Experimental determination of noble gases and SF6, as tracers of CO2 flow through porous sandstone

Abstract

A B S T R A C TIn order to label the CO 2 injected underground for geological storage and allow it to be differentiated from natural sources, a panoply of additive chemical tracers have been proposed. Yet, the transport of these tracers relative to CO 2 in pore space is currently poorly constrained. This leads to uncertainty as to whether tracers will act as an early warning of CO 2 arrival, or be preferentially retained in the pore space making them ineffective. Here, we present the factors affecting transport of noble gases and SF 6 relative to CO 2 in a porous rock. Using a porous sandstone core, each of the tracers were loaded into a sample loop and injected as discrete gas pulses into a CO 2 carrier stream at five different experiment pressures (10-50 kPag upstream to ambient pressure downstream). Tracer arrival profiles were measured using a quadrupole mass spectrometer. Significantly, our results show that peak arrival times of helium were slower than the other noble gases at each pressure gradient. The differences in peak arrival times between helium and other noble gases increased as the pressure gradient along the system decreased and the curve profiles for each noble gas differ significantly. The heavier noble gases (Kr and Xe) along with SF 6 show an earlier arrival time and a wider curve profile compared to He and Ne curves through the CO 2 carrier gas stream. This shows that Kr and Xe could be substituted for SF 6 , a potent greenhouse gas, in tracer applications. For comparison, CO 2 pulses were passed through a N 2 carrier gas resulting in significantly slower peak arrival times compared to those of noble gases and SF 6 . Hence, all investigated tracers when co-injected with CO 2 could potentially act as early warning tracers of CO 2 arrival, though we find that Kr, Xe and SF 6 will provide the most robust advance warning. Analysis of our experimental results shows that they cannot be explained by a simple one dimensional flow model through a porous medium. We outline a conceptual model that incorporates different preferential flow paths depending on flow velocities of individual gas streams. This model can explain the observed dataset and shows that the flow of noble gases and SF 6 tracers is influenced by pore-scale heterogeneity.

Transport of perfluorocarbon tracers in the Cranfield Geological Carbon Sequestration Project

“…, for the PMCH tracer, arrival times vary tremendously for different depths within each observation well. There are distinct flow pathways, created by sinuous fluvial channels between the injector and the observation wells, which respond differently to the pressure responses caused by changes in injection rate (similar to the effects seen in the core‐scale experiments by Kilgallon et al …”

“…Fluvial depositional systems clearly have a dramatic impact on solute transport in the subsurface in both single and multiphase flow problems because the preferential pathways account for the majority of mass transport. 7,8,34,[49][50][51][52][53][54][55] Recently, Kilgallon et al 56 compared the travel times of noble gases and SF 6 tracer in sandstone core samples, and found that the heterogeneity play a key role at the core-scale as well. They also found that different molecules had different travel times at core-scale level.…”

Using SF6 and Xe to Monitor Gas Migration Through Explosion‐Generated Fracture Networks

“…Sulfur hexafluoride (SF 6 ) is a common tracer gas that has been used as a surrogate for RNGs in the past (e.g., Carrigan et al, and ; Stauffer et al, ) because of its relatively low cost to obtain and to analyze, low background concentration (9 parts per trillion) in the atmosphere, and the very small subsurface‐gas samples needed for analysis (e.g., 100 cc) to achieve high detection sensitivity in the parts‐per‐trillion range. A laboratory study by Kilgallon et al () shows similar breakthrough times for both xenon and SF 6 mixed with CO 2 carrier gas passing through sandstone samples. However, a recent laboratory study involving adsorption of both xenon and SF 6 gases on rock surfaces under near‐vacuum conditions suggests that xenon has higher adsorption while moving through the rock mass (e.g., Paul, ; Paul et al, ) causing its breakthrough (i.e., reaching a sampling station) to be retarded at a given concentration compared to SF 6 .…”

Characterizing Tracer Transport Behavior in Two‐Phase Flow System: Implications for CO2 Geosequestration

“…Heat jackets were placed around both the core holder and the syringe pumps to maintain the system at the required test temperature. The fluid was sampled at the outlet of the core and subsequently analyzed with a quadrupole mass spectrometer (QMS), which measures gas concentration (Freifeld & Trautz, 2006; Kilgallon et al, 2018).…”

“…Chemical tracers have been used extensively to study the movement and interactions of gases, liquids, and solids in a wide range of fields, including environmental science (Myers et al, 2013). Recently, chemical tracers have been applied to carbon dioxide (CO 2 ) geosequestration studies (Bachaud et al, 2011; Fagerlund et al, 2013; Flude et al, 2016; Freifeld, 2005; Freifeld & Trautz, 2006; Györe et al, 2015; Hovorka et al, 2013; Kilgallon et al, 2018; Lu et al, 2012; Rasmusson et al, 2014; Roberts et al, 2017; Soltanian et al, 2018; Stalker et al, 2015; Strazisar et al, 2009; Watson & Sullivan, 2012; Zhang et al, 2011). Sulfur hexafluoride (SF 6 ), perfluorocarbon tracer (PFT), and noble gases, such as krypton (Kr) and xenon (Xe), have been widely used in CO 2 geosequestration studies due to their unreactive nature.…”