Simultaneous Characterization of Water Content and Distribution in

Apr 20, 2016 - High-water-cut crude oil is currently the main form of underground oil in China. The high water content and the distribution of oil and...
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Simultaneous Characterization of Water Content and Distribution in High-Water-Cut Crude Oil Yan Song,†,‡ Hong L. Zhan,*,†,‡ Kun Zhao,*,†,‡ Xin Y. Miao,‡ Zhi Q. Lu,‡ Ri M. Bao,‡ Jing Zhu,‡ and Li Z. Xiao† †

State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China Beijing Key Laboratory of Optical Detection Technology for Oil and Gas, China University of Petroleum, Beijing 102249, China

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ABSTRACT: High-water-cut crude oil is currently the main form of underground oil in China. The high water content and the distribution of oil and water have an important impact on the rheological property and exploration efficiency. Terahertz (THz) waves are extremely sensitive to hydrogen bonds and intermolecular forces, and consequently, the responses of oil, water and air exhibit obvious differences. In this work, a series of crude oils with high water content were investigated by using THz timedomain spectroscopy (THz-TDS). Linear models were built between amplitudes of THz-TDS and the absorbance and water content from 50.05% to 100.00% with correlation coefficients (R) of 0.99 and >0.93, respectively. In addition, oil and water were determined to be distributed at different positions, based on THz signals in a series of oriented scans. Moreover, the transparent parts for visible light are identified by scanning the local points. The positions with low and high THz intensities refer to water and air regions, respectively. Therefore, THz technology is expected to act as a supplementary method to characterize highwater-cut crude oil, which will promote the efficiency and safe operation of pumping units and oil pipelines.

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INTRODUCTION Oil, which is also known as the “blood” of industrial production, has become one of the most important energy resources for the past 100 years. However, the high price of crude oil and future worldwide energy demands make it necessary to enhance oil recovery processes as well as the oil production capacity.1 In order to meet this demand, secondary recovery strategies have been implemented, which rely on maintaining or increasing the reservoir pressure by injecting water into the reservoir. The water injection schemes are quite successful in enhancing hydrocarbon production; these strategies lead to the generation of water content in crude oil.2,3 The measurement of water content in crude oil is of great significance. The existence of large quantities of water in crude oil can introduce serious problems in the petroleum refining process, transportation, and other related chemical or petroleum engineering processes. The increase of oily sewage in the petroleum production process and damage to the lubrication films on mechanical equipment can cause the equipment to corrode, as well as cause instability of the distillation operation for the petroleum refining process. In addition to these problems, high-water-content oil causes issues regarding the distribution of oil, water, and the remaining oil. The distribution of oil and water becomes more complex in the ground, and the remaining oil is highly dispersed but locally enriched in space. Thus, the high water content has an influence on the characteristics of oil, which has generated widespread consideration of oil−water mixtures. It has been previously difficult to accurately measure the water content of crude oil as a result of several factors including the time-variant mixture state of crude oil and water, the multiflow pattern, the complex ingredients, the poor oil field environment, etc. Many measurement technologies, have been developed for determining water content in crude oil, such as the electrical © XXXX American Chemical Society

desorption, the dehydration, the distillation, the Karl Fischer titration, the capacitance, and the ray method. These methods are employed in different situations according to specific needs. The distillation, the Karl Fischer titration and the ray methods are preferred for the sake of high precision. In order to operate easily, the electrical desorption and the ray methods are competitive. Besides, the capacitance method has advantage over others in economy and maintainability. Nevertheless, the aforementioned methods have their respective limitations under different conditions.4−8 Moreover, some new techniques have been proposed in recent years. For example, a noncontact optical method, called all-optical detection, has been proposed to measure the water content ranging from 0 to 100% in crude oil by a noncontact laser source and receiver (adaptive laser interferometer).9 A system with capacitance to detect phase angle conversion is designed to measure ultralow water content in crude oil, where the resolution can reach ±50 ppm.10 Terahertz (THz) spectroscopy is a developing spectral technique bridging the gap between microwave and infrared spectroscopy and is becoming a hot research topic, because of its unique advantages.11−13 It has been shown that some lowenergy events such as molecular torsions or vibrations, as well as intramolecular or intermolecular hydrogen bonding in part intrinsically correspond to THz frequencies.14−17 In addition, differences in molecular configuration and the polarity of oil, water, and gas make the THz time-domain spectroscopy (THzTDS) a technique that can distinguish between them readily. Therefore, THz-TDS technology has attracted significant attention for applications in the energy field such as in the petroleum industry.18−20 The THz-TDS technology, combined Received: February 15, 2016 Revised: April 12, 2016

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DOI: 10.1021/acs.energyfuels.6b00340 Energy Fuels XXXX, XXX, XXX−XXX

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at the focus of the lens and the THz wave is vertically incident to the surface of the plastic plates.

with multivariate statistical methods, has been shown to be an effective method to identify crude oils from different oil fields and pattern transitions of oil−water two-phase flow, which are the principal components of natural gas.21−25 Because of the different absorption properties of oil, water, and gas in the THz range, their respective distributions in crude oil can be determined qualitatively and quantitatively. Recently, THz-TDS was used to measure the water content for crude oil with low water content ranging from 0.01% to 25.00% (w/w).26 However, because of the strong absorption of the THz pulse in water, the method has trouble measuring the water content (as a percentage) for high-water-content oils. In this research, the oil−water mixtures with different water contents (50.05%− 100.00%) were studied by THz-TDS. Initially, the water content was quantitatively determined by linear models based on THz peak intensities and absorbance values. By comparing THz intensities at different locations, the distribution of oil and water, especially water or air, were clearly distinguished. This investigation shows that THz spectroscopy is an effective technique for the comprehensive detection of underground high-water-cut crude oil.

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RESULTS AND DISCUSSION The water content of crude oil in oil exploitation, transportation, and storage is a vital parameter to assess crude oil production accurately. Because of the water flooding technique currently used in oil exploitation, the water content in the majority of mined oil is high. Accurately measuring the water content of high-water-content crude oil has historically been difficult, because of the oil medium conditions, working conditions, measurement technologies and instruments, and other factors.27−29 Several points were measured on the oil− water mixture samples with a distribution that was almost symmetric, because of the uneven distribution of oil and water. The peak intensity (Ep) value of each point were extracted from the THz time-domain spectra. As shown in Figure 2, the

EXPERIMENTAL METHODS

A conventional transmission THz-TDS system was used in this research. The principal measurement of this setup was described in a previous report.23As shown in Figure 1, the collimated THz pulse was

Figure 2. THz peak intensity (EP) values of the oil−water mixtures with water content ranging from 50.05% to 100%. The inset shows the time-domain waveforms corresponding to different oil content. The table lists the parameters of linear fitting.

Figure 1. Experimental setup for the detection of oil−water distributions with transmission terahertz time-domain spectrometry (THz-TDS).

smaller black points represent the peak intensities of the timedomain spectra measured at different sampling locations on the oil−water samples. The bigger colored stars are a function of the water content acquired by averaging all the values of the measured peak intensities. The average peak value decreased from 140 mV to 74 mV with increasing water content from 50.05% to 100.00%. The peak intensities were linearly fitted to the equation y = − 0.00139x + 0.21328

focused and reflected by a lens (L1) and mirror (M1), respectively, and transmitted through the oil−water mixture. The sample-encoded THz beam then reached the ZnTe crystal detector. The THz signal was transformed to an electrical signal by the electro-optic effect and was detected by a lock-in amplifier. The experiment was operated at a constant temperature of 294 ± 0.3 K. The water−oil mixtures with varying water content used in this experiment were obtained by mixing different proportions of crude oil to water. The crude oil samples used in this work were Brazilian crude oil with a water content of 0.1%. The sample cells consisted of two plastic plates with a thickness of ∼40 μm, to ensure a high enough signal amplitude, since the water has a strong absorption in the target THz frequency range. The sample cells had dimensions of 5 cm × 5 cm. The oil and water was added dropwise into the sample cell using syringes. The total number of droplets was eight, with the number of oil droplets decreased from four to zero while water droplets increased from four to eight. Therefore, oil−water mixtures were obtained with water content from 50.05% to 100.00%. Since oil and water are not miscible, the distribution of oil and water was not uniform. As a result, measurements were taken at several locations on the sample with nearsymmetrical distributions. In each measurement, the mixture is located

and had a correlation coefficient (R) of ∼0.99, which is shown as the pink line in Figure 2. This indicates that the linear fit represents the data well. This is explained by noting that water has a significantly stronger absorption in the THz range compared with crude oil. The THz time-domain spectra of the mixtures with varying water content were obtained using the spectra of the points whose peak intensities were closest to the average values. As shown in the inset in Figure 2, the peak intensities of the THz waveform decreased with increasing water content in the mixture. It can be seen that there is a phase shift as the water content increased. Therefore, THz-TDS can provide information about water content, and it can be used to measure the water content for oil exploitation, transport, or storage. B

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points), and Right (11 points), as shown in Figure 4. The Left sampling line only transmits through one point at 1.4 cm from

Both phase and amplitude information on the THz pulses after propagating through the reference (empty sample cell) and oil−water mixtures were measured using THz-TDS.30 By comparing the pulse of the reference with the mixtures and applying a numerical fast Fourier transform (FFT), the frequency domain spectra were obtained. The absorbance spectra were calculated using the definition of absorbance, A = log (I0/I), where I0 and I are the values of the THz frequency domain spectrum (FDS) of empty sample cell and sample, respectively. The absorbances of the mixtures were obtained using the method described above. The principal behavior of the absorbances of the different mixtures investigated was very similar to the THz peaks, which increased with increasing frequency. The absorbance spectra of the mixtures are shown in the inset of Figure 3. The slopes of the absorbance spectra

Figure 4. THz peak intensity (EP), as a function of the sampling point’s position from the bottom to the top along the y-axis. The arrows in the inset indicate the trend of sampling.

the bottom of the sample, which is shown in Figure 4 as corresponding to the one strong decrease of Ep. The Middle sampling line had two transparent points at 1 and 3.2 cm with an obvious decrease in the Ep. The Right sampling line had a single transparent point at 3.2 cm, and a deep decrease in Ep occurred. These phenomena indicate that THz-TDS technology effectively realized the identification of water content via comparing the THz time-domain spectra of the mixtures, and it is capable of distinguishing between water and oil distributions. In addition, another 28 points dispersed in three horizontal directions were measured. As shown in Figure 5, these points Figure 3. Absorbance of the oil−water mixtures with mentioned water content at five randomly selected THz frequencies. The inset graph shows the absorbance spectra for 0.5−1.2 THz.

increased with higher water content in the mixtures. The available dynamic range of the system was approximately limited to the values from 0.5 THz to 1.2 THz. The sphere symbols in Figure 3 represent the absorbance of the mixtures obtained from the absorbance spectra at select frequencies (0.47, 0.55, 0.70, 0.87, and 1.00 THz). The results show that the absorbance increased as the water content increased, and the corresponding linear fitting lines are shown with a correlation of R > 0.93. This linear relationship strongly indicates that the water content in oil−water mixtures can be estimated using the relationship between THz absorbance and water content. It is important to accurately measure the concentration of oil, water, and gas to monitor the production status of oil wells, predict oil reserves, optimize the method of oil exploitation, and manage the production process of crude oil.31−34 Because of the inhomogeneous distribution of oil and water and the obvious difference between the absorption of water, gas, and oil in the THz range, the 50.05% water content oil−water mixture was used to investigate the distribution of oil, water, and gas for the mixture. The distribution of oil was studied first. A total of 33 points were measured from the bottom of the sample to the top along three vertical lines, denoted as Left (11 points), Middle (11

Figure 5. THz peak intensity (EP), as a function of sampling point locations along the x-axis. The inset shows the sampling direction from left to right.

were measured from left to right and were named Top (9 points), Middle (9 points), and Bottom (10 points). It can be seen that the extracted peak intensity (EP) values of the THz time-domain spectra for these points show a distinction between the water and oil regions. The Top sampling line transmitted through three transparent points at the left, middle, and right of parts of the line. Transmitted light corresponded to C

DOI: 10.1021/acs.energyfuels.6b00340 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels the two deep decreases of EP at 2.1 and 3.7 cm in Figure 5 but only a minor decrease at the left point. It was determined that the transmission from the left transparent region was a gas by later experimentation. The Middle sampling line had no transparent points, and correspondingly, no obvious decrease in EP was observed. The Bottom sampling line had one transparent point in the middle of the line at 2.6 cm, and a corresponding deep decrease in EP was observed. With regard to the measurement in horizontal and vertical directions, the EP values of the total of 9 intersections show the distribution of the oil or moisture phase. Taking the intersection of the vertical Left and the horizontal Middle direction as an example, EP = 0.13592 V in Figure 4 and EP = 0.13942 V in Figure 5. By comparing the EP values of the crossing points, the THz absorption of an oil−water mixture is similar to each other in same area. As discussed above, Figures 4 and 5 clearly display the primary distributions of water and oil, compared to gas. The positions with large and small THz intensities corresponded to oil and water regions, which was in agreement with the physical inspection of color differences in the sample. However, a new phenomenon was observed from the relatively large THz intensity decreases at the first two measurement points on the up-horizon line, which were also observed to be transparent in the photograph. It was determined that both water and air molecules existed in the transparent regions. To determine the contents of these transparent regions, the five transparent regions were designated as Labels 1−5 and were carefully measured. Three positions were chosen for Label 1, located approximately at the center, the left edge, and the right edge of this region. Because of the size of Label 2, only one position at the center was measured. The points measured for Label 3 were located about the tail of this region, the center, and the border of the inner circle. Two points were measured for Labels 4 and 5 located at the core of the embedded circle, as well as the bottom of the region for Label 4 and the upper middle for Label 5, respectively. All chosen points for each region are illustrated in Figure 6a. The raw experimental transmission

time-domain spectra are plotted in Figure 6b, with time values from 17 ps to 23 ps and correspond to the five transparent regions. According to THz-TDS, a difference in the spectra from the sampling points in one transparent region (Label 3) can be clearly observed. Different responses in the THz range were observed, depending on the position of the sample point in each transparent region. It was apparent that the THz signal intensities deviated in Labels 1, 3, and 4 more so than those in Labels 2 and 5. Figure 6c illustrates the EP value of the THz time-domain spectra shown in Figure 6b. For Labels 1 and 4, the EP values of the points measured in the marginal annular section were lower than that of the middle section. In contrast, the EP values of the points located on the edge were relatively greater than those in the middle section of Label 3. For Label 5, there were no obvious differences between EP values in the spectra. Since the size of Label 2 is small, only one point was measured, and EP = 104 mV. The decrease of the peak intensities is due to the existence of water. These features are in good agreement with this fact and show that water and gas can be differentiated using THz spectroscopy. As for Labels 1−5, the sampling points are measured randomly and some of them are close to the measurement regions in Figures 4 and 5. The left location of Label 1 is taken as an example in this part, and EP = 0.11014 V and 0.10544 V when measured vertically in Figure 4. The results suggest that this technology has high repeatability. The measurement of water content in high-water-content crude oil has been problematic for a long time. The ability of THz-TDS to characterize the water content for crude oil emulsions with low water content (0.01%−25.00%) has been shown previously. A linear relationship was found between the absorption coefficient or the amplitude of the THz pulse and the water content of crude oil emulsions in the observed range. This relationship was used to evaluate the water content in oil emulsions.26 In this work, the potential of THz-TDS to accurately determine the water content in high-water-content crude oil and the distribution of oil, water, and gas has been shown. The hydrogen-bond collective network formed by water molecules changes on a picosecond (ps) time scale, thereby causing the THz spectrum to be sensitive to the fluctuations of the water dipole moments.35,36 Because the main component of petroleum is hydrocarbons, which are mostly nonpolar molecules, the absorption of oil from THz radiation is relatively small. Thus, in the crude oil production process, because of the strong attenuation of THz radiation by water, THz radiation becomes a highly sensitive, noncontact probe of water content in high-water-content oil−water mixtures, as well as in the distinction between oil, water, and gas. The repeatability has been proved in this paper, thus the THz technique allows us to follow the online examination of water content for oil exploitation and should be regarded as a complementary method for qualitative and quantitative characterization, especially for the online determination of water content of crude oil in the oil industry.

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CONCLUSION In summary, the terahertz (THz) time-domain spectra of crude oil mixtures with water contents from 50.05% to 100.00% were measured. The experimental results indicate that both the amplitude and absorbance of the THz pulse can be used to characterize the water content in water−oil mixtures. The mixture with a water content of 50.05% was also measured to

Figure 6. Qualitative analysis of water and gas in transparent regions of the sample: (a) the photograph identifying specific locations of each sample point in the transparent regions, which are denoted as Labels 1−5; (b) the THz time-domain spectra of the sampling points at the five distinct transparent regions; and (c) the THz peak intensities of each transparent region for each sampling point with the color corresponding to the spectra in panel (b). D

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study the distribution of oil, water, and gas. The results demonstrated that the water content in high-water-cut crude oil can be precisely characterized using linear fits. The distribution of oil, water, and air is directly predicted with THz signal intensities. The THz technique can not only provide an accurate method for quantitative analysis of water content in crude oil but also proves to be a promising technique for determining the distribution of oil, water, and gas.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H. L. Zhan). *Tel.: +86-10-89732270. E-mail: [email protected] (K. Zhao). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Grant No. 2014CB744302), the Specially Funded Program on National Key Scientific Instruments and Equipment Development (Grant No. 2012YQ140005), and the National Nature Science Foundation of China (Grant Nos. 11574401 and 61405259).

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DOI: 10.1021/acs.energyfuels.6b00340 Energy Fuels XXXX, XXX, XXX−XXX