Article pubs.acs.org/EF
Ultrasonic Detection and Analysis of Wax Appearance Temperature of Kingfisher Live Oil Hao Chen,*,†,‡ Shenglai Yang,†,§ Xiangrong Nie,† Xiansong Zhang,§,∥ Wei Huang,† Zhilin Wang,† and Wei Hu† †
Key Laboratory of Petroleum Engineering of the Ministry of Education, China University of Petroleum, Beijing 102249, People’s Republic of China ‡ Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States § State Key Laboratory of Offshore Oil Exploitation, Beijing 100027, People’s Republic of China ∥ China National Offshore Oil Corporation (CNOOC) Research Institute, Beijing 100027, People’s Republic of China S Supporting Information *
ABSTRACT: Wax crystallization is a serious problem because it may result in the plugging of wellbores, production facilities, and transportation pipelines during oil production. This study describes the use of a novel high-pressure ultrasonic detection system as a reliable technique to evaluate the wax appearance temperature (WAT) and the wax precipitated curve (WPC) of both crude oil [stock tank oil (STO)] and live oil at pressures up to 22.87 MPa. The oil sample was from Kingfisher (KF) oilfield, which is a typical high waxy reservoir. Transit time and amplitude were demonstrated to be good indicators for WAT determination with the ultrasonic method, which could not only self-verify but also distinguish the different stages of the complex process of wax crystallization. In addition, differential scanning calorimetry (DSC) and the rheometry method under ambient pressure were also presented to match the ultrasonic method. The obtained WAT data are comparable to results from DSC and rheometry, which could be used to further optimize the modeling of wax deposition. Moreover, the amount of wax precipitated could also be estimated by the ultrasonic method, and the WPC is very close to the curve obtained by DSC. It is also concluded that WAT experiments on live oils must be conducted because the measured dead oil WAT may provide too conservative and unrealistic estimates because of the composition and pressure difference. Thus, for oil production and transportation, the addition of solution gas or just to maintain the dissolution gas in contact with the crude oil may reduce the risk of wax precipitation and then drop all of the costs related to wax removal. For high-waxy oilfields, such as KF reservoirs, multiphase production and transport may be an economic solution.
1. INTRODUCTION During oil production and transportation, the high-molecularweight paraffins of the crude oil will separate from the supersaturated solution and start to crystallize because of the temperature drops. The onset temperature of wax crystals that appears because of the heat loss to the surroundings is called the wax appearance temperature (WAT).1 This phenomenon not only affects oil production and transportation but also causes large production loss because of the ensuing downtime and remediation operations. In addition, wax deposition becomes increasingly serious with the new development shift driven by harsh environments, more challenging fluid properties, and probably much longer pipelines.2 It will inevitably increase the cost of remediation. Therefore, wax precipitation and wax-related problems, such as increased viscosity and pressure drops at temperatures below the WAT, must therefore be clearly identified and eliminated at the initial stages of engineering and facility design to optimize production and transportation of the crude oil. Otherwise, the outcome of this continuous precipitation will result in severe economic losses, even abandonment of production wells.3−6 An understanding of the rheology and crystallization behavior of the crude oil is crucial to deal with the consequences or make effective precautions of wax deposition © 2014 American Chemical Society
problems. The WAT and amount of wax precipitated are two of the most important parameters of the crystallization behavior of crude oil, which indicate whether wax gelation and deposition will occur. Estimation of these two factors is essential to the pipeline design of production and transportation as well as the preparation of wax control.2 Thus far, several techniques are available for the measurement of the WAT and amount of wax precipitated for crude oil, including ASTM D2500-88 or IP 219/82, differential scanning calorimetry (DSC),2,7−9 light transmission (LT), coldfinger, visual observation,10,11 near-infrared (NIR) spectroscopy,12,13 centrifugation, filter plugging (FP),14,15 rheometry,3,16 viscometry,3,8,9 cross-polarization microscopy (CPM),3,8 and densitometry.17 Monger-McClure et al.14,18 summarized and compared the different techniques of WAT measurements. It is found that different methods have different advantages and weaknesses. For example, the ASTM methods depend upon visual observation of the wax crystals, which are intuitive and credible, but they require transparent fluids. Thus, they are not appropriate to determine the WAT of opaque or dark oils. Received: January 7, 2014 Revised: March 6, 2014 Published: March 7, 2014 2422
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decrease is inversely proportional to the carbon atom number and closely relates to the chemical nature of the components.21 In 2006, Lionetto et al. analyzed the evolution of viscoelastic properties during oil gelation with the combination of rheological and ultrasonic methods. The ultrasonic wave propagation has demonstrated to be a powerful tool for monitoring the sol−gel transition in waxy crude oils.1 Although the WAT method varies and the effect of the pressure on the WAT and amount of wax precipitated is wellrecognized, the WAT and amount of wax precipitated still need comprehensive analysis and finally to be determined with different methods to obtain a reliable result.22−26 Further, there have not been many studies on the dynamic process and quantitative analysis of live oil wax crystallization. Consequently, this work proposed a high-pressure ultrasonic detection system with a magnetically coupled impeller mixer to investigate the WAT and amount of wax precipitated of Kingfisher (KF) live oil samples, which are from a typical high waxy oil reservoir. The tests were performed at pressures from stock tank conditions to the initial reservoir conditions (0.1, 4.5, 8.8, 10.56, 15.84, and 22.87 MPa) for both crude oil and live oil. Benchmark tests, such as the WAT and amount of wax precipitated, were measured and compared. In addition, to verify the accuracy and reliability of the ultrasonic detection method, the measured data were compared as a check to results from conventional DSC and rheometry under atmospheric pressure.
CPM, DSC, and viscometry are good candidates for dark oil. In addition, most of these methods are conducted under atmospheric conditions. The pressure and dissolved gas contained in the live oil are just ignored. Actually, a significant amount of wax could be dissolved because of the existence of lighter components under reservoir conditions. A growing body of research shows that both the WAT and the amount of wax precipitated will be reduced by the dissolution gas. Determination of these two values is vital to not only formulation of appropriate flow assurance strategies for new oilfield development but also preventing problems in existing operations. Therefore, the development of novel methods or the improvement of routinely used laboratory techniques to detect wax crystallization that is more representative of live oil is crucial and meaningful. In other words, to obtain more accurate estimation of the reservoir fluid WAT and better assessment of the risk for wax deposition, measurements must be carried out at the relevant operating conditions.2 Because of the uncertainties associated with different techniques, at least two independent methods for the WAT test of crude oil are recommended.19,20 Typically, DSC and rheometry are two of the most conventional techniques to measure the WAT of crude oil under atmospheric pressure. The sensitivity of these two methods is dependent upon the amount of wax precipitated. Theoretically, DSC involves the measurement of the heat transition as the first wax crystal appears in the liquid phase during the cooling process under isobaric conditions.3,8,12,16 The WAT is characterized by the appearance of an exothermic peak in the DSC thermogram. Rheometry involves viscosity measurement with a viscometer or rheometer during the decrease of the temperature. The point where the curve of viscosity VS temperature obtained under constant shear deviates from Arrhenius-type behavior is taken as the WAT. In addition, other techniques, such as LT and CPM, are also often used. LT could be applied to both stock tank oils (STOs) and live oil. The WAT can be seen as an abrupt drop in a relation curve of light power received and temperature. It depends upon the number of wax crystals. CPM involves visual observation of the appearance of wax crystals when the sample is cooled on a microscopic slide and observed under polarized light. It depends upon the size of the wax crystals. Generally, the size and number of crystals are influenced by the cooling rate. A higher cooling rate results in the formation of a larger number of nucleation sites and smaller crystals. Conversely, a lower cooling rate leads to a smaller number of larger crystals.14,18 Similar to the LT technique, the ultrasonic signal can be used to travel through the crude oil sample and received at a transducer. The velocity of the ultrasonic wave, which depends upon the density of the medium, changes significantly with wax precipitated. In addition, the growth and further association between wax crystals causes a dissipation of acoustic energy; therefore, the crude oil WAT could also be indicated by the variations of the wave attenuation.2 For the past few decades, the use of the ultrasonic method has aroused some interest of researchers, but most of the studies found in the literature focus on the wave velocity. Because of the complexity of wave attenuation, it is generally reported qualitatively or just ignored. On the basis of this theory, in 1993, Meray et al. designed ultrasonic equipment to determine the pressure effect and compared the influence of light components and WAT of two different crudes to the DSC method. It demonstrates that light components decrease the WAT proportionally, and this
2. EXPERIMENTAL SECTION 2.1. Materials. Both the crude oil (STO) and live oil samples were collected from KF oilfield during drill stem test (DST), with the properties of low sulfur content, mediate viscosity, and high pour point. The details of the physical−chemical properties of the studied oil sample were listed in Table 1. The bubble point pressure of the
Table 1. Physical−Chemical Properties of the Studied Crude Oil parameter
standard test method
value
viscosity (mPa s) API gravity (deg) pour point (°C) gas/oil ratio (m3/m3) WAT (°C) wax (wt %)
rheology (85 °C, 0.1 MPa) table ASTM D SY/T 0541-2009 SY/T5542 DSC DSC
9.52 31.50 44.80 38.76 65.67 29.67
reservoir fluid was measured as 8.8 MPa at the reservoir temperature of 85 °C. The initial reservoir pressure is 22.87 MPa. The DSC and rheological properties have been studied in the temperature range of 20−70 and 45−85 °C, respectively. Before the experiments, both the crude oil and live oil samples have been heated at 85 °C for 24 h to make sure that all of the wax has been completely dissolved. 2.2. Experimental Apparatus. 2.2.1. High-Pressure (HP) Ultrasonic Detection System. The HP ultrasonic detection system, developed by the Key Laboratory of Petroleum Engineering of the Ministry of Education, China University of Petroleum, Beijing, China, in cooperation with Yangzhou Guangling Special Ultrasonic Equipment Factory, as is shown in Figure 1, consists mainly of the following devices: (1) The key components are two specifically developed piezoelectric transducers, propagating longitudinal waves at 1 MHz. The transducers were integrated with the inlet and outlet ports to withstand high pressure during the test. The face of the inlet and outlet ports was polished to reduce the attenuation. The improvements were devised to avoid common problems of ultrasonic detection, such as poor pressure-bearing capacity for sensors, extensive wave diffraction 2423
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Figure 1. Schematic of the experimental setup used for ultrasonic detection of the WAT of the oil sample. along a steel holder, large energy loss, and low test accuracy of equipment. The transducers are electrically connected with a pulse generator and receiver and are used in direct contact with the oil sample. The emitter receives an electrical pulse train and then converts them to ultrasound; after the waves travel through the oil sample, they reach the receiver and are converted back to electrical signals. On the basis of an amplifier and analogue/digital conversion, the transit time t and amplitude A of the waves traveled through the oil sample are calculated and recorded automatically by a high-frequency (HF) data acquisition system, averaged over many determinations for each temperature point. (2) The cell (Haian; P, 10 000 psi; T, 150 °C), horizontally placed, was designed to be cylindrical considering the pressure. It was modified from the cell used for interfacial tension (IFT) measurement. Five of the eight valves are used in these experiments. Two inlet valves are connected to an Isco pump, one on the left and the other on the top of the cell. Two outlet valves are connected to a back-pressure pump, one on the right and the other on the bottom of the cell. Both the inlet and outlet valves of the cell are connected to the pressure transducers. The pressures during the tests are measured with a digital pressure indicator. Another valve is connected to an automatic temperature control unit. There are two holes along the cell to place two heater rods. In addition, the cell is coated with aluminum foil to avoid the temperature gradient during the experiments. The principle of this design is for better pressure and temperature control, which are two core parameters for the tests. However, it is very important to prevent the leakage problem because of the five values. (3) A data acquisition system was used for real-time data acquisition and transfer. The transit time and amplitude of the first arrival wave, which transmitted through the oil sample, are two main physical quantities that can be tested with the ultrasonic technique. For each point, 10 times are measured to reduce the uncertain; the average values of the transit time and amplitude with the relative deviation less than 0.02 and 0.05 would be accepted, respectively. (4) An ultrasonic transit-receive unit was used [Yangzhou, China; voltage, 250, 500, and 1000 V; pulse width, 3 μs, 2 μs, 1 μs, 500 ns, and 200 ns; frequency range, from 500 kHz to 5 MHz; attenuation range, 1−79 db, with 1 db step by step; time range, 0−1000 μs; time accuracy, 0.0625 μs; analog to digital (A/D) resolution, 12 bit; amplitude range, from −4.8 to 4.8 V, with amplitude increments of 0.05 V]. (5) Two 100DX syringe pumps [Teledyne Isco, Lincoln, NE; flow range, 0.001−60 mL/min; flow accuracy, 0.5% of the set point; pressure range, 0−10 000 psi; standard pressure
accuracy, 0.55% full scale (FS); optional pressure accuracy, 0.1% FS] were used to displace STO/live oil samples and control back pressure. (6) Two HP stainless-steel cylinders (ZR-3; 16−70 MPa; ≤150 °C; 200−1000 mL; Huaan, China) were used to store and deliver the STO and live oil samples. (7) An automatic temperature control unit CN76000 microprocessor-based temperature/process controller was used (temperature accuracy, 0.1 °C). A stir bar, aluminum foil, and rapidly responding thermocouple are used avoid the temperature gradient during the ultrasonic test. (8) A back-pressure regulator was used to maintain the pre-specified pressure inside the cell during the tests (Huaan, China; pressure range, 0−5800 psi; pressure accuracy, 0.1%). 2.3. Experimental Procedure. Both the crude oil (STO) and live oil were homogenized at 85 °C in a temperature-controlled oven for 24 h prior to starting each run, to ensure complete dissolution of wax. Approximately 20 mL of the crude oil (STO) was added to the cell continually until the oil discharged from the outlet ports to eliminate the air in the cell, followed by pressurization using the Isco pump to the desired pressure (0.1, 4.5, 8.8, 10.56, 15.84, and 22.87 MPa), respectively. The cell is then cooled from 85 °C to a temperature of 35 °C (below the pour point) with a rate of 2 °C/h and measured isothermally at the test temperature. The time delay of the ultrasonic system is 3.5 μs under the conditions of 500 V emission voltage, 1 time compression ratio, and 1 μs pulse width for the crude oil (STO) WAT test. Waveforms were measured, and ultrasonic parameters, such as transit time and amplitude, were obtained during the desired temperature point. After the WAT test of crude oil (STO), the oil sample was heated isobarically (22.87 MPa) to 85 °C for 24 h, and then the live oil was injected to the cell with the velocity of 1 mL/min until the original gas/oil ratio (GOR) (38.76 m3/m3) was reached. The above operation of the ultrasonic test was repeated. Then, the pressure with the Isco pump decreased to the desired pressure (15.84, 10.56, 8.8, and 4.5 MPa), and the ultrasonic test was repeated. 2.4. Interpretation of the Measured Data. Theoretically, the crystallization process of the oil sample is actually an ordered solid structure continually precipitating from a disordered phase as the temperature decreases. It typically involves two distinct stages, namely, nucleation and growth.27 Specifically, with the cooling of the oil sample to its WAT, the molecular motion starts to be progressively hindered and the randomly tangled molecules tend to move closer together and form clusters of contiguous aligned chains. This process 2424
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Figure 2. DSC thermogram of crude oil under 0.1 MPa. of the continuous attachment and detachment of the paraffin molecules from the ordered sites will continue until the clusters reach a critical size and stable state. The procedure is called nucleation, and the clusters are termed nuclei. As soon as the stable formation for the nuclei with the WAT is kept or lowered, more and more molecules will successively attach to the nucleation sites and become part of the growing lamella structure, which means that the growth process occurs.18 It should be noted that both nucleation and growth are a function of the temperature and almost occur simultaneously at each temperature during the cooling process because of the wide molecular weight distribution of paraffins. This explains why the decrease of the transit time curve is progressive during the overall experiment without presenting a well-defined and clearly detectable onset. On the basis of the above analysis, the measured ultrasonic data could be explained as follows: 2.4.1. Transit Time. The time delay of the ultrasonic wave traveling through a certain size media (L) depends upon the density ρ and the elasticity β of this media. For this study, only longitudinal waves could be transmitted through the oil sample.21 The transit time is given by the Laplace law.
t = L ρβ
These two parameters change with the temperature; typically, the transit time decreases during the cooling process of the oil sample. We assume that the solid phase disperses homogeneously in the fluids for the two phase media after the wax crystallization; the above equation will remain valid on the condition that the dimensions of the dispersed particles are smaller in comparison to the wavelength of the ultrasonic wave. If ϕ is the volume fraction of the solid phase of the oil sample, then the density and the adiabatic compressibility are given by (2)
β = φβS + (1 − φ)βL
(3)
(4)
β ≈ (1 − φ)βL
(5)
⎛ t ⎞2 φ≈1−⎜ ⎟ ⎝ tL ⎠
(6)
Because of the apparition of a solid phase during the cooling process, the overall density and elasticity of the oil sample would be changed accordingly, which, in turn, causes the variations of the transit time. In other words, it is reasonable to determine the formation of paraffin crystals by the ultrasonic method. In addition, the accumulate wax fraction during the cooling process could also be estimated according to variations of the transit time. 2.4.2. Amplitude. Generally, the ultrasonic detection could be indentified as the equivalent of a loss factor in a dynamic mechanical process.1 Typically three regimes can be recognized. For the temperatures higher than the WAT (before crystallization occurs), the amplitude increases slightly or moderately because of the slowing of the molecular motion and reduction of the molecular distance, which gently reduce the absorption losses. After that, crystallization occurs (below WAT), the nucleation process appears, the molecular motion becomes increasingly hindered, and the randomly tangled molecules tend to move closer together and form clusters of adjacently aligned chains.18 More and more oil fractions with high molecular weight start to arrange in an ordered way, presenting a greatly reduced mobility, which leads to a tremendous decrease of scattering attenuation and, thus, increases the wave amplitude significantly. However, this increase will continue until the growth process dominates, when increasing molecules are laid down successively on the nucleation sites and become part of the growing lamella structure.18 In that case, the rapid growth, aggregation, and formation of many crystals lead to an increase of the attenuation as a consequence of absorption and scattering losses, which finally results in a great decline of the amplitude. Consequently, it is obvious that the amplitude curve could be another indicator of the onset of crystallization according to the distinctly different growth or decline during the cooling process.
(1)
ρ = φρS + (1 − φ)ρL
ρ ≈ ρL
where the indices S and L refer to the solid and liquid, respectively. During the paraffin crystallization, the change of density is modest, while the variation in compressibility is significant for the sample transformed from the liquid to the solid state; therefore, the following approximation can be considered acceptable:21 2425
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3. RESULTS AND DISCUSSION 3.1. DSC. A DSC thermal analyst (Auto TA-Q20, TA Instruments, New Castle, DE) was used to determine the WAT of the crude oil from the onset of the DSC exotherm. The sample was scanned from 70 to −20 °C at a scan rate of 2 °C/ min. DSC results were used as a check for the ultrasonic method. The DSC technique is also widely applied to quantify wax precipitation. It is a fast simple technique for routine essays, which can detect bimodal or multiple wax events and estimate the amount of wax precipitated. In DSC analysis, the temperature of an oil sample is varied and heat transfer is recorded, so that the phase transition can be quantified on the basis of the heat change involved.4,28,29 In this study, DSC was used to determine the WAT, the simultaneous wax precipitated, and the accumulative wax precipitated at different temperatures of the crude oil (STO) under atmospheric pressure. The DSC thermogram displayed in Figure 2 shows two distinct wax crystallization events for this oil sample. Obviously, the WAT estimated from this test is 65.67 °C. Further, the total heat produced because of the phase change can be obtained by the difference between the DSC curve and the baseline. On the basis of the specific melting heat, the corresponding mass can be converted.28 The data from DSC were also used to determine the amount of wax precipitated as a function of the temperature during the cooling process. Figure 3 is the
Figure 4. Accumulative wax precipitation of the crude oil under 0.1 MPa.
Figure 5. Viscosity profile of the crude oil under 0.1 MPa.
the linear behavior of the oil viscosity curve. On the basis of the Arrhenius theory,30 from the ln η−T−1 curve of the crude oil, as shown in Figure 6, the WAT of the crude oil (STO) is determined to be 63.74 °C, which is comparative to the DSC results.
Figure 3. Simultaneous wax precipitation of the crude oil under 0.1 MPa.
simultaneous wax precipitated curve, while Figure 4 is the accumulative wax precipitated curve. Through comparative analysis of the results obtained for uncertainty estimation, the relative deviation is about 0.03. 3.2. Rheology. A controlled stress rheometer (Haake RS600, ThermoGap, Karlsruhe, Germany) was used to measure the viscosity of the oil sample under normal pressure. It is imperative to test with a low shear rate to avoid or minimize degradation of the wax crystals under shear.2 The cooling rate is 1 °C/min. The WAT were determined on the basis of the point where deviation from linearity occurs as the oil sample is cooled. In other words, the temperature at which the viscosity deviates from the Newtonian Arrhenius-type behavior can be considered the WAT. Results obtained were used as another check. The viscosity profile of the crude oil (STO) under three shear rates (5, 20, and 50 s−1) for this study is demonstrated in Figure 5 as well as the Supporting Information. The WAT was identified to be around 65 °C by the obvious departure from
Figure 6. ln η−T−1 curve of the crude oil under 0.1 MPa.
3.3. Ultrasonic Detection. The ultrasonic detection test of both crude oil (STO) and live oil has been conducted at 1 MHz in transmission mode. As reported in Figure 7, it can be observed as a shift toward a lower transit time during the cooling experiments. The increase of sample elastic properties by gelation is responsible for the reduction of the transit time 2426
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Figure 8. Transit time and volume fraction of accumulative wax precipitated as a function of the temperature for the crude oil (STO) cooled at 2 °C/h under 0.1 MPa.
Figure 7. Evolution of the transit time and amplitude with the temperature for the crude oil (STO) cooled at 2 °C/h under 0.1 MPa.
because the ultrasonic wave takes less time to go through the gelling sample than through the fully liquid sample. Obviously, the transit time curve is characterized by a nonlinear growth, indicating the progressive sample stiffening with a decreasing temperature. For the initial stage of the experiment, the transit time decreases moderately as a consequence of the temperature reduction. Below the vicinity of the WAT obtained by DSC and rheology, the transit time curve presents a slightly faster decrease, which is indicative of the change of elastic properties caused by the wax crystallization and crystal aggregation in clusters. On the basis of the least-squares method, two sections of the transit curve were selected and the intersection point of the fitting curves is taken as the onset of the WAT. As shown in Figure 7, the WAT of the crude oil (STO) cooled at 2 °C/h under 0.1 MPa is 64.14 °C, which agrees well with the results obtained by DSC and rheology, which are 65.67 and 63.74 °C, respectively. Generally, the DSC method is more accurate according to the principle of measurement. In addition, the amplitude curve reported in Figure 7 presents a bell shape, in which three regimes can be recognized. For the temperature above the WAT, the amplitude increases slightly because of the reduction of scattering losses. While for the temperature below the WAT, there are mainly two stages. For the nucleation dominant stage, the amplitude increases significantly because of the limit of molecular motion, which leads to a greater decline of scattering attenuation. After that, the increase of amplitude continues until growth process dominates. As more and more molecules lie down successively on the nucleation sites and become part of the growing lamella structure, both scattering attenuation and absorption attenuation begin to increase and then the amplitude declines abruptly. The obvious variations of these three stages could be another indictor for detecting the onset of the WAT. Further, according to eq 6, a rough estimation for the volume fraction of the accumulated wax precipitated could be obtained under different temperatures, as shown in Figure 8. For this study, under 55 and 35 °C, the value is 2.18 and 9.69%, respectively. Considering the density of wax (about 900 kg/ m3), the data from the ultrasonic method are very close to the results obtained by DSC. The WATs of both KF crude oil (STO) and live oil determined at six pressures (0.1, 4.5, 8.8, 10.56, 15.84, and 22.87 MPa) are summarized in Figure 9. Obviously, for the crude oil, the effect of the pressure, as expected, was to increase
Figure 9. WAT versus pressure for both the KF crude oil (STO) and live oil.
the WAT at 0.202 °C/MPa according to the slope of the fitting curve (dT/dP) through the six points. The results are comparable and can be explained by the Clausis−Clayperon equation and studies reported by Brown et al.22 and Meray et al.21 The reason is that the lighter fractions are compressed much more severe than the heavier fractions as the pressure increases. Then, paraffins become less soluble with less apparent solvent. Consequently, the WAT of crude oil (STO) tends to be higher at elevated pressures. For the live oil, the WAT will decrease as the pressure increases within certain realms because of the gas dissolution. This process will continue until the saturation pressure is reached because a further pressure increase cannot add more solvent.26,31 After that, as the pressure increases, the WAT will also increase. This is a simple thermodynamic effect that an increase in pressure always shifts the equilibrium in favor of the denser phase, in this case the wax phase.32 The WAT is reduced by 9.57 °C because of the gas dissolution under the saturation pressure for the sample of KF crude oil. The WAT of the dead oil was 65.86 °C, while that of the live oil could be 56.31 °C under a pipeline operating pressure of approximately 10.56 MPa. Thus, if production systems are designed using STO WAT values, it may overestimate the fluid properties in a flow-line or riser, which will finally lead to a potential for overdesign.31
4. CONCLUSION The newly developed HP ultrasonic detection system opens up new doors for experimental determination of the WAT and is 2427
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especially useful for corroborating results obtained by DSC, rheometry, CPM, or other methods. It is also a self-verification analysis because both the transit time and amplitude could be obtained simultaneously during the WAT test. In addition, a rough amount of wax precipitated could also be estimated. Furthermore, both crude oil (STO) and live oil with high pressure can be studied. This type of information is extremely valuable for oil production and transportation, flow assurance, etc. In summary, the following conclusions can be drawn from this study: First, it is concluded that WAT experiments on live oils must be conducted. The reason is that the measured dead oil WAT may provide conservative and unrealistic estimates because of the composition and pressure difference. Therefore, it seems to be a reasonable choice to add certain solution gas or just keep the gas dissolved in the crude oil to reduce the risk of wax precipitation. From an economic point of view, probably the requirement of a pour point depressant (PPD) or paraffin treatment may be reduced or completely eliminated. Thus, the total costs related to waxy deposits will drop. In addition, a new apparatus and methodology have been developed and can be successfully used for handling live oil samples and determining WAT at different pressures, which are appropriate for most crude oils and gas condensate fluids, and the obtained WAT values are comparable to results from DSC and rheometry. The data could also be used to further optimize the predictive models for wax deposition. Furthermore, it demonstrates that the ultrasonic method is a potential technique for determining the WAT and amount of wax precipitated. It presents the advantage of distinguishing the different stages of the complex crystallization process of wax through the interpretation of transit time and amplitude. However, it should be noted that the present ultrasonic study could be further improved by matching the transducer frequency and bandwidth to the specific oil sample to enhance the resolution and sensitivity, so that a clearer onset of crystal aggregation from the ultrasonic technique can be detected.
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ASSOCIATED CONTENT
S Supporting Information *
Viscosity data with temperature under different shear rates (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-135-0123-4602. E-mail: chenhaomailbox@ 163.com. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National High Technology Research and Development Program of China (Grant 2011ZX05032-002), the National Program on Key Basic Research Project (Grant 2011CB707304), and the Science Foundation of the China University of Petroleum, Beijing (Grant 2462013YXBS003).
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REFERENCES
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dx.doi.org/10.1021/ef500036u | Energy Fuels 2014, 28, 2422−2428
