Article pubs.acs.org/EF
Characterization of Produced and Residual Oils in the CO2 Flooding Process Shuhua Wang,† Shengnan Chen,*,† and Zhaomin Li‡ †
Chemical and Petroleum Engineering, University of Calgary, Calgary T2N 1N4, Canada College of Petroleum Engineering, China University of Petroleum, Qingdao 266580, People’s Republic of China
‡
ABSTRACT: In immiscible CO2 flooding processes, the injected CO2 extracts light and intermediate components from reservoir hydrocarbons, changing the properties of both the produced and residual oils. CO2 injection also increases asphaltene precipitation, which further causes adverse effects on the oil effective permeability, operational facilities, and the oil recovery. In this study, six CO2 flooding followed by blowdown processes are carried out in the laboratory under four CO2 injection pressures and two temperatures, and the properties of produced and residual oils are further characterized. It is found that uneven residual oil distribution occurred due to the coupled effects of viscous fingering and oil properties changes during CO2 flooding process. Characterization of the produced oil shows that both the produced oil density and viscosity decrease when CO2 injection pressure increases. The asphaltenes content of the oil produced after CO2 breakthrough is found to be relatively lower than that of the oil samples collected before CO2 breakthrough. In addition, the produced oil becomes lighter as the CO2 injected volume increases. The saturates content in the residual oil decrease considerably in comparison with original crude oil in the CO2 miscible flooding process. Furthermore, this study discovers that asphaltene component in produced oils is more stable than that in residual oils based on asphaltene-to-resin ratio (ATR) analysis. Finally, incremental oil recoveries of 2.78%−11.72% can be experimentally achieved through blowdown processes that are carried out after a 30 min soaking period. The laboratory study not only shows that blowdown processes can be successfully applied to improve the performance of CO2 flooding processes, but also provides a deep understanding of properties changes of produced and residual oils in CO2-enhanced oil recovery processes.
1. INTRODUCTION Carbon capture, utilization, and storage (CCUS) have been attracting more and more attention from the petroleum industry, as well as governments all over the world.1−4 It is not only an effective means of reducing greenhouse gas emissions, but it is also a promising enhanced oil recovery (EOR) technique.5,6 In 2012, 114 miscible and 9 immiscible ongoing CO2-EOR projects provided a total oil production of 284 193 barrels per day.7 In Canada, two commercial CO2EOR projects are being operated, in Alberta and Saskatchewan.8 The major CO2 processes proposed in the field or the laboratory include continuous CO2 injection, simultaneous water-CO2 injection, water-alternating CO2, CO2 huff-and-puff, and CO2 cyclic injection. The commonly recognized CO2 enhanced oil recovery mechanisms mainly include interfacial tension reduction, oil viscosity reduction, oil-swelling effect, light-hydrocarbons extraction, CO2-rock reaction, wettability alternation, and miscible displacement.9,10 Laboratory experiments of CO2 flooding started as early as the 1950s. The initial experimental efforts have been made to investigate the properties of CO2−oil mixtures using pressure/ volume/temperature (PVT) tests. Welker and Dunlop11 studied the solubility, swelling, and viscosity behavior of CO2/dead crude oil systems using a high-pressure visual cell. Chung et al.12 presented experimental results for the physical properties of heavy oils before and after CO2 saturation. Li et al.13 developed the experiments to examine the enhanced swelling effect and viscosity reduction of CO2-saturated heavy oil with the addition of solvents. It has been well-recognized that oil viscosity reduction and swelling effect are the principal © XXXX American Chemical Society
mechanisms contributing to the improvement of oil recovery by immiscible CO2 displacement. In addition, the see-through window high-pressure cell and pendant drop technique are applied to investigate the interfacial interactions of CO2−crude oil systems. Yang et al.14 argued that the equilibrium interfacial tension between CO2 and crude oil decreases as the pressure increases, whereas it increases as the temperature increases. They also found that initial strong light hydrocarbon extraction occurs between CO2 and crude oil when the pressure is greater than the critical pressure of CO2. This indicates that the interfacial tension reduction and strong light hydrocarbon extraction mechanisms make a great contribution to oil recovery when the CO2 injection pressure is high. However, one common problem during CO2 injection is asphaltene instability, leading to asphaltene precipitation and deposition, which leads to adverse effects on the effective permeability, operational facilities, and oil recovery improvement.15 Field and laboratory data confirm that the asphaltene solubility is lower in the light oil and asphaltene has a tendency to precipitate more easily from light oil than heavy oil.16 Zanganeh et al.17 studied the asphaltene deposition mechanism during CO2 injection at different pressures, temperatures, and compositions through a high-pressure pressure−volume−temperature (PVT) cell. Their results show that CO2 injection increases asphaltene deposition in all of the pressure ranges, in comparison with the tests without CO2 injection. Besides the studies of the physical Received: August 10, 2015 Revised: December 7, 2015
A
DOI: 10.1021/acs.energyfuels.5b01828 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Schematic diagram of the CO2 flood experimental apparatus.
properties for CO2−crude oil systems, many laboratory experiments are conducted to evaluate the performance of CO2 injection in carbonate reservoirs,18 thin heavy oil reservoirs,19 naturally fractured reservoirs,20 and tight oil formations.21,22 Li and Gu8 recommended that the optimum timing for starting miscible CO2 tertiary flooding is when water flooding reaches half of its maximum secondary oil recovery factor in tight sandstone formation. Han and Gu23 evaluated the WAG slug size for CO2 WAG injection and found that the optimum WAG slug ratio is ∼1:1 for the Bakken tight formation. Abedini and Torabi24 studied the recovery performance of immiscible and miscible CO2 huff-and-puff processes in light crude oil samples. Although laboratory experiments have been conducted to evaluate the properties of CO2−crude oil mixtures in a PVT cell and study the CO2 flooding performance, few attempts have been made to investigate the properties of the produced and residual oil at different CO2 injection pressures and reservoir temperatures. It is well-recognized that, in a miscible or near-miscible CO2 flooding process, the injected CO2 is capable of extracting light and intermediate hydrocarbons from the reservoir oil; therefore, the composition and properties of the reservoir oil are constantly changing over time,.25,26 The oil components and asphaltene/resin ratio alteration during CO2 flooding may cause the asphaltene precipitation and, thereby, its deposition in the reservoir. The possibilities of asphaltene precipitation must be assessed for CO2 flooding projects, especially in light oil reservoirs. Therefore, it is important, fundamentally and practically, to evaluate the oil properties over time during the CO2 injection process, as well as under different injection pressures and reservoir temperatures. In this study, six CO2 flooding tests are carried out under different injection pressures and two reservoir temperatures. Blowdown processes are conducted after a 30 min soaking period. The oil recovery factor at CO2 breakthrough, at the end of CO2 flooding, and the blowdown phases in terms of original
oil in place (OOIP), and the total oil recovery are used to evaluate the performance of CO2-EOR processes. Subsequently, the density, viscosity, and asphaltenes content of the oil produced before and after CO2 breakthrough are further characterized. Furthermore, the residual oil is extracted from the inlet, middle, and outlet of the residual oil sands in the sandpacks. The residual oil saturation is calculated and the asphaltenes content of the residual oil is measured using saturates, aromatics, resins, and asphaltenes (SARA) analysis.
2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. A schematic diagram of the CO2 coreflood apparatus is shown in Figure 1. Sandpacks 30 cm in length and 2.5 cm in diameter are used for the CO2 flood and blowdown tests. Two automatic displacement pumps (with a flow accuracy of
