It was ideal for implementations in the upstream oil and gas sector, where there is a pressing need for techniques to quantify unsteady methane emissions from primary venting sources such as casing gas vents and liquid storage tanks. The sensor could record casing gas venting or time-resolved gas production sufficient to determine real-time gas-oil ratios (GOR) when combined with tank level or any other measurement of generated oil volumes.
The updated sensor significantly lowered the thermally induced mass flow velocity drift from 0.44 m/s/K to 0.015 m/s/K and also improved measurement uncertainties on mass flow velocity from 0.15 to 0.10 m/s. It reduced the methane mass flow rate uncertainty from ±2.55 kg/h to ±0.40 kg/h in the scenario where mass flow velocity and methane fraction were independently variable.
An inadequately measured GOR is one of the primary reasons for underestimating venting in current inventories.
The researchers believe the new methane flux sensor can effectively remove this obstacle in accurately reporting methane emissions.
S.P. Seymour, S.A. Festa-Bianchet, D.R. Tyner, M.R. Johnson (2022). Reduction of Signal Drift in a Wavelength Modulation Spectroscopy-Based Methane Flux Sensor. Sensors. https://www.mdpi.com/1424-8220/22/16/6139/htm