Crude Oil Electrical Conductivity Measurements at High Temperatures

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Crude Oils Electrical Conductivity Measurements at High Temperature: Introduction of Apparatus and Methodology Rafael Mengotti Charin, Gabriela Muniz Telo Chaves, Khalil Kashefi, Robson Pereira Alves, Frederico Wanderley Tavares, and Márcio Nele Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03237 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017

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Crude Oils Electrical Conductivity Measurements at High Temperature: Introduction of Apparatus and Methodology Rafael Mengotti Charin†, Gabriela Muniz Telo Chaves‡, Khalil Kashefi‡, Robson Pereira Alvesᵻ, Frederico Wanderley Tavares†‡, Márcio Nele†‡ †

Programa de Engenharia Química, COPPE, Universidade Federal do Rio de Janeiro, CEP 21945-970, Rio de Janeiro, RJ, Brazil ‡

Escola de Química, Universidade Federal do Rio de Janeiro, Cidade Universitária, CEP 21949-900, Rio de Janeiro, RJ, Brazil

ᵻ

Centro de Pesquisas e Desenvolvimento Leopoldo Américo Miguez de Mello, Cidade Universitária, Rio de Janeiro 21941-915, RJ, Brazil ABSTRACT In this communication, the electrical conductivity measurements of four types of crude oils at reservoir conditions were carried out using an in-house developed pressurized cell. The newly introduced apparatus has the capability of performing electrical conductivity measurements of various reservoir fluids at HPHT conditions. The cell can

be pressurized with an inert gas to be far enough above the bubble points (Pb) of crude oils. The alternate voltage (AV)cycle methodology was applied to perform the experiments. In this method, the subtraction of electrical currents when voltage alternated between two specified values (0 and 2 V in this work) was used to calculate the conductivity. The reliability of the methodology and the equipment in making conductivity tests has been verified by comparing the conductivity results with the “rest conductivity” values measured via a standard method. Variation of crude oils electrical conductivity showed a strong relation to the physical and chemical properties of the reservoir fluids including the electrical nature of compounds in oils and viscosity of fluids. The experimental data were fitted well to the Arrhenius model with a clear break point at certain temperature for each oil which can be linked to the structural change of the fluids at the mentioned temperatures. KEYWORDS: Crude Oil Conductivity, Electronic Hopping Mechanism, Alternate Voltage

Corresponding Author ACS Paragon Plus Environment

*Corresponding author. Email: [email protected]

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INTRODUCTION Electrical conductivity is often obtained to characterize aqueous solutions that show charge transfer substantially affected by the presence of electrolytes and charged particles. Importance of the conductivity measurement in aqueous media lies on applications such as performance of water demineralization. Nevertheless, instruments for obtaining the conductivity of aqueous solutions are useless to measure the conductivity of crude oil samples, because both magnitude of measurement and mechanism of charge transport are different. At standard conditions for temperature, the electrical conductivity of aqueous solutions usually is about10-3 to 100 S/m (Siemens per Meter),while the crude oil electrical conductivity may vary between 10-9 and 10-8 S/m. Turning to charge transport mechanisms, solubilized electrolytes and ionic compounds are responsible for the electrical properties in aqueous media. Alternate current (AC) is therefore applied for avoiding electrode polarization in aqueous solutions while the conductivity measurement takes place. On the other hand, electrical charge transfer for crude oil is characterized by the “electron hopping” mechanism[1],[2]. It is believed that hopping of the electrons from the cyclic π bond of aromatic compounds is responsible for the electrical charge transfer[1]. Oil and water are concurrently produced during onshore and offshore operations. Because of several factors[3], water in oil emulsions are formed in oil production units. Since the presence of water in crude oil causes operational problems and affects the specification of oil, separation of water and oil is essential, and this goal is achieved by mechanical, chemical, thermal and electrostatic separations, or combination of these techniques[4]. Due to both economical and environmental reasons, electrocoalescence is an attractive method to remove water emulsified in oil. Over the years, industrial electrocoalescers have developed[5],[6] and can work under different conditions of temperature, pressure and electric field[7]. According to Maruskaet al.[1], one of the important parameter for the electrocoalescence process is the oil conductivity, where direct current (DC) and alternate current (AC) responses are essential to define the intrinsic limitations of electrocoalescers. Specifically, high conductivity of oils is associated with electric field decay[4],which can reduce the benefits of electrostatic method in oil dehydration. Crude oil conductivity is therefore valuable information in

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order to select the proper separation technique and to design an efficient electrical arrangement of industrial electrocoalescers. Despite the great advances in petroleum analysis[8], there are possibilities for development of new methodologies. Particularly, the literature lacks of crude oil conductivity measurements at high temperatures which are in need to design the industrial electrocoalescers. Not only the magnitude of the applied electric field matters when describing the electrocoalescence process. There are many parameters that influence the attraction between the coalescing drops such as the inter-drop separation, size of drops, shape distortion besides the permittivity, viscosity, interfacial tension, all fluid properties[9]. Less and Vilagines[4] and Noïk et al. [10] cited four electrical forces related to the electrocoalescence process in water in crude oil emulsions, which are the dielectrophoresis, electrophoresis, dipolar attraction and droplet deformation caused by electrical stresses. Each one of these forces is affected by the oil permittivity, which is directly related to the oil conductivity. An electric field can only be used to increase the coalescence rate of a dispersed phase in an emulsion when the continuous phase has a much lower permittivity than the dispersed phase[11]. Therefore, electrostatic coalescers work by applying an electrostatic field strength affecting the conductive water-droplets in an insulating oil media. The conductivity measurements indicate the insulating property of the crude oil. One concludes that the conductivity of crude oil affects the electrocoalescence process[4, 9]. Hence, this relation between conductivity and electrocoalescence process should be investigated, what may cope with the design and

improvement

of

new

and

existing

correlations

for

modeling

the

electrostaticcoalescer behavior. Also, the conductivity data could promote conceptions for phenomenological models. Thus, the introduction of an apparatus and a methodology to access the electrical properties of crude oil would be interesting for oil processing industry. In this work, an in-house built setup and a methodology capable of characterizing crude oil samples with respect to electrical conductivity for a wide range of temperatures is introduced. The conductivity measurements were performed on various crude oils at high temperature conditions and above bubble point of the fluids. The relation between electrical conductivity and properties of fluids were also discussed.


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EXPERIMENTAL SECTION Properties of Crude Oils The crude oils tested, with the specifications, were provided by Petrobras, Brazil. The samples were received and quartered at 80 °C in equal volumes of 400 mL. Three volumes of the same sample were used to perform the triplicates. The main physical and chemical properties of the four crude oils that were used to perform these series of electrical conductivity tests are shown in Table 1. Crude oils H1 and H2 are considered as heavy oils whileW1 and W2 are medium oils. W1 and W2are rich in paraffin, and therefore they are classified as waxy crude oils. The crude oil W2 has the most amount of polar constituents (resins and asphaltenes) and crude oil H1 is the most viscous fluid. The density and viscosity were measured using the SVM 3000 model of Anton Paar viscometer (densimeter) and the SARA tests were performed with chromatography technique. The water content was determined by Karl Fisher (ASTM E-203).

Table 1 – Properties of crude oils Crude Oils Properties Density (kg/m3) at 25 ºC Viscosity (mPa s) at 25 ºC Saturate (wt %) Aromatic (wt %) Resin and Asphaltene (wt %) Water Content (wt %)

H1

H2

W1

W2

941 684 52.9 36.2 10.9 0.05

915 160 55.4 33.0 11.6 0.25

884 71 52.6 35.8 11.6 0.5

907 178 55.1 31.8 13.1 0.16

Experimental Setup A setup was developed in-house to measure the conductivity of various crude oils at reservoir conditions. The experimental apparatus consists of a jacketed high pressure vessel (SS316L) with maximum working pressure and temperature of 130 bar and350°C, respectively. A high pressure syringe pump (Teledyne Isco, model 260D) was employed to maintain the pressure of the cell using an inert gas (nitrogen).


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Thermostatic bath (Thermo Haake, model A28F) was used to control the temperature of the cell with a silicone oil as circulating fluid. Silicone oil (Polydimethylsiloxane, viscosity =200 cSt), provided by Silicones Paulista, was used in order to omit any impact of electrical interference caused by the circulating fluid. A picoammeter (Keithley, model 6485) was utilized as power supply with ampere and voltage ranges of up to 25 µAand500 V, respectively. The experimental setup is shown in Figure 1.

Figure 1 - Experimental setup for measurement of petroleum electrical conductivity. (1) Conductivity cell; 1.1, Petroleum sample; 1.2, Temperature transducer (Pt100); 1.3, Pressure gauge; 1.4, Discharge valve; 1.5, Magnetic bar; 1.6, Heating jacket. (2) Syringe pump; 2.1, Valve, 2.2, Inert gas (N2). (3) Picoammeter; 3.1, Internal electrode; 3.2, External electrode. (4) Magnetic stirrer. (5) Bath for controlling temperature with circulating silicone oil. (6) Temperature indicator.

The cell is a HPHT vessel consisting of four main parts: the cup, the cap, the jacket and the internal electrode. The volume of the cup is 473 ml and approximately 2/3 of this volume was filled by the sample during the experiments (about 315 ml). The cap of the pressure cell contains entries for electrode, temperature transducer (Pt100)to measure the temperature inside the pressure vessel, pressurizing connections, pressure


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gauge and discharge valve. The oil conductivity is obtained by using the picoammeter and application of DC type (direct current). The picoammeter is connected to the internal electrode and cup of the cell (as external electrode) and all the connections are entered electrically insulated through the cell. On the surface of internal electrode, some holes were made for improving the fluid circulation inside the cell and between the electrodes. Electrical connections are made by special low noise cables and the connectors are made of gold to improve the quality of the measurements.

Methodology and Procedure By using the picoammeter, a small voltage was applied and the resultant current was measured. A very small current ammeter (picoammeter) is needed because of the high electrical resistance of the sample. The distance between the internal electrode and the cup (external electrode) is 7 mm. By applying the transport equation for the geometry between the two electrodes (approached by the concentric cylinders arrangement),the conductivity was calculated by using the recorded current and the applied voltage. The surface area of the holes that were made on the internal electrode was discounted in the area integration between the inner radius (r1) and the outer radius (r2). A caliper rule was used to obtain the geometric measurements. The equation used to obtain the conductivity is a primitive form of the second Ohm’s law: =

Where I is electrical current, σ represents conductivity, A is transversal section area, V is voltage and r is radius. The crude oil sample was first placed in a beaker and the temperature is measured. The EMCEE Electronics instrument (ASTM D 4308-89)[9]was used to evaluate the electrical conductivity of the sample at room temperature. The EMCEE used herein is a hand held, battery operated, instrument for determining the conductivity of hydrocarbon samples. This conductivity meter device (EMCEE) is originally designed for aviation fuel, with special aim of avoiding explosions causes by static energy[13], but there are devices for special resolutions that match with crude oil measurement requirements. The EMCEE device is comprised by a console and a stainless steel probe, which is


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immersed in the liquid sample. The probe is a concentric-cylindrical arrangement of inner and outer electrodes that are separated by an insulator. Since the probe is made of steel, with a low calorific capacity, it should rest inside the liquid for a period of time in order to equilibrate the temperature between the liquid and the steel probe. The EMCEE instrument provides the “rest conductivity” which comes from the current crossing the geometry when the sample is discharged, i.e, during the first moment of the voltage application[9]. There are other EMCEE devices manufactured for special resolutions that match the magnitude order of the petroleum sample electrical conductivity. As it is mentioned above, the “rest conductivity” values that were measured by the EMCEE device, uses the resistivity response of the first instant of voltage application for calculating the conductivity, therefore the capacitance influence (rotation effect of the molecules due to molecular polarity asymmetry) would not affect the measurement. But, the methodology applied in the present study has the capability to take into account the capacitance influence in measurements of crude oils electrical conductivity. Despite the fact that the standard method of EMCEE device gives different electrical conductivity mode from the proposed apparatus, it could be used for comparison reasons to assure the reliability of the procedure. Since the electrical conductivity is highly dependent to temperature (exponential dependence), it is essential to collect the conductivity values when the temperatures in both setups are the same. A great care was made in temperature check and the same temperature transducer (Pt100) was used in these two measurements (EMCEE and the proposed apparatus). When these two measurements were consistent, the setup was ready to start recording the data for higher temperatures. These series of experiments showed that the “rest conductivity” measuring by the EMCEE device and the “alternate voltage (AV) conductivity” obtaining by the in-house built HPHT setup were comparable which could confirm the reliability of the procedure and apparatus. To carry out the measurements in HPHT apparatus, a special magnetic bar was placed inside the cell to keep the fluids thermally and compositionally homogenized during the course of experiments. Then the dehydrated sample was loaded in the cell which is thermally isolated. The circulating bath was then turned on in order to pump the silicone oil towards the pressure cell jacket for making the analyte reaching the same


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temperature as the one recorded for the previous measurement using the EMCEE device. As commented before, the temperature is recorded during the experiments using a temperature transducer-Pt100, with an electrically insulated well. The Pt100 is positioned within the cell between the two electrodes. To assure that the temperature of the testing sample is homogeneous, in addition to the magnetic mixing, special for viscous fluids, and the holes made in the internal electrode to improve the circulation, the heating procedure is stepwise and slow, and the temperature manipulation is particular ultra-slow (~0.1 °C/minute) in the vicinities of the target temperature. The repeatability of the measurements is part of the methodology applied, as depicted below. Three measurement repetitions are averaged in the methodology. Moreover, the experimental reproducibility was evaluated in independent triplicates, which generate the standard deviation bars. The alternate voltage (AV) measurements were carried out using 3 cycles of 30 seconds alternating between 2 V(higher voltage)and 0 V (lower voltage), as shown in Figure 2.The Ohm’s first law was consistent (the steady state current is linear with the voltage)with the applied voltages.

Figure 2–Diagram of applied voltage versus time for the AV measurement.

During a cycle, the electrical currents measured for zero voltage were subtracted from the correspondent measurements at “higher voltage”. This procedure had the aim of canceling background currents which is a result of capacitance effects. This could improve the accuracy of conductivity measurements. Also, the instrumental noise was


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omitted by averaging three cycles. The resulting current for one cycle (subtracted result) is shown in Figure 3.

Figure 3 - The subtracted current response from one AV cycle. For this experiment a voltage of 1 V was applied. (Vh= 1 V).

The AV measurement can be modeled as a DC transient circuit of capacitor-resistor, arranged in parallel, as shown in Figure 4. Perini et al. [14]has modeled the response of crude oil samples using this circuit for in-situ impedance spectroscopy analysis. The equation for the current is: =

,

Where, I(t) presents the current flow through the oil between the electrodes as a function of time, Vh is the DC voltage applied to the sample, R and C are the resistance and capacitance of the sample, respectively. The curve shape for the two cycles of AV measurement is shown in Figure 4. According the mentioned graph, for the first instant of voltage application, the molecules are not polarized, and therefore the response for the applied voltage (Vh = 2 V) is only due to resistance. Over time, the molecules likely to polarize start to rotate and the response to the applied voltage is divided between the resistive and capacitive effects. The current decreases because part of the charge now stays in the electrode for


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maintaining the molecules polarized. Finally, “the capacitor” charges completely and the resultant current for the voltage applied remains constant over time. When the system relaxes at 0 V, the picoammeter detects the current returning from the charged electrodes, and that’s why this current is negative (opposite direction). Then, the current reaches the zero because there is no tension applied and no electrodes charged.

R

I0

C

Figure 4 - Diagram of applied voltage and measured current versus time, where I0 is the starting peak current and Vh is the applied “high voltage”. The inside figure shows the electric model for the system.

RESULTS and DISCUSSION Electrical conductivity measurements were carried out with the aid of the newly developed apparatus and also four different crude oils were chosen to perform the experiments.

Mechanism of Electrical Charge Transfer in Crude Oil Information about the mechanisms of electrical charge transport in crude oil samples are scarce in literature. However, one can assert that the electrical conductivity magnitude


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of crude oil sample is very low and is known to be electrically insulating, so the conductivity measurement could be very delicate.

Figure 5 shows the results of an experiment that indicates the mechanism of electrical charge transfer for a heavy crude oil. The conductivity cell was filled with a sample of crude oil, at room temperature, and the voltage of 1V (electrical field of 143 V/m) was applied for 4 hours. The result presented in Figure 5indicatesthat the current remains constant, near 60 nA. This experiment is inspired by Maruskaet al.[1]. The sample tested in Maruska et al.[1] was a bottom stream from a distillation tower and the applied DC electrical field was 106 V/m, a much greater field than the one tested here. They call this test as “long term current stability measurement”. In Maruska et al. [1], the current remained unchanged for almost three months. Both experiments, the one was performed in this work (shown in Figure 5) and the one was accomplished by Maruska et al.[1], show the charge transfer mechanism for crude oil. The constant current demonstrates that no polarization happened in electrodes because there is no creation of additional resistances. This result indicates that the molecules participating in the charge transport process allow the electrons to pass through them, so this is an electronic transport mechanism. The electrons hopping from one molecule to another which this characterizes the charge transport. The transport mechanism is related with the weak cyclic π bounding of aromatic compounds. Forster[15]showed this feature of aromatic compounds, when conducting electricity clearly. Two conductivity cells were filled with benzene and cyclohexane, and they were subjected to a constant voltage of 1kV for 1500 hours. A constant current was recorded for benzene but not for cyclohexane. In the latter case the resistance increased during time due to the charge accumulation in electrode. This was caused by impurities, responsible for the charge conduction in cyclohexane, moving towards the electrode. Thus, in benzene case, the charge transport was performed by electrons, while electrode polarization indicated that impurities are the charge carrier of the aliphatic sample (cyclohexane).


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Figure 5 - Current stability tests for a heavy oil sample, the current maintained almost constant during four hours.

Crude Oils Electrical Conductivity Four crude oil samples were analyzed by the in-house built apparatus with the application of the AV mode. The conductivity tests were carried out using two samples classified as heavy oils (H1 and H2) and two samples classified as waxy crude oils (W1 and W2).Electrical conductivity results are shown in Figure 6 and a better view of these series of data at temperatures below 60 °C is illustrated in Figure 7.For the range of temperatures tested, it was noticed that the conductivity measured can vary three orders of magnitude. Samples H1, H2 and W2 were tested between 30 and 130 °C and the recordings occurred every 20 °C. Data for the sample W1were collected every 10°Cfrom 40 to 150 °C.


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Figure 6 - Data of petroleum electrical conductivity along with the error bars (that were made in triplicates). A better view of the temperatures below 60 °C is illustrated in Figure 7.

Figure 7–Experimental results of crude oils electrical conductivity at temperatures below 60 °C (for close look to the data at lower temperatures).


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The electrical conductivity results of heavy crude oils(H1 and H2) appeared to be close while the waxy oils showed electrical conductivity with quite different values. The sample W2 presented the highest electrical conductivity which could be due to the highest amount of polar compounds (Table 1).At room temperature, the lowest conductivity was recorded for the sample H1, and this was also confirmed by the EMCEE instrument. The sample W1 at lower temperatures, showed higher electrical conductivity than the samplesH1 and H2 in this work (Figure 7).At temperature ranges above 50 °C, the lowest conductivity ranges were measured for the W1 sample. The scope of this work is not to discuss the properties of the crude oils, but some analysis can be made in this respect. The effect of higher density (heavier crude oil) in conductivity can be observed by comparing the results for the two waxy oils, where the W2 is heavier than W1, the electrical conductivity is also much higher.W2 also showed the highest electrical conductivity among the tested fluids which could be due to higher asphaltenes and resin contents (as polar compounds). Since H1 and H2 are both heavier than W2, this shows that it is not only the presence of heavy compounds that affects the conductivity values, but the nature of the compounds in respect to their ability to transfer the charges also play an important role. The viscosity also can have influence in determining the electrical charge transport for petroleum [1],[16]. According to Forster [17], for poly-nuclear aromatic compounds, the electronic hopping charge transfer is possible whenever a planar overlap of πorbitals occurs and this situation is facilitated in low viscosities, which means higher viscosity normally causes lower conductivity. This can be confirmed by comparison the viscosity of H1, which is the highest one, with the viscosity of H2 and W2, which are almost the same values. But this statement cannot be verified regarding the crude oil W1, which has the lowest viscosity value and the lowest electrical conductivity, in particular at high temperatures. The reason could be due to diminishing the influence of viscosity on electrical conductivity at high temperatures. The viscosity values of crude oils drop drastically at high temperatures and as a result, the difference in viscosity values becomes less, which leads to decrease the viscosity effect on conductivity. This justification can be verified by considering the electrical conductivity at low temperatures (Figure 7), where the crude oil W1, with the lowest viscosity, showed higher electrical conductivity than the samples H1 and H2. In low range temperatures, viscosity can be considered as key factor in determining conductivity.


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Above discussion shows that the electrical conductivity of crude oils can be function of various physical and chemical properties. So, in addition to heavy fraction content and viscosity, other competing variables, such as the nature of each compound in regard to its effect on charge transfer may govern the conductivity of crude oils. Because the crude oil conductivity varies exponentially with temperature, Arrhenius dependence is plotted in Figure for all samples. Results suggest a good possibility of interpolation and extrapolation of data. This graph shows that the slope of Arrhenius plot has changed for all crude oils, at about 70 to 90 oC. Petroleum presents a degree of organization arising from its micro-structure. This shows that the molecules tend to organize structures which govern petroleum properties. The slope change in Figure 8 could mean a transition in the petroleum micro-structure, what governs the macroscopic properties. This result evidences that this apparatus will be useful for providing a better knowledge about petroleum.

Figure 8 - Arrhenius plot for the AV measurement up to 150 °C.A slope change of Arrhenius plot also is observed.


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CONCLUSIONS In present study, an in-house developed apparatus and a methodology for measuring crude oils electrical conductivity are introduced. The setup is capable of performing electrical conductivity tests at reservoir temperatures and under a pressure, well above the bubble points of fluids. An alternate voltage (AV) cycle methodology with the application of three cycles of 2 to 0 V was used to calculate this property of the reservoir fluids. The technique and equipment showed comparable results to the standard electrical conductivity measurement method (rest conductivity). Four different crude oils were utilized to perform the experimental study. As it was expected, the crude oil electrical conductivity against temperature showed exponential behavior. The importance of physical and chemical properties of the crude oils in altering the magnitude of electrical conductivity was observed. Crude oils rich in polar compounds tend to be more conductive than other crude oils. However, at lower temperatures, the effect of crude oils viscosity showed to be significant. The results suggest that this apparatus could provide a valuable insight on crude oil micro-structures. This could be seen in conductivity-temperature trend change (break point) when they are illustrated in Arrhenius plot. The data presented in this study are particularly useful as input variable for electrocoalescers design and operation.

ACKNOWLEDGMENTS The authors thank CNPq, CAPES, FAPERJ and Petrobras for scholarships and ﬁnancial support.


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[10] Noïk, C.; Chen, J.; Dalmazzone, C. Electrostatic Demulsification on Crude Oil: A State-Of-The-Art Review. In: SPE International Oil & Gas Conferenceand Exhibition, Beijing, China, 5–7 December 2006. [11] Eow, J. S.; Ghadiri, M.Electrostatic enhancement of coalescence of water droplets in oil:a review of the technology. Chemical Engineering Journal, 2002,85, 357368. [12] Standard test method for electrical conductivity of liquid hydrocarbons by precision meter. ASTM International: West Conshohocken, 2014; D4308-13. [13] Ludt, W. C. Conductivity measurement system. U.S. Patent 3,820,014, June 25, 1974. [14] Perini, N., Prado, A. R., Sad, C. M. S., Castro, E. V. R., Freitas, M. B. J. G. Electromechanical impedance spectroscopy for in situ petroleum analysis and water-in-oil emulsion characterization, Fuel, 2012,91, 224-228 [15] Forster, E. O.Electric conductance in liquid hydrocarbons. IEEE Transactions on Electrical Insulation,1967, 2, 10-18. [16] Lesaint , C.; Spets, O.; Glomm, W. R.; Simon, S.; Sjoblom, J. Dieletric response as a function of viscosity for two crude oils with different conductivities. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2010, 369, 20-26. [17] Forster, E. O. On the activation energy for conductance in aromatic hydrocarbons. ElectrochemicaActa, 1964, 9, 1319-1327.


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Crude Oil Electrical Conductivity Measurements at High Temperatures

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