Detecting Supercritical CO2 in Brine at Sequestration Pressure with

Article pubs.acs.org/est

Detecting Supercritical CO2 in Brine at Sequestration Pressure with an Optical Fiber Sensor Bo Bao,† Luis Melo,‡ Benjamin Davies,‡ Hossein Fadaei,† David Sinton,†,* and Peter Wild‡ †

Department of Mechanical and Industrial Engineering, Center for Sustainable Energy, University of Toronto, 5 King’s College Road, Toronto, Ontario, M5S 3G8, Canada ‡ Department of Mechanical Engineering, Institute for Integrated Energy Systems, University of Victoria, Victoria, British Columbia, V8W 3P6, Canada S Supporting Information *

ABSTRACT: Monitoring of sequestered carbon is essential to establishing the environmental safety and the efficacy of geological carbon sequestration. Sequestration in saline aquifers requires the detection of supercritical CO2 and CO2-saturated brine as distinct from the native reservoir brine. Here we demonstrate an all-optical approach to detect both supercritical CO2, and saturated brine under sequestration conditions. The method employs a long-period grating written on an optical fiber with a resonance wavelength that is sensitive to local refractive index within a pressure- and temperature-controlled apparatus at 40 °C and 1400 psi (9.65 MPa). The supercritical CO2 and brine are clearly distinguished by a wavelength shift of 1.149 nm (refractive index difference of 0.2371). The CO2-saturated brine is also detectable relative to brine, with a resonance wavelength shift of 0.192 nm (refractive index difference of 0.0396). Importantly, these findings indicate the potential for distributed, all-optical monitoring of CO2 sequestration in saline aquifers.

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INTRODUCTION Geological sequestration of anthropogenic CO2 presents an opportunity to reduce global fossil fuel CO2 emissions by 20− 40% by 2050.1 Deep saline aquifers are considered ideal sequestration sites due to large collective capacity (100−10 000 GtCO2), wide distribution with proximity to emission pointsources worldwide, high formation pressure and favorable geochemistry.2 Establishing the environmental safety and efficacy of this approach will require widespread monitoring of the process in the deep subsurface. Detection of the CO2 plume and CO2-saturated brine is required both to inform the process and ultimately to detect any possible CO2 leakage on the long-term.3,4 Existing subsurface CO2 monitoring techniques include seismic,5−8 geoelectric9,10 and geochemical methods.11−13 Seismic monitoring technologies have detected CO2 saturation in brine by measuring the compressional wave velocity in formations in the Sleipner Project.5 However, the vertical resolution of seismic data was low, ∼17 and 12 m for brine saturated rocks and CO2-saturated rocks, respectively.14 Geoelectrical sensing using a vertical electrical resistivity array indicated an increase of electrical resistivity of about 200% due to CO2 injection, however the electrical noise was found to limit the technique.10 Geochemical analysis of reservoir fluid samples has also been used to study CO2 transport and reactivity in saline formations. However, this analysis is © 2012 American Chemical Society

performed at the surface and is not well-suited to in situ monitoring.11 The massive scale of sequestration formations will require monitoring approaches that are capable of widely distributed in situ measurement of CO2 at sequestration conditions. Sensors based on optical fibers have several inherent advantages including immunity to electromagnetic interferences, compact size, high sensitivity, robustness, low signal loss and capacity for distributed in situ sensing over large distances.15,16 Leveraging these advantages, optical fiber sensors have been successfully used to measure parameters such as temperature, pressure, pH and chemical composition.16 Applications include aerospace instruments,17 biomedical devices,18,19 chemical detection devices,20 structural health monitoring,21 and energy and environmental processes.22,23 In the context of subsurface energy and environmental processes, there is precedent for optical fiber based sensing of pressure, temperature, vibration and flow rate.22,24 Special Issue: Carbon Sequestration Received: Revised: Accepted: Published: 306

September 4, 2012 November 14, 2012 November 15, 2012 November 15, 2012 dx.doi.org/10.1021/es303596a | Environ. Sci. Technol. 2013, 47, 306−313

Environmental Science & Technology

Article

Optical fiber sensors have also been developed for measurement of CO2.25,26 A chalcogenide glass fiber based sensor was developed to detect CO2 gas based on its characteristic optical absorption spectrum.25 A related absorption-based approach was developed to detect CO2 dissolved in water under atmospheric conditions.26 These sensors were applied to gaseous and dissolved CO 2 but not supercritical CO 2 (scCO2), which is the relevant state under sequestration conditions (i.e., temperatures and pressures exceeding the CO2 critical point of 1070 psi (7.38 MPa) and 31 °C). Although not in an environmental monitoring context, optical and spectroscopic methods have been successfully applied to the study of scCO2.27,28 An optical probe was employed to determine the refractive indices of scCO2 and scCO2-ethanol mixture by measuring the reflected light from the end tip.27 Infrared spectroscopy has been applied to study forsterite carbonation in the presence of scCO2 and water.28 In-situ X-ray diffraction and NMR technique have also been used to study reactions between rock formations and scCO2.29,30 Although these related technologies show promise, an optical fiber method suitable for the detection of CO2 under sequestration conditions has not been demonstrated to date. In this paper, a long-period grating optical fiber sensor is presented for the detection of scCO2 and CO2-saturated brine relative to brine at sequestration temperature and pressure. The resonance wavelength in the transmission spectrum of the optical fiber sensor is sensitive to the refractive index of the surrounding medium and, thus, to the chemical species and phase in the grating vicinity. A pressure- and temperaturecontrolled apparatus was developed to test the optical fiber sensor at 40 °C and 1400 psi (9.65 MPa). Both scCO2 and CO2-saturated brine were detected relative to brine through the resonance wavelength shift of the optical fiber sensor. This approach indicates potential for distributed, all-optical monitoring of CO2 sequestration in saline aquifers using optical fibers.

Figure 1. Schematic of the CO2 sensing method developed here. As light traveling through the fiber encounters the long-period grating, specific resonant wavelengths are scattered outward into the surrounding medium. The resulting transmission spectrum displays an attenuation band. A contrast between the native brine solution and either scCO2 or CO2-saturated brine is detected through a resonance wavelength shift.

brine. Polymer fiber, for instance, is not suitable. The diffusion of scCO2 into polymers generally causes swelling and changes the physical and optical properties.35 Here, the fiber core and the cladding are both made of silica, which is resistant to scCO2. Silica materials have been used extensively in microreactors and micromodels for the study of scCO2 and brine.36,37 In addition, silica-based optical fibers have been successfully employed in other scCO2 applications.38,39 In most commercial fibers, the cladding material is surrounded by an additional coating layer for mechanical protection. The silica core and silica clad fiber used here was initially coated with polymer, however, this coating was removed at the sensing section prior to testing with scCO2 and brine. As discussed earlier, a key advantage of optical fiber based sensing is the low attenuation coefficient and, thus, potential for monitoring over long distances. The maximum specified attenuation for Corning SMF 28e fiber is ∼0.20 dB/km at λ = 1550 nm. Given these loss characteristics, the optical loss will be 0.4−1.2 dB for the 2−6 km round-trip fiber length scales of sequestration operations (1−3 km deep). This optical loss is much smaller than the 30 dB magnitude of the measured resonance attenuation (i.e., to significantly reduce the 30 dB signal requires over 100 km of transmission). Thus, optical fiber signal transmission is well-suited to the length scales of sequestration operations. High-Pressure Apparatus. An apparatus was developed to characterize and test the optical fiber sensor, as shown in Figure 2. The apparatus integrated a stainless steel test chamber, a flow path, an optical path and a data collection unit. The test chamber was used to immobilize and submerge the optical fiber sensor in high-pressure samples. Ports C (inlet) and D (outlet) of the test chamber were connected to the flow path (labeled as thickened gray lines), while ports A and B were linked to the optical path (labeled as black lines). Stainless steel tubing (1/16'' diameter) built a linkage among the test chamber, cylinders, pumps and valves as shown. Through the inlet (Port C), the test chamber could be filled with the test sample from either of the two vertically positioned cylinders (Swagelok, 40 cm3, 1800 psi (12.41 MPa)) using a 3way valve, V6. The Cylinder-1 was connected to the brine supply and to Pump-1 (Simplex, 10,000 psi (68.95 MPa), 20

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EXPERIMENTAL SECTION Optical Fiber Sensor. The optical fiber sensor was developed based on a long-period grating in which the refractive index of the fiber core is periodically modulated over a small distance along the long axis of the fiber.31 The physics of optical gratings has been detailed in previous studies.31−33 The concept, as applied in this work, is schematically illustrated in Figure 1. Briefly, as light traveling through the fiber encounters the grating, specific resonant wavelengths are scattered outward into the surrounding medium. These wavelengths depend on the period of the grating, the refractive index of the cladding material surrounding the fiber core, and the refractive index of the medium surrounding the fiber. Thus, the resulting transmission spectrum displays an attenuation band directly influenced by the refractive indices of the chemical constituents surrounding the grating. This sensing capacity has been exploited previously in a range of applications.31−34 Brine solutions, CO2-saturated brine, and scCO2, all have different refractive indices, which provides the mechanism for detection in this work. The long-period grating (Technica SA) was fabricated in 9/ 125 single mode silica optical fiber (Corning SMF-28e). The grating had a period of 400 μm, a total length of 20 mm and attenuation of 30 dB at the resonant wavelength of ∼1557.7 nm, in air. For this work, appropriate selection of optical fiber material is critical to ensure its resistance to high-pressure scCO2 and 307

dx.doi.org/10.1021/es303596a | Environ. Sci. Technol. 2013, 47, 306−313


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Figure 2. Schematic of the high-pressure apparatus used to characterize and test the optical fiber sensor. (a) The setup employed for detecting scCO2 relative to brine solution, and (b) the contents of the cylinders used for detecting CO2-saturated brine relative to brine. The pressure of 1400 psi (9.65 MPa) was monitored by pressure gauges (G1 to G3) while the temperature of 40 °C was controlled by the water bath.

°C was taken as 1.0002.40 The test chamber was filled with air and DI water separately. The apparatus was controlled at 40 °C and kept at atmospheric pressure. The transmission spectrum was detected and the resonance wavelength was recorded. The responses of the optical fiber sensor to temperature and pressure were characterized separately. In the temperature characterization, the test chamber was filled with DI water at atmospheric pressure and the data were collected between 35 and 45 °C, at increments of 1 °C. The temperature control provided by the water bath demonstrated a stability of 0.1 °C. In the pressure characterization, the optical fiber sensor was submerged in DI water at pressure points from 1000 psi (6.89 MPa) to 1800 psi (12.41 MPa) with increments of 100 psi (0.69 MPa). Pressure was controlled by Pump-1 and monitored by the pressure gauges G1 and G3, while the temperature was held constant at 40 °C. The resonance wavelength was tracked and recorded for each temperature and pressure point. Detection of scCO2 Relative to Brine. The optical fiber sensor was tested for its ability to distinguish between scCO2 and brine. The purpose of this test was to demonstrate that the optical fiber sensor could distinguish a localized plume of scCO2 from native reservoir brine solution. Test samples of brine and scCO2 were loaded into Cylinder1 and Cylinder-2 of the high-pressure apparatus, respectively (Figure 2a). The brine was a 3 M sodium chloride solution (NaCl) prepared with DI water. Both samples were pressurized to 1400 psi (9.65 MPa) and the temperature was controlled at 40 °C, under which conditions the CO2 becomes supercritical. The test procedure was designed to detect alternating test samples of brine and scCO2 within the test chamber. Cycles 1, 3, 5, and 7 corresponded to brine injections while Cycles 2, 4, and 6 corresponded to scCO2 injections. Each replacement cycle included a 10 min time interval for sample replacement and stabilization as well as a 5 min period for data collection. Injection of the samples into the test chamber was performed over approximately 10 s. This injection ensured that the test chamber was fully occupied by the sample. A pressure drop of about 500 psi (3.45 MPa) was observed as a result of each injection operation. However, the test chamber was immedi-

cc). The Cylinder-2 was connected to the CO2 tank (800 psi (5.52 MPa)) and to Pump-2 (High Pressure Equipment, 30 cc, 10,000 psi (68.95 MPa)). Through the outlet (Port D), the test chamber could be safely purged. The flow of test samples was controlled by the ball valves, V1 to V5. Details of the test chamber structure and its sealing features can be found in Supporting Information. The optical path guided the signals between the optical fiber sensor and an optical interrogator (Micron Optics, SM125). This unit supplied a broadband input signal, in the 1510−1590 nm range, and detected the transmission spectrum through a single channel. The incident and transmitted optical signals of the optical fiber sensor were coupled into this single channel through an optical fiber circulator (Thorlabs, 6015-3-APC). The transmission spectrum was detected at a frequency of 1 Hz by the interrogator. A computer with supporting software package (Micron Optics, ENLIGHT) recorded the transmission spectrum for subsequent analysis. The pressure and temperature of test samples were controlled and monitored in all tests. The pressures in the test chamber and both of the cylinders were monitored by pressure gauges G1, G2 and G3 (2000 psi (13.80 MPa) range, 5 psi (0.03 MPa) accuracy). The test samples and the optical fiber sensor were maintained at a constant temperature by immersion of the test chamber and the two cylinders in a water bath (Fisher Scientific, Isotemp, 5 L capacity, 0.1 °C accuracy), shown as the dashed line in Figure 2a. Figure 2a also shows the contents of the cylinders for the tests involving brine and scCO2. Figure 2b shows the contents of the cylinders for the tests involving brine and CO2-saturated brine. Characterization of the Optical Fiber Sensor. The longperiod grating based optical fiber sensor approach was chosen based its sensitivity to the refractive index of the surrounding medium. To characterize the sensor, two samples were used as refractive index standards, deionized (DI) water and air. The refractive index of DI water was measured by an Abbe refractometer (Officine Galileo) at 40 °C controlled by circulating water between the refractometer and a heating circulator bath (Haake Fisons). The refractive index of air at 40 308



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ately repressured to 1400 psi (9.65 MPa) using either Pump-1 or Pump-2, as appropriate. The process of sample replacement, including injection and repressurization, took approximately 3− 4 min. Detection of CO2−Saturated Brine Relative to Brine. The optical fiber sensor was used to detect CO2-saturated brine relative to pure brine. The purpose of this test was to determine if the sensor could distinguish between CO2-saturated brine and the native brine solution, based on refractive index. The test samples of brine and CO2-saturated brine were prepared and loaded into the high-pressure apparatus, with cylinder contents as shown in Figure 2b. A 3M-brine was loaded into Cylinder-1 and the test chamber. Brine and scCO2 were both loaded into Cylinder-2 and allowed to equilibrate. The brine and the scCO2-brine solutions were both pressurized to 1400 psi (9.65 MPa) with the temperature controlled at 40 °C. The scCO2-brine solution in Cylinder-2 was considered saturated when no pressure drop was observed at gauge G2 after valve V3 was temporarily closed. Initially, the test chamber was filled with brine and data were collected from t = 0−5 min. At t = 5 min, V6 was switched to Cylinder 2, and V5 was temporarily opened. This procedure injected scCO2-brine solution from Cylinder-2 into the test chamber. The full system was repressurized to 1400 psi (9.65 MPa), compensating for the pressure drop caused by the injection. Data were collected from t = 15−20 min. Subsequently, data were collected for 5 min periods at 15 min intervals, with the pressure maintained. The new saturation equilibrium was considered to be re-established if no pressure drop was observed at gauge G2 nor G3 when valve V3 was temporarily closed. Lastly, the test chamber was injected with the original brine solution (t = 125 min).

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RESULTS AND DISCUSSION Characterization of the Optical Fiber Sensor. Figure 3a presents the transmission spectrum of the optical fiber sensor obtained during the calibration tests. The resonance wavelengths of the sensor surrounded by DI water and air at 40 °C are 1556.102 and 1557.705 nm, respectively, which corresponds to a resonance wavelength shift of 1.603 nm. In comparison, the refractive index of DI water at 40 °C is 1.3309 RIU and the refractive index of air at 40 °C is 1.0002,40 corresponding to a refractive index difference of 0.3307. The resonance wavelength shift of long-period gratings has been shown to be approximately linear in the region of refractive indices from approximately 1−1.4 RIU.31 Therefore, the sensitivity of the optical fiber sensor to changes in refractive index is calculated to be 4.847 nm/RIU. Figure 3b shows the resonance wavelength shift of the transmission spectrum as a function of pressure and temperature. A linear relationship is observed between the resonance wavelength and the temperature with a correlation coefficient (R2) of 0.9959. The sensitivity to temperature is 0.054 nm/°C in the range of 35−45 °C. A linear correlation is indicated between the resonance wavelength and the pressure with a correlation coefficient (R2) of 0.9881. The sensitivity to pressure is 0.026 nm/100 psi (0.69 MPa) in the range of 1000−1800 psi (6.89−12.41 MPa). Detection of scCO2 Relative to Brine. Figure 4 shows the resonance wavelength shift corresponding to the alternating cycles of brine and scCO2. Each cycle consisted of a 10 min period for sample replacement and stabilization, and a 5 min period for data collection (1 Hz sampling frequency). Each 5

Figure 3. (a) The transmission spectrum of the optical fiber sensor corresponding to DI water and air at 40 °C. The resonance wavelengths are found to be 1556.102 nm in DI water and 1557.705 nm in air. Given the refractive index value of air (1.0002 RIU) and DI water (1.3309 RIU) at 40 °C, the sensitivity to refractive index (between 1.00 and 1.33) is determined to be 4.847 nm/RIU. (b) The resonance wavelength shift as a function of pressure and temperature. The resonance wavelength shift shows linear correlations with both pressure and temperature, with the correlation coefficients (R2) of 0.9881 and 0.9959, respectively. The sensitivities to pressure and temperature are 0.026 nm/100 psi (0.69 MPa) (1000−1800 psi, or 6.89 to 12.41 MPa) and 0.054 nm/°C (35 − 45 °C), respectively.

min data collection period gives 300 data points indicating either brine, labeled as red rectangles, or scCO2, labeled as green diamonds. A moving average of 20 data points was applied to each 5 min data collection period. A dashed trend line over the 10 min sample replacement and stabilization period is provided as a guide for the eye. As shown, the scCO2 and brine can be clearly distinguished based on resonance wavelength shift. The average value of the wavelength shift corresponding to each of the measurement intervals are, in chronological order: 0.000, 1.125, −0.063, 1.111, 0.018, 1.199, and 0.063 nm. The absolute values of the differences between adjacent periods are 1.125, 1.188, 1.174, 1.093, 1.181, and 1.136 nm, giving an average shift of 1.149 nm which demonstrates a significant contrast between scCO2 and brine. The repeatability of the optical fiber sensor was investigated by analyzing the standard deviations of all cycles corresponding 309



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Figure 4. The resonance wavelength shift corresponds to the alternate cycles of test samples between brine (red rectangles) and scCO2 (green diamonds). Each cycle includes a 10 min time gap for sample replacement and stabilization plus a 5 min period for data collection. A moving average curve is shown as a solid line, and a dashed trend line between data collections is shown as a guide for the eye. The brine and scCO2 are distinguished repeatedly and significantly in terms of the resonance wavelength shift, 1.149 nm as the average value.

Figure 5. The resonance wavelength shift corresponding to CO2-saturated brine solution (blue circles) as compared to the original brine solution (red rectangles). Data are collected in 5 min periods at 15 min intervals. A moving average is plotted as a solid line, and a dashed line provides a guide for the eye between data sets. The saturation equilibrium was achieved after 100 min from t = 5 min when scCO2-brine solution replaced the initial brine. A detectable resonance wavelength shift of 0.192 nm is observed for CO2-saturated brine relative to the original brine solution.

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eye between data sets. The resonance wavelength shift corresponding to brine (t = 0−5 min) provides a reference. The resonance wavelength observed while injecting the CO2saturated brine solution increased from t = 15 min and finally approached a stable level (t = 105−125 min). This increase in resonance wavelength is consistent with a decrease in refractive index accompanying CO2 dissolution into brine. The resonance wavelength of the CO2-saturated brine is a detectable 0.192 nm higher than the original brine solution. This is the first reported optical measurement of CO2-saturated brine. It is important to note that the sensor is responsive to carbon dioxide within the solution only so far as CO2 modifies the refractive index of the solution. At the given conditions of temperature and brine salinity, saturation with CO2 results in a low pH solution (pH 3.0), in which the majority of the dissolved inorganic carbon is in the form of aqueous CO2 (aq). Specifically, according to the data for CO2 solubility47 and the equilibrium model for dissolved inorganic carbon,48 the expected saturation concentrations in the test are as follows: 0.6916 mol/kg CO2 (aq), 0.009 mol/kg HCO3− and negligible CO32‑. As shown, CO2 (aq) represents 99.87% of the total dissolved inorganic carbon (0.6925 mol/kg), and is, therefore, chiefly responsible for measured resonance wavelength shift (0.192 nm) caused by the refractive index change. Unlike the tests in Figure 4, where one pure solution replaces another within the test chamber, the replacement fluid in Figure 5 is initially a CO2-saturated mixture. The solution replacement process necessarily disrupts this scCO2-brine equilibrium (producing CO2 bubbles) and longer times are required for the solution in the test chamber to re-equilibrate. According to Henry’s law, the pressure drop (at relatively constant temperature) would cause a decrease of solubility so that some of the dissolved CO2 would be temporarily released from the solution. Although the solution was repressurized within a few minutes, it takes time for the new solution in the test chamber to recover the original equilibrium composition. During this period, the temperature was maintained by the water bath and the test chamber pressure was maintained by Pump-2. It is important to note that this delay is a function of the solution replacement process, local equilibrium chemistry, and the geometry of test chamber. The inherent sensor response time is governed only by the speed of light and the optical instrumentation and is effectively instantaneous. After 125 min, the CO2-saturated brine solution was replaced with the original brine solution. The corresponding resonance wavelength measurements are shown at the far right of Figure 5. The resonance wavelength shift between the brine (recorded during t = 0 through 5 min) and the reintroduction of the brine was −0.039 nm. This difference is small, compared to the intermediate measurements corresponding to CO2 saturation. Using these results, the refractive index of CO2-saturated brine can also be estimated. By dividing the measured resonance wavelength shift of 0.192 nm by the established refractive index sensitivity of 4.847 nm/RIU, the refractive index difference between CO2-saturated brine and brine is calculated to be 0.0396 RIU (at 40 °C and 1400 psi (9.65 MPa)). Therefore, given that the refractive index of the brine, under the set test conditions, is 1.3559 RIU, the refractive index of the CO2-saturated brine, under the same conditions, can be determined to be 1.3163 RIU. To our knowledge, this is the first published value of the refractive index of CO2-saturated brine solution, a critical parameter for the design of optical

to the same test sample. The standard deviations of the resonance wavelength shifts related to brine and scCO2 are 0.045 and 0.039 nm, respectively. Both deviations are below 4% of the resonance wavelength shift observed between scCO2 and brine (1.149 nm). These results demonstrate repeatability and, therefore, feasibility in detecting scCO2 relative to brine using the method developed here. As further validation, the observed wavelength shifts are compared to those expected from the measured sensitivity and literature values for refractive index. Given the measured refractive index sensitivity of 4.847 nm/RIU, the observed refractive index difference between the brine and scCO2 corresponds to 0.2371 RIU. The refractive index of the brine (3M) is 1.3559 RIU as measured by the Abbe refractometer at 40 °C. Therefore, by subtracting the refractive index difference from the refractive index of the brine, the refractive index of scCO2 is determined to be 1.1188 RIU at 40 °C and 1400 psi (9.65 MPa). This value is within 1.4% of the refractive index of scCO2 reported previously41 at the same temperature and pressure condition (1.1347 RIU). The refractive index of pure brine at 1400 psi (9.65 MPa) is assumed here to be that measured at atmospheric pressure. The pressure effect on refractive index of water has been investigated previously.42 Specifically, measurements up to 54 MPa indicate a change in refractive index with respect to pressure of ∼1.5 × 10−4 RIU/MPa.42 Thus the test pressure of 1400 psi (9.65 MPa) is expected to change the refractive index by ∼0.0014 RIU. Given the sensitivity of the long-period grating (4.847 nm/RIU), this pressure effect is expected to induce a resonance wavelength shift of approximately 0.007 nm, which is small compared to the measured shifts of 1.149 and 0.192 nm, percentages of 0.61% and 3.63%, respectively. Alternatively, the refractive index of liquid can be related to its density by Lorentz−Lorenz formula.43−45 Specifically, the density of NaCl solution changes 1.4% when pressure increases to 20 MPa,46 which indicates a change of ∼0.7% in brine density for the pressure employed here which would correspond to a ∼ 0.0028 RIU shift in refractive index. While it is important to note the effect of pressure on refractive index, from both estimation approaches discussed above, the expected influence is small compared to the shifts measured here. The potential influence of changes in temperature and pressure on the measurement can also be quantified. Sensor characterization indicated sensitivities of 4.847 nm/RIU, 0.054 nm/°C and 0.026 nm/100 psi (0.69 MPa) to refractive index, temperature and pressure, respectively. The temperature control of the water bath has an accuracy of ±0.1 °C which could generate a fluctuation in resonance wavelength of ±0.005 nm. The pressure is controlled within an error of approximately ±25 psi (±0.17 MPa) which would correspond to a fluctuation in resonance wavelength of ±0.006 nm. Therefore, the maximum possible error caused by the sensitivities to temperature and pressure would be up to ±0.011 nm, which is less than 1% of the measured resonance wavelength shift of 1.149 nm. Detection of CO2-Saturated Brine Relative to Brine. Figure 5 shows the measured resonance wavelength shift corresponding to CO2-saturated brine as compared to the original brine solution. The test chamber was initially filled with the brine solution and after 5 min, this solution was replaced with the CO2-saturated brine solution using the cylinder configuration in Figure 2b. A solid line in Figure 5 indicates the moving average, and the dashed line provides a guide for the 311



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instrumentation related to carbon sequestration in saline aquifers. The detection of CO2-saturated brine also indicates potential to detect CO2 concentrations in brine at levels below saturation. Given the ability to differentiate resonance wavelength shifts over 0.001 nm, and the measured shift of 0.192 nm corresponding to saturated CO2 concentrations (∼0.6916 M), the ultimate limit of detection is on the order of ∼0.0036 M. It is important to note, however, that in practice the minimum concentration is likely much larger than this ultimate minimum due to associated errors in, for instance, temperature and pressure controls. In the context of future deployment within or adjacent to geologic sequestration sites, several aspects of the technology are noteworthy. With respect to potential weaknesses, it will be critical to ensure the physical protection of the fiber in the harsh downhole environment while still exposing the periodic sensors to reservoir fluids. Fortunately there is precedent for optical fiber-based measurements in other downhole applications.24 It will also be necessary to measure local temperature separately and, potentially, pressure using additional parallel or in-line fiber optic sensors, as demonstrated previously.49 In application, the local solution refractive index is expected to change from reservoir-specific chemistries as a result of dissolution and precipitation of minerals (in addition to the presence of inorganic dissolve carbon as detected here). Thus similar to controlling for temperature and pressure of a reservoir, reservoir-specific chemistry will need to be considered in practice. In-lab testing as applied here, but with both the applicable reservoir rock and brine, could be used to assess or calibrate for reservoir-specific geochemistries. With respect to strengths, the optical fiber based detection method offers high sensitivity, immunity to electromagnetic interferences, compact size, robustness, low signal loss andperhaps most important in the context of large scale carbon sequestration operationscapacity for multiplexed sensing over large distances. The efficacy presented here indicates the potential for distributed, all-optical monitoring of CO 2 sequestration in saline aquifers.

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ASSOCIATED CONTENT

S Supporting Information *

More details about the structure and sealing features of the test chamber in Figure 2 can be found in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1 416 978 1623; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge funding from the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Networks of Centres of Excellence Program, Carbon Management Canada, Theme C. We also gratefully acknowledge experimental help from Fei Ye of Prof. Li Qian’s lab and summer student Haiyi Wang, and helpful discussions with Prof. Don Lawton, and Prof. David Risk.

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REFERENCES

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IMPLICATIONS The most significant implication of this work is the demonstration that both scCO2 and CO2-saturated brine are detectable at sequestration conditions using optical fiber sensors. Importantly, the established benefits of optical fiber based sensing for environmental monitoring can be leveraged for monitoring carbon sequestration operations. These results indicate feasibility for deployment in observation wells in test injection sites and full-scale commercial projects. An example of a candidate test injection site with observation wells is the current pilot project in Ketzin, Germany.50 Planned full-scale commercial projects with drilled observation wells, include the Bell Creek Project (ten wells) and the Shell Quest Project (seven wells).51 In such operations, the sensor could report both increasingly CO2-saturated brine, as would be expected far from the injector in advance of the CO2 front, as well as a scCO2 plume, as would be expected close to an injection well, or through a leakage pathway/fracture. The detection of saturated brine is particularly significant as it could serve as an early indication of the advancing CO2 front, and also serve to inform models and policy surrounding carbon sequestration operations. 312



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Detecting Supercritical CO2 in Brine at Sequestration Pressure with

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