Article pubs.acs.org/EF
Characterization and Comparison of Dissolved Organic Matter Signatures in Steam-Assisted Gravity Drainage Process Water Samples from Athabasca Oil Sands Rajesh G. Pillai,*,† Ni Yang,† Steven Thi,† Jannat Fatema,† Mohtada Sadrzadeh,† and David Pernitsky*,†,‡ †
Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada Suncor Energy Inc., P.O. Box 2844, 150 Sixth Avenue SW, Calgary, Alberta T2P 3E3, Canada
‡
S Supporting Information *
ABSTRACT: Steam-assisted gravity drainage (SAGD) process water contains high concentrations of dissolved organic and inorganic matter. A wide range of analytical techniques including electrospray ionization mass spectrometry, gas chromatography−mass spectrometry, Fourier transform infrared spectrometry, and fluorescence spectrophotometry have been utilized for the identification and measurement of dissolved organic matter (DOM) in oil sands process-affected water. The composition of DOM in the SAGD water is relatively complex, and thus one plausible method for its analysis is the fractionation of DOM into hydrophilic and hydrophobic portions using suitable resin columns and the characterization of these fractions using standard analytical methods. Comparing the fractionation and characterization of the SAGD produced water from different plant sites can provide considerable insight into better management, recycle, and reuse of this process water. Also, a detailed knowledge of the chemical composition of the SAGD produced water provides guidelines for identifying the constituents that are responsible for scaling and fouling at various stages of the SAGD process. This study aims at developing a systematic approach for the fractionation and characterization methods of SAGD process water samples.
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INTRODUCTION Steam-assisted gravity drainage (SAGD) is a thermally enhanced heavy oil recovery method that is broadly practiced for bitumen extraction from oil sands in Alberta, Canada. In this process, steam is injected through a horizontal well into the bitumen-containing formation to decrease the viscosity of the bitumen and affect its extraction. An emulsion of steam condensate and heated bitumen flows down along the periphery of the steam chamber to the production well which is located below the injection well. This emulsion is then pumped to the surface where the bitumen and water are separated. Two methods of bitumen−water separation are used: the diluted bitumen process and the high-temperature separation process (also known as the inverted process). At typical produced emulsion temperatures (120−170 °C), the density of Athabasca bitumen and water are similar,1 making gravity separation difficult. In the diluted process, a low-density petroleum product such as naphtha or natural gas condensate is added to the produced emulsion to “dilute” the bitumen and reduce its density. The diluted bitumen then floats on the water and is removed by gravity in a free water knockout vessel. The lower-density diluted bitumen product can be tailored to meet downstream pipeline and refinery specifications. The diluent can be recovered at the SAGD facility and/or after pipelining and reused. In the high-temperature separation process, the produced emulsion is heated to between 200 and 230 °C. At these temperatures, bitumen is heavier than water1 and sinks in the gravity high-temperature separator, hence the term “inverted” © XXXX American Chemical Society
separation. Typically the separated bitumen is kept hot and pipelined offsite for further processing or blending. The selection of the diluted or inverted separation process is made on a site-specific basis considering heat integration, pipelining, and final product destination considerations. Water treatment process and boiler operation and overall water reuse rates are independent of the separation process used. After separation, the produced water is treated onsite for the removal of free oil, hardness, and silica. Free oil is removed via gravity skim tanks, gas flotation vessels, and/or media filters. Silica and hardness are then removed in a warm or hot lime softener. Final polishing steps are performed by media filtration to remove the remaining particulates and ion exchange to remove the remaining dissolved hardness. This treated water is then used as boiler feedwater (BFW) for robust oilfield oncethrough steam generators (OTSGs) capable of operating with high salinity BFW (up to 8000 to 10 000 mg/L total dissolved solids), and producing 70−80% steam quality. Steam enters a steam separator, and dry steam is sent to the injection well, and the liquid blowdown is sent for recycle and/or disposal. In the current industrial practice, a portion of the boiler blowdown (BBD) is recycled back to the warm lime softener, and the rest is disposed of either by deep well injection, an onsite zero liquid discharge facility, or to an off-site third-party waste management company. The amount of recycle is set to maintain the overall boiler feedwater specifications for total dissolved solids Received: February 17, 2017 Revised: June 9, 2017 Published: June 21, 2017 A
DOI: 10.1021/acs.energyfuels.7b00483 Energy Fuels XXXX, XXX, XXX−XXX
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solubility than higher molecular weight compounds. Low molecular weight, short-chain alkyl phenols were found to be extremely water-soluble. Although similar studies have not been done on Athabasca bitumen, similar trends would be expected. As bitumen and crude oil characteristics alone are inadequate to characterize the dissolved organics present in SAGD produced waters, direct analytical studies are required. Heaton et al. used a variety of analytical methods to examine Athabasca SAGD deoiled produced water (DOW), boiler feedwater (BFW), boiler blowdown water (BBD), and low-pressureseparator flash blowdown samples from four oil sands producers.8 Phenols and organic acids were the most significant chemical classes recovered. BTEX, thiophenes, ketones, alkylsubstituted benzenes and phenols, dimethylpyridine and indanes, and some residual aliphatic and aromatic hydrocarbons were also found. Petersen and Grade conducted a direct evaluation of dissolved organics from SAGD process waters using a two-dimensional gas chromatography (GC)/electrospray ionization time-of-flight (ESI-TOF) MS technique.10 Three primary groups of compounds were identified: saturated aliphatics, aromatics, and polar compounds. Saturated aliphatic hydrocarbons, both straight chain and branched, ranging from C6 to C18 were found at relatively low concentrations, as expected due to their low aqueous solubility. Low concentrations of aromatics such as benzene, alkylbenzenes, and naphthalenes were identified. Polar species were much more prevalent. These included carboxylates and naphthenic acids, ketones, phenols, thiophenes, and oxidized sulfur-containing species. Methyl- and ethyl-phenols were prevalent constituents. Kawaguchi et al. used GC/MS to conduct a detailed analysis of SAGD produced water samples and identified a wide variety of organic compounds.11 Volatile organic compounds (VOCs) such as acetone and 2-butanone predominated. Significant amounts of polar compounds with carboxylic and phenolic functional groups and trace amounts of polycyclic aromatic hydrocarbons (PAHs), e.g., naphthalene and phenanthrene, were also found. Organic acids and VOCs were found to be major constituents in the produced water, deoiled water, and softened water samples, whereas organic acids were most abundant in the blowdown and recycle water samples. Naphthenic acids were present in all process water samples. As the water treatment progresses, the total concentrations of organic acids and phenols increased, whereas PAHs and VOCs decreased significantly. Pereira et al. analyzed SAGD boiler blowdown using liquid chromatography followed by ESI Orbitrap MS and found over 3000 elemental compositions corresponding to a range of heteroatom-containing homologue classes (Ox: x = 1−6, NOx: x = 1−4, SOx: x = 1−4, NO2S, N, and S).7,12 The O2 species detected in the ESI negative mode were chemically distinct from the corresponding species in the ESI positive mode. This study also found distinct differences between the organic compounds identified in the SAGD sample and those of a process-affected water sample from an oil sands surface mine. The higher temperature of the SAGD process compared to mined bitumen extraction is most likely responsible for the differences in the number and type of dissolved organics found between these two sources. The detailed, direct analytical studies reported above have shown that SAGD produced water contains thousands of individual compounds. Because of the large number of individual compounds present in SAGD waters and because the importance of every single compound in the fouling process is unknown, alternate analysis schemes have been developed to
(TDS), Si, Ca, Mg, and TOC. Overall, 80−90% of the produced water is recycled as BFW. In some process configurations, evaporators are used to desalinate the boiler feedwater, allowing the use of drum boilers and higher water recycle rates. A schematic view of a typical conventional plant is included in Figure S1. SAGD produced water contains high concentrations (200− 1000 mg/L) of dissolved organic carbon (DOC), and the conventional deoiling, lime softening, and ion exchange water treatment processes used in a typical SAGD plant do not remove more than approximately 10−15% of this DOC. High DOC in the SAGD process water is suspected to be implicated in the formation of fouling deposits in heat exchangers and the tubes of the OTSGs, leading to increased cleaning frequencies and equipment failures,2−5 although the exact fouling mechanisms are not entirely understood. The research work described in this paper focuses on the characterization of the DOC present in SAGD produced water. The information gained from this investigation is intended to be used to better understand the mechanisms of equipment fouling and the role that DOC may play in that fouling. A useful starting point for investigation of the dissolved organic compounds present in SAGD produced water is an examination of the nature of the bitumen in the producing formation. The organic compounds found in the bitumen will migrate to varying degrees into the hot steam condensate in the producing SAGD formation. Athabasca bitumen chemistry is well-studied,1 and it is known to contain organic acids, which would be expected to partition into the produced water phase to varying degrees. These organic acids have been shown to include the naphthenic, carboxylic, and sulfonic acids typically found in crude oil,6 as well as significant fractions of humic acids.7 Majid and Ripmeester reported that these humic acids were similar to typical coal humic acids, with relatively high sulfur content and significant aromatic content (29−38% of the carbon present). They also showed the presence of phenolic, carboxylic, amide, ester, aldehyde, and ketonic functional groups in these humic acids.7 Light hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX) can also partition into the produced water from diluents that are added during processing. Nevertheless, it is important to note that the organic characterization of source bitumen or crude oil, in general, has been found to be an incomplete predictor of the characterization of the organic compounds found in produced water.8,9 In a detailed study of the water solubility of heavy crudes, Stanford et al. found that the abundance of a particular compound class in the parent oil did not directly relate to the aqueous solubility of that compound class.9 Rather, it was found that the increased presence of polar functional groups, oxygenated functional groups, alkyl branching, and aromatization increased aqueous solubility. The parent-oil heteroatomic classes that showed significant aqueous solubility were found to be the ones that contained functional groups that form strong hydrogen bonds with water. For example, Ox and OxS compounds were found to be the most water-soluble. Pyrrolic (acidic nitrogen) compounds showed poor aqueous solubility. Nitrogen-containing species exhibited aqueous solubility only if an oxygenated functional group was present in combination with a pyrrolic group (i.e., NOx classes). Sulfur-containing organics showed similar trends; only O3S and O4S compounds showed appreciable water solubility. Lower molecular weight compounds present in the parent oil showed higher aqueous B
DOI: 10.1021/acs.energyfuels.7b00483 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. FEEMS peaks of various industrial waters from the research literature. Coal bed methane produced water,15 coal coking wastewater,16,17 refinery wastewater,18 OSPW and SAGD produced water.19
classify SAGD DOC on the basis of more general chemical properties. These schemes allow the identification of groups or families of compounds that have similar physicochemical properties. Two indirect characterization techniques that have been investigated are fluorescence excitation−emission matrix spectroscopy (FEEMS) and resin fractionation. FEEMS has been used to characterize and compare the types of dissolved organic matter present in a wide variety of water samples, from natural surface and ground waters to refinery wastewaters and produced waters.13,14 The technique provides a “fingerprint” of the dissolved organics present, based on their fluorescence properties. The fluorescence response can then be compared to that of other samples or the response of known reference compounds. The pioneering work in using FEEMS to characterize dissolved organics was conducted on fresh and marine water samples and municipal wastewater effluents, and five standard response regions have been defined based on the principal components in those waters: aromatic proteins (regions I and II), fulvic acids (region III), soluble microbial byproducts (region IV), and humic acids (region V).13 In addition to natural water samples, the technique has been used to analyze the organic compounds found in produced water and industrial wastewater samples. Figure 1 shows the FEEMS results from the analysis of several industrial water samples overlaid on the five standard response regions. Although the specific compounds found in the industrial and natural water samples may differ, this figure can provide some insight into the likely size and structure of the dissolved organics present in the industrial wastewaters. Resin fractionation techniques have also been used to study the characteristics of natural organic matter on the basis of their adsorption on various ion exchange resins. Through wellestablished techniques, dissolved organics can be separated into hydrophobic and hydrophilic fractions of acid, base, and neutral compounds. The types of compounds typically found in each resin fraction have been well studied, and are summarized in Table 1. The resin fractionation technique has been used to separate the DOC in SAGD process water samples as described above.20
Table 1. Characterization of Dissolved Organics by Resin Fractionation14 fraction hydrophobic acids hydrophobic bases hydrophobic neutrals hydrophilic acids hydrophilic bases hydrophilic neutrals
abbreviation HPoA
representative compounds
HPoB
humic acids, fulvic acids, phenols, tannins, medium-MW alkyl carboxylic acids proteins, aromatic and high-MW amines
HPoN
hydrocarbons, aldehydes, alkyl alcohols
HPiA
hydroxy-acids, low-MW alkyl carboxylic acids
HPiB
amino acids, low-MW alkyl amines
HPiN
polysaccharides, low-MW alkyl alcohols, aldehydes
Additionally, the DOC was fractionated based on size by filtering through a series of membranes with increasingly tighter molecular weight cut-offs of 10, 3, and 0.5 kDa. FEEMs, specific UV absorbance (SUVA), and FTIR were used to characterize the water samples and the different fractions. The ion exchange fractionation revealed that the DOC contained a high level of hydrophobic acids (39.0%) and hydrophilic neutrals (28.5%). The different ion exchange fractions showed distinct fluorescence excitation−emission (Ex/Em) signatures. The fluorescence EEM spectra of the permeate samples from the membrane fractionation, on the other hand, were not significantly different, suggesting that the hydrophilic and hydrophobic constituents of the DOC could not be separated based on the molecular sieve property of the membranes. Membrane filtration results showed that the majority of the SAGD DOC was less than 500 Da, which is smaller than typical aquatic natural organic matter and smaller than bitumen asphaltenes and resins. Thakurta et al. also used FEEMS to examine the individual fractions from the resin separation technique;20 these data are included in Figure 1. It can be seen that SAGD produced waters, Athabasca oil sands process-affected waters (OSPW), and refinery wastewater all contain some compounds with fluorescence responses that are similar to each other, and which C
DOI: 10.1021/acs.energyfuels.7b00483 Energy Fuels XXXX, XXX, XXX−XXX
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reagent, ≥ 99.8%) and diethyl ether (anhydrous, ACS reagent, ≥ 99.0%) were purchased from Sigma-Aldrich and were used as received. Standard Analysis. A TOC analyzer (Shimadzu, TOC-L) was used to measure the dissolved total carbon (TC) and inorganic carbon (IC). The TOC was calculated by subtracting IC from TC. The dissolved organic carbon (DOC) was also determined by the TOC analyzer after filtering the water samples through a 0.45 μm PVDF membrane filter (Millex, EMD Millipore). Before TOC analysis, the fractionated samples were stored at 4 °C for less than a week. The measurement of pH was conducted using a pH meter (Fisher Scientific, Accumet Excel XL60). A portable conductivity meter (Hach HQ40d) was used for conductivity measurements and was calibrated with conductivity calibration standard solutions (Fisher Scientific, Traceable CRM). The pH and conductivity measurements of raw water samples were performed without any filtration or dilution. UV absorbance of the water fractions was measured at 254 nm wavelength using a UV−vis spectrophotometer (Cary 50, Varian), after filtering through a 0.45 μm PVDF membrane filter (Millex, EMD Millipore). The specific UV absorbance (SUVA254) of the samples was calculated by the corresponding DOC values by the following equation: SUVA254 = (Abs)254nm × 100/DOC.24 The total solids (TS) and TDS values were measured by gravimetric analysis. TS and TDS were calculated by using the mass of solid residue left after baking 10 mL of unfiltered and filtered process water samples, respectively, dried to a constant weight at 105 °C for TS and 180 °C for TDS. The mass of total solid was measured using an analytical balance with 0.1 mg readability. Resin Fractionation. The separation of DOM from the water samples was performed using a slightly modified procedure of the previously reported resin fractionation method.20 The details of the resin fractionation method are provided in the Supporting Information. Concisely, this method includes (1) cleaning the resins by Soxhlet extraction, (2) preparing respective resin columns while considering the TOC concentration of the sample solution, (3) conditioning the resin columns by a standard procedure, prior to the introduction of SAGD water samples, to ensure the TOC and conductivity of the eluted DI water were minimized and within the acceptable limit, (4) diluting the raw water samples and adjusting the TOC within 50−70 mg/L, and finally (5) performing the fractionation, as shown in Figure S2, to obtain three hydrophilic and hydrophobic fractions. Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Elemental concentrations in the produced water samples were determined by using a quadrupole inductively coupled plasma mass spectrophotometer (ICP-MS) (PerkinElmer, Elan 600, USA). The ICP-MS data were obtained with 1300W ICP RF power under dual detector mode where bismuth, scandium, and indium were used as internal standards. The sample uptake rate was 1 mL/min. The concentrations were calculated from the corresponding calibration curves. Fluorescence Excitation Emission Matrix Spectroscopy (FEEMS). Fluorescence excitation−emission contours of the water samples were acquired using a fluorescence spectrophotometer (Cary Eclipse, Varian). The excitation−emission spectra were obtained over a wavelength range of 200−500 nm with 10 nm excitation intervals. Water samples were diluted to a 20 mg/L TOC concentration to minimize any inner filtration effects due to excessive concentration.19,25 The pH of all water samples was adjusted to 11 to minimize any effect of pH changes on fluorescence. Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (ESI-FT-ICR-MS). Ultrahigh resolution ESI-MS was performed using a Bruker Apex-De 9.4T FTICR mass spectrometer (Bruker Daltonics, Billerica, MA). The SAGD water samples were extracted with dichloromethane at pH 2.0. These sample extracts were mixed with methanol and toluene before ESI-MS analysis. The injection flow rate was 2 μL/min. Data were collected over the mass to charge ratio (m/z) range of 150−2000. Mass accuracy across the full mass range was around 2 ppm (RMS). SAGD water contains a high concentration of salts. Because these inorganic salts strongly suppress the ionization efficiency, the organics were extracted with CH2Cl2.26 Approximately 10 mL DOW or BFW
are similar to the responses of phenol, cresol, and naphthenic acid reference compounds. Although SAGD DOC is not identical to that found in natural waters, the DOC present exhibited Ex/Em signatures similar to those found in humic and fulvic acids, suggesting that some components of SAGD DOC have a similar molecular structure. This result is not surprising, as humic and fulvic acids, which originate from the decomposition of organic matter, have been found in oil sands bitumen, as mentioned above.7 Humic acids are large, complex molecules having a wide variety of components, including phenol, catechol, carboxylic, and amine moieties. Their polar nature will result in their partitioning into the produced water during SAGD extraction. Oil-refinery wastewater DOC characterization studies have also shown the presence of lower molecular weight fulvic and humic acids.21 The present work involves the characterization and comparison of three types of process water samples received from four SAGD water treatment sites in the Athabasca Oil Sands region of Alberta, Canada. Using a resin fractionation method, the DOM in these water samples was separated into six hydrophobic and hydrophilic fractions (HPoA, HPoB, HPoN, HPiA, HPiB, and HPiN). The chemical characterization of the process water samples and the resin fractions was investigated using various analytical methods including gas chromatography−mass spectrometry (GC-MS), electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS), inductively coupled plasma mass spectrophotometry (ICP-MS), TOC, SUVA, and fluorescence excitation emission matrix spectroscopy (FEEMS) analysis.
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MATERIALS AND METHODS
Samples and Materials. Three types of process water samples were received from four SAGD water treatment sites located in the Athabasca Oil Sands region (Alberta, Canada). All four sample sites use conventional SAGD water treatment consisting of deoiling, lime softening, and weak acid cation exchange processes followed by OTSGs, which produce high-pressure steam with 70−80% steam quality. Approximately 40−50% of the resulting blowdown is recycled back to the lime softener for these plants. All plants employed the diluted bitumen separation process except Plant B, which employed high-temperature separation. DOW, BFW, and BBD water samples were collected from three different locations within each plant. The DOW sample point was selected as a baseline sample that would capture the differences in dissolved organics from each plant’s bitumen formation after the removal of free oil. The BFW sample was selected to capture any changes in the dissolved organics resulting from the softening and ion exchange processes and to characterize the organics entering the OTSGs. The BBD sample was intended to capture any high-temperature transformations that may occur in the boiler. The Plant B BFW sample was not obtained due to field logistics. It should be noted that in the full-scale SAGD water treatment process, fluids are kept at a temperature of 80−90 °C in a relatively oxygen-free environment. Samples were collected from hot process streams using a sample cooler and were placed into epoxy-lined, steel containers with zero headspace. Exposure to oxygen during and after sampling can cause chemical transformations to occur.22,23 These effects were mitigated in this study by minimizing air exposure during sample collection and maintaining a nitrogen blanket on the samples during storage. Samples were stored at room temperature. Three types of resins were purchased from Sigma-Aldrich, including Supelite DAX-8 (adsorbent resin, moderate polarity), Dowex 50WX8 (cation exchange resin, strong acid), and Amberlyst A21 (anion exchange resin, weak base). Deionized water (Millipore, 18.2 MΩ·cm) was used for all sample preparation and dilution. Methanol (ACS D
DOI: 10.1021/acs.energyfuels.7b00483 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 2. General Analysis of SAGD Produced Water Samples Plant A
Plant B
Plant C
Plant D
parameters
DOW
BFW
BBD
DOW
BBD
DOW
BFW
BBD
DOW
BFW
BBD
pH conductivity (mS/cm) TSS (mg/L) TDS (gravimetric, mg/L) TOC (mg/L) UV absorbance (254 nm) SUVA (254 nm) color
9.1 1.8 210 1,820
10.1 2.6 307 2,217
11.4 12.9 367 11,710
7.9 1.9 290 5,483
11.1 15.2 2,313 36,807
7.6 1.5 30 2,452
9.4 2.7 5 2,729
11.3 11.2 1,467 27,333
7.1 2.7 47 2,560
9.8 5.6 3,930 3,670
11.2 19.2 5,878 51,665
544 0.67
534 0.70
2,184 0.87
512 0.68
2,602 0.72
401 0.77
426 0.84
2,997 0.62
278 0.51
554 0.56
3,296 0.55
4.37 reddish brown
5.21 black
2.90 dark brown
4.81 black
3.85 dark brown
4.20 reddish brown
3.10 black
2.55 dark brown
2.80 reddish brown
2.75 black
3.34 dark brown
Table 3. Elemental Concentrations (mg/L or ppm) in SAGD Produced Water Samples Measured Using ICP-MS Plant A
Plant B
Plant C
Plant D
analyte (mg/L)
DOW
BFW
BBD
DOW
BBD
DOW
BFW
BBD
DOW
BFW
BBD
Na K Li Ca Mg Al Fe Zn Mn Si P B
402 20.99 0.50 12.66 2.21 0.25 0.08 0.18 0.01 98.25 0.35 20.05
620 26.26 0.55 1.44 0.03 0.06 0.22 0.15 0.00 36.94 0.13 20.61
3439 142.21 3.04 19.45 0.44 0.25 0.66 0.02 0.02 256.68 0.51 99.73
1935 35.18 1.12 20.24 6.37 0.02 0.20 0.16 0.01 72.55
