Multi-Channel Auto-Dilution System for Remote Continuous Monitoring of High Soil-CO2 Fluxes

Abstract

SummaryGeological sequestration has the potential capacity and longevity to significantly decrease the amount of anthropogenic CO 2 introduced into the atmosphere by combustion of fossil fuels such as coal. Effective sequestration, however, requires the ability to verify the integrity of the reservoir and ensure that potential leakage rates are kept to a minimum. Moreover, understanding the pathways by which CO 2 migrates to the surface is critical to assessing the risks and developing remediation approaches. Field experiments, such as those conducted at the Zero Emissions Research and Technology (ZERT) project test site in Bozeman, Montana, require a flexible CO 2 monitoring system that can accurately and continuously measure soil-surface CO 2 fluxes for multiple sampling points at concentrations ranging from background levels to several tens of percent. To meet this need, researchers at Pacific Northwest National Laboratory (PNNL) are developing a multi-port battery-operated system capable of both spatial and temporal monitoring of CO 2 at concentrations from ambient to at least 150,000 ppmv.The system consists of soil-gas sampling chambers and a sensing unit based on an infrared gas analyzer (IRGA). Headspace gas from the chambers is continuously pumped through a relay-driven manifold that directs gas from individual chambers into the IRGA. A unique feature of the system is its ability, based on feedback from the IRGA, to automatically dilute the sample gas stream with N 2 using a network of gas-flow controllers, thus allowing measurement of CO 2 levels beyond the normal IRGA limit of 3000 ppmv. The entire system is controlled by a programmable datalogger that also stores the output from the IRGA and gas-flow controllers. The system consists of 27 sampling chambers, thus allowing detailed spatial monitoring of a moderately sized test area (~100 m 2 ). Power can be provided either by an AC source or by an off-grid power-generation system consisting of six deep-cycle batteries charged by both wind and photovoltaic power sources. Dilution gas is provided by a liquid N 2 dewar containing the equivalent of 135,000 L of N 2 (g), enough for 4 weeks of continuous operation. The system is remotely controlled and monitored by connecting the datalogger communication port to a cellular-based modem and antenna.On 27 August 2008, a shallow test injection of CO 2 was started at the ZERT site using a horizontal well located about 2 m below the surface and 1 m below the water table. The PNNL team set up a 27-chamber sampling grid directly above and extending northwest of the injection well. Concentrations were monitored continuously in twenty of these steady-state flux chambers during 13 days of injection (308 h) and for 33 days post injection. These concentration data can be converted directly to flux data using a flow-rate specific constant.With the 20-chamber setup, data were collected on a 2.7-h cycle continuously for nearly 7 weeks. We centered the southeastern side of the sampling grid on a known leakage spot, and this ...

Novel MVA tools to track CO2 seepage, tested at the ZERT controlled release site in Bozeman, MT

“…Tank CO 2 broke through the soil surface 10 h after CO 2 injection began (Amonette and Barr 2009;Lewicki et al 2009a;Humphries et al 2008). The variations that occur in the generally downward trend after 7 h may be due partially or entirely to natural diurnal variations in soil 222 Rn flux (Wilkening and Hand 1960) which have been attributed to atmospheric pressure changes that cause the soil gases to advectively pump out of the soil (Gast and Stolz 1982;Singh et al 1990).…”

A gas-flow funnel system to quantify advective gas emission rates from the subsurface

“…The collar is pushed into the soil to ensure an air-tight connection to the soil and the chamber is placed on top. By this, gas emanating from the subsurface can accumulate in the closed chamber and the measurement of the gas accumulation within the chamber, i.e., increase in gas concentration over a certain time period, is used to determine the mass flux (Amonette and Barr 2009;Chiodini et al 1998;Elío et al 2012). The gas analyzers are equipped with gas-specific sensors typically covering a particular concentration span depending on the expected gas emanation concentrations.…”

Spatiotemporal changes in CO2 emissions during the second ZERT injection, August–September 2008

“…As described by Amonette and Barr (2009), the steadystate flux measurement system used in this study consisted of 20 soil surface flux chambers and a central measurement and control system. The steady-state continuous-purge flux chambers were mounted on collars that extended 15 cm into the soil, and were connected to the central system by 9.14-m lengths of 0.63-cm-OD tubing to transfer gas samples from the chamber interior to the analyzer.…”

“…Field experiments to characterize these pathways, such as those conducted for the near-surface region at the Zero Emissions Research and Technology (ZERT) project test site in Bozeman, Montana, require a flexible CO 2 monitoring system that can accurately and continuously measure soil surface CO 2 fluxes for multiple sampling points at concentrations ranging from background levels to several tens of percent. To meet this need, a steady-state, multi-port, battery-operated system capable of both spatial and temporal monitoring of CO 2 fluxes by measuring concentrations from ambient to at least 150,000 ppmv is being developed (Amonette and Barr 2009).…”