Testing and validation of a low-cost, regional approach for continuous monitoring of CH4 emissions from natural gas production and storage using long-distance dual-comb spectrometry

Much scientific and industry interest is focused on the amount and nature of methane emissions along the natural gas supply chain, and, in particular, how to quickly, cheaply and effectively identify fugitive emissions, or leaks. This is partly due to the well-documented possibility that emissions in excess of a threshold percentage of supply chain throughput would mean that the near-term climate impacts of natural gas as an energy source are greater than those of coal. Also, the operators who produce, transport, and store natural gas have an interest in minimizing product losses incurred by fugitive emissions. A suite of new technologies and methods aimed at monitoring methane emissions have emerged in recent years, in response to growing awareness of the need for reliable, continuous (because oil and gas methane emissions are known to be intermittent), and inexpensive leak detection. We present one such technology, its validation through a series of single-blind controlled leak tests, and its comparison with existing leak detection and repair (LDAR) practices in cooperation with industry operators in the field. The monitoring technique employs a dual frequency comb laser spectrometer for continuous, autonomous leak detection and quantification over several square kilometer areas. The spectrometer is situated within the area to be monitored (e.g., field of production pads or storage entities) and samples a series of beam paths created using retroreflectors placed up to a kilometer or more away, which direct the laser light back to a detector. This configuration allows us to derive path-integrated concentrations of atmospheric trace gases covering the monitored region. The laser light spans 1620-1680 nm with 0.002 nm line spacing, measuring thousands of individual absorption features from multiple species. The result is high-stability measurement of trace gases (here CH₄, CO₂, and H₂O) over long (1 km+) open paths through the atmosphere. Measurements are used in an atmospheric inversion to estimate the location, rate, time variability and uncertainty of emissions from pads and components in the field. We also measure and solve explicitly for background concentrations, which vary rapidly in fields of active oil and gas production and other regions with local sources. Multiple weeks of single-blind tests, of varying levels of complexity, have been performed at the Methane Emissions Test and Evaluation Center (METEC), which was built to mimic oil and gas production. The single-blind tests presented here demonstrate the detection, location and quantification of leaks that span a range from smaller than 1 g/min to greater than 30 g/min (which approaches common definitions of so-called "super-emitters"), from a distance of >1 km away. Similarly, we show direct comparison of leak detection and attribution capabilities against existing LDAR practices in coordination with an industry LDAR team, in an area of active oil and gas production in the Denver Julesburg Basin. We will discuss the system capabilities demonstrated by the single-blind testing in the context of equivalency with current LDAR practices, as well as how our system has been integrated with and directly compared to existing LDAR practices in cooperation with oil and gas operators in the field.