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

International Journal of Greenhouse Gas Control 41 (2015) 1–10

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Evaluation of the applicability of four different radon measurement techniques for monitoring CO2 storage sites J. Elío a,∗ , M.F. Ortega a , B. Nisi b , L.F Mazadiego a , O. Vaselli c,d , J. Caballero a , L.S. Quindós-Poncela e , C. Sainz-Fernández e , J. Pous a,f a

Technical University of Madrid (UPM), Madrid, Spain CNR-IGG Institute of Geosciences and Earth Resources, Pisa, Italy c Department of Earth Sciences, Florence, Italy d CNR-IGG Institute of Geosciences and Earth Resources, Florence, Italy e Radon Group, University of Cantabria, Santander, Spain f Sacyr S.A., Madrid,Spain b

a r t i c l e

i n f o

Article history: Received 23 March 2015 Received in revised form 18 June 2015 Accepted 23 June 2015 Keywords: CO2 storage Monitoring Leakages Radon Natural analogues

a b s t r a c t Four different techniques for measuring radon activity in soil gases were analyzed in the natural analogue of Campo de Calatrava, Spain, to study its application in superficial monitoring of CO2 storage sites. The following methods were applied: (i) alpha spectroscopy monitors (SARAD, model RTM-2100), (ii) pulse ionization chamber detector (RADON v.o.s. model RM-2), (iii) solid-state nuclear track (CR-39) detectors and (iv) Lucas cell detectors (EDA model RD-200). Measurement protocols and selection criteria were defined according to the different phases of a CCS project, i.e., pre-injection, injection and post-injection. Radon measurements and CO2 fluxes were determined at 36 points around a natural CO2 leak. The concentrations of 222 Rn in the soil gases by ionization chamber detectors (RM-2) and alpha spectrometry monitors (SARAD RTM-2100) showed consistent data, whereas passive detectors (CR-39) did not produce realistic 222 Rn activity values although anomalous concentrations related to the CO2 leak were recognized. This implies that the latter can be used to create a control grid to estimate whether an increase of radon concentration due to a possible CO2 leak occurs. The Lucas cell (EDA RD-200) produced the greatest analytical uncertainty and, in the present configuration, this method appears the least appropriate for monitoring CO2 storage sites. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Emanometry and, in general, radiometric methods, have been used over recent decades in a large number of different scientific fields. The very first applications were restricted to the prospecting for uranium ore deposits (Sutton and Soonawala, 1975). Subsequently, they were used as excellent complementary tools for characterizing fractured media with the aim to evaluate potential underground repositories of medium to highly active radioactive wastes (Gates et al., 1990; Reimer 1992). Emanometry is also used for recognizing geothermal emanations (Cox, 1980), as geochemical monitoring tool of seismogenetic ˜ and Fernández, 1987) and in the study of fractured faults (Duenas

∗ Corresponding author. Fax: +34 913 366 948. E-mail address: [email protected] (J. Elío). http://dx.doi.org/10.1016/j.ijggc.2015.06.021 1750-5836/© 2015 Elsevier Ltd. All rights reserved.

media (Walia et al., 2010). Similarly, radiometric methods have also been used as indirect indicators in oil and/or gas accumulation areas (Klusman and Voorhees 1983; Morse et al., 1982; Mazadiego 1994) and for characterizing contaminated soils by hydrocarbons (Davis et al., 2002; García-González et al., 2008). Radon measurements at the soil-atmosphere interface are considered useful for detecting leakages in monitoring programs of geological storage of CO2 (Klusman, 2011), and they have been applied to some projects, for example IEA GHG Weyburn CO2 Monitoring and Storage Project (Riding and Rochelle, 2005; Strutt et al., 2003; Wilson and Monea, 2004) and the Frio Brine Pilot Project (Nance et al., 2005). Recently, investigations on natural analogues have proliferated to assess the relationship between CO2 and 222 Rn (radon) and 220 Rn (thoron) isotopes since CO2 (as well as CH4 ) 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-

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J. Elío et al. / International Journal of Greenhouse Gas Control 41 (2015) 1–10

Fig. 1. (A) Sketch map of the Campo de Calatrava Volcanic Field (modified after González-Cárdenas and Ubaldo González Rey, 2004). (B) Sampling grid near the CO2 bubbling pool.

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). In this regards, radon is a good tracer to discriminate leakage areas and can overcome the limitations of others geochemical methods; e.g., ı13 C–CO2 or helium isotope ratios. 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 CO2 might leak from pressurized sequestration reservoirs (e.g., Walia et al., 2010). Radon measurements are also easy to be performed, effective and low-cost. Furthermore, they have relatively small seasonal variations at a given location when compared to those of CO2 (Klusman 2011). All of these characteristics make of Rn an important parameter to be taken into account when monitoring CO2 injection sites. Several environmental (e.g., De Jong et al., 1994; Vaupotic et al., 2007; Klusman 1993) and instrumental (e.g., Ishikawa 2004; Papastefanou 2007) factors can affect the radon concentrations measured at the surface. Furthermore, a large number of different techniques dedicated to the measurement of radon in soil gases (e.g., Giammanco et al., 2009) is available. This therefore requires measurement protocols, which are expected to: (i) minimize the potential variations, (ii) compare different techniques for evaluating the actual differences among the different technical approaches; (iii) assess which one(s) can provide the best analytical response and best fits the specific objectives in monitoring activities related to CO2 geological storage programs, with particular reference to pre-, during and post-injection phases. The main goals of this paper are aimed to compare four radiometric methodologies for measuring radon in soil gases to validate the different techniques and propose the cheapest, most efficient and quickest methodology for surface radiometric monitoring in CO2 storage sites. It should be noted that a single analytical technique is not to be regarded as exclusive since the different phases of Carbon Capture and Storage (CCS) projects may require different approaches. Thus, a detailed investigation was carried out on which kind of radon measurement technique is able to furnish the best response (identifying “weaknesses” in the system), when it should be used (e.g., large scale campaigns, detailed campaigns), which other technique can be coupled with (e.g., diffuse CO2 soil flux, stable isotope analysis, water chemistry, and so forth), which is the cheapest and the quickest method to obtain information and determination of the analytical uncertainty associated to each methodological approach.

In this study, we focused on those techniques that may be more effective in application to CCS projects, i.e., active and continuous methods and integrated methods. The study of the different techniques was performed in the natural analogue of Campo de Calatrava (Ciudad Real, Spain). Natural analogues enable us to evaluate the behavior of soil gases and to validate leak detection technologies in a quick and straightforward manner. 2. Geological summary The Campo de Calatrava Volcanic Field (CCVF), located SE the province of Ciudad Real (Spain), within the district of Ciudad Real (Ancochea, 1983; Hernández-Pacheco, 1932 Fig. 1a), is characterized by a series of scattered vents and outcrops of lava flows and pyroclastic deposits of alkaline composition distributed in an area of about 4000 km2 within the Iberian Hercynian Massif, close to the external sectors of the Alpine Betic Cordilleras. The CCVF is an intracontinental plate consisting of a magmatic association of leucitites, melilitites, nephelinites and olivine basalts extruded during the late Miocene to Quaternary. Numerous CO2 -rich bubbling and dry gases and soda and Fe-rich thermal springs occur throughout ˜ the whole CCVF (Melero Cabanas, 2007; Piedrabuena, 1987; Vaselli et al., 2013). The geochemical composition of the discharged gases indicates a high concentration of CO2 (up to 98.5% by vol.), while the helium and carbon isotopic ratios suggest a magmatic source (Pérez et al., 1996; Vaselli et al., 2013). The fluid discharging sites are preferentially aligned along well-defined lineaments, i.e., NW-SE and NNW-SSE and subordinately, ENE-WSW. It is worth to mention that during the drilling of domestic wells gas blasts have occasionally occurred. These events are likely indicative of the presence of a geopressurized (CO2 -rich) reservoir (Vaselli et al., 2013). The region of CCVF, together with that of La Selva-Empordà (NE Spain), likely represents the largest area of CO2 discharge in Spain (e.g., Vaselli et al., 2013). Elío et al., (2015) recognized a correlation between CO2 leakage and radon concentration. The study area is characterized by the presence of a superficial clay layer, which likely regulates the release of diffuse CO2 soil fluxes, since strikingly high variations were observed just a few decimeters across (Elío et al., 2015). 3. Materials and methods The measurements of radon activity in the soil were conducted by alpha spectroscopy monitors (SARAD, model RTM-2100), pulse ionization chamber detectors (RADON v.o.s. model RM-2), Solid State Nuclear Track Detectors – SSNTD- (CR-39) and Lucas cell

J. Elío et al. / International Journal of Greenhouse Gas Control 41 (2015) 1–10

detectors (EDA model RD-200). All the measurement devices were calibrated in the laboratory of the Radon Group of the Cantabria University (Gutiérrez et al., 2012). Carbon dioxide soil fluxes were measured by the accumulation chamber technique (Chiodini et al., 1998; Elío et al., 2012 and references therein). Together with the emanometric measurements and CO2 fluxes, the following atmospheric variables were recorded: temperature, barometric pressure, wind speed and relative humidity. As a consequence, in the case of abrupt changes or sudden rainfall during the field campaign, the measurements were suspended since comparable measurements were not guaranteed. 3.1. Measurements of radon (222 Rn) 3.1.1. Solid-state nuclear track detector -SSNTD- (CR-39) Solid-state detectors or track-etch detector are made up of a sample of solid material (photographic emulsion, polymer, glass) sensitive to nuclear radiation. The basis of this method is that charged particles produce perturbations on the surface of the solid detector, in the form of nanometric traces. Thus, for a certain type of particle, the length of the trace produced on the detector demonstrates a direct relationship with its energy. In this project CR-39 detectors were used, which are photosensitive films made from allyl diglycol carbonate C12 H18 O7 ). This detector measures the exposure to radon due to its presence in the close surroundings. They have low sensitivity to intermittent changes in concentration. Thus, they integrate radon concentrations during the exposure time, minimizing the effects of wind or humidity, as well as those resulting from either thermal changes or different solar intensity (diurnal and nocturnal variations). In contrast, no instantaneous measurements can be provided, since in addition to the exposure time in the field (normally between 7 and 21 days for a standard radon concentration in the soil gas) a laboratory processing is required. Once exposed, the detectors were subjected to a developing process in the laboratory of the Radon Group of the Faculty of Medicine (University of Cantabria, Spain). The number of traces was read using an optical microscope (Sainz-Fernández et al., 2014). The developing procedure consists on positioning the film in a bath of a solution of distilled water with NaOH at 90 ◦ C for 4 h. As a control measure, three readings were taken at each detector under controlled laboratory conditions of temperature and humidity, attributing the average value (trace mm−2 ) as measurement of trace density. The exposure was determined by multiplying the trace density of each detector by a calibration coefficient and, on the basis of the number of days at which each detector was exposed, the average concentrations were calculated. The calibration coefficient was determined in the laboratory using a set of calibration detectors, which were subjected to different controlled exposures in a closed chamber (including very low background values). A 50 cm deep hole with a diameter of 6 cm was used for the temporary installation of each CR-39 detector (Fig. 2a). A PVC tube was inserted to protect against potential sideways sliding of the hole walls and to guide the detectors. Each tube was arranged in such a way that all of them were in equivalent positions to the vertical of the terrain. In order to reduce the risk from intense rainfall and prevent flow through the PVC tube to the atmosphere, the PVC tube was filled with paper and the upper opening was covered. The time set as standard exposure time was of 18 days. The saturation exposure of the detectors is critical since an optimization of the radon measurement is required. In the case of saturation, the tracks produced by the alpha particles would indeed not be counted under the microscope. In our case, the CR-39 saturation exposure was in the order of 8000–9000 kBq m−3 h. This means that for an exposure time of 18 days, the maximum integrated concentration

3

that could be measured is 20 kBq m−3 . It is noteworthy to point out that the concentration of radon in the soil tends to increase with the depth and the values can thus be considered representative of the area where they were located. 3.1.2. Lucas cells detectors (EDA RD-200) This apparatus uses a scintillation cell detector to measure the radon activity. The walls of each cell are covered with ZnS(Ag). When an alpha particle, originated from the disintegration of the radon progeny, impacts with this substance an ultraviolet photon is emitted. This energy is measured with a photomultiplier where it is converted into electrical impulses, which are measured in a count system. The advantage of this method is the rapid acquisition of the data. Furthermore, small sample quantities are required, minimum distortion in the soil gas is produced and the concentration of radon is measured in-situ. Nevertheless, the main disadvantage of this technique is related to interpretation of the data. In the first three minutes of detection the number of counts (equivalent to the number of the detected alpha particles) grows when the radiation is derived by 238 U, while a decrease after the first minute is observed if the radiation is due to 232 Th. This effect could be used to distinguish thoron (220 Rn) from radon (222 Rn) if three sequential one-minute long recordings are taken. When the recording from the first minute is equal or slightly less than the third minute, the value is associated with 222 Rn. If it is 2.5 times greater, it will be associated with 220 Rn. The radon measurements can be affected by the physical conditions of the soil and the atmospheric parameters. This implies that the radon measurements may not satisfactorily produce reproducible results. Other disadvantages are related to the detector, which can be saturated and give erroneous values, and the hardness of the terrain, as reliable measurements cannot be performed. The resulting measurements are also to be corrected in order to minimize the noise from the device. In this work we used a “mixed correction”, which gave the best results when the contrast between the peak values of anomalies and the regional base values was small. Specific formulas to apply this correction are available, e.g., the Morse formula (Mazadiego 1994; Llamas and Mazadiego, 2000) The “mixed correction” corresponds to the following equation:

M3C = M3 × (

M1 − B1 ) M1

where M3c is the corrected value of the counts at the third minute, M1 and M3 are the values of the counts at the first and third minute, and B1 is the value of the recording in the first minute without introducing the probe into the soil, i.e., the “blank”. Two procedures for measuring radon with EDA were followed, a 6 suction method (EDA-NC) and a continuous method (EDA-C). The EDA-NC procedure is, as follows (Fig. 2b): (a) drill a hole of approximately 40 cm deep, using a stake and manually striking the top, (b) introduce clean air into the device and measure the number of counts in the first minute (B1 ), (c) remove the stake driven into the ground and insert the hollow rod, which is connected to the scintillation chamber via PTFE tubes, (d) pump a fixed amount of soil gas, defined by the same number of (6) suctions using a manual pump, into the interior of the chamber and measure the number of counts resulting in the first (M1 ) and third minute (M3 ) and (e) apply the mixed correction to determine the value of the measurement at the selected point (M3c ). The EDA-C procedure is similar to the EDA-NC described above, in which air is constantly pumped while measuring B1 , M1 and M3 . It is worth to mention that an open Lucas Cell was used with the EDA RD-200, unlike those commonly used for this device. This makes the sampling procedure quicker since the Lucas Cell does

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Fig. 2. Measurement procedures: (a) Passive Track Etch detectors (CR-39); (b) Lucas cell detectors (EDA RD-200); (c) Alfa spectroscopy monitors (SARAD RTM-2100); (d) Pulse ionization chamber detectors (RM-2).

not need to be pre-evacuated, and the soil gas sample can directly be transferred to the cell itself. However, the sensitivity of the instrument is lowered since external air could enter the cell and the number of counts, i.e., the number of disintegrations produced inside the Lucas Cell during the period of measurement, cannot be converted into kBq m−3 . 3.1.3. Alpha spectroscopy monitors (SARAD RTM-2100) With this device, the concentration of radon is measured by using progeny radionuclides originating from the decay of radon (218 Po/214 Po). These isotopes are ionized and collected on the surface of a semiconductor detector in a way that the radioactive decomposition of the isotopes can be recorded in a multichannel analyzer (alpha spectrometry), and it can be transformed into radon concentration (Bq m−3 ). The device only allows the radon gas (222 Rn and 220 Rn) to enter the detection chamber. The 222 Rn isotope, during the disintegration process, produces alpha emissions and the daughter 218 Po is attracted by the surface of the negatively charged detector. The progeny nuclides are also radioactive and decompose emitting alpha radiation, which is also measured by the device. The nuclide 214 Po originating from the disintegration of 218 Po can be used to measure more accurately the concentration of radon. If only 218 Po were taken into account, the time necessary to measure the radon concentration would be of 10–15 min. Using 218 Po and 214 Po the measure time is up to 150 min, although it could be reduced to 60 min by applying a non-linear model to extrapolate the equilibrium Rn concentration in soil air (García-González et al., 2008). The measurement protocol (Fig. 2c) to determine radon in the field consists in inserting a 120 cm long hollow steel probe into the ground by striking it on the top, with a steel arrow tip on the bottom, up to a depth of 75–100 cm in order to minimize the influence of atmospheric factors. Once the desired depth is achieved, a solid rod is slid through the space inside the hollow probe, then struck to release the lower arrowhead and to enable the aspiration of the soil gas (García-González et al., 2008). If necessary, to drive the steel probe inside the soil an electric hammer can be used to overcome the presence of pebbles or rock boulders. The device, equipped with an internal pump, is connected to the hollow probe as the sampling is performed. During the measure-

ment, the soil gas passes through a tube containing drierite (97% CaSO4 + 3% CoCl2 ) and a Teflon® hydrophone filter to maintain the humidity contents inside the instrument at acceptably low levels and to remove the presence of particulate material, respectively. The measurements of 222 Rn are acquired by setting the device in Thoron mode with integration times of 1 min and recording duration of 20 min. The time that has to elapse in order to consider the measurement of 222 Rn as the value of the “radon” measured by the device is 15 min. The instrumental detection limit is 1 kBq m−3 , √ and the standard error (1␴) is defined as 100%/ N, where N is the number of counts (disintegrations) of the recorded radionuclide (218 Po/214 Po) during the integration time. For a concentration of 1 kBq m−3 of radon, or greater, the error is lower than 10%. After each measurement the internal pump switches to high flow mode. To clean the detection chamber before analyzing the next location air is pumped through the instrument for 20–30 min. Whenever possible, attempts to measure Rn in the soil gas with a low to high concentration sequence was carried out to shorten the cleaning time. 3.1.4. Pulse ionization chamber detectors (RM-2) These devices consist of an ionization chamber where a potential difference is applied between the positively charged, metallic exterior and the electrode situated along the longitudinal axis (0 V). When the soil gas is introduced into the chamber, the internal air is ionized by the emission of alpha particles, which are generated by 222 Rn, thoron and progeny. Thus, the ions are moved towards the electrodes of opposite polarity. The resulting ionization current is measured and converted into radon concentration (Bq m−3 ). In this work a “Radon detector RM-2” by RADON v.o.s. corporation was used. It is robust with individual low cost chambers and sufficient sensitivity to measure radon in the soil gas. The sampling procedure is quick and straightforward. The disadvantage with respect to alpha spectrometry monitors is that 222 Rn and 220 Rn cannot be distinguished. The soil gas sampling is carried out with a hollow probe inserted into the ground at 75–100 cm depth. Then, the arrowhead is removed and a 150 mL syringe connected to the hollow probe to pump the internal air away. Finally, 150 mL of soil gas are introduced into the evacuated ionization chamber (250 mL). Since the

J. Elío et al. / International Journal of Greenhouse Gas Control 41 (2015) 1–10

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Table 1 Results of radon activity in soil gas and CO2 flux obtained by using the four different techniques. ID

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

EDA NC [count]

EDA C [count]

45.00 57.78 101.92 113.06 79.06 115.99 94.84 73.52 99.24 66.84 58.45 99.54 81.09 126.43 90.00 35.61 46.00 95.40 64.54 126.55 44.29 77.88 83.42 47.77 82.69 80.79 209.43 56.00 19.69 58.43 75.79 72.39 83.72 99.29 91.67 77.00

123.00 233.23 326.39 245.70 369.61 238.78 553.85 808.82 204.00 136.81 112.64 320.90 304.01 373.01 264.70 195.44 518.43 236.06 425.99 1186.49 252.46 527.51 309.52 141.02 212.67 604.66 346.41 600.40 587.87 180.95 232.50 163.10 261.02 341.11 352.03 516.20

222

SARAD Rn [kBq m−3 ]

SARAD 220 Rn [kBq m−3 ]

NF

NF

NF 73.9 29.0 NF NF

NF 27.0 9.0 NF NF

29.0 29.0 NF 11.6 46.0 NF <2.4 18.0 NF 22.0 32.0 195.0 89.5 NF 26.1 NF NF 12.9 173.1 NF NF NF NF NF

28.5 15.5 NF 17.8 10.5 NF <0.8 11.2 NF 12.3 30.8 35.2 31.6 NF 19.4 NF NF 24.0 18.4 NF NF NF NF NF

15.3 NF NF

13.9 NF NF

volume of the syringe is smaller than that of the ionization chamber and to ensure that the pressure in the chamber equalizes to that of the atmospheric pressure, external air was allowed to enter into the chamber. Similarly to the procedure used for the alpha spectrometry the humidity and particulate material were removed by Drierite and a particle filter, respectively. The radon measurement requires two minutes, although 15 min of delay after introducing the soil gas into the chamber were required to ensure that the thoron activity was negligible due to its short half-life (51.50 s). Therefore, only the 222 Rn activity is measured. The detection limit is 5 kBq m−3 , while the uncertainty of radon concentration (1␴) is 0.33 (Crn )0.5 , where Crn is the concentration of radon (Crn ± ␴). Thus, the error of radon measurement using the RM-2 instrument is below 15%.

RM-2 Rn [kBq m−3 ]

222

NF 32.4 NF 92.3 29.3 NF NF 28.2 20.1 22.3 NF 12.9 48.1 NF 49.7 19.3 NF 23.0 35.7 170.0 92.7 NF 22.4 NF NF 13.5 194.0 NF NF NF NF NF 7.8 19.2 NF NF

TRACK ETCH Rn [kBq m−3 ]

222

5.8 1.7

5.1

5.2 13.7 3.3

6.0 6.2 Saturated(>20.0) 15.6 3.1

10.6 Saturated (>20.0) 3.5

2.7 5.8 7.1

CO2 Flux [g m−2 d−1 ] 0.36 1.36 1.13 43.67 5.34 2.93 2.47 21.85 12.27 −1.10 (
the flux of CO2 , which is released from the soil. The CO2 flux in g m−2 d−1 is calculated, as follows: ˚(g × m−2 × day−1 ) =

dC V 86400 P × PM × × × A 1000 R×T dt

where dC/dt [ppm s−1 ] is the gradient of the curve of concentration versus time, V [m3 ] the net volume of the chamber (including the volumes of the sensor, pump and connection tubes), A [m2 ] the area of the accumulation chamber, P [bar] the atmospheric pressure, PM the molecular weight of the gas (for CO2 = 44 g mol−1 ), R the constant of the ideal gases (0.08314510 bar l K−1 mol−1 ) and T [K] the air temperature. The device used to determine the flux is manufactured by the company West Systems (WS-LI820) and is equipped with a LICOR-820 brand infrared detector (West Systems 2009). 4. Results

3.2. Measuring CO2 flux Diffuse CO2 soil fluxes were determined by the accumulation chamber method (e.g., Chiodini et al., 1998; Cardellini et al., 2003). The lower part of an open chamber is placed in direct contact with the soil, previously cleaned following the process described in Elío et al. (2012), allowing the accumulation of the soil gas inside the chamber itself. The soil gas is transferred with a pump to an infrared detector, which measures the concentration of carbon dioxide and then, is re-injected into the accumulation chamber to minimize the disturbance induced by the pumping effect. The time-related change in CO2 concentration inside the chamber can be related to

• In total, thirty-six points, distributed on a regular grid (20 m spacing) and close to a CO2 -bubbling pool, were analyzed (Fig. 1b). The radon data obtained using the four different radiometric methods and the CO2 flux values are listed in Table 1. The presence of a clay-rich layer, up to a depth of at least 50–70 cm, did not allow to collect any soil gas sample from 17 points. Such a layer acted as an impervious barrier, which prevented the gas release. These points are indicated as NF (No flow) in Table 1 and were characterized by the lowest values of CO2 flux in the study area. • The CO2 fluxes were from −2.10 (
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Table 2 Pearson correlation coefficients calculated for the four different radiometric techniques. Radimetric methods

R2

RM 2–SARAD EDA RD200 – RM 2 EDA RD200 – SARAD Track Etch-RM2 Track Etch-SARAD Track Etch- EDA RD-200 EDA RD-200 (“EDA-NC”) – EDA RD-200 (“EDA C”)

0.9814 0.4832* 0.4182* 0.185 0.163 0,066* 0.0220

(44.65 g m−2 d−1 ) and 4 (43.67 g m−2 d−1 ) also have high CO2 flux values and are possibly be related to a deep source. The measured CO2 flux in the remaining points was between −2.10 (20 kBq m−3 , the latter being the maximum measurable value using these detectors when left for an exposure time of 18 days. • The 222 Rn data by EDA are expressed in total number of counts in the third minute (M3c ) and therefore, a qualitative observation related to presence of 222 Rn was applied. Count readings were varying from 112 to 1,186 with the continuous method (EDA-C) and from 35.61 to 209.43 with the 6 suctions method (EDA-NC). 5. Discussion 5.1. Comparison of devices The linear models of the radon data obtained using each technique are shown in Table 2. A significant correlation for the 222 Rn measurements is only achieved when the RM-2 and SARAD RTM2100 devices are used. In this case the result of the linear model is a straight line, with a slope coefficient close to 1 and R2 of 0.98 (Table 2 and Fig. 3; level of significance of 0.01). Consequently, the

RM-2 and the SARAD RTM-2100 are equally reliable for 222 Rn measurements. The choice between them depends on different factors, e.g., length of the measurements, number of personnel involved in the sampling, financial cost or the need to also measure 220 Rn. The other techniques do not show any linear relationship (Table 2). For a spatial interpretation of the results, the data were plotted according to the quantile ranges (Fig. 4). As shown in Fig. 4, the same anomalous areas are indeed detected, although no linear relationship is observed between each technique. The maximum and minimum values with the RM-2 and SARAD RTM-2100 techniques coincide in the same sample points, except for point 15 for which the results are 48.1 kBq m−3 and <2.4 kBq m−3 , respectively. The discrepancy at this point is likely due to a problem of air flow from the soil during sampling, the low permeability recorded in some points suggests that the flow required for collecting the gas samples is insufficient, or even absent, resulting in erroneous measurements. This effect is more evident with the SARAD instrumentation due to the continuous pumping for 20 min. The measurement at point 15 using the SARAD is indeed close to the detection limit of the instrument, therefore it was not considered in the interpretation of the results and it was labeled as “no flow” point. In the case of EDA-200, no linear relationship between the two methodologies (EDA-C and EDA-NC) was recorded. The counting values observed with the continuous methodology (EDA-C) were higher than in 6 suctions method (EDA-NC). Also the values range was highest with EDA-C. Additionally, no spatial relationship was observed between the EDA measurements (regardless of the method) and those by RM-2 or SARAD (Fig. 4, Table 1). One of the main consequences of these discrepancies among the different methods (EDA-C and EDA-NC) is the need to establish a new correction formula adapted to the values expected in CCS projects. In CCS small variations in the radon activity relative to the baseline have to be detected to confirm the existence of possible leakages. 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

Fig. 3. Relationship between radon measurement with SARAD and RM-2: (a) linear fitting and (b) boxplot diagram.

J. Elío et al. / International Journal of Greenhouse Gas Control 41 (2015) 1–10

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Fig. 4. Spatial relationship of the radiometric measurements.

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. As previously mentioned, a loss in the sensitivity of the EDA RD200 was produced for the use of an open Lucas Cell. Nevertheless, a lower sensitivity is negligible in the monitoring of medium to highly radioactive waste storage sites or in uranium prospecting surveys. In these cases, if a leakage occurs it can be expected that the radon values can be of orders of magnitude higher than the

baseline and, therefore, easily detectable. Vice versa, in the case of monitoring of CCS projects, this can be a limiting factor since for early detection of a CO2 leakage small variations in radon activity relative to the baseline are expected. Nevertheless, it appears to be useful to improve this technique as a screening method since radon data can quickly be produced. The radon values by passive detectors (Track Etch; TE) were clearly lower than those obtained using SARAD/RM-2 (Table 1). In this respect, the SARAD/RM-2 measurement is interpreted as being

Medium Yes No No Yes No Yes Yes Yes

Easy

Medium

Low

High

1

No No No No No No No Yes

Yes Yes No Yes Yes No Yes Yes No Yes No No Yes Yes No Yes Yes No No No Yes 2 2 2 Easy Easy Easy

RM-2 SARAD EDA RD 200 (Continuo) EDA RD 200 (No Continuo) TRACK ETCH

Method

High Slow Very high Very high Slow

Low Low Low

1

Good Good No Yes Yes No

Correlation with high CO2 flux values Correlation with low CO2 flux values Applicability to densification campaigns (detect pathways for gas leaks) Applicability to baseline campaigns Applicability Applicability Applicability to pre-campaigns to identify to detect hot spots spatial (general trends recognition) Affected by atmospheric conditions Manageability Number of operators required Measuring Financial speeds cost Properties

Table 3 Applicability of the four different radiometric techniques in monitoring surveys of CO2 storage sites.

No

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Overall correlation with CO2 flux

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the most representative of radon activity since a specific amount of soil gas is pumped, whereas Track Etch measurements represent average concentrations in the soil during the exposure period in the hollow drilled in the ground. Thus, the radon values are likely more related to exhalation, understood as the quantity of radon that escapes through the walls of the hollow and therefore, lower concentrations were recorded due to the low permeability of the soil. In the points where high CO2 fluxes were measured, the total radon activity is the sum of exhalation and that carried by CO2 . The influence of atmospheric conditions creates more uncertainty in the results of Track-Etch than with the other techniques. First, the Track-Etch detectors were located only at 50 cm depth, where the influence of the atmospheric conditions can be high (e.g., King and Minissale, 1994). Secondly, the walls of the PVC tube installed to protect the hollow could act as a preferential pathway for the release of gas, thus increasing the influence due to the atmospheric variations. These effects enhance the variability of the radon measurements by the Track-Etch detectors. As far as the spatial relationship is concerned, the detectors at points 20 and 27, where the highest radon values by SARAD and RM2 were measured, saturated (Rn > 20 kBq m−3 ). They represented the highest radon activities obtained with these methods (saturation exposure and exposure time of 18 days). A high value at 21 by Track-Etch was also measured (15.6 kBq m−3 ), although it was 6 times less than those recorded by SARAD and RM-2 (89.5 and 92.7 kBq m−3 ; respectively). Radon activities of 13.7 kBq m−3 were also measured at 13 by Track-Etch whereas SARAD and RM-2 produced values of 46.0 and 48.1 kBq m−3 , respectively. Eventually, medium (5.8 kBq m−3 ) and high (73.9 and 92.3 kBq m−3 ) values were measured by Track-Etch, SARAD and RM-2, respectively. Contradictory data occurred at 26 and 35. In the former, the radon concentration by SARAD and RM-2 was relatively low: 12.9 and 13.5 kBq m−3 , respectively, while one of the highest measurements was obtained by Track-Etch: 10.6 kBq m−3 . In the latter, the absence of gas flow did not produce satisfactory results with SARAD and RM-2, whereas 7.1 kBq m−3 were measured with the TrackEtch method. Nevertheless, our results appear to prove that unless the radon concentrations between Track-Etch and SARAD/RM-2 are not comparable in magnitude, the area of radon anomaly was detected close to the hot spring where a CO2 -rich gas phase is bubbling, indicating the complementarities between these techniques. Another important fact to be considered when using the TrackEtch method is the estimation of exposure time in the ground. In the CCVF, once the measurements were evaluated it can be considered that the estimated exposure time of 18 days was rather high, with several detectors becoming saturated. Low exposure times should be considered at these points and the measurement correlation with other techniques might be improved. This would require a pre-assessment survey to adjust the exposure time to the specific field conditions. The concentration of radon in the soil gas is depending on the depth at which the soil gas is collected. The lesser the depth the higher the influence of the atmospheric variables; this increases the uncertainty. In our case, the radon measurements were not carried at the same depth for operational reasons (Track-Etch: 50–60 cm, EDA: 50–60 cm, SARAD: 75–100 cm, RM-2: 75–100 cm; Fig. 2). This could affect the interpretation of the correlation among the different radon measurements. However, the main differences between the Track-Etch and EDA methods and between them and SARAD and RM-2, lead one to believe that this depth effect may be negligible relative to the difference in methodology and the measurement system. Nevertheless, it seems convenient to increase the sample depth up to 75–100 cm for the Track-Etch detectors. However, the loss of time necessary to increase the sample depth with EDA detector does not justify its application.

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Table 4 Advantages and disadvantages of the four different radiometric techniques. Method

Advantages

Disadvantages

SARAD RTM-2010

- Can measure 222 Rn and 220 Rn - In-situ measurements, allow adaptive sampling - Representative measures of the system

- Takes time to measure and purge (about 0.5–1 h per measure) - Requires at least two people for measuring - Intermittent leaks could be not detected

RM-2

- Fast (15 min per measure), able to measure various point at the same time - In-situ measurements, allow adaptive sampling - Representative measures of the system

- Only measure 222 Rn - Requires at least two people for measuring - Intermittent leaks could be not detected

EDA RD-200

- Fast (15 min per measure) - In-situ measurements, allow adaptive sampling - One person can use the apparatus

- Inaccurate - Quantitative measurements (counts) - Difficult to differentiate between 222 Rn and 220 Rn, some equipment could perform a mathematical estimation - Intermittent leaks could be not detected

CR-39 passive detectors (TRACK ETCH® )

- Integrate radon concentration during the exposition time, minimizing atmospheric variation influence - Intermittent leaks could be detected - fast once the sampling grid has been installed, a large number of detectors can be installed in one day

- High financial cost - Two field campaigns are needed (one to install and other to collect the detectors after 7–21 days) - Laboratory treatment is required to count the number of traces - Do not allow adaptive sampling - Do not differentiate between 222 Rn and 220 Rn

5.2. Applicability of radiometric techniques in CCS According to the investigations carried out in the CCVF natural analogue, the applicability of the different radiometric techniques for the detection of CO2 leakages is presented in Table 3, while in Table 4 the advantages and disadvantages of each technique applied in this work are reported. These tables allow to compare the four different methodological approaches and to facilitate their selection on the basis of specific monitoring objectives related to the geological storage of CO2 . The possible application of the different techniques analyzed here could be implemented in three phases:

• Preliminary phase; in this phase inexpensive and quick radiometric techniques can be applied. The objective is to analyze soil gas samples over the entire monitoring area in a short time span. This is to provide a first qualitative view of the radon values, and recognize were the highest radon activities are concentrated. The RM-2 technique is likely the most suitable for this purpose. • Research or study phase; in this phase, the areas of interest identified in the preliminary phase shall be characterized, increasing the density of the radon measurements. The objective is that to quantify the concentration of radon in the soil, delimiting weakness and high permeability areas, e.g., faults. The baseline is to be obtained before injection. In this phase, ionization chamber (RM-2) and alpha spectrometry techniques (SARAD) can be used. The latter, though not quick, has the advantage to distinguish between 222 Rn and 220 Rn contents. Therefore, the deployment of SARAD instrumentation at selected (with high to low Rn values) locations to measure 220 Rn and 222 Rn could be used to obtain a qualitative indication about the depth at which the carrier gas transporting radon was formed. Finally, the use of Track Etch could be introduced for key points, where for example radon shows the lowest and highest values. Thus, a permanent sampling grid can be generated. • Monitoring phase: the aim of this phase is to identify possible variations in the radon concentrations relative to the baseline measurements. The main applicable methods are the pulse ionization chambers (RM-2) and the alpha spectrometry chamber (SARAD RTM-2100). Passive detectors (Track-Etch) could also be useful, since they integrate the radon concentration according

to the exposure time such that intermittent leakages could be identified.

6. Conclusions • The natural analogue of Campo de Calatrava has provided a good scenario for testing different radiometric technique to be applied as monitoring tools in CCS. Radon measurement protocols were established. Furthermore, a selection criterion was defined according to the different phase of the project: pre-injection, injection and post-injection. • The most reliable 222 Rn measurements were obtained by using the ionization chamber detectors (RM-2) and alpha spectrometry monitors (SARAD RTM-2100) as no significant differences between these two methods were observed. RM-2 however cannot be applied to measure 220 Rn and therefore, if necessary, thoron can only be analyzed by alpha spectrometry monitors (such as SARAD). • The result of the 222 Rn measurement using passive detectors (CR39) did not coincide with the more realistic values measured by either SARAD or RM-2. However anomalous values related to the CO2 leak were detected, suggesting that they cannot be used to quantify the 222 Rn contents but they can be useful to create control grids and estimate whether an increase of radon concentration due to a possible CO2 leak occurs. Another advantage is that this method is relatively fast once the sampling grid has been prepared, since a large number of detectors can be installed in one day. Passive detectors integrate the signal during the exposure time, which could enable to detect intermittent leaks of CO2 . However, it is not possible to conduct an adaptive sampling and this technique is relatively expensive. • The Lucas cell (EDA RD-200) technique has provided the highest uncertainty in terms of Radon concentrations by using either the continuous measurement (EDA-C) or the 6 suctions (EDA-NC) mode. Therefore, this instrumental approach appears to be the least appropriate for monitoring geological storage of CO2 . This is likely due to the device configuration, which has been optimized for uranium prospecting and detecting leakages from geological reservoir were radioactive wastes have been stored. Different configurations may produce more reliable data, although the measurements would likely be slower. However, the data acquisition is relatively fast as it allows to measure a large number

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