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Fiber refractometer to detect and distinguish carbon dioxide and methane leakage in the deep ocean
International Journal of Greenhouse Gas Control 31 (2014) 41–47
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Fiber refractometer to detect and distinguish carbon dioxide and methane leakage in the deep ocean Geoff Burton a,∗ , Luis Melo a , Stephen Warwick a , Martin Jun a , Bo Bao b , David Sinton b , Peter Wild a a
Department of Mechanical Engineering, Institute for Integrated Energy Systems, University of Victoria, Victoria, BC, Canada V8W 3P6 Department of Mechanical and Industrial Engineering, Institute for Sustainable Energy, University of Toronto, 5 King’s College Road, Toronto, ON, Canada M5S 3G8 b
a r t i c l e
i n f o
Article history: Received 8 August 2014 Received in revised form 11 September 2014 Accepted 15 September 2014 Keywords: Carbon dioxide Methane Fiber optic tip Refractometer Fresnel reflection Sequestration
a b s t r a c t Deep ocean injection of carbon dioxide into subsea geologic formations carries a risk of carbon dioxide leakage as well as leakage of methane from petroleum bearing formations that injection wells may penetrate. Therefore, leakage monitoring technology for this application must detect carbon dioxide and distinguish it from methane. Here we demonstrate an all-optical approach to detect and differentiate between liquid carbon dioxide and supercritical methane bubbles in synthetic seawater at 9.65 MPa. This method employs fiber tip refractometry, a fiber optic measurement technique that is sensitive to the refractive index of the surrounding medium. Carbon dioxide and methane bubbles are clearly detected as they pass the sensor tip and these species are clearly distinguished from each other. Interferometric signals are also observed in association with the transition of bubbles onto and off of the sensor tip. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Deep ocean injection of carbon dioxide into subsea geologic formations, such as is planned for the Santos Basin offshore of Brazil (Melo et al., 2011), carries a risk of carbon dioxide leakage during and after injection. At these sites, there is also a risk of methane leakage from the petroleum bearing formations that injection wells may penetrate. Therefore, leakage monitoring technology for this application must both detect carbon dioxide and distinguish it from methane. Monitoring carbon dioxide storage has been demonstrated at the Sleipner injection site located in the North Sea (Furre and Eiken, 2014; Chadwick et al., 2006). In this pilot test, seismic and gravimetric sensors were used to monitor a subsurface plume of carbon dioxide with a limit of detection of 500 t of carbon dioxide (Chadwick et al., 2006). Methods have also been demonstrated to detect carbon dioxide rising as a stream of bubbles or droplets in the water column. Acoustic tomography has been used to detect such a rising stream based on the difference in the speed of sound propagating in different media (i.e. water and carbon dioxide) (Brewer et al., 2006).
∗ Corresponding author. Tel.: +12507216295; fax: +12507216323. E-mail address: [email protected] (G. Burton). http://dx.doi.org/10.1016/j.ijggc.2014.09.015 1750-5836/© 2014 Elsevier Ltd. All rights reserved.
Brewer et al. (2002) demonstrated detection of a rising carbon dioxide stream using a camera mounted to a remotely operated vehicle. Brewer also used pH and conductivity sensors to detect carbon dioxide enriched seawater adjacent to the rising carbon dioxide stream (Brewer et al., 2005). Similarly, Shitashima et al. (2013) developed pH/pCO2 (partial pressure of carbon dioxide) sensors which were installed in an autonomous underwater vehicle for detection of carbon dioxide leakage. An alternative method to monitor carbon dioxide leakage from subsea injection sites is based on refractive index measurements. The refractive index (RI) of a liquid or gas varies with species, temperature, pressure and, in the case of solutions and mixtures, concentration. As a result, RI measurement is a broadly applicable method to determine the values of these properties. In recent years, a range of fiber optic methods to determine RI has emerged, including grating based methods (Fang et al., 2010; James and Tatam, 2003), interferometric methods (Wang and Tang, Jan 2012) and intensity based methods (Vurek et al., 1983; Munkholm et al., 1988; Cartellier, 1992; Goyet et al., 1992; Degrandpre, 1993; Hamad et al., 1997; Cartellier and Barrau, 1998; Neurauter et al., 2000; Chang et al., 2002; Chang et al., 2003; Ertekin et al., 2003; Fortunati et al., 2003; Avdeev et al., 2004; Kim and Su, 2004; Enrique Juliá et al., 2005; Lee et al., 2007; Zhao et al., 2009; Prada et al., 2011; Wang and Tang, Jan 2012; Xu et al., 2013). Fiber optic methods allow the sensing element to be located at a distance from
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the interrogation system and, as a result, sensing of RI in environments where conventional methods are difficult to apply (e.g. high pressure, high temperature). For measurements that require high accuracy, gratings used in conjunction with fine resolution wavelength interrogation is preferred. However, for applications in which the objective is to discriminate between different substances or between different states or phases of the same substance which have distinctly different values of RI, an intensity-based method, fiber tip refractometry (FTR) offers a relatively simple and low cost alternative. The basis for FTR is Fresnel reflection. When light exits a medium with RI, n1 , and enters another with RI, n2 , the intensity of light reflected back into the first medium is dependent on the refractive indices of both media, as shown in Eq. (1). Hence, if n1 is known, then the intensity of the reflected light, I, is a direct measure of n2 . I=
(n1 − n2 )2 (n1 + n2 )2
(1)
An FTR comprises the distal end of an optical fiber that is immersed in a target medium. Light propagating in the fiber undergoes Fresnel reflection at the interface between the fiber tip and the target medium. The intensity of this reflected signal is as defined above for the general case of Fresnel reflection with n1 and n2 being the RIs of the fiber and the target medium. FTR has been applied by a number of researchers to measure RI of various fluids under static (i.e. no flow) conditions. For example, Xu et al., 2013) measured the RI of (i.e. benzene, ethanol, methanol, acetone glycerol) with a broadband light source centered at 1550 nm. Kim and Su (2004) performed similar experiments but measured RI at different wavelengths. Other studies present the use of FTR to measure salinity of aqueous solutions (Chang et al., 2002; Zhao et al., 2009). Two studies have been identified in which FTR has been used for static RI measurements of carbon dioxide at high pressures. Avdeev et al. (2004) measured the RI of carbon dioxide in the gas, liquid and super critical states as well as mixtures of carbon dioxide and organic solvents. Prada et al. (2011) measured the RI of liquid carbon dioxide at pressures up to 5.5 MPa under static conditions. No FTR-based RI measurement of methane at elevated pressures, its solutions or mixtures have been identified. There are several examples of FTR-based sensors that make use of either fluorescent or colorimetric coatings that react with carbon dioxide (Degrandpre, 1993; Ertekin et al., 2003; Goyet et al., 1992; Munkholm et al., 1988; Neurauter et al., 2000; Vurek et al., 1983). These methods have been demonstrated in aqueous solutions at atmospheric pressures to detect dissolved carbon dioxide rather than distinct phases. These methods have not been applied at high pressure Interrogation of FTR systems is typically intensity-based. As a result, these systems offer fast response times, relative to gratingbased systems, which typically depend upon wavelength-based interrogation. As a result, FTR has been used by a number of researchers to make dynamic measurements in two-phase flows (Hamad et al., 1997; Fortunati et al., 2003; Enrique Juliá et al., 2005; Chang et al., 2003; Lee et al., 2007; Cartellier, 1992; Cartellier and Barrau, 1998). Hamad et al. (1997) used FTR to distinguish kerosene droplets from water in two-phase pipe flow. Kerosene droplets were entrained in the flow and the optical probe was used to detect the passage of droplets. In another study, the size and velocity of air bubbles in pipe flow of water were measured with a four-point FTR device by Guet et al. (Fortunati et al., 2003). This system provided higher accuracy velocity measurements than a single point system. Chang et al. (2003), Enrique Juliá et al. (2005) and Lee et al. (2007) all used FTR to measure void fraction (i.e. bubble fraction) in entrained bubble flow. Cartellier et al. (Chang et al., 2003; Lee et al.,
2007) did extensive work on velocity measurement in entrained bubble flow in an air-water system. In most previous FTR studies in flowing liquids or gases, the fiber orientation is parallel to the flow (Hamad et al., 1997; Fortunati et al., 2003; Enrique Juliá et al., 2005; Chang et al., 2003; Lee et al., 2007; Cartellier, 1992; Cartellier and Barrau, 1998). Only one study was identified in which an effective FTR probe was mounted normal to the direction of flow (Rojas and Loewen, 2007). It is important to note, however, that this study focused on high speed, turbulent, aerated flows so the bubble direction was not necessarily normal to the fiber tip. In the current study, an FTR probe is mounted normal to the flow and flush with a pipe wall, to monitor the passage of liquid carbon dioxide and supercritical methane bubbles in synthetic seawater at 9.65 MPa (1400 psi) and 20 ◦ C. Throughout this paper, the term bubble is used to refer to a small and distinct volume of liquid or supercritical fluid and all pressures are gauge pressures. These experiments simulate the use of FTR to monitor leakage during and after deep (i.e. 950 m) ocean injection of carbon dioxide into subsea geologic formations. 2. Materials and methods 2.1. Sensor The optical fiber used in this work is SMF 28e (Corning, USA). A length of this fiber is mounted in a standard FC/PC zirconia ferrule (F12070, Fiber Instrument Sales) and secured with epoxy (Thorlabs, F120) (see Fig. 1). The ferrule and fiber are epoxied to one end of a stainless steel tube with outer diameter 1/8 (3.13 mm) and wall thickness 0.028 (0.71 mm). The distal end of the fiber is cleaved normal to the fiber axis and is mounted in the ferrule so that the
Fig. 1. Schematic of sensor tip with cutaway at the distal end.
Fig. 2. Schematic of flow cell and sensor tip.
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Fig. 3. Schematic of the high pressure test system. Seawater, methane and CO2 are directed from their respective sources through the flow cell and out the relief valve at 9.65 MPa.
distal surface of the fiber is slightly proud of the end of the ferrule. The fiber tip is then polished flush with the ferrule end. This assembly is fixtured in a compression fitting (SwagelokTM , SS-200-1-2BT) which is mounted to the flow cell, as shown in Fig. 2. 2.2. Flow cell The flow cell is comprised of a vertically-oriented glass throat (0.125 in., 3.13 mm inside diameter) housed in an aluminum body (see Fig. 2). The sensor is aligned normal to the axis of the throat and the tip of the sensor is set in a hole in the wall of the throat such that the distal surface of the sensor is approximately flush with the inner surface of the throat. The flow cell includes a sight glass which enables viewing of the throat in the region of the sensor tip.
2.4. Interrogation Two optical interrogation systems are used. The first system (Fig. 4(a)), consists of a broadband light source with wavelength range 1528 nm to 1570 nm (BBS) (BBS1550, AFC Technologies) coupled to an optical splitter (Blue Road Research, 35S). The splitter is coupled to the sensor and to an optical diode with range 1280 nm to 1580 nm (410 WDM, Eigenlight). The optical diode measures the reflected light from the tip and converts this to an analogue voltage
2.3. High pressure test system The flow cell is integrated into a high pressure test system, shown in Fig. 3. A metering pump (EldexTM , USA) pressurizes the synthetic seawater and pumps it through the flow cell at a constant flow rate. The synthetic seawater, (hereafter referred to as seawater) comprises 24.72 g sodium chloride, 2.18 g magnesium chloride, 1.03 g calcium chloride, 0.67 g potassium chloride and 3.07 g magnesium sulfate per 1 L deionized water, as per Marine Biological Laboratory specifications (Möller et al., 2006). Methane and carbon dioxide are released into the inlet chamber from high pressure tanks using needle valves (valves 1 and 2). The carbon dioxide tank is pressurized to 9.79 MPa (1420 psi) with a helium headspace. This tank is fitted with a liquid withdrawal siphon to ensure carbon dioxide is withdrawn. The methane tank is fitted with a high pressure regulator through which supercritical methane is withdrawn at 9.79 MPa (1420 psi). The high pressure relief valve (SS-43RA-BU, SwagelokTM ) is set to open at 9.65 MPa (1400 psi) and reseal at 9.48 MPa (1375 psi). This valve allows fluid to exit the system as seawater, methane or carbon dioxide is introduced while maintaining relatively constant pressure (i.e. 9.65 ± 0.070 MPa). The system pressure was set to 9.65 MPa (950 m depth) because, at this pressure, carbon dioxide is a liquid and is positively buoyant in seawater. Carbon dioxide is a liquid at depths greater than approximately 500 m (depending on the water temperature), and is neutrally buoyant at approximately 2700 m (Aya et al., 1999). Methane exists as a gas or supercritical fluid for the entire range of pressures and temperatures in the ocean.
Fig. 4. Schematic of (a) broadband light source interrogation system and (b) SmartScan interrogation system.
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which is read by the data acquisition module (USB6210, National Instruments) in the PC. The BBS system acquires high resolution intensity data but no spectral information. The second interrogation system (Fig. 4(b)) consists of a SmartScan 04 (Smart Fibres, UK) connected to the sensor. The SmartScan unit was used to obtain spectral data over the wavelength range 1528–1568 nm. Custom software was written to enable acquisition of this spectral data at 25 Hz.
or vice versa), the system was purged using the fluid for the subsequent test. During the purging, system pressure dropped below test pressure (i.e. 9.65 MPa). The system was brought back to test conditions by running the metering pump for approximately 5 min prior to starting the next test. During this process, data was not collected. This transition data is not shown as it is significantly affected by pressure swings. 3. Results
2.5. Characterization 3.1. Static characterization The sensor response to RI was first calibrated using seawater, air, and isopropanol. The calibration was done by dipping the tip sensor in each fluid at atmospheric pressure and 20 ◦ C. Between each test, the tip was rinsed with isopropanol and cleansed with a fiber optic connector cleaner (OptipopTM ). The RI of seawater was determined with an Abbe refractometer to be 1.3383. The sensor was calibrated in tests with methane in which pressure was varied to obtain a range of RI values. In these tests, the high pressure relief valve (Fig. 3) was replaced with a shut-off valve. Air was purged from the flow cell with methane and the shut-off valve was then closed. Methane was used to pressurize the flow cell up to the set point of the regulator. Data was recorded for one minute at each of 3.45 MPa (500 psi), 6.89 MPa (1000 psi), 10.34 MPa (1500 psi) and 13.79 MPa (2000 psi). The sensor was tested with carbon dioxide at one pressure only, 10.34 MPa (1500 psi). For reference, the RI of carbon dioxide was also calculated with the Lorentz-Lorentz equation, Eq. (2) (Song et al., 2003; Sinko et al., 2009), where, n, is the refractive index of the medium, R, is the molar refractivity, M, is the molar mass and, , is the density. n2 − 1 R = M n2 + 2
The static characterization of the sensor is presented in Fig. 5(a) and (b). These experimental results are compared with theoretical intensities calculated with the Fresnel equation (Eq. (1)). Intensity data is normalized with respect to the measured and calculated intensities for air. The maximum difference between measured and calculated intensities are 12% and 6% in Fig. 5(a) and (b), respectively. 3.2. Dynamic testing In the dynamic tests, seawater was in contact with the sensor tip except when a bubble of either carbon dioxide or methane
(2)
Reference data for the RI of methane was taken from Achtermann et al. (1992). 2.6. Testing Dynamic tests were conducted to assess the ability of the sensor to detect the passage of carbon dioxide and methane bubbles. The sensor probe was removed from the flow cell, cleansed and then reinstalled. The flow cell was then purged of air using carbon dioxide. Valves 1 and 4 were then closed and both the methane and carbon dioxide lines were pressurized to 9.65 MPa (1400 psi). Seawater was pumped through the flow cell for 5 min to ensure that the flow cell contained only seawater at 9.65 MPa (1400 psi). For the carbon dioxide tests, the carbon dioxide tank was pressurized to 140 kPa (20 psi) above the set point pressure of the relief valve to ensure flow. Valves 1 and 4 were opened for 30 s to allow carbon dioxide to partially fill the inlet chamber of the flow cell. Valve 4 was then closed and the metering pump was set at 0.25 mL/min. The flow of seawater carried entrained carbon dioxide bubbles from the inlet chamber, past the sensor tip and out through the relief valve. For each methane test, valves 2 and 4 were opened for 30 s allowing methane to fill the inlet chamber region. The regulator on the outlet of the methane tank was set 140 kPa (20 psi) above the set point pressure of the relief valve to ensure flow. Valve 4 was then closed and the metering pump was set to run continually at 0.25 mL/min. The flow of seawater carried entrained methane past the sensor tip and out through the relief valve. Visual confirmation of the bubble passage was provided by images taken through the high pressure sight glass with a Canon 1DX high speed camera. Both methane and carbon dioxide tests were conducted for 90 s. Between each test (i.e. transition from methane to carbon dioxide
Fig. 5. (a) Characterization of sensor response to seawater, isopropanol, air, liquid CO2 and supercritical CH4 . (b) Characterization of sensor response to gaseous and sc-CH4 . Intensity data is normalized with respect to intensity for air. For both plots, the intensity data is normalized with respect to the intensity for air.
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Fig. 8. Typical spectral response for water, methane and for transition methane bubble onto the sensor.
Fig. 6. (a) and (b) CO2 bubble approach, contact and departure from the sensor tip. Numbers noted in (a) correspond to the bubble locations shown in (b).
made contact with the sensor tip. At these times, both the sensor response and photographic images were recorded, the latter with a high speed camera. Fig. 6(a) shows the initial approach (1), contact (2), exposure (3–5) and departure (6) of a liquid carbon dioxide bubble on the tip sensor. Fig. 6(b) shows the response to the bubble at the times corresponding to these images.
Typical time domain results from dynamic testing are presented in Fig. 7. This figure shows data from four consecutive tests, alternating between sets of carbon dioxide injections and sets of methane injections. As noted earlier, data between each test (i.e. during transition from methane to carbon dioxide or vice versa) was not collected. The Smart Scan interrogation system was used to determine the spectral response of the sensor during the passage of bubbles. These results show that the spectral responses for carbon dioxide and for methane are relatively uniform and at a consistent level for each substance, as shown in Fig. 8. However, when the bubble is in transition into or out of contact with the sensor, a sinusoidal signal was occasionally observed, also shown in Fig. 8. Note that the vertical scale in Fig. 8 does not align with the vertical scale in Fig. 7 because the Smart Scan interrogation unit has a less powerful laser than the BBS and, as a result, has both a higher noise floor and a lower signal
Fig. 7. Intensity data recorded during passage of CO2 , CH4 bubbles (a) and (c) correspond to passage of CO2 and (b) and (d) correspond to passage of methane.
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to noise ratio. As a result, the normalized intensities for methane and water acquired with the Smart Scan system (Fig. 8) differ from those acquired with the BBS (Fig. 7). 4. Discussion The intensity data gathered during the high pressure tests with moving bubbles of carbon dioxide and methane show that the tip sensor is able to clearly detect the passage of these bubbles and, furthermore, that it is able to distinguish between the passage of carbon dioxide and methane bubbles. Most previous dynamic testing of FTRs has been done with the fiber axis oriented parallel to the flow (Hamad et al., 1997; Fortunati et al., 2003; Enrique Juliá et al., 2005; Chang et al., 2003; Lee et al., 2007; Cartellier, 1992; Cartellier and Barrau, 1998) to allow the fiber to pierce moving bubbles. In this study, the fiber axis was oriented perpendicular to the flow and flush to the wall of the channel. Preparatory work by the authors demonstrated that orientation of the fiber perpendicular to the flow, but not flush with the wall of the channel, yielded poor results. Small bubbles adhered to the fiber tip and blocked the passing bubbles from directly contacting the tip. Subsequently, the fiber was potted in a zirconia ferrule and the fiber and ferrule were polished to a flush surface. It was observed that for this configuration, the tendency of bubbles to adhere to the fiber was significantly reduced. The pseudo-sinusoidal spectral data gathered during the passage of bubbles is believed to be due to the interferometric effect of a thin film on the sensor tip. These interferometric effects occurred only at the transition between the two fluids. It is thought that, as a carbon dioxide or methane bubble passes over the sensor tip, a thin film of water is created between the carbon dioxide or methane of the bubble and surface of the sensor tip. Other researchers have observed similar effects at the transition between two phases or fluids. Chang et al. (2003) and Sakamoto and Saito (2012) observed intensity fluctuations just prior to a tip sensor contacting an air bubble in a flow of water. In static experiments, Avdeev et al. (2004), also observed intensity fluctuations during evaporation of water from a fiber tip. Chang and Avdeev attribute these signal fluctuations to an interferometric effect caused by a film of water in contact with the fiber tip. In the current study, this phenomenon was not observed for every transition of a bubble onto or off of the sensor tip. This intermittency is believed to be due to the short duration of the transition event and the relatively slow rate of data acquisition for the spectral data. It is expected that, were this spectrum to be sampled at higher frequency, the thinning and eventual removal of the film from the sensor tip would be observed. At the test conditions, the densities of carbon dioxide and methane are 0.85 kg/L and 0.12 kg/L, respectively. These densities are sufficiently high, relative to the density of seawater, that the low velocities of the bubbles through the narrow tubes of the apparatus, if driven solely by buoyancy, prevented completion of the tests in a timely manner. The dynamic tests were, therefore, run with a low flow of seawater that yielded a bubble velocity of approximately 3.2 cm/s. For reference, Brewer et al. (2002) demonstrated that carbon dioxide bubbles will travel between 10–14 cm/s at depths of 800 m to 300 m in seawater. The tests conducted in this study were performed at 20 ◦ C. However, in the deep ocean, temperatures are typically on the order of 2 ◦ C. At these temperatures, clathrates of both carbon dioxide and methane form and will likely hinder measurements. White et al. (2006) conducted deep sea measurements of both methane and carbon dioxide using a heater to melt the clathrates to either a liquid, gaseous or supercritical state. Implementation of the FTR to monitor carbon dioxide and methane leakage from a deep ocean injection site would likely require similar measures.
A significant obstacle to the widespread implementation of FTR technologies is their susceptibility to contamination. The two main sources of contamination are marine bio-fouling and nonbiological film deposition (e.g. oils). Marine bio-fouling has been inhibited successfully with UV bio-fouling inhibitors, such as the UV-XchangeTM (AML Oceanographic, Canada). These inhibitors keep a specified area clean of algae and other marine flora through UV irradiation. Non-biological film deposition may be dealt with using high pressure fluid jets. The fluid jet is directed toward the sensor at regular intervals, removing foreign matter that is adhering to the sensor. Water jet sensor cleaning technologies have been implemented by Analyticon (Germany) and DKK-OKK (Japan). For field implementation of the FTR device described here, methods to prevent bio-fouling and non-biological film deposition, such as those noted above, may be required. 5. Conclusions In this study, an FTR system was used to detect and to distinguish between the passage of methane and carbon dioxide bubbles in seawater. The tests were conducted at 9.65 MPa (1400 psi) and 20 ◦ C to simulate monitoring conditions at deep sea carbon dioxide injection sites. Test data shows that these sensors perform with a maximum 12% deviation of intensity from that predicted by Fresnel reflection theory. Pseudo-sinusoidal spectral signals were occasionally observed as methane or carbon dioxide bubbles transition onto or off of the surface of the sensor tip. These effects are attributed to thin films of these substances adhering to the surface of the sensor. Acknowledgments The authors gratefully acknowledge funding provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) (grant no. RGPIN/194490-2010), Carbon Management Canada (CMC) (grant no. C233) and the Korean Institute of Geoscience and Mineral Resources (KIGAM). References Achtermann, H.J., Hong, J., Wagner, W., Pruss, A., 1992. Refractive index and density isotherms for methane from 273 to 373 K and at pressures up to 34 MPa. J. Chem. Eng. Data 39 (4), 414–418. Avdeev, M.V., Konovalov, A.N., Bagratashvili, V.N., Popov, V.K., Tsypina, S.I., Sokolova, M., Ke, J., Poliakoff, M., 2004. The fibre optic reflectometer: a new and simple probe for refractive index and phase separation measurements in gases, liquids and supercritical fluids. Phys. Chem. Chem. Phys. 6 (6), 1258. Aya, I., Yamane, K., Shiozaki, K., 1999. Proposal of self sinking CO2 Sending System: COSMOS. Greenhouse Gas Control Technologies, pp. 269–274. Brewer, P.G., Peltzer, E.T., Rehder, G., 2002. Experimental determination of the fate of rising CO2 droplets in seawater. Environ. Sci. Technol. 24 (36), 5441–5446. Brewer, P.G., Chen, B., Warzinki, R., Baggeroer, A., Peltzer, E.T., Dunk, R.M., Walz, P., Dec 2006. Three-dimensional acoustic monitoring and modeling of a deep-sea CO2 droplet cloud. Geophys. Res. Lett. 33 (23), L23607. Brewer, P.G., Peltzer, E.T., Walz, P., Aya, I., Yamane, K., Kojima, R., Nakajima, Y., Nakayama, N., Haugan, P., Johannessen, T., 2005. Deep ocean experiments with fossil fuel carbon dioxide: creation and sensing of a controlled plume at 4 km depth. J. Mar. Res. 63 (Jan (1)), 9–33. Cartellier, A., Barrau, E., 1998. Monofiber optical probes for gas detection and gas velocity measurements: optimised sensing tips. Int. J. Multiphase Flow 24 (8), 1295–1315. Cartellier, A., 1992. Simultaneous void fraction measurement, bubble velocity, and size estimate using a single optical probe in gas–liquid two-phase flows. Rev Sci. Instrum. 63 (11), 5442. Chadwick, A., Arts, R., Eiken, O., Williamson, P., Williams, G., 2006. Geophysical Monitoring of the CO2 Plume at Sleipner, North Sea: An outline review. In: Lombardi, S.E.B.S., Altunina, L.K. (Eds.), Advances in the Geological Storage of Carbon Dioxide. Springer, Dordrecht, The Netherlands, pp. 303–314. Chang, K.-A., Lim, H.-J., Su, C.B., 2003. Fiber optic reflectometer for velocity and fraction ratio measurements in multiphase flows. Rev. Sci. Instrum. 74 (7), 3559. Chang, K.-A., Lim, H.-J., Su, C.B., 2002. A fibre optic Fresnel ratio meter for measurements of solute concentration and refractive index change in fluids. Meas. Sci. Technol. 13 (Dec (12)), 1962–1965.
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Munkholm, C., Walt, D.R., Milanovich, F.P., 1988. A fiber-optic sensor for CO(2) measurement. Talanta 35 (Mar (2)), 109–112. Neurauter, G., Klimant, I., Wolfbeis, O.S., 2000. Fiber-optic microsensor for high resolution pCO2 sensing in marine environment. Fresenius J. Anal. Chem. 366 (Mar (5)), 481–487. D. Prada, C. Martelli, C. C. Kato, A. M. B. Braga, and M. S. P. Gomes, “CO2 Phase Study Using an Optical Fiber Refractometer,” 21st Int. Conf. Opt. Fiber Sensors, vol. 7753, pp. 775366-775366-4, May 2011. Rojas, G., Loewen, M.R., 2007. Fiber-optic probe measurements of void fraction and bubble size distributions beneath breaking waves. Exp. Fluids 43 (Jul (6)), 895–906. Sakamoto, A., Saito, T., 2012. Computational analysis of responses of a wedgeshaped-tip optical fiber probe in bubble measurement. Rev. Sci. Instrum. 83 (Jul (7)), 075107. Shitashima, K., Maeda, Y., Ohsumi, T., 2013. Strategies for detection and monitoring of CO2 leakage in sub-seabed CCS. Energy Procedia 37, 4283–4290. Sinko, J.E., Korman, V., Hendrickson, A., Defense, K., Solutions, S., 2009. A Fiber-Optic Sensor for Leak Detection in a Space Environment. In: AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit., pp. 1–8. Song, Y., Nishio, M., Someya, S., Ohsumi, T., 2003. Measurement on CO2 solution density by optical technology. J. Vis. 6 (1), 41–51. Vurek, G.G., Feustel, P.J., Severinghaus, J.W., 1983. A fiber optic PCO2 sensor. Ann. Biomed. Eng. 11 (Jan (6)), 499–510. Wang, J.-N., Tang, J.-L., 2012. Photonic crystal fiber Mach–Zehnder interferometer for refractive index sensing. Sensors (Basel) 12 (Jan (3)), 2983–2995. White, S.N., Dunk, R.M., Peltzer, E.T., Freeman, J.J., Brewer, P.G., 2006. In situ Raman analyses of deep-sea hydrothermal and cold seep systems (Gorda Ridge and Hydrate Ridge). Geochem. Geophys. Geosyst. 7, http://dx.doi.org/10. 1029/2005GC001204, Q05023. Xu, W., Huang, X.G., Pan, J.S., 2013. Simple fiber-optic refractive index sensor based on Fresnel reflection and optical switch. IEEE Sens. J. 13 (May (5)), 1571–1574. Zhao, J.R., Huang, X.G., Chen, J.H., 2009. A Fresnel-reflection-based fiber sensor for simultaneous measurement of liquid concentration and temperature. J. Appl. Phys. 106 (8), 083103.
Contents lists available at ScienceDirect
International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Fiber refractometer to detect and distinguish carbon dioxide and methane leakage in the deep ocean Geoff Burton a,∗ , Luis Melo a , Stephen Warwick a , Martin Jun a , Bo Bao b , David Sinton b , Peter Wild a a
Department of Mechanical Engineering, Institute for Integrated Energy Systems, University of Victoria, Victoria, BC, Canada V8W 3P6 Department of Mechanical and Industrial Engineering, Institute for Sustainable Energy, University of Toronto, 5 King’s College Road, Toronto, ON, Canada M5S 3G8 b
a r t i c l e
i n f o
Article history: Received 8 August 2014 Received in revised form 11 September 2014 Accepted 15 September 2014 Keywords: Carbon dioxide Methane Fiber optic tip Refractometer Fresnel reflection Sequestration
a b s t r a c t Deep ocean injection of carbon dioxide into subsea geologic formations carries a risk of carbon dioxide leakage as well as leakage of methane from petroleum bearing formations that injection wells may penetrate. Therefore, leakage monitoring technology for this application must detect carbon dioxide and distinguish it from methane. Here we demonstrate an all-optical approach to detect and differentiate between liquid carbon dioxide and supercritical methane bubbles in synthetic seawater at 9.65 MPa. This method employs fiber tip refractometry, a fiber optic measurement technique that is sensitive to the refractive index of the surrounding medium. Carbon dioxide and methane bubbles are clearly detected as they pass the sensor tip and these species are clearly distinguished from each other. Interferometric signals are also observed in association with the transition of bubbles onto and off of the sensor tip. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Deep ocean injection of carbon dioxide into subsea geologic formations, such as is planned for the Santos Basin offshore of Brazil (Melo et al., 2011), carries a risk of carbon dioxide leakage during and after injection. At these sites, there is also a risk of methane leakage from the petroleum bearing formations that injection wells may penetrate. Therefore, leakage monitoring technology for this application must both detect carbon dioxide and distinguish it from methane. Monitoring carbon dioxide storage has been demonstrated at the Sleipner injection site located in the North Sea (Furre and Eiken, 2014; Chadwick et al., 2006). In this pilot test, seismic and gravimetric sensors were used to monitor a subsurface plume of carbon dioxide with a limit of detection of 500 t of carbon dioxide (Chadwick et al., 2006). Methods have also been demonstrated to detect carbon dioxide rising as a stream of bubbles or droplets in the water column. Acoustic tomography has been used to detect such a rising stream based on the difference in the speed of sound propagating in different media (i.e. water and carbon dioxide) (Brewer et al., 2006).
∗ Corresponding author. Tel.: +12507216295; fax: +12507216323. E-mail address: [email protected] (G. Burton). http://dx.doi.org/10.1016/j.ijggc.2014.09.015 1750-5836/© 2014 Elsevier Ltd. All rights reserved.
Brewer et al. (2002) demonstrated detection of a rising carbon dioxide stream using a camera mounted to a remotely operated vehicle. Brewer also used pH and conductivity sensors to detect carbon dioxide enriched seawater adjacent to the rising carbon dioxide stream (Brewer et al., 2005). Similarly, Shitashima et al. (2013) developed pH/pCO2 (partial pressure of carbon dioxide) sensors which were installed in an autonomous underwater vehicle for detection of carbon dioxide leakage. An alternative method to monitor carbon dioxide leakage from subsea injection sites is based on refractive index measurements. The refractive index (RI) of a liquid or gas varies with species, temperature, pressure and, in the case of solutions and mixtures, concentration. As a result, RI measurement is a broadly applicable method to determine the values of these properties. In recent years, a range of fiber optic methods to determine RI has emerged, including grating based methods (Fang et al., 2010; James and Tatam, 2003), interferometric methods (Wang and Tang, Jan 2012) and intensity based methods (Vurek et al., 1983; Munkholm et al., 1988; Cartellier, 1992; Goyet et al., 1992; Degrandpre, 1993; Hamad et al., 1997; Cartellier and Barrau, 1998; Neurauter et al., 2000; Chang et al., 2002; Chang et al., 2003; Ertekin et al., 2003; Fortunati et al., 2003; Avdeev et al., 2004; Kim and Su, 2004; Enrique Juliá et al., 2005; Lee et al., 2007; Zhao et al., 2009; Prada et al., 2011; Wang and Tang, Jan 2012; Xu et al., 2013). Fiber optic methods allow the sensing element to be located at a distance from
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the interrogation system and, as a result, sensing of RI in environments where conventional methods are difficult to apply (e.g. high pressure, high temperature). For measurements that require high accuracy, gratings used in conjunction with fine resolution wavelength interrogation is preferred. However, for applications in which the objective is to discriminate between different substances or between different states or phases of the same substance which have distinctly different values of RI, an intensity-based method, fiber tip refractometry (FTR) offers a relatively simple and low cost alternative. The basis for FTR is Fresnel reflection. When light exits a medium with RI, n1 , and enters another with RI, n2 , the intensity of light reflected back into the first medium is dependent on the refractive indices of both media, as shown in Eq. (1). Hence, if n1 is known, then the intensity of the reflected light, I, is a direct measure of n2 . I=
(n1 − n2 )2 (n1 + n2 )2
(1)
An FTR comprises the distal end of an optical fiber that is immersed in a target medium. Light propagating in the fiber undergoes Fresnel reflection at the interface between the fiber tip and the target medium. The intensity of this reflected signal is as defined above for the general case of Fresnel reflection with n1 and n2 being the RIs of the fiber and the target medium. FTR has been applied by a number of researchers to measure RI of various fluids under static (i.e. no flow) conditions. For example, Xu et al., 2013) measured the RI of (i.e. benzene, ethanol, methanol, acetone glycerol) with a broadband light source centered at 1550 nm. Kim and Su (2004) performed similar experiments but measured RI at different wavelengths. Other studies present the use of FTR to measure salinity of aqueous solutions (Chang et al., 2002; Zhao et al., 2009). Two studies have been identified in which FTR has been used for static RI measurements of carbon dioxide at high pressures. Avdeev et al. (2004) measured the RI of carbon dioxide in the gas, liquid and super critical states as well as mixtures of carbon dioxide and organic solvents. Prada et al. (2011) measured the RI of liquid carbon dioxide at pressures up to 5.5 MPa under static conditions. No FTR-based RI measurement of methane at elevated pressures, its solutions or mixtures have been identified. There are several examples of FTR-based sensors that make use of either fluorescent or colorimetric coatings that react with carbon dioxide (Degrandpre, 1993; Ertekin et al., 2003; Goyet et al., 1992; Munkholm et al., 1988; Neurauter et al., 2000; Vurek et al., 1983). These methods have been demonstrated in aqueous solutions at atmospheric pressures to detect dissolved carbon dioxide rather than distinct phases. These methods have not been applied at high pressure Interrogation of FTR systems is typically intensity-based. As a result, these systems offer fast response times, relative to gratingbased systems, which typically depend upon wavelength-based interrogation. As a result, FTR has been used by a number of researchers to make dynamic measurements in two-phase flows (Hamad et al., 1997; Fortunati et al., 2003; Enrique Juliá et al., 2005; Chang et al., 2003; Lee et al., 2007; Cartellier, 1992; Cartellier and Barrau, 1998). Hamad et al. (1997) used FTR to distinguish kerosene droplets from water in two-phase pipe flow. Kerosene droplets were entrained in the flow and the optical probe was used to detect the passage of droplets. In another study, the size and velocity of air bubbles in pipe flow of water were measured with a four-point FTR device by Guet et al. (Fortunati et al., 2003). This system provided higher accuracy velocity measurements than a single point system. Chang et al. (2003), Enrique Juliá et al. (2005) and Lee et al. (2007) all used FTR to measure void fraction (i.e. bubble fraction) in entrained bubble flow. Cartellier et al. (Chang et al., 2003; Lee et al.,
2007) did extensive work on velocity measurement in entrained bubble flow in an air-water system. In most previous FTR studies in flowing liquids or gases, the fiber orientation is parallel to the flow (Hamad et al., 1997; Fortunati et al., 2003; Enrique Juliá et al., 2005; Chang et al., 2003; Lee et al., 2007; Cartellier, 1992; Cartellier and Barrau, 1998). Only one study was identified in which an effective FTR probe was mounted normal to the direction of flow (Rojas and Loewen, 2007). It is important to note, however, that this study focused on high speed, turbulent, aerated flows so the bubble direction was not necessarily normal to the fiber tip. In the current study, an FTR probe is mounted normal to the flow and flush with a pipe wall, to monitor the passage of liquid carbon dioxide and supercritical methane bubbles in synthetic seawater at 9.65 MPa (1400 psi) and 20 ◦ C. Throughout this paper, the term bubble is used to refer to a small and distinct volume of liquid or supercritical fluid and all pressures are gauge pressures. These experiments simulate the use of FTR to monitor leakage during and after deep (i.e. 950 m) ocean injection of carbon dioxide into subsea geologic formations. 2. Materials and methods 2.1. Sensor The optical fiber used in this work is SMF 28e (Corning, USA). A length of this fiber is mounted in a standard FC/PC zirconia ferrule (F12070, Fiber Instrument Sales) and secured with epoxy (Thorlabs, F120) (see Fig. 1). The ferrule and fiber are epoxied to one end of a stainless steel tube with outer diameter 1/8 (3.13 mm) and wall thickness 0.028 (0.71 mm). The distal end of the fiber is cleaved normal to the fiber axis and is mounted in the ferrule so that the
Fig. 1. Schematic of sensor tip with cutaway at the distal end.
Fig. 2. Schematic of flow cell and sensor tip.
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Fig. 3. Schematic of the high pressure test system. Seawater, methane and CO2 are directed from their respective sources through the flow cell and out the relief valve at 9.65 MPa.
distal surface of the fiber is slightly proud of the end of the ferrule. The fiber tip is then polished flush with the ferrule end. This assembly is fixtured in a compression fitting (SwagelokTM , SS-200-1-2BT) which is mounted to the flow cell, as shown in Fig. 2. 2.2. Flow cell The flow cell is comprised of a vertically-oriented glass throat (0.125 in., 3.13 mm inside diameter) housed in an aluminum body (see Fig. 2). The sensor is aligned normal to the axis of the throat and the tip of the sensor is set in a hole in the wall of the throat such that the distal surface of the sensor is approximately flush with the inner surface of the throat. The flow cell includes a sight glass which enables viewing of the throat in the region of the sensor tip.
2.4. Interrogation Two optical interrogation systems are used. The first system (Fig. 4(a)), consists of a broadband light source with wavelength range 1528 nm to 1570 nm (BBS) (BBS1550, AFC Technologies) coupled to an optical splitter (Blue Road Research, 35S). The splitter is coupled to the sensor and to an optical diode with range 1280 nm to 1580 nm (410 WDM, Eigenlight). The optical diode measures the reflected light from the tip and converts this to an analogue voltage
2.3. High pressure test system The flow cell is integrated into a high pressure test system, shown in Fig. 3. A metering pump (EldexTM , USA) pressurizes the synthetic seawater and pumps it through the flow cell at a constant flow rate. The synthetic seawater, (hereafter referred to as seawater) comprises 24.72 g sodium chloride, 2.18 g magnesium chloride, 1.03 g calcium chloride, 0.67 g potassium chloride and 3.07 g magnesium sulfate per 1 L deionized water, as per Marine Biological Laboratory specifications (Möller et al., 2006). Methane and carbon dioxide are released into the inlet chamber from high pressure tanks using needle valves (valves 1 and 2). The carbon dioxide tank is pressurized to 9.79 MPa (1420 psi) with a helium headspace. This tank is fitted with a liquid withdrawal siphon to ensure carbon dioxide is withdrawn. The methane tank is fitted with a high pressure regulator through which supercritical methane is withdrawn at 9.79 MPa (1420 psi). The high pressure relief valve (SS-43RA-BU, SwagelokTM ) is set to open at 9.65 MPa (1400 psi) and reseal at 9.48 MPa (1375 psi). This valve allows fluid to exit the system as seawater, methane or carbon dioxide is introduced while maintaining relatively constant pressure (i.e. 9.65 ± 0.070 MPa). The system pressure was set to 9.65 MPa (950 m depth) because, at this pressure, carbon dioxide is a liquid and is positively buoyant in seawater. Carbon dioxide is a liquid at depths greater than approximately 500 m (depending on the water temperature), and is neutrally buoyant at approximately 2700 m (Aya et al., 1999). Methane exists as a gas or supercritical fluid for the entire range of pressures and temperatures in the ocean.
Fig. 4. Schematic of (a) broadband light source interrogation system and (b) SmartScan interrogation system.
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which is read by the data acquisition module (USB6210, National Instruments) in the PC. The BBS system acquires high resolution intensity data but no spectral information. The second interrogation system (Fig. 4(b)) consists of a SmartScan 04 (Smart Fibres, UK) connected to the sensor. The SmartScan unit was used to obtain spectral data over the wavelength range 1528–1568 nm. Custom software was written to enable acquisition of this spectral data at 25 Hz.
or vice versa), the system was purged using the fluid for the subsequent test. During the purging, system pressure dropped below test pressure (i.e. 9.65 MPa). The system was brought back to test conditions by running the metering pump for approximately 5 min prior to starting the next test. During this process, data was not collected. This transition data is not shown as it is significantly affected by pressure swings. 3. Results
2.5. Characterization 3.1. Static characterization The sensor response to RI was first calibrated using seawater, air, and isopropanol. The calibration was done by dipping the tip sensor in each fluid at atmospheric pressure and 20 ◦ C. Between each test, the tip was rinsed with isopropanol and cleansed with a fiber optic connector cleaner (OptipopTM ). The RI of seawater was determined with an Abbe refractometer to be 1.3383. The sensor was calibrated in tests with methane in which pressure was varied to obtain a range of RI values. In these tests, the high pressure relief valve (Fig. 3) was replaced with a shut-off valve. Air was purged from the flow cell with methane and the shut-off valve was then closed. Methane was used to pressurize the flow cell up to the set point of the regulator. Data was recorded for one minute at each of 3.45 MPa (500 psi), 6.89 MPa (1000 psi), 10.34 MPa (1500 psi) and 13.79 MPa (2000 psi). The sensor was tested with carbon dioxide at one pressure only, 10.34 MPa (1500 psi). For reference, the RI of carbon dioxide was also calculated with the Lorentz-Lorentz equation, Eq. (2) (Song et al., 2003; Sinko et al., 2009), where, n, is the refractive index of the medium, R, is the molar refractivity, M, is the molar mass and, , is the density. n2 − 1 R = M n2 + 2
The static characterization of the sensor is presented in Fig. 5(a) and (b). These experimental results are compared with theoretical intensities calculated with the Fresnel equation (Eq. (1)). Intensity data is normalized with respect to the measured and calculated intensities for air. The maximum difference between measured and calculated intensities are 12% and 6% in Fig. 5(a) and (b), respectively. 3.2. Dynamic testing In the dynamic tests, seawater was in contact with the sensor tip except when a bubble of either carbon dioxide or methane
(2)
Reference data for the RI of methane was taken from Achtermann et al. (1992). 2.6. Testing Dynamic tests were conducted to assess the ability of the sensor to detect the passage of carbon dioxide and methane bubbles. The sensor probe was removed from the flow cell, cleansed and then reinstalled. The flow cell was then purged of air using carbon dioxide. Valves 1 and 4 were then closed and both the methane and carbon dioxide lines were pressurized to 9.65 MPa (1400 psi). Seawater was pumped through the flow cell for 5 min to ensure that the flow cell contained only seawater at 9.65 MPa (1400 psi). For the carbon dioxide tests, the carbon dioxide tank was pressurized to 140 kPa (20 psi) above the set point pressure of the relief valve to ensure flow. Valves 1 and 4 were opened for 30 s to allow carbon dioxide to partially fill the inlet chamber of the flow cell. Valve 4 was then closed and the metering pump was set at 0.25 mL/min. The flow of seawater carried entrained carbon dioxide bubbles from the inlet chamber, past the sensor tip and out through the relief valve. For each methane test, valves 2 and 4 were opened for 30 s allowing methane to fill the inlet chamber region. The regulator on the outlet of the methane tank was set 140 kPa (20 psi) above the set point pressure of the relief valve to ensure flow. Valve 4 was then closed and the metering pump was set to run continually at 0.25 mL/min. The flow of seawater carried entrained methane past the sensor tip and out through the relief valve. Visual confirmation of the bubble passage was provided by images taken through the high pressure sight glass with a Canon 1DX high speed camera. Both methane and carbon dioxide tests were conducted for 90 s. Between each test (i.e. transition from methane to carbon dioxide
Fig. 5. (a) Characterization of sensor response to seawater, isopropanol, air, liquid CO2 and supercritical CH4 . (b) Characterization of sensor response to gaseous and sc-CH4 . Intensity data is normalized with respect to intensity for air. For both plots, the intensity data is normalized with respect to the intensity for air.
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Fig. 8. Typical spectral response for water, methane and for transition methane bubble onto the sensor.
Fig. 6. (a) and (b) CO2 bubble approach, contact and departure from the sensor tip. Numbers noted in (a) correspond to the bubble locations shown in (b).
made contact with the sensor tip. At these times, both the sensor response and photographic images were recorded, the latter with a high speed camera. Fig. 6(a) shows the initial approach (1), contact (2), exposure (3–5) and departure (6) of a liquid carbon dioxide bubble on the tip sensor. Fig. 6(b) shows the response to the bubble at the times corresponding to these images.
Typical time domain results from dynamic testing are presented in Fig. 7. This figure shows data from four consecutive tests, alternating between sets of carbon dioxide injections and sets of methane injections. As noted earlier, data between each test (i.e. during transition from methane to carbon dioxide or vice versa) was not collected. The Smart Scan interrogation system was used to determine the spectral response of the sensor during the passage of bubbles. These results show that the spectral responses for carbon dioxide and for methane are relatively uniform and at a consistent level for each substance, as shown in Fig. 8. However, when the bubble is in transition into or out of contact with the sensor, a sinusoidal signal was occasionally observed, also shown in Fig. 8. Note that the vertical scale in Fig. 8 does not align with the vertical scale in Fig. 7 because the Smart Scan interrogation unit has a less powerful laser than the BBS and, as a result, has both a higher noise floor and a lower signal
Fig. 7. Intensity data recorded during passage of CO2 , CH4 bubbles (a) and (c) correspond to passage of CO2 and (b) and (d) correspond to passage of methane.
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to noise ratio. As a result, the normalized intensities for methane and water acquired with the Smart Scan system (Fig. 8) differ from those acquired with the BBS (Fig. 7). 4. Discussion The intensity data gathered during the high pressure tests with moving bubbles of carbon dioxide and methane show that the tip sensor is able to clearly detect the passage of these bubbles and, furthermore, that it is able to distinguish between the passage of carbon dioxide and methane bubbles. Most previous dynamic testing of FTRs has been done with the fiber axis oriented parallel to the flow (Hamad et al., 1997; Fortunati et al., 2003; Enrique Juliá et al., 2005; Chang et al., 2003; Lee et al., 2007; Cartellier, 1992; Cartellier and Barrau, 1998) to allow the fiber to pierce moving bubbles. In this study, the fiber axis was oriented perpendicular to the flow and flush to the wall of the channel. Preparatory work by the authors demonstrated that orientation of the fiber perpendicular to the flow, but not flush with the wall of the channel, yielded poor results. Small bubbles adhered to the fiber tip and blocked the passing bubbles from directly contacting the tip. Subsequently, the fiber was potted in a zirconia ferrule and the fiber and ferrule were polished to a flush surface. It was observed that for this configuration, the tendency of bubbles to adhere to the fiber was significantly reduced. The pseudo-sinusoidal spectral data gathered during the passage of bubbles is believed to be due to the interferometric effect of a thin film on the sensor tip. These interferometric effects occurred only at the transition between the two fluids. It is thought that, as a carbon dioxide or methane bubble passes over the sensor tip, a thin film of water is created between the carbon dioxide or methane of the bubble and surface of the sensor tip. Other researchers have observed similar effects at the transition between two phases or fluids. Chang et al. (2003) and Sakamoto and Saito (2012) observed intensity fluctuations just prior to a tip sensor contacting an air bubble in a flow of water. In static experiments, Avdeev et al. (2004), also observed intensity fluctuations during evaporation of water from a fiber tip. Chang and Avdeev attribute these signal fluctuations to an interferometric effect caused by a film of water in contact with the fiber tip. In the current study, this phenomenon was not observed for every transition of a bubble onto or off of the sensor tip. This intermittency is believed to be due to the short duration of the transition event and the relatively slow rate of data acquisition for the spectral data. It is expected that, were this spectrum to be sampled at higher frequency, the thinning and eventual removal of the film from the sensor tip would be observed. At the test conditions, the densities of carbon dioxide and methane are 0.85 kg/L and 0.12 kg/L, respectively. These densities are sufficiently high, relative to the density of seawater, that the low velocities of the bubbles through the narrow tubes of the apparatus, if driven solely by buoyancy, prevented completion of the tests in a timely manner. The dynamic tests were, therefore, run with a low flow of seawater that yielded a bubble velocity of approximately 3.2 cm/s. For reference, Brewer et al. (2002) demonstrated that carbon dioxide bubbles will travel between 10–14 cm/s at depths of 800 m to 300 m in seawater. The tests conducted in this study were performed at 20 ◦ C. However, in the deep ocean, temperatures are typically on the order of 2 ◦ C. At these temperatures, clathrates of both carbon dioxide and methane form and will likely hinder measurements. White et al. (2006) conducted deep sea measurements of both methane and carbon dioxide using a heater to melt the clathrates to either a liquid, gaseous or supercritical state. Implementation of the FTR to monitor carbon dioxide and methane leakage from a deep ocean injection site would likely require similar measures.
A significant obstacle to the widespread implementation of FTR technologies is their susceptibility to contamination. The two main sources of contamination are marine bio-fouling and nonbiological film deposition (e.g. oils). Marine bio-fouling has been inhibited successfully with UV bio-fouling inhibitors, such as the UV-XchangeTM (AML Oceanographic, Canada). These inhibitors keep a specified area clean of algae and other marine flora through UV irradiation. Non-biological film deposition may be dealt with using high pressure fluid jets. The fluid jet is directed toward the sensor at regular intervals, removing foreign matter that is adhering to the sensor. Water jet sensor cleaning technologies have been implemented by Analyticon (Germany) and DKK-OKK (Japan). For field implementation of the FTR device described here, methods to prevent bio-fouling and non-biological film deposition, such as those noted above, may be required. 5. Conclusions In this study, an FTR system was used to detect and to distinguish between the passage of methane and carbon dioxide bubbles in seawater. The tests were conducted at 9.65 MPa (1400 psi) and 20 ◦ C to simulate monitoring conditions at deep sea carbon dioxide injection sites. Test data shows that these sensors perform with a maximum 12% deviation of intensity from that predicted by Fresnel reflection theory. Pseudo-sinusoidal spectral signals were occasionally observed as methane or carbon dioxide bubbles transition onto or off of the surface of the sensor tip. These effects are attributed to thin films of these substances adhering to the surface of the sensor. Acknowledgments The authors gratefully acknowledge funding provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) (grant no. RGPIN/194490-2010), Carbon Management Canada (CMC) (grant no. C233) and the Korean Institute of Geoscience and Mineral Resources (KIGAM). References Achtermann, H.J., Hong, J., Wagner, W., Pruss, A., 1992. Refractive index and density isotherms for methane from 273 to 373 K and at pressures up to 34 MPa. J. Chem. Eng. Data 39 (4), 414–418. Avdeev, M.V., Konovalov, A.N., Bagratashvili, V.N., Popov, V.K., Tsypina, S.I., Sokolova, M., Ke, J., Poliakoff, M., 2004. The fibre optic reflectometer: a new and simple probe for refractive index and phase separation measurements in gases, liquids and supercritical fluids. Phys. Chem. Chem. Phys. 6 (6), 1258. Aya, I., Yamane, K., Shiozaki, K., 1999. Proposal of self sinking CO2 Sending System: COSMOS. Greenhouse Gas Control Technologies, pp. 269–274. Brewer, P.G., Peltzer, E.T., Rehder, G., 2002. Experimental determination of the fate of rising CO2 droplets in seawater. Environ. Sci. Technol. 24 (36), 5441–5446. Brewer, P.G., Chen, B., Warzinki, R., Baggeroer, A., Peltzer, E.T., Dunk, R.M., Walz, P., Dec 2006. Three-dimensional acoustic monitoring and modeling of a deep-sea CO2 droplet cloud. Geophys. Res. Lett. 33 (23), L23607. Brewer, P.G., Peltzer, E.T., Walz, P., Aya, I., Yamane, K., Kojima, R., Nakajima, Y., Nakayama, N., Haugan, P., Johannessen, T., 2005. Deep ocean experiments with fossil fuel carbon dioxide: creation and sensing of a controlled plume at 4 km depth. J. Mar. Res. 63 (Jan (1)), 9–33. Cartellier, A., Barrau, E., 1998. Monofiber optical probes for gas detection and gas velocity measurements: optimised sensing tips. Int. J. Multiphase Flow 24 (8), 1295–1315. Cartellier, A., 1992. Simultaneous void fraction measurement, bubble velocity, and size estimate using a single optical probe in gas–liquid two-phase flows. Rev Sci. Instrum. 63 (11), 5442. Chadwick, A., Arts, R., Eiken, O., Williamson, P., Williams, G., 2006. Geophysical Monitoring of the CO2 Plume at Sleipner, North Sea: An outline review. In: Lombardi, S.E.B.S., Altunina, L.K. (Eds.), Advances in the Geological Storage of Carbon Dioxide. Springer, Dordrecht, The Netherlands, pp. 303–314. Chang, K.-A., Lim, H.-J., Su, C.B., 2003. Fiber optic reflectometer for velocity and fraction ratio measurements in multiphase flows. Rev. Sci. Instrum. 74 (7), 3559. Chang, K.-A., Lim, H.-J., Su, C.B., 2002. A fibre optic Fresnel ratio meter for measurements of solute concentration and refractive index change in fluids. Meas. Sci. Technol. 13 (Dec (12)), 1962–1965.
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