Method for the determination of residual carbon dioxide saturation using reactive ester tracers

Applied Geochemistry 27 (2012) 2148–2156

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Method for the determination of residual carbon dioxide saturation using reactive ester tracers Matthew Myers a,c,⇑, Linda Stalker a, Andrew Ross a, Christopher Dyt a, Koon-Bay Ho b a

CSIRO Earth Science and Resource Engineering, 26 Dick Perry Avenue, Kensington, WA 6151, Australia National Measurement Institute, 26 Dick Perry Avenue, Kensington, WA 6151, Australia c University of Western Australia, School of Biomedical, Biomolecular and Chemical Sciences, Crawley, WA 6009, Australia b

a r t i c l e

i n f o

Article history: Received 13 April 2012 Accepted 12 July 2012 Available online 21 July 2012 Editorial handling by R. Fuge

a b s t r a c t The mechanisms for storage of CO2 in rock formations include structural/stratigraphic, mineral, solubility and residual trapping. Residual trapping is very important in terms of both containment security and storage capacity. However, to date, the contribution from residual trapping (i.e. immobilisation of supercritical fluid via capillarity in pore spaces) is still relatively difficult to quantify accurately. Using a laboratory-based testing program, this study demonstrates the feasibility of using reactive ester tracers (i.e. triacetin, propylene glycol diacetate and tripropionin), which partition between a mobile water phase and a stationary supercritical CO2 phase, to quantify the residual CO2 saturation, Sgr, of a rock formation. The proposed single-well test involves injecting these tracers into the subsurface, followed by CO2 saturated water, where the ester tracers slowly hydrolyse to form products with differing partition coefficients. After a suitable period of time, allowing for partial hydrolysis, water containing the tracer mixture is produced from the subsurface and analysed using gas chromatography mass spectrometry (GCMS). A numerical simulator of the tracer behaviour in a reservoir is used to explain the differential breakthrough of these tracer compounds during water production to estimate Sgr. Computer modelling suggests that the use of esters tracers to determine CO2 residual saturation is a potentially robust method. The supercritical CO2/water partition coefficients directly dictate the amount of time that each tracer spends in the CO2 and water phases. As such for modelling of tracer behaviour and estimating Sgr, knowing the tracer partition coefficient is essential; in this paper, the first laboratory study to determine the partition coefficients of these reactive ester tracers is described. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction Carbon capture and storage (CCS) technologies are increasingly being refined and tested in pilot, field and commercial-scale demonstrations around the world. Examples include Weyburn, Canada (Wilson, 2004); the Frio Brine Project, USA (Hovorka et al., 2006); the Otway Project, Australia (Sharma et al., 2007); In Salah, Algeria (Ringrose, 2009); Cranfield, USA (Hovorka et al., 2009; Han et al., 2010) and Sleipner, Norway (Torp and Gale, 2004). One of the primary goals of these projects is to better understand and define the dynamics (i.e. plume evolution and changes over time in the various storage mechanisms) of CO2 storage. The storage capacity of the target reservoir in these projects is limited by the trapping efficiency of CO2 in the pore space of the rock matrix. Trapping efficiency and the contribution from each type of CO2 trapping ⇑ Corresponding author at: CSIRO Earth Science and Resource Engineering, 26 Dick Perry Avenue, Kensington, WA 6151, Australia. Tel.: +61 8 6436 8708. E-mail address: [email protected] (M. Myers).

remain the biggest uncertainties in storage capacity assessments (Bachu et al., 2007; Michael et al., 2010). Therefore, a more comprehensive understanding of the trapping mechanisms of CO2 in the rock matrix can produce accurate models with improved prediction of migration and storage potential. This allows for a calculation of the overall capacity of potential sites and reduces uncertainties related to containment security. Proposed storage scenarios most commonly involve injecting CO2 into rock formations at depths where the pressure and temperature exceeds its critical point of (7.38 MPa, 31.1 °C), typically greater than 800 m. There are four primary trapping mechanisms for long term storage of CO2 in saline aquifers or depleted natural gas fields: structural/stratigraphic, mineral, solubility and residual trapping (Metz et al., 2005; Suekane et al., 2008). Over the lifetime of a CO2 storage site, the contribution from each mechanism continually changes (see Fig. 1). Residual trapping is considered to be of greatest importance in the first several hundred years (Han et al., 2010; Metz et al., 2005); however, the contribution of residual trapping in current storage capacity estimates constitutes a

0883-2927/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.07.010

M. Myers et al. / Applied Geochemistry 27 (2012) 2148–2156

Fig. 1. Storage security depends on a combination of physical and geochemical trapping. Over time, the physical processes of residual CO2 trapping and geochemical processes of solubility trapping and mineral trapping increase. This figure is reproduced from Carbon Dioxide Capture and Storage, IPCC, 2005, Cambridge University Press, UK, Fig. 5.9 on p. 208 with permission.

large uncertainty. Quantifying the residual CO2 saturation of a rock formation is, therefore, important in assessing the viability of large volume/long term storage reservoirs. This study demonstrates the feasibility of using reactive ester tracers (i.e. triacetin, propylene glycol diacetate and tripropionin) to quantify the amount of residually trapped CO2 through an integrated program of laboratory experiments and computer simulations. Accurately determining the tracer partition coefficients and understanding the hydrolysis kinetics of these tracers is essential for both determining the feasibility of the proposed tracers in field studies and any subsequent modelling of the tracer behaviour to estimate residual saturation. Laboratory high pressure/high temperature sampling experiments were implemented to determine the CO2/water partition coefficients and understand the in situ hydrolysis kinetics. This tracer research program was designed to replicate the reservoir conditions at the Otway Project in Victoria, Australia to assess the potential for a field trial as a component of the CO2CRC Stage 2B Residual Saturation and Dissolution Test (Zhang et al., 2011).

1.1. Background This paper is focused on methods to quantify the mechanism of residual CO2 saturation, also known as residual CO2 trapping, using reactive tracers. Residual CO2 trapping occurs when the supercritical fluid displaced by water is immobilised via capillarity in pore spaces. The non-wetting phase (i.e. supercritical CO2) is trapped in pore spaces by the wetting phase (i.e. water) in the pore throats. Estimates of residual trapping range from 20% to 40% of overall trapped CO2 (Bachu et al., 2007; Flett et al., 2007; Bennion and Bachu, 2008; Suekane et al., 2008). Laboratory studies that mimic or replicate subsurface conditions using core floods have been an invaluable tool in studying the various trapping mechanisms of supercritical CO2 in rock formations (Muller, 2011). Suekane et al. (2008) attempted to quantify residual and solubility trapping using the well-characterised Berea Sandstone. They were able to estimate maximum trapped gas saturation of 25–28%, over a range of temperatures (38–50 °C) and pressures (7.6–10.0 MPa), representing depths from 750 to 1000 m.

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To date, measurement or prediction of residual trapping beyond the core scale in a particular formation remains difficult due to abundant subsurface heterogeneities. According to Han et al. (2010), research in this area is hampered by three key factors: (i) not all trapping mechanisms are considered simultaneously in the models, (ii) simplification of the models is such that heterogeneities are effectively neglected and (iii) heterogeneity and associated data from field trials are either absent or not used. The primary limitation of core-flood tests to determine rock properties is that the core is only representative of the very near-well environment and that extrapolations of these results is unlikely to be accurate. A pulsed neutron test or reservoir saturation test (RST) can be used to estimate the amount of displaced formation water and from this to infer the residual CO2 saturation; however, this test, like laboratory tests, is also only representative of the near well environment (Hovorka et al., 2006). To better understand rock behaviour on a larger scale, several CCS projects have used chemical tracers. They have been used to distinguish native and injected CO2 and to aid in the measurement of the first arrival of CO2 in the formation (Freifeld et al., 2005; Boreham et al., 2011; Johnson et al., 2011). Inert perfluorocarbon, Kr and SF6 were used in the Frio Brine Project, USA (Freifeld et al., 2005) and for In Salah, Algeria (Ringrose, 2009). These tests have yielded information which has improved the understanding of CO2 migration and has also provided constraints for models. For the CO2CRC Otway Project in the Otway Basin in Victoria, Australia, SF6, Kr, perdeuterated CH4 and 1,1,1,2-tetrafluoroethane were used to observe the evolution in CO2 saturation (Stalker et al., 2009; Boreham et al., 2011). Results to date have shown discrete changes throughout the reservoir with differences in CO2 arrival times and gas/tracer compositions at varying sampling depths. 1.2. Rationale A single well ‘‘push–pull’’ test using Kr and Xe as tracers is currently being considered at the CO2CRC Stage 2B test (Zhang et al., 2009, 2011) to characterise reservoir heterogeneity and quantify residual gas saturation. Single well ‘‘push–pull’’ inert gas tests consist of first injecting a suite of tracers dissolved in formation water (e.g. SF6 or noble gases) into the target reservoir and then producing water samples from the same well used for injection (see Fig. 2). In addition, for ‘‘push–pull’’ tests, each inert tracer is exposed to approximately the same volume of rock during both injection and production. If the supercritical CO2–water partition coefficients of each tracer are known, then the relative differences in the dispersion seen in the breakthrough profiles (see Fig. 4) between tracers can be correlated to the residual gas saturation, Sgr, using a simulation of tracer behaviour (Zhang et al., 2011). However, these models are susceptible to large errors due to several factors, including a lack of partitioning coefficient data for many species in different formation waters with variable concentrations of dissolved cations and anions and varying pH, temperature and pressure (Curren and Burk, 2000). Formation heterogeneities, flow channelling, diffusion and other phenomena also contribute to considerable errors in these models. Multiple tracers are typically injected for these experiments and the breakthrough curves for each tracer are compared; the risk is that the differences in these curves will be minor and the observed retention times might be nearly identical (see Fig. 4). Nevertheless, modelling of these differences is the basis for determining residual saturation, potentially leading to large errors in the estimates (Zhang et al., 2009, 2011). This paper proposes an alternative to this method using reactive ester tracers which, after injecting the primary or ‘‘parent’’ tracer compounds, undergo a subsurface hydrolysis reaction to generate ‘‘daughter’’ tracers (see Fig. 3). Amides, which are known to hydrolyse, are not typically used in this type of test due to the much

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(a)

(a)

(b)

(b)

(c)

(c)

= CO2 partitioning tracer

= CO2 partitioning tracer converting to water partitioning tracer

= CO2 partitioning tracer

= water partitioning tracer

Fig. 2. Idealised schematic of inert tracer plumes in the subsurface. (a) Injection of two inert tracers (blue and red) into the formation. (b) ’Soak’ for inert tracers. (c) Production for inert tracers (blue and red) from formation.

harsher conditions (very low or very high pH, high temperature) necessary for the hydrolysis reaction. This technique using ester tracers is based on a patent by Deans (1971) where esters (e.g. ethyl acetate) were used to determine residual oil saturation in depleted oilfields (i.e. one that is at residual oil saturation). This patent details a field experiment where ethyl acetate is injected into a depleted oilfield, followed by water, and then allowed to ‘soak’ for several days while the ethyl acetate partially hydrolyses to yield ethanol and acetic acid. During water production, the unreacted and more hydrophobic ethyl acetate preferentially partitions into the oil phase, while the more hydrophilic ethanol preferentially partitions into the water phase. In this case, compared to the inert tracers, there is a significant difference in the retention time allowing for a more sensitive and accurate determination of the residual saturation (see Fig. 4). Chromatographic/reaction models of the concentration profiles, in which the movement of analytes is primarily dictated by phase partitioning, were shown to be effective in determining the residual oil saturation level in this reservoir (Deans, 1971; Tomich et al., 1973).

Fig. 3. Idealised schematic of reactive tracer plumes in the subsurface. (a) Injection of parent reactive tracer (blue) into formation. (b) ’Soak’ for reactive tracers in which the parent reactive tracer (blue) partially hydrolyses forming the water partitioning tracer (green). (c) Production for reactive tracers (blue and green) from formation.

The use of multiple tracers combining both inert and reactive tracers is potentially a powerful tool to reduce the ambiguities caused by using only inert tracers (Tomich et al., 1973; Field and Pinsky, 2000; Geyer et al., 2007). The purpose of this study was to identify several reactive ester tracers and test their usefulness for determining the quantity of residually trapped CO2 in geological formations under a wide range of conditions. The reactive ester tracers will have a higher partition coefficient into CO2 compared to water and are referred to in this paper as CO2 partitioning tracers. The corresponding hydrolysis products of the esters will, compared to the parent esters, be partitioned into the water more and are referred to in this paper as water partitioning tracers.

1.3. Selection and characterisation of tracers The laboratory characterisation of selected tracers was undertaken at pressure and temperature regimes appropriate for injec-

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INERT

0

0

Concentrattion

Concentrattion

REACTIVE

0

0

Time

Time

= CO2 partitioning tracer = water partitioning tracer = CO2 partitioning tracer converting to water partitioning tracer Fig. 4. Idealised schematic of how (a) inert (or non-reactive) partitioning tracers vs. (b) reactive partitioning tracers may behave in a push–pull test. Time zero is taken to be the start of the pull section of the test where fluids are withdrawn from the well after a soaking period. In (a) the process mainly relies on subtle chromatographic effects (due to the inert nature of the tracer) and reservoir heterogeneity; while in (b) the initial parent tracer returns at a different rate compared to the daughter products with different chemistry and functionality.

tion into the target reservoir at the CO2CRC field site. The critical parameters set for assessing the suitability of individual esters as tracers in field trials are as follows: 1. 2. 3. 4. 5.

Environmental safety. High solubility in water. Detectability of tracers at low concentrations using GCMS. Suitable hydrolysis kinetics. Differential partitioning of parent and daughter products between supercritical CO2 and water. 6. Minimal effects on the surface properties of the rock matrix. Once a suite of suitable chemicals had been identified an experimental program to determine their supercritical CO2/water partition coefficients was set-up. This required the design and commissioning of a high-pressure cylinder which allowed for the sampling of fluids at reservoir conditions. The selection criteria for the proposed tracers will be addressed in detail in the Discussion section of this manuscript. 2. Materials and methods Two triglycerides (triacetin and tripropionin) and propylene glycol diacetate were tested against the above listed criteria to determine their suitability as tracers for use in field trials. The experimental program was designed to assess the hydrolysis of these parent tracers and to determine the supercritical CO2/water partition coefficients for the parent tracers and their suite of daughter products. 2.1. Chemicals Triacetin (Aldrich, W200700, >98.5%, food grade), tripropionin (Aldrich, W328618, food grade) and propylene glycol diacetate (Al-

drich, 528072, >99.7%) were used as received. Distilled water and 99.99% grade CO2 (BOC) were used for laboratory experiments. 2.2. Sampling from high-pressure apparatus A high-pressure and high-temperature chamber (see Fig. 5) was designed to allow the experimental fluids to react at reservoir conditions. Pressure was maintained between 14 and 17 MPa (2050– 2450 psi) and temperature at 62 °C. The vessel was constructed of alloy 316 stainless steel with a capacity of 2.0 L and 1=4 in. (6.35 mm) NPT threading on both top and bottom. A piston compression vessel (also 316 alloy stainless steel) with a compression capacity of 800 mL was attached with 1=4 in. (6.35 mm) NPT threading (see Fig. 5). Water was used in the piston pump to compress fluids in the reservoir vessel to the desired pressure. Both vessels were equipped with H83 series Swagelok valves. For the partitioning studies, the system was filled with 1.9 L of distilled water and 2.00 (±0.05) g of each parent tracer. The vessel was pressurised with CO2 (99.99% grade) to approximately 5 MPa from a G-size cylinder and the cylinder left open overnight to allow CO2 to diffuse into the water and reach equilibrium. After overnight equilibration, the vessel was sealed from the gas cylinder and the temperature raised to 62 °C using heating tape and a custom built PID (proportional, integral, derivative) type temperature controller. After heating for 1 h, a piston pump with a pressure proportional feedback loop controller was used to increase the pressure of the vessel. After the pump had stopped moving indicating that the pressure/temperature equilibrated to the desired conditions, the valve between the sampling vessel and the piston pump was closed. Once the temperature was attained, the fluids were allowed to equilibrate and partially hydrolyse over a period of 6–20 days. During this time, fluid samples were acquired and analysed in pairs, one from the supercritical CO2 phase (taken at the top of the ves-

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SAMPLE LOOP RINSE

PRESSURE RELIEF VALVE

PISTON PUMP

WATER INJECTION

SAMPLE LOOP CO2

PRESSURE RELIEF VALVE

WAT W TER AND A D CO2 RE VE PRESSUR ESSEL E NG TAPE HEA ATIN

V SEL W VESS WITH H SE EPAR RATO OR FOR COM MPRE ESSING FLUIDS USIN NG PIST TON PUM MP

SAMPLE COLLECTION

CO2 CYLINDER

PRESSURE GAUGE SAMPLE COLLECTION SAMPLE LOOP H2O

SAMPLE LOOP RINSE

WATER REMOVAL

Fig. 5. A schematic of apparatus used to determine tracer hydrolysis rates and partition coefficients.

sel) and one from the water phase (taken at the base of the vessel). The water sample was obtained by opening the main valve between the high-pressure vessel and a second immediately adjacent high pressure valve, allowing the void volume (which was measured independently) between the two valves to fill up in the manner of a sample loop. After 5 min (to allow for any equilibration) the main valve was closed and the water collected into a vial. The sample loop was then flushed with acetone to remove any residual sample into the vial. After sample collection, the sample loop was flushed again with acetone and then dried thoroughly with compressed air. The samples from the supercritical CO2 phase were collected in a similar manner. The upper main valve to the high-pressure vessel was opened where the fluid was allowed to equilibrate in the volume between the two valves, then the main vessel was sealed again. To control the high gas pressure, a needle valve was used in combination with a ball valve to bubble the gas slowly through 5 mL of acetone. The sample loop was then flushed with acetone to remove any residual sample into the vial. After sample collection, the sample loop was flushed again with acetone and then dried with compressed air. After collection, both samples were refrigerated (4 °C) before subsequent preparation for GCMS analysis.

2.3. GCMS analysis Each pair of samples were analysed using two different GCMS methods: the first method to accurately quantify the parent tracers (i.e. triacetin, propylene glycol diacetate and tripropionin) and the second method to quantify the daughter products. 2.3.1. Method to quantify parent tracers A standard aliquot of 200 lL of the water sample in acetone was further diluted to 1 mL with acetone and injected onto the column. The gas extracts in acetone were directly injected without further dilution. Analysis was performed in full scan mode with an Agilent 6890 GC/Agilent 5973 inert MSD fitted with a Grace AT-AquaWaxDA column (30 m  0.25 mm i.d., 0.25 lm film thickness). The carrier gas was He with a constant column flow rate of 1.3 mL/min. Injection temperature was 300 °C and injection volume was 1 lL (via an injector in splitless mode). An initial oven temperature of 40 °C was held for 3.0 min then ramped at 15 °C/min to 160 °C. The oven temperature was then held at 160 °C for 6 min before being ramped to 200 °C at 40 °C/min and held at that temperature for a further 12 min. The total run time was 30 min. The MSD conditions were typically: ionisation energy 70 eV, source temperature 230 °C and electron multiplier voltage 1700 V.

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2.3.2. Method to quantify the daughter products This method required the removal of most water from the sample to allow for complete derivatisation (a necessary condition for quantification). The water samples in acetone and the gas extracts in acetone were treated identically. 0.5 mL of the sample was vortex mixed with 0.5 mL of acetone and 100 lL of dimethyl sulphoxide in a 2 mL reaction vessel fitted with a cap. Using a gentle flow of N2, the volume was concentrated by heating to 30–40 °C for 2– 3 h. This was then vortex mixed with 0.5 mL of acetone. The products were derivatised by the addition of 200 lL of N,O-bis(trimethylsilyl)acetamide (BSA). The mixture was vortex mixed again and then incubated at 70 °C for 2 h. After functionalisation, the sample was made up to 1 mL volume with acetone and then vortex mixed. 200 lL of the functionalised sample was diluted to 1 mL with acetone for injection onto the column. Analysis was performed in full scan mode with an Agilent 6890 GC/Agilent 5973 inert MSD fitted with a J&W DB-5MS column (30 m  0.25 mm i.d., 0.50 lm film thickness.). The carrier gas was He with a constant column flow rate of 1.3 mL/min. Injection temperature was 300 °C and the injection volume was 1 lL (via an injector in pulsed splitless mode at 25 psi for 0.30 min). The initial oven temperature was held at 40 °C for 1.0 min then programmed at 15 °C/min to 180 °C. The oven temperature was immediately ramped to 230 °C at 7 °C/min then to 320 °C at 30 °C/min. The oven temperature was held at 320 °C for a further 3.0 min. The total run time was 23.48 min. The MSD conditions were typically: ionisation energy 70 eV, source temperature 230 °C and electron multiplier voltage 1700 V. Concentrations of standards used in constructing the calibration curves ranged from 2 to 70 lg/mL. Recovery of a blank solution spiked with propylene glycol and glycerol at 11–12 lg/mL was carried through the process in the second GCMS method and ranged from 83–111%. Compounds were identified by library matches and calibration. Standard peak areas were established using an automatic integration function followed by manual baseline adjustment when required. 2.4. Computational modelling A two-dimensional finite difference simulator had been previously developed to determine residual oil saturation (Tomich et al., 1973) and used extensively for the past several decades to model single well chemical tracer tests (Wellington et al., 1994). A similar simulator written in FORTRAN that uses a radial grid was used here. In this simulator, at each time step the velocity field for the tracer movement/partitioning and the tracer hydrolysis kinetics are solved. This model can account for radial flow during injection and production, reaction kinetics in the subsurface, dispersion and tracer partitioning. The model assumes that (a) the amount of residually trapped fluid is constant, (b) the residually trapped fluid is stagnant, (c) the tracer only partitions between the water and supercritical CO2, (d) the tracer concentrations in the water and supercritical CO2 are always at equilibrium and (e) the simulated formation is isotropic. A very small reaction rate constant is implemented with a large ‘‘soak’’ time so that the amount of reaction occurring during tracer injection and production is negligible. A 2-dimensional radial grid with only one vertical layer and 200 concentric circles with a spacing of 0.04 m and 8 equally spaced angles was used. The dimensionless partition coefficients based on mass concentration (i.e. 5 and 0.2 for the CO2 partitioning tracer and water partitioning tracer, respectively) are used in this model. Furthermore, the reservoir porosity is set to 28%, the reservoir thickness to 7 m, the volumes of water for tracer injection to 5 tonnes, the volume of water for tracer production to 48 tonnes and the residual CO2 saturation to 20%.

3. Results 3.1. GCMS tracer analysis and hydrolysis kinetics GCMS analysis of the samples from the high pressure hydrolysis/partitioning study showed that a 0.1 wt.% (or 1000 ppm) initial concentration of the parent tracers was sufficient for quantitative detection of the parent tracers and their daughter products. Despite the complexities of both the hydrolysis reaction in the water phase and the partitioning between the supercritical CO2 and water phases acting on the tracers, the GCMS results (results not shown) demonstrated for water and supercritical CO2 phases a decrease in concentrations of triacetin, propylene glycol diacetate and tripropionin over time and an increase in concentration of acetic acid, propionic acid and other daughter products. Furthermore, consistent with literature reported hydrolysis rate constants in water, the rate of hydrolysis of tripropionin is much slower compared to the hydrolysis of either propylene glycol diacetate or triacetin (Washburn).

3.2. Partition coefficients Partition coefficients were determined using a previously published method predicated on separately measuring the equilibrium concentration of each species in both phases using GCMS (Timko et al., 2004). For comparison over a range of pressures (or supercritical CO2 densities), partition coefficients are typically reported x on a solute/solvent mole-fraction and represented as ki . However, the partition coefficients, ki, used in determining residual supercritical CO2 capacity and determined by the ratio of the GCMS rex sponses for the two phases, are based on mass concentrations. ki x qw Mc and ki are related by ki ¼ q Mw ki , where qw and qc are the densities c of water and supercritical CO2, respectively, and Mw and Mc are the molecular weights of water and CO2, respectively. As such the measured partition coefficients are corrected to a constant pressure using this relationship. Furthermore, due to the ongoing hydrolysis reaction occurring, the concentrations of each of the chemical species is changing as well. For the purposes of averaging the meaTable 1 Partition coefficients for parent compounds and tracer compounds at 62°. The reported partition coefficients and associated errors were determined by analysing five different sets of samples. Compound

Partition coefficient based on mole fraction of solute x in solvent, ki

Partition coefficient based on concentration at 15 MPa, ki

Propylene glycol diacetate Propylene glycol monoacetate 1 Propylene glycol monoacetate 2 Triacetin Diacetin 1 Diacetin 2 Monoacetin 1 Monoacetin 2 Tripropionin Dipropionin 1 Dipropionin 2 Monopropionin 1 Monoproprionin 2 Acetic acid Propionic acid Glycerol Propylene glycol

54.5 ± 5.3 1.04 ± 0.12 9.79 ± 2.22 27.7 ± 5.6 0.837 ± 0.214 0.769 ± 0.107 0.876 ± 0.272 0.163 ± 0.057 313 ± 58 5.09 ± 0.85 4.83 ± 1.06 8.72 ± 1.64 0.349 ± 0.089 0.914 ± 0.256 1.50 ± 0.60

8.77 ± 0.86 0.167 ± 0.019 1.57 ± 0.36 4.46 ± 0.90 0.135 ± 0.034 0.124 ± 0.017 0.141 ± .044 0.0263 ± 0.0093 50.3 ± 9.3 0.820 ± 0.137 0.778 ± 0.170 1.40 ± 0.26 0.0562 ± 0.0143 0.147 ± 0.041 0.241 ± 0.097

a

a

a

a

a Glycerol and propylene glycol were not detected in the supercritical carbon dioxide phase. This is presumably due to their low solubility in supercritical carbon dioxide.

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Fig. 6. Hydrolysis pathways for the triglycerides (triacetin and tripropionin) (top) and propylene glycol diacetate (bottom).

sured partition coefficients from different samples (see Table 1), the change in concentration is assumed to have a negligible effect. The justification for this assumption and the correction of the partition coefficients for varying fluid densities is discussed below (see Section 4.2). At a pressure of 15 MPa, the partition coefficients, ki, for the parent compounds range from 4.46 for triacetin to 50.3 for tripropionin (see Table 1). Likewise, the partition coefficients for the daughter products of propylene glycol diacetate range from 0.17 to 1.6, the daughter products of triacetin range from 0.026 to 0.14 and the daughter products of tripropionin range from 0.056 to 1.4. It is notable for both monoacetin and monopropionin that the two different isomers have dramatically different partition coefficients (0.88 vs. 0.16 and 8.72 vs. 0.35, respectively) suggesting that differences in H-bonding between the two alcohol groups in the two isomers has a substantial effect (see Table 1). 4. Discussion 4.1. Environmental safety, water solubility and GCMS detectability Triacetin and tripropionin are considered food derivatives (MSDS information from Aldrich) and propylene glycol diacetate has low toxicity and is fully biodegradable (product safety assessment from Dow Chemical). Triacetin is commonly used as a food additive and has a water solubility of 72 g/L at 25 °C. Triproprionin (water solubility of 2.6 g/L at 25 °C) and propylene glycol diacetate (water solubility of 100 g/L at 25 °C) are used as a solvents or diluents for flavour and fragrance agents. As a consequence, it is considered that large volumes can potentially be utilised to perform well tests without significant health and environmental hazards, both at the surface during well test preparation and in the subsurface after injection. Choosing substances that will not impact on potable aquifers (if leakage were to occur) fulfils regulatory requirements as a part of the strict conventions relating to the application of CCS in different parts of the world. The sensitivity of the GCMS and the effect of the water matrix on detection dictate the quantity of tracer necessary for injection into a formation for a notional field trial. For example, for a concentration of 1000 ppm, 150 kg of tracer would need to be injected into the formation, followed by 150 tonnes of CO2 saturated water (CO2 saturated water is required to prevent dissolution of residually trapped CO2). These compounds have the potential to impact on the subsurface system by either changing the water properties

or by adhering to surfaces, so minimising the amount of tracer is important (see Section 4.3). 4.2. Hydrolysis kinetics and partition coefficients The sequential hydrolysis of triglycerides is represented by A3 (triglyceride) ? A2 (diglyceride, both isomers) ? A1 (monoglyceride, both isomers) ? A0 (glyercol). Similarly, the sequential hydrolysis of propylene glycol diacetate is represented by A2 (propylene glycol diacetate) ? A1 (propylene glycol monoacetate, both isomers) ? A0 (propylene glycol) (see Fig. 6). For both the triglycerides and propylene glycol diacetate, the rate constant for each hydrolysis step is nearly equivalent (Washburn). In this case, the multi-exponential mathematics of sequential first order reaction kinetics predicts that there are relatively large time frames in which detectable quantities of each tracer (i.e. parent tracer and daughter products) can be attained. This is confirmed by the presence of both parent and all daughter products for hydrolysis times (or ‘soak’ times) ranging from 6 to 20 days. This results in considerable field trial design flexibility. Furthermore, in a similar setup, the equilibration time required for steady state diffusion between supercritical CO2 and water has been shown previously less than 24 h (Bahar and Liu, 2008). As the reaction kinetics is slow in comparison (6–20 days), the assumption that the system is at equilibrium during sampling is valid. Timko et al. (2004) showed for a wide variety of compounds that the error in the partition coefficient due to pressure (or density) interpolation is less than 20% and is much lower if the differences in pressure are small. However, the measured partition coefficient was markedly affected when the concentration is comparable to the saturation level in either of the phases. For the present studies, the initial concentration of the parent compounds (triacetin, propylene glycol diacetate and tripropionin) of 0.1 wt.% is much lower than the solubility in both supercritical CO2 and water. Furthermore, the change in pressure of the fluid during the various rounds of sampling is less than 20%. As such, the error due to sampling at different pressures and species concentrations is expected to be smaller than other sources of experimental error (e.g., sampling and GCMS analysis). 4.3. Tracer effects on the subsurface formation Residual trapping is the immobilisation of a fluid via capillarity in formation pore spaces. The threshold capillary pressure, in

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which a non-wetting phase (supercritical CO2) begins to flow in porous formation with a wetting phase (water) is given by 2cbrine;CO cosh 2 P th where cbrine;CO2 is the brine-supercritical CO2 interc ¼ R facial tension, h is the water contact angle in the presence of supercritical CO2 and R is the size of the largest connected pore throat in the formation (Chalbaud et al., 2009). In determining CO2 residual saturation, it is important that the tracers have a minimal effect on fluid viscosity, the brine-supercritical CO2 interfacial tension and the water-rock contact angle. High pressure studies on CO2–water–alcohol mixtures show that at a temperature of 65 °C and pressure of 15.51 MPa, there is less than a 20% decrease in interfacial tension for a 5 wt.% solution of ethanol in water relative to pure water (Chun and Wilkinson, 1995). The proposed organic tracers and their daughter products are not surfactants and are expected to have a similar effect on the interfacial tension as ethanol; at very low concentrations (i.e. 1000 ppm or 0.1 wt.%) the change in interfacial tension should be minimal. Furthermore, the expected change in viscosity as predicted by the Refutas equation should be negligible at these low concentrations (Maples, 2000). Sandstones are the primary rock material for CO2 injection. The adsorption of organic materials onto clay minerals in sandstones is significant and has an effect on wettability (Pan et al., 2005). Depending on the clay content of a sandstone formation, the adsorption of the organic tracers onto the formation rock may have an effect on the contact angle; however, the dilute concentrations proposed for use in field trials should limit this effect. However, if the formation has a high adsorption capacity for organics, tracer recovery during production may be very low; in this scenario, the usefulness of this technique may be very limited. 4.4. Modelling tracer behaviour to determine residual saturation The single well reactive ester tracer test for determining residual oil saturation was developed by Tomich et al. (1973). Computational modelling of the chromatographic behaviour of the reactive ester tracers was validated in field trials and demonstrated that robust and accurate determinations of residual oil saturation could be obtained. For a representative set of tracers (the partition coefficients are 5 and 0.2 for the CO2 and water partitioning tracer, respectively) in a reservoir at 20% residual CO2 saturation, this simulator yields the breakthrough curves in Fig. 7. Comparing the breakthrough curves for the CO2 (blue1 line) and water partitioning tracer (red line) simulates an inert tracer test. Here the retention times are almost identical and the breakthrough curve for the CO2 partitioning tracer is only slightly broader than the curve for the water partitioning tracer. Comparing the curves for the CO2 partitioning tracer (blue line) and the hydrolysis product (green line) which simulates a reactive ester test, the expected chromatographic separation is seen. Modelling this separation in the breakthrough curves has been used successfully and extensively with reactive ester tracers to estimate residual oil saturation in the near-well reservoir volume (Wellington et al., 1994). In this study (laboratory experiments and computation modelling), the adsorption of organic compounds onto a rock formation has not been considered, but has the potential to significantly affect the partitioning of tracers and may limit tracer recovery from the subsurface for analysis. Likewise, for the reactive ester tracers that have previously used ethyl acetate to determine residual oil saturation, this has not been considered (Tomich et al., 1973). With the exception of possible organic material (e.g. clays) within the reservoir, the sorption of the tracers used in this study is expected

1 For interpretation of colour in Figs. 1–5 and 7, the reader is referred to the web version of this article.

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Fig. 7. Simulated concentration (vertical axis, maximum normalised to 1) vs. produced water volume (horizontal axis, tonnes) curves using a simple chromatographic model (top) and using a TOUGH2 model (bottom). The blue curves are for the CO2 partitioning tracer which is injected and then produced from the same well. The red curves are for the water partitioning tracer which is injected and then produced from the same well. The green curves are for the reactive tracer in which the CO2 partitioning tracer is injected and then the water partitioning tracer is produced from the same well.

to be minimal and reversible; as such, it will have a secondary effect (i.e. minor change in the shape of the breakthrough curve) on the tracer breakthrough profile (Myers et al., 2012). Given the robust nature of the modelling used here, it is expected that the effect on the estimated residual saturation capacity will be minimal. 4.5. Methodology for using reactive tracers in a field experiment A summary of the proposed program for using reactive ester tracers to determine residual saturation is as follows: 1. Establishment of a reservoir at residual saturation (methods for doing this are discussed elsewhere (Zhang et al., 2011)). 2. Injection of a single pulse of tracers followed by CO2-saturated water (to maintain residual saturation) into the same formation. The volumes of tracer and water used are dependent on geological parameters (e.g. porosity), size of the perforated interval and the desired volume around the well bore to be interrogated. Laboratory experiments suggest that the amount of tracer added should be 0.1% (or 1000 ppm) of the amount of water added. 3. Allowance of ‘soak’ time (approx. 6–20 days) for the injected tracer/water mixture, to enable partial hydrolysis of the tracers to occur. 4. Production and sampling of water from the same well used for injection. 5. Laboratory analysis of tracers in water and subsequent computational modelling to determine residual saturation. 5. Conclusions The tracer partition coefficients suggest that the choice of tracer is important in terms of the amount of geological formation sampled and the hydrolysis rate of the tracer compound. If the partition coefficient is low relative to Sgr, a relatively small chromatographic separation will be observed and the prediction of Sgr will be prone to significant errors. Likewise, if the partition coefficient is high relative to Sgr, the tracer’s movement will be highly retarded and a relatively small amount of the formation will be sampled resulting in a prediction of Sgr that may not be indicative of the formation. Furthermore, the differing hydrolysis kinetics between the tracers (i.e., propionate esters hydrolyse slower than acetate esters) must be considered in terms of formation tempera-

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ture and pH when choosing a tracer. It is important then to match the conditions (i.e. temperature, residual supercritical CO2 capacity, pH, ‘soak’ time) with the choice of parent tracer compound to ensure that an appropriate amount of the parent tracer has hydrolysed. Due to differences in hydrolysis rates, the acetate esters may be more appropriate for lower temperature regimes or when the time frame for the ‘soak’ period is more limited, while the propionate esters may be more appropriate for higher temperature regimes. The amount of tracer that needs to be added depends on several parameters including recovery from the subsurface, volume of water injected and the GCMS quantification/detection limits for the organic tracers. When there is little knowledge beforehand of the downhole conditions, it is attractive to use a suite of tracers (e.g. triacetin, propylene glycol diacetate and tripropionin) with differing partition coefficients and hydrolysis kinetics so that adequate chromatographic separation and suitable kinetics will occur with at least one of the tracer compounds. In the few cases where the daughter products are identical for separate parents tracers (i.e., glycerol, acetic acid and propionic acid), the concentrations profiles for these daughter products are not used to determine estimates of residual saturation and instead the other daughter product (e.g., diacetin, propylene glycol monoacetate and dipropionin) are used. The conditions for this study have been chosen to closely mimic the conditions (i.e. pressure and temperature) for the planned CRC Stage 2B field test. It is expected, based on previous studies, that both the temperature and water composition will have an effect on the hydrolysis kinetics. The wide range of suitable set-in times (or reaction times) in which both the parent compounds and their daughter products can be detected demonstrates that the effects of pH and temperature on kinetics can be easily compensated by adjusting the choice of tracer and/or ‘soak’ time. Acknowledgements We would like to acknowledge CSIRO and NMI for their funding and permission to publish this manuscript, the CO2CRC for releasing carbon dioxide storage field trial details allowing us to design appropriate experiments, Keyu Liu for permission to use his high pressure apparatus and Karsten Michael and Allison Hortle for advice and editing of the manuscript. We would also like to thank our CSIRO internal reviewers for their valuable contributions in improving this manuscript for publication. References Bachu, S., Bonijoly, D., Bradshaw, J., Burruss, R., Holloway, S., Christensen, N.P., Mathiassen, O.M., 2007. CO2 storage capacity estimation: methodology and gaps. Int. J. Greenhouse Gas Control 1, 430–443. Bahar, M., Liu, K., 2008. Measurements of the diffusion coefficient of CO2 in formation water under reservoir conditions: implication for CO2 storage. In: 2008 SPE Asia Pacific Oil & Gas Conference and Exhibition. Society of Petroleum Engineers, Perth, Australia, SPE 116513. Bennion, D.B., Bachu, S., 2008. Drainage and imbibition relative permeability relationships for supercritical CO2/brine and H2S/brine systems in intergranular sandstone, carbonate, shale, and anhydrite rocks. SPE Reservoir Eval. Eng. 11, 487–496. Boreham, C., Underschultz, J., Stalker, L., Kirste, D., Freifeld, B., Jenkins, C., EnnisKing, J., 2011. Monitoring of CO2 storage in a depleted natural gas reservoir: gas geochemistry from the CO2CRC Otway Project, Australia. Int. J. Greenhouse Gas Control 5, 1039–1054. Chalbaud, C., Robin, M., Lombard, J.M., Martin, F., Egermann, P., Bertin, H., 2009. Interfacial tension measurements and wettability evaluation for geological CO2 storage. Adv. Water Resour. 32, 98–109. Chun, B.-S., Wilkinson, G.T., 1995. Interfacial tension in high-pressure carbon dioxide mixtures. Ind. Eng. Chem. Res. 34, 4371–4377.

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