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Experimental investigation on enhanced oil recovery of extra heavy oil by supercritical water flooding Qiuyang Zhao, Liejin Guo, Zujie Huang, Lei Chen, Hui Jin, and Yechun Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03839 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018
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Energy & Fuels
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Experimental investigation on enhanced oil recovery of extra heavy oil
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by supercritical water flooding
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Qiuyang Zhao, Liejin Guo∗, Zujie Huang, Lei Chen, Hui Jin, Yechun Wang
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State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi’an, 710049, P. R. China
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Abstract: Exploitation of deep extra heavy oil is a challenging work due to its high viscosity and high reservoir
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pressure. Supercritical water is firstly proposed as an injection agent, considering its favorable physiochemical
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properties. A novel flooding experimental system with the design temperature up to 450oC and pressure up to 30 MPa
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was developed to demonstrate the feasibility of supercritical water flooding (SCWF) technology. A sand pack core with
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adiabatic boundary was used to eliminate heat unbalance. The experimental results indicated that SCWF is a promising
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enhanced oil recovery technology. SCWF could significantly enhance oil recovery when compared with steam flooding
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and hot water flooding and reduce the oil viscosity simultaneously. SCWF at 25 MPa and 400oC raised the recovery
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efficiency to 97.07%, and reduced oil viscosity by 36.9%. The mechanism is attributed to the extraction heavy oil
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components into water rich phase by supercritical water and the formation of miscible flooding.
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Key words: Supercritical water flooding, Heavy oil, Enhanced oil recovery.
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1. Introduction
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With the depletion of conventional oil resources, heavy oil and bitumen become increasingly important because
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there is an estimated 5.6 trillion barrels resources and they contribute about 10% of world oil production 1. Various
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thermal recovery technologies have been proposed, improved and applied to exploit heavy oil in shallow and middle ∗
Corresponding author: TEL.: +86-29-82663895; FAX: +86-29-82669033 E-mail address:
[email protected] (Liejin Guo)
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2-5
since Perry and Warner
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stratum
issued the first patent to reduce oil viscosity by electrical heating in 1865.
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However, there is few mature technologies for developing extra heavy oil in the deep stratum. For example, Tuha
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extra heavy oil reservoir in China with the depth of 2100 m and the viscosity of more than 50 Pa·s at 70oC 7 has not
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yet been developed. So it is urgent to seek new ways to exploit such oil reservoirs.
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Thermal methods can improve the mobility of heavy oil because heavy oil viscosity decreases rapidly with the
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temperature increasing. Hot water flooding (HWF), cycle steam stimulation, steam flooding (SF) and steam assisted
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gravity drainage have been certificated to be efficient technologies, and the researches focus on mutual integration of
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these technologies at present. It can be inferred that the key principle of these thermal technologies is to reduce the oil
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viscosity by injection of thermal fluid. It is logical to improve water parameters in order to exploit these deep extra
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heavy oil reservoirs with high initial pressure and high viscosity, because steam cannot be injected into these deep oil
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reservoirs and the performance of hot water flooding is poor.
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It is generally known that water exists in the supercritical state when the pressure and temperature exceed 22.1 MPa
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and 374.3oC simultaneously. Supercritical water possesses the characteristics of both gas and liquid 8-14. The favorable
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physical and chemical properties of supercritical water enable itself to act as a solvent, reactant or catalyst in many
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reactions, such as the oxidation, pyrolysis and hydrolysis. Supercritical water can completely dissolve nonpolar
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compounds and still maintain high solubility to polar compounds and ionic compounds by adjusting its pressure and
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temperature 15.
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Smith et al.
19
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reported that supercritical water upgrades petroleum as a green chemical process. Watanabe et al.
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proposed that supercritical water inhibits coke formation by providing hydrogen and diluting coke precursors
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conceptually. Takanohashi et al. 18 treated Canadian oil sand bitumen in an autoclave with three kinds of reaction media
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supercritical water, nitrogen and supercritical toluene, and attributed the upgrading effect in supercritical water to the
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physical dispersion effect. Fatemi et al. 19 found that supercritical water acts as the hydrogen donor by applying isotope
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label technique and regarded it as the evidence of the chemical effect. Based on the above reports, supercritical water
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can not only carry heat into reservoir, but also act as a solvent and upgrade heavy oil. Therefore, it is theoretically
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possible to enhance extra heavy oil recovery by supercritical water flooding (SCWF).
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Experimental investigation is an effective and reliable method to verify the feasibility of this novel technology SCWF
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and to reveal the enhanced oil recovery (EOR) mechanism. There are two types of experimental systems, one dimension
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linear core model and three dimension scaled model. Three dimension model scaled on the target reservoir can predict
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the dynamic process, and it is particularly suitable to develop proposals for pilot test. But it must meet the requirement
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of similarity principle and the experimental operations are complex
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experiments have been widely performed to evaluate the performance of various flooding experiments and to reveal the
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basic principles 22-26. Consequently, it is wise to demonstrate the feasibility of SCWF by linear flooding experiments.
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But there are still two restrictions to perform SCWF experiment on these existed systems. One restriction is that pressure
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and temperature of these systems are lower than the critical parameters of water. Based on research of literature, the
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maximum parameters of flooding platforms that developed by State Key Laboratory of EOR in China are only 20 MPa
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and 350oC
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injected thermal fluid all the time. It would bring about either heat loss or excessive compensation to the core. What’s
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more, thermal reaction would take place once the core is preheated to the injection temperature. Based on the above
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20, 21
. On the contrary, one dimension flooding
. The other restriction is that the existed linear systems need to work at the same temperature with the
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discussion, it is necessary to develop a new sand pack core with an adiabatic boundary in order to keep thermal balance.
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The overall objective of this study is to demonstrate the feasibility of SCWF technology. Firstly, a SCWF
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experimental system was developed in State Key Laboratory of Multiphase Flow in Power Engineering. Steam
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flooding and hot water flooding were carried out on this system to verify the reliability of this experimental system.
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Then Tuha extra heavy oil was flooded by supercritical water under constant conditions of the sand pack core. The
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recovery efficiency and recovered oil viscosity was reported and analyzed. Finally, the EOR mechanism of SCWF
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was proposed.
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2. Experimental apparatus
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The schematic diagram of SCWF experimental system is illustrated in Fig. 1. It consists of a supercritical water
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generator system, a test section of sand pack core, a production system, and a data acquisition system. This platform is
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designed for the pressure up to 30 MPa and the temperature up to 450oC. All pressure equipment and pipelines are made
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of 316 stainless steel. The positive displacement pump supplies deionized water with the maximal flow rate of 40 g/min.
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Supercritical water, steam or hot water is produced by the supercritical water generator, which is similar to a
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once-through electric boiler. A piston pushes the oil from one side of the intermediate crude oil container into the sand
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pack core when the deionized water is pumped into the other side. The temperature of produced liquid is controlled
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within the range of 70~80oC through a heat exchanger, in order to avoid failure of the downstream equipment and
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blockage in the pipelines. The temperature distribution within the sand pack core is measured by sheathed K-type
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thermocouples with diameter of 1.0 mm. Pressure and pressure difference in this system are measured by Rosemount
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3051S transducers. Certainly, hot water and steam flooding experiments can be carried out in this system.
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The test section of sand pack is a horizontally mounted cylindrical pressure vessel with the inner diameter of 40 mm
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and length of 420 mm, as shown in Fig. 2. Two metal foam filters are installed in the entrance and exit of the sand pack
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in order to prevent the quartz sand from escaping. The sand pack is heated by six 500 W electric heaters soiled around
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its outer surface. Every electric heater is controlled independently by two temperature transducers and one
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proportional–integral–derivative (PID) temperature controller.
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As shown in Fig. 3a, there are two kinds of control modes for thermal boundary conditions of the sand pack core.
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When the switch is placed in position I, the PID controller adjusts heating power according to the outer surface
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temperature Tout and the preset value that equals to the initial reservoir temperature. Then the boundary condition of
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constant temperature is constructed. When the switch is located in position II, heating power depends on the
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temperature difference of the outer and inner surface and the preset value. The adiabatic boundary would be built
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theoretically by setting the temperature difference to zero. Whereas, excessive heat compensation would happen since
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thermal conductivity coefficient of stainless steel (λss≈16.6 W/(m*K)) are far higher than that of quartz sand
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(λSiO2≈7.70 W/(m*K)). So it is indispensable to determine the preset value of the PID controller by pre-test.
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Here the sand pack core is regarded as an open system in flooding process. Based on the mass conservation Eq. (1),
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the mass difference between inflow and outflow is equal to the mass increment of fluid within the sand pack core.
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Based on the law of energy conservation Eq. (2), an adiabatic boundary condition is created when the power of the
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electric heater just offsets the heat loss and the storage heat of the sand pack, i.e. the accumulative thermal enthalpy
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difference between the inlet fluid and outlet fluid is equivalent to the internal heat increment of the sand pack core.
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∫Q
in
t − ∫ Qout t = ρ f Vp − ρiVp
(1)
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h t − ∫ Qout hout t = ( ρ f h f − ρi hi ) V p + mS cS (Tf − Ti )
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∫Q
2
The test results of injecting supercritical water of 25 MPa and 400oC are exhibited in Fig. 3b as an example. The
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parameter q, as shown in Eq. (3), represents the extent of heat loss or over-compensation and it refers to the ratio of net
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inflow energy of the sand pack core and the total energy carried by the injected thermal fluid. The net inflow energy is
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calculated by subtracting the internal heat increment of the sand pack from the accumulative enthalpy difference of
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the inflow and outflow. So, q0 signify over-compensation and under-compensation of heat, respectively. The
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value of the temperature difference should be set as -4.5oC in this case.
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in in
q=
∫Q
h t − ∫ Qout hout t − ( ρ f h f − ρ i hi ) V p − mS cS (T f − Ti )
in in
∫Q
(2)
(3)
h t
in in
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Materials and analysis methods: It is necessary to soak high purity quartz sand in hydrochloric solution, flush it by
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deionized water and then bake it in the oven. Extra heavy oil produced from the well 503H of Tuha oilfield in China is
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degassed, filtered and dehydrated in turn. Water and oil content of the output fluid are obtained by centrifugal seperation.
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Oil viscosity are measured in parallel and plate system at 60oC by the rheometer Anton paar MCR302 with the shear
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speed of 20 s-1.
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3. Results and discussions
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Flooding experiments by injecting supercritical water, steam and hot water were carried out in this study. Table 1
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lists the experimental conditions, including injection parameters and seepage parameters. All parameters, except
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pressure and temperature, are kept consistent among these flooding experiments, and the relative errors of porosity,
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absolute permeability and initial oil saturation are all less than 1.0%. Table 2 shows the properties of Tuha extra heavy
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oil used in this paper. In this study, the mass flow rate of steam, water and supercritical water are equivalent to each
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other. The mass flow rate is calculated as 5 g/min to eliminate the end effect of sand pack in water flooding and gas
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flooding.
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3.1 Test feasibility of this system
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Steam flooding and hot water flooding have been widely investigated in theory and experiment, and their basic
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principles are well-known. Therefore, both flooding experiments are performed to justify the reliability of this novel
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system. The results are analyzed from three aspects: pressure difference of displacement, core temperature, and
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recovery efficiency.
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The pressure at the sand pack entrance Pin and the displacement pressure difference between the entrance and exit
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of the sand pack DP are shown in Fig. 2. The influence of dimensionless volume of injection steam on pressure and
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displacement pressure difference are presented in Fig. 4. There are three stages for the displacement pressure
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difference in steam flooding process. At the first stage, a greater driving force is produced due to the poor mobility of
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extra heavy oil. The displacement pressure difference increases rapidly from 2.0 MPa at 0 PV and finally to a peak 3.6
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MPa at 0.75 PV. Then, the continuous heat injection causes the decrease of both oil saturation and oil viscosity, so the
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displacement pressure difference reduces quickly until the initial value 2.0 MPa. At the last stage, the displacement
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pressure difference keeps constant, because stable flow channels form after 1.0 PV, where more than 40% of the
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saturated oil has been recovered, as shown in Fig. 6. The variation of pressure Pin and displacement pressure
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difference DP is consistent during the whole flooding process, whereas slight fluctuation of the pressure Pin is
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observed, which is attributed to the intermittent outflow of oil and water from the back pressure regulator. Songyan Li
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et al. 28 have certificated this changing process of pressure field in different flooding experiments including both steam
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flooding and fuel gas assisted steam flooding.
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Steam overlapping is a typical phenomenon occurred during steam flooding process. The temperature of the upper
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layers would be higher than that of the lower layers in oil reservoir because steam tends to flow upward under gravity.
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Temperature distribution versus the dimensionless volume of injection steam and the density curve of water at 2 MPa
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are both displayed in Fig. 5a. The accumulated steam at the top of the core could be considered as a heat source, and any
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spherical shell centered on the heat source point could be approximately regarded as an isothermal surface. Hence arcs
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with the heat source point refer to isotherms in cross-section as shown in Fig. 5b, and the size relationship of
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temperatures shown in Eq. (4) signifies the occurrence of steam overlapping. The overlapping character becomes weak
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with the increase of the distance to injection position, because the steam releases latent heat to reservoir and condenses.
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During the flooding process, heavy oil would gradually be replaced by steam, and the phenomenon of steam
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overlapping would disappear once the steam fills whole pore of the cross-section. Therefore it is a typical steam
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overlapping phenomenon.
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TUP > TMID > TRIGHT > TDOWN
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It is commonly accepted that the recovery efficiency of steam flooding is higher than that of hot water flooding at
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the same temperature. The results indicated that steam flooding with 2 MPa and 350oC enhances extra heavy oil
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recovery from 74.20% to 80.40% compared with hot water flooding with 25 MPa and 350oC. Willman et al.
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already certificated that steam flooding is significantly superior to hot water flooding due to multiple effects of solvent
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extraction, steam distillation 29 and gas flooding by laboratory tests.
(4)
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have
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The feasibility of this experimental system is verified by above experimental results. Therefore, it is capable of
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simulating the pressure and temperature field distribution during flooding process and predicting recovery efficiency
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correctly on this system.
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3.2 EOR performance of SCWF
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Fig. 6 shows the recovery efficiency ER and average weighted temperature of the sand pack core Tw versus
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dimensionless injection volume PV. The results illustrate that the recovery efficiencies all increase linearly when
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injection volume is less than about 1.5 PV and the differences among three flooding experiments are small. The
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efficiencies do not continue to increase after 3.5 PV. The final recovery efficiencies of SCWF, steam flooding and hot
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water flooding are 97.07%, 80.40% and 74.20%, respectively. It is obvious that SCWF technology has remarkable
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predominance in EOR.
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SCWF owns another advantage in respect of thermal efficiency when compared with steam. Effective enthalpy he is
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introduced in Eq. (5) to signify the available thermal energy brought into the sand pack core by steam, hot water or
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supercritical water. The parameters h and hi refer to the enthalpy of water under the condition of injection temperature
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and that under the condition of initial core temperature, respectively. So the thermal efficiency γ is proportional to the
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ratio of recovery efficiency ER and effective enthalpy he, as shown in Eq. (6). Table 3 displays the ratio of thermal
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efficiency for SCWF, steam flooding and hot water flooding is about 3.0:2.0:4.0.
17
he = h − hi
18
γ∝
(5)
ER he
(6)
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In addition to enhance oil recovery, it is generally known that thermal recovery methods could reduce viscosity of
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the recovered oil. The viscosity ratio µr, which refers to the viscosity ratio of the recovered oil to the initial extra heavy
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oil, is used to indicate the decline of oil viscosity herein. The viscosity of initial extra heavy oil is 50.879 Pa·s at 60oC.
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The average viscosity ratio of the recovered oils were decreased by 36.9%, 47.1% and 56.0% for SCWF, steam flooding
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and hot water flooding, as shown in Table 4.
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3.3 EOR mechanism of SCWF
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SCWF shows advantage in EOR when Tw reaches the critical temperature, where the injected thermal fluid is about
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1.5 PV. Before that, the recovery efficiencies have little difference among three experiments and its size relations are
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consistent with average weighted temperatures. At the early flooding stage, the temperature of the sand pack core
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increases with the amount of the injected heat. Because the enthalpy of steam at 2 MPa and 350oC is higher than that
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of supercritical water at 25 MPa and 400oC with the same mass flow rate, the recovery efficiency of steam flooding
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would be higher than that of supercritical water flooding. The heat transfer process within the sand pack core is
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simplified as three procedures, thermal conduction in water phase (including water, steam and supercritical phase),
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thermal convection between water and oil in phase interface and thermal conduction in oil phase. The rate of thermal
15
conduction can be represented by thermal diffusion coefficient α. It can be inferred from Table 5 that α of steam is
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much higher than that of hot water or supercritical water. As far as this study concerned, the heat transfer in water-oil
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phase interface belongs to laminar thermal convection and the Nusselt number (Nu) is a constant. Under the
18
assumption that the area of water-oil interface is two thirds power of injection volume of water, the heat exchange
19
amount Ф between oil and water phase is proportional to the conduction coefficient λ of water and the temperature
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differences of these two phase, and inversely proportional to 2/3 power of water density, as shown in Eq. (7). The ratio
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of heat exchange amount Ф for SCWF, steam flooding and hot water flooding is calculated as about 1.8:3.7:2.1.
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Because the thermal conduction in oil phase are similar among three flooding experiments, steam flooding owns the
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most amount of heating rate, then SCWF, hot water flooding is in the third place.
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φ∝
λH O (T − T ) ρ 2/3 in i 2
(7)
H 2O
6
The outstanding EOR performance of SCWF should be attributed to the favorable properties of supercritical water,
7
because ER of SCWF continuously increases even when Tw exceeds the critical temperature. The mechanism of heavy
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oil upgrading in supercritical water has been studied and elucidated
9
that is, physical properties effect (solvation and dispersion) and chemical effect (hydrogen donor). The physical
17-19, 30-33
17, 18, 30-34
, and it can be divided into two aspects,
10
properties of supercritical water has get widespread agreement
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existing, the asphaltenes and aromatics in oil rich phase would form coke via condensation and polymerization
12
reactions. However, large amounts of organic compounds, including coke precursor, would be extracted into water
13
rich phase under the condition of supercritical water, which improves the light liquid hydrocarbon yield through
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suppressing coke formation. There is controversy about the chemical action of supercritical water 18, 19. Fatemi et al. 19
15
asserted that the chemical role of supercritical water as hydrogen-donor solvent had been verified when temperature
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exceeds 425oC by applying heavy water (D2O). But the results that there existed C-D bonds in the products can only
17
state that supercritical water is involved in chemical reaction, because H-exchange would produce the same outcomes.
18
Takanohashi et al. 18 analyzed gaseous product, middle distillate, distillation residue, and coke after treating bitumen in
. Under the condition of hot water or steam
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SCW and nitrogen. The results indicate that the H/C atomic ratios of middle distillate, distillation residue and coke are
2
almost the same when using water and nitrogen at 450oC, and only 0.2% feed water is converted to gaseous products.
3
It is suggested that supercritical water does not chemically affect the yield of products. Therefore, it is reasonable to
4
ascribe the EOR performance of supercritical water to its physical action rather than its hydrogen-donor role in this
5
study.
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Morimoto et al.
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contrasted the dielectric constants ε and Hansen solubility parameters (HSP) δ [Eq. (8)] of
7
typical good and poor solvents to heavy oils and concluded that 2.2≤ε≤10.4, δp
