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Vaporization of Crude Oil by Supercritical SC-CO2 at Different Temperatures and Pressures: Example from Gorm Field in the Danish North Sea Svetlana Rudyk, Pavel Spirov, Premkumar Samuel, and Sanket J. Joshi Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017
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Energy & Fuels
Vaporization of Crude Oil by Supercritical SC-CO2 at Different Temperatures and Pressures: Example from Gorm Field in the Danish North Sea
Svetlana Rudyk1*, Pavel Spirov2, Premkumar Samuel3, Sanket J. Joshi3, 1
Oil and Gas Research Centre, Sultan Qaboos University, Muscat, Oman,
[email protected] 2
Petroleum Engineering Department, Soran University, Kurdistan Region of Iraq
3
Central Analytical and Applied Research Unit, Sultan Qaboos University, Muscat, Oman
Abstract The effect of temperature on supercritical carbon dioxide (SC-CO2) extraction of crude oil from the Gorm oil field of the North Sea was investigated at temperatures of 40-70°C and pressures of 20-60 MPa. MALDI-TOF, GC-MS and NMR analyses were conducted in order to characterize the oil. Generally, the total recovery and vapor recovery increased while the liquid recovery decreased when the temperature was increased from 40 to 70°C. At lower temperatures (40 and 50°C) the liquid recovery increased from a minimum of 10% at 30 MPa to a maximum of 60% at 60 MPa whereas the vapor recovery was 16% on average over the entire pressure range. In contrast, at higher temperatures (60 and 70°C) the liquid recovery was 20% on average at all pressures while the vapor recovery increased from 20% at 30 MPa to 58% at 60 MPa on average. This sharp increase of vapor recovery at higher pressures signified retrograde vaporization. This observed phenomenon suggested that the extraction of some types of oil from high pressure reservoirs could be achieved by vaporization using SC-CO2 .
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Key words: supercritical, carbon dioxide, extraction, crude oil
1. Introduction 1.1 Crude Oil Extraction by Supercritical Carbon Dioxide The Dan, Halfdan, and Gorm fields are three giant oil fields in the Danish North Sea. The average annual decline of these Danish giants is found to be 6.7%, which is lower than the 13%, estimated for the Norwegian sandstone giant reservoirs in the North Sea. The lower depletion rates are explained by the generally lower permeability of the Danish chalk reservoirs.1 However, this lower permeability limits the application of many Enhanced Oil Recovery (EOR) methods that could be successfully applied. In this regard, carbon dioxide (CO2) injection was considered as the most feasible EORtechnology for the Danish oil fields. The interaction of CO2 and crude oil is described as immiscible at lower pressures and miscible at higher pressures. Miscibility between a reservoir fluid and an injection gas usually develops through a dynamic process of mixing, with component exchange controlled by phase equilibria. The process by which a molecule becomes part of a supercritical (SC) phase can be described as dissolution since it involves solute-solvent interaction or as vaporization in as much as molecules shift from a condensed phase to an expanded phase.2 Oil vaporizes into CO2, which eventually condenses into oil. At the changing conditions of temperature and pressure oil undergoes a phase split calculated as a vaporization ratio (K-value). The oil viscosity and interfacial tension (IFT) decreases at higher temperature, which leads to a higher degree of solubility/miscibility with CO2. The isobaric decrease of
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density and viscosity of SC-CO2 occurs at higher temperatures while their isothermal increase occurs at higher pressures. In addition to immiscible and miscible displacements, crude oil can be produced through the process of vaporization by injected gas, a mechanism that was rarely investigated in last few decades. This approach has been employed for vaporization of retrograde condensates by injection of different dry gases for several years as a standard practice to prevent the loss of valuable retrograde liquids.3-6 Pirson suggested the production of oil as a vapor using high pressure injection of carbon dioxide into a partially depleted reservoir for the purpose of causing the residual oil to vaporize. The vaporization of crude oil by CO2 cyclic repressuring was efficient in a highly fractured formation where the oil was contained in the nonfractured porosity with low permeability.7 In gas cycling, the total oil recovery includes vaporized products from the immobile oil in addition to oil produced by displacement.8 Standing et al.3 concluded that the heavier components from a gas cap could be completely recovered from the swept portions of the reservoir by sufficient quantities of dry gas. The vaporization ratios ( Kvalues) of crude oil (35o API) and carbon dioxide were investigated at the pressure range of 4-58.6 MPa and at (4, 49 and 94.4) oC of temperature. 9 Katz et al. stated that at high pressures (20-27 MPa) and relatively high temperatures (75-90°C), "when natural gas is brought into contact with crude oil not only does gas dissolve in the oil, but constituents of the oil travel over into the gas”.10 Such oil can flash mainly to gas with a relatively low liquid content even at moderate pressure depletion. The phenomenon of increase of vapor phase at increasing pressure is known as a retrograde vaporization. The amount of oil vaporized may range from almost 0 to 100 percent. Cook et al. concluded that the most favorable factors for high recovery of immobile oil by vaporization were high API gravity and
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high reservoir pressure and temperature.8 However, no simple and reliable method has been presented in the literature for calculating oil vaporization because the number of components in reservoir oils is too large, and no two reservoir oils are exactly alike.8 As a result, experimental studies of mechanisms of oil recovery by SC-CO2 are necessary.
1.3 Characterization of Crude Oil by Various Techniques Various analytical techniques and methods exist to characterize different properties of crude oil and oil distillation fractions. The distribution of single carbon number (SCN) groups or compounds are obtained using different methods of chromatography. For example, the PIONA/PONA analyser is used for the analysis of Paraffins, Isoparaffins, Olefins, Napthenes and Aromatics by a GC-FID system using a special column. Crude oil can be also divided at components including Saturates, Aromatics, Resins, and Asphaltenes (SARA) according to their polarizability and polarity.11 SARA analysis is often used as one of the screening criteria for asphaltene stability of reservoir fluids. Users of such data assume that the SARA fraction values obtained by different chromatographic methods are essentially interchangeable. However, different SARA analytical techniques do not necessarily produce identical results, as demonstrated by Kharrat et. al.12 Accurate content of aliphatic and aromatic hydrogen and carbon in the oil, and other components such as CH2 and CH3 can be determined by Nuclear Magnetic Resonance (NMR). A variety of structural parameters calculated using data of elemental analysis and NMR for petroleum characterization were developed and can be found elsewhere.13 Empirical correlations linking NMR data, SARA properties, and API gravity have been suggested by several authors.11, 14, 15 However, oil characterization by NMR data is still scarce in the literature.
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The current study describes SC-CO2 extraction of Gorm crude oil in the range of different pressures and temperatures. An unusually high level of oil vaporization has been observed at higher temperatures of 60 and 70°C and higher pressures of 50 and 60 MPa compared to 30 and 40 MPa that has not previously been observed in the experiments with crude oil from Dan and Halfdan fields.16-19 The Gorm crude oil was also investigated using Gas Chromatography-Mass Spectroscopy (GC-MS), Nuclear Magnetic Resonance (NMR) spectroscopy and Matrix Associated Laser Desorption Ion-Time of Flight (MALDI-TOF) spectroscopy.
2. Materials and Methods 2.1 Materials Crude oil from the Gorm oilfield was supplied by the Maersk Oil Company, Esbjerg. 99.9% pure carbon dioxide was procured from Strandmollen A/S, Denmark. The oil density was 0.8641 kg/L at 15 °C (ASTM D 4052). Viscosity at 20 oC and 40 oC were 9.163 and 5.138 mm2/s, respectively (ASTM D 445). The initial boiling point was 750 °C (ASTM D 86). Sulphur content was 0.261% (ASTM D 2622), nitrogen content was 1368 mg/kg (ASTM D 5762). Using PNAanalyser, the PIONA at 65oC: n-Paraffins 42.6%; i-Paraffins 47.3%; Olefins
