CO 2 and Rn degassing from the natural analog of Campo de Calatrava (Spain): Implications for monitoring of CO 2 storage sites

Abstract

a b s t r a c tNatural analogs offer a valuable opportunity to investigate the long-term impacts associated with the potential leakage in geological storage of CO 2 .Degassing of CO 2 and radon isotopes ( 222 Rn-220 Rn) from soil, gas vents and thermal water discharges was investigated in the natural analog of Campo de Calatrava Volcanic Field (CCVF; Central Spain) to determine the CO 2 -Rn relationships and to assess the role of CO 2 as carrier gas for radon. Furthermore, radon measurements to discriminate between shallow and deep gas sources were evaluated under the perspective of their applicability in monitoring programs of carbon storage projects.CO 2 flux as high as 5000 g m −2 d −1 and 222 Rn activities up to 430 kBq m −3 were measured; 220 Rn activities were one order of magnitude lower than those of 222 Rn. The 222 Rn/ 220 Rn ratios were used to constrain the source of the Campo de Calatrava soil gases since a positive correlation between radon isotopic ratios and CO 2 fluxes was observed. Thus, in agreement with previous studies, our results indicate a deep mantle-related origin of CO 2 for both free and soil gases, suggesting that carbon dioxide is an efficient carrier for Rn. Furthermore, it was ascertained that the increase of 222 Rn in the soil gases was likely produced by two main processes: (i) direct transport by a carrier gas, i.e., CO 2 and (ii) generation at shallow level due to the presence of relatively high concentrations of dissolved U and Ra in the thermal aquifer of Campo de Calatrava.The diffuse CO 2 soil flux and radon isotopic surveys carried out in the Campo de Calatrava Volcanic Fields can also be applicable to geochemical monitoring programs in CCS (Carbon Capture and Storage) areas as these parameters are useful to: (i) constrain CO 2 leakages once detected and (ii) monitor both the evolution of the leakages and the effectiveness of subsequent remediation activities. These measurements can also conveniently be used to detect diffuse leakages.

Evaluation of the applicability of four different radon measurement techniques for monitoring CO2 storage sites

“…The region of CCVF, together with that of La Selva-Empordà (NE Spain), likely represents the largest area of CO 2 discharge in Spain (e.g., Vaselli et al, 2013). Elío et al, (2015) recognized a correlation between CO 2 leakage and radon concentration. The study area is characterized by the presence of a superficial clay layer, which likely regulates the release of diffuse CO 2 soil fluxes, since strikingly high variations were observed just a few decimeters across (Elío et al, 2015).…”

“…In this respect, the "mixed correction" formula has demonstrated its efficacy when the contrast between the peak values of radon anomalies and regional background were small, obviating the thoron concentration (Mazadiego, 1994). However, in the soil gas of Campo de Calatrava, thoron is not negligible and, occasionally its concentration was higher than that of 222 Rn (e.g., Elío et al, 2015). Therefore, 220 Rn has to be considered in the correction formula.…”

“…Recently, investigations on natural analogues have proliferated to assess the relationship between CO 2 and 222 Rn (radon) and 220 Rn (thoron) isotopes since CO 2 (as well as CH 4 ) commonly acts as the carrier gas for radon. Therefore, radon can be transported for relatively greater distances with respect to what would be expected by considering the short half-lives of its iso- topes and only taking into account the diffusive effects (Elío et al, 2015;Etiope and Martinelli, 2002;Etiope et al, 2005;Giammanco et al, 2007;Voltattorni et al, 2009;Michel-lepierres et al, 2010).…”

“…Furthermore, radon measurements may avoid "false positives" (Elío et al, 2015). Besides, baseline Rn surveys can be useful for identifying permeable zones and pathways, which are considered as preferential sites from where CO 2 might leak from pressurized sequestration reservoirs (e.g., Walia et al, 2010).…”

Mapping the internal structures of fault zones of the sedimentary cover: a tectonophysical approach applied to interpret TDEM data (Kovykta gas condensate field)

“…Это объясняется тем, что подобные структуры являются путями миграции для природных газов, поднимающихся из глубоких горизонтов земной коры, к которым вблизи поверхности присоединяется и радон. В качестве такихкарьерных для радона -газов выделяются углерод, азот, водород, метан и другие [Etiope, Martinelli, 2002;Elío et al, 2015;Yuce et al, 2017;Kumar et al, 2018;Shuleikin, 2018;Yang et al, 2003].…”