ARTICLE pubs.acs.org/IECR
Analysis of Steam Assisted Gravity Drainage Produced Water using Two-Dimensional Gas Chromatography with Time-of-Flight Mass Spectrometry Matthew A. Petersen* and Hans Grade GE Global Research Center, General Electric Company, 1 Research Circle, Niskayuna, New York 12309, United States
bS Supporting Information ABSTRACT: The recycling of water coproduced during in situ bitumen production is one of the primary operating challenges for steam assisted gravity drainage (SAGD) operations in oil sand reserves. Produced water that is either recycled for steam production or disposed of may be subject to different water quality requirements for the purposes of plant operations or environmental regulations. The organic components dissolved and suspended in the produced water are a fingerprint of the bitumen-in-place, the processing conditions utilized during production, and the chemicals used by operators during operations. Analysis of SAGD produced water using two-dimensional gas chromatography coupled with electron ionization and time-of-flight mass spectrometry (2DGC-TOFMS) showed a wide variety of organic constituents within the water sample. Compounds ranging from C6 to C18 straight chain and branched aliphatics to more polar, water-soluble, oxygen- and sulfur- heteroatomic species were tentatively identified. Methyl- and ethyl-phenols were prevalent constituents eluting in the heteroatomic region of the chromatographic contour plot. Sample extraction conditions that enhanced partitioning of polar organic species resulted in a significantly larger amount of compounds being detected by this approach. These results show how this method is complementary to more widely used analytical techniques, which cannot provide a comprehensive view of the broad range of compounds within oilfield produced water using a single method.
’ INTRODUCTION Oil sand deposits in Alberta, Canada represent the second largest reservoir of hydrocarbons in the world.1 The development of oil sand reserves has been performed to date primarily using mining techniques to extract bitumen from material located near the ground surface.2 Recently, factors such as increasing oil prices, expanding demand for petroleum, and the large relative amount of bitumen-in-place of the deeper oil sands deposits have made the pursuit of in situ bitumen production using the steam assisted gravity drainage (SAGD) technology more attractive. The steam-to-oil ratio in a SAGD operation is typically around 3 to 1, but can significantly increase during the life of the plant.2 This dictates that a large volume of water be utilized and recycled over the lifetime of a field. As the oil sands industry expands, technical and regulatory constraints on the water resources available for steam generation have driven water recovery and treatment to be one of the principal design and operational constraints of SAGD plants. Typical SAGD plants consist of multiple well pads connected to a central processing facility where the coproduced bitumen and water are separated. The recovered bitumen is transported to an upgrading facility, and a large fraction of the remaining water (produced water) is recycled to generate steam for reinjection at a well pad. A conceptual block flow diagram of plant operations is presented in Figure 1. Typically the fluids produced from the well pads are separated in primary oil separation unit processes, such as gravity separator and heater-treater. Free and emulsified oil are separated from the produced water, which still contains dissolved and some residual colloidal organics. These residual organic compounds r 2011 American Chemical Society
are removed from the produced water in a deoiling unit operation train (induced gas floatation and walnut shell filtration) before being fed to a boiler feedwater pretreatment system. The deoiled produced water is pretreated to remove hardness and dissolved silica using processes such as warm lime softening, weak acid cation exchange, and thermal evaporators prior to steam generation.3 The presence of significant amounts of oil in the boiler pretreatment equipment, boilers, or blowdown condensate management units can lead to equipment reliability and maintenance issues. Oil concentrations are routinely tracked through a plant using techniques that lump hydrocarbons into an operationally defined parameter.4 Examples of these techniques include total oil and grease (TO&G) and total organic carbon (TOC). Although these methods are useful for performance monitoring, more powerful techniques to elucidate the composition of the organics in produced water can provide valuable information for plant design and troubleshooting activities. Multidimensional gas chromatography coupled with mass spectrometry is an attractive platform for analysis of complex mixtures that is becoming more widely utilized in academic, industrial, and public-sector entities.5,6 The basis for two-dimensional gas chromatography (2DGC) is the separation of an entire sample over two orthogonally characterized stationary phases.7 This allows analytes to be separated by two properties, which Received: March 18, 2011 Accepted: September 19, 2011 Revised: August 10, 2011 Published: October 07, 2011 12217
dx.doi.org/10.1021/ie200531h | Ind. Eng. Chem. Res. 2011, 50, 12217–12224
Industrial & Engineering Chemistry Research
ARTICLE
’ MATERIALS AND METHODS
Figure 1. Conceptual high-level flow diagram of a SAGD central processing facility.
dramatically increases the separation resolution compared to traditional one-dimensional GC methods. Analysis of the entire sample is accomplished by synchronizing a thermal modulator that continuously focuses and pulses sample segments received from the first column into the second column, with a relatively fast separation through the secondary column. This provides a distinct advantage over traditional GC GC heart-cutting techniques that only resolve select portions of the sample. Many publications are available that document a detailed composition of produced water from conventional hydrocarbon production operations.8 13 A wide range of organic species that encompass several functional families are likely to be present in SAGD produced water based on the comprehensive analysis of heavy oils and bitumen14 18 Previous detailed reports of oil sand associated produced water have focused on the composition of oil sands process water (OSPW), originating from surface mining activities and stored in tailings ponds.19 24 The composition of OSPW is likely to consist of more weathered and recalcitrant organic compounds compared to SAGD produced water due to environmental factors such as biodegradation and volatilization that readily degrade or remove compounds susceptible to those processes. OSPW is generated from the bitumen extraction process that utilizes warm alkaline water to separate bitumen from the sand. The water may be partially reused in the process without supplemental treatment, although continuously deteriorating water quality may lead to operational problems.25 Alternatively, SAGD produced water is treated and continuously reused to generate steam for reinjection into the subsurface. These differences in origin and use may both impact the organic fingerprint of the water and support a separate motivation for pursuing a more comprehensive identification of the compounds present in the produced water. In addition to bitumen components, the organic fingerprint of the produced water may also include production chemicals such as emulsion breakers and reverse breakers, scale and corrosion inhibitors, and biocides that are added to enhance producer well productivity and the water treatment process. The objective of this study was to better define the dissolved and suspended organic species, and families of species, in a sample of SAGD produced water collected from a central processing facility using 2DGC with time-of-flight mass spectrometry detection (TOFMS). Several liquid liquid extraction conditions were evaluated to provide a comprehensive view of the ensemble of organics present in the sample amenable to gas chromatography-based analysis. This technique may not be sensitive to high molecular weight/ boiling-point compounds, those that are poorly ionized, or that are thermally labile, which may be present in heavy oil produced waters.17 The analysis procedure was designed to minimize sample preparation steps (e.g., no derivatization) in order to retain method simplicity and limit sample preparation-induced interferences, while still capturing a large breadth of compounds.
Chemicals. Mixed hexanes (Optima grade, Fisher), methylene chloride (ACS grade, Fisher), and hydrochloric acid (ACS Plus grade, Fisher) were used in this study without further purification. Hydrochloric acid was diluted with deionized water (>18 MΩ cm) to a 6 M concentration for acidifying samples. Water Sample. A sample of produced water from a SAGD central processing facility deoiling unit operation was collected from an operator in Alberta, Canada. Sampling was performed after the primary oil water separation (skim tank), but before deoiling operations. The water was shipped to Niskayuna, NY in a sealed polyethylene 45-gal drum. Upon receiving the sample four 1-L aliquots were collected and stored in glass bottles with Teflon-lined screw caps at 4 °C in the dark. Water was transferred from the storage drum to the 1-L bottles using a peristaltic pump and Teflon tubing. The water was circulated in the drum for approximately 5 min to suspend solids and homogenize the sample before filling the 1-L bottles. General water quality parameters of the produced water are listed in Table 1. The total organic carbon in the sample was primarily made up of the nonvolatile fraction. A stable foam would form during mild shaking of the sample, which indicated the potential presence of dissolved organic species containing surfactant-like properties. Sample Preparation and Extraction. Four samples of the SAGD produced water were prepared and extracted using different combinations of acidification and organic solvent extraction. The extraction conditions are summarized in Table 2. Two target pH values (2.0 and neutral) and extraction solvents were used in the study. Sample pH was adjusted to a value of 2.0 using 6 M HCl prior to the extraction in preweighed samples that were approximately 1 L in volume. The acid was added dropwise while monitoring the sample solution using an Orion 3-star pH meter. The pH meter was calibrated using pH 4.0, 7.0, and 10.0 buffers. Less than 10 mL of dilute acid was required to adjust the pH to less than 2 for each 1-L sample analyzed. The organic fraction of acidified and unacidified samples was extracted from the aqueous samples using either hexane or methylene chloride (MC) in a continuous extraction apparatus. Batch extraction methods were evaluated using a separatory funnel. This method was not selected due to a significant metastable emulsion that formed during the batch extraction procedure. Continuous extractions allowed for better control over the organic aqueous liquid mixing conditions than the batch extractions, and did not tend to form problematic oil/water emulsions with the extraction liquid. MC extractions were performed in a glass extraction apparatus for extraction solvents with a specific gravity greater than the sample liquid (Ace Glass, Inc., product 6848). Hexane extractions were performed in a glass extractor designed for solvents with a specific gravity less than 1 (Ace Glass, Inc., product 6846). Each apparatus was fitted with a water-cooled Alihn condenser and electrically heated reflux flask. The reflux flask temperature was controlled using a heating mantle attached to a rheostat. During the extraction the rheostat was adjusted until the solvent began to gently boil. The extraction apparatus condenser temperature for the MC and hexane extractions was set at 27.5 and 32 °C, respectively. Once reflux was initiated, the extractions were carried out for 24 h. Approximately 300 400 mL of the extraction solvent was added to the apparatus so that the reflux flask did not completely 12218
dx.doi.org/10.1021/ie200531h |Ind. Eng. Chem. Res. 2011, 50, 12217–12224
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. SAGD Produced Water Quality Parameters parameter
value
method
pH
7.11
Orion pH probe (4, 7, 10 calibration buffers)
specific conductivity
1.54 mS/cm
conductivity probe
mean drop/particle size 500 nm
dynamic light scattering
total organic carbon
232 mg/L
catalytic combustion oxidation (Shimadzu TOC-5050)
nonpurgeable total
214 mg/L
catalytic combustion oxidation (Shimadzu TOC-5050) of acidified sample purged with zero-grade air for 10 min
organic carbon total oil and grease
29.1 mg/L
EPA 1664a
bicarbonate alkalinity dissolved silica
120 mg/L as CaCO3 102 mg/L as SiO2
titration to bromocresol green methyl red end point silicomolybdate method (LaMotte Test 3687-SC)
chloride
292 ppm
ion chromatography with conductivity detection (IC)
sulfate
12 ppm
phosphate
