Plasmonic optical sensing probe for online measurement of crude oil viscosity

Abstract

The present application relates to an optical probe, in particular to a kind of plasma optical sensing probe for measuring crude oil viscosity on-line.The probe is composed of microstructure optical fiber and nanometer gold wire;Air hole in the cladding of optical fiber is provided with outer layer air hole and inner layer air hole, wherein the number of inner layer air hole is 12, and it is arranged in regular hexagon, the number of outer layer air hole is 18, and it is arranged in regular hexagon;The number of nanometer gold wire is two, and it is embedded in the two inner layer air holes close to the core of Y axis;The center distance between the two adjacent outer layer air holes, the two adjacent inner layer air holes and the two adjacent outer layer air holes and inner layer air holes parallel to Y axis is equal;Inner layer air hole and outer layer air hole are alternately arranged in the direction along the side of regular hexagon.The highest sensitivity of the probe for detecting single crude oil viscosity can reach 36063CP / RIU, and the measurement range is wide, up to 4-20000 cP, which can cover a wide range of types from light oil to heavy oil.

Description

Technical Field

[0001] This invention relates to an optical probe, specifically a plasma optical sensing probe for online measurement of crude oil viscosity. Background Technology

[0002] Crude oil viscosity, also known as dynamic viscosity, directly characterizes the ratio of internal shear stress to shear rate, and is a key parameter for measuring crude oil flow resistance. Excessively high viscosity indicates strong intermolecular forces and difficult flow, while excessively low viscosity may lead to sealing failure and leakage risks. Therefore, the accuracy of viscosity data directly determines the reliability of subsequent development plans and technical pathways. Heavy oil at room temperature can have a dynamic viscosity tens to hundreds of times higher than conventional crude oil, resulting in extremely poor fluidity and a recovery rate typically only one-third that of conventional crude oil. To improve its fluidity, the industry commonly employs methods such as heating, dilution, or adding chemical viscosity reducers to lower viscosity and improve extraction and transportation efficiency. Crude oil viscosity is not constant but is affected by various factors such as temperature, pressure, and composition, with temperature having the most significant effect, forming the so-called "viscosity-temperature law," meaning that liquid viscosity decreases as temperature increases, and most of the viscosity reduction occurs in the initial heating stage—approximately 70% of the reduction occurs within this range. More importantly, temperature changes not only affect viscosity but also simultaneously cause significant changes in the refractive index of crude oil, providing a theoretical basis for indirect viscosity measurement.

[0003] Surface plasmon resonance (SPR) is highly sensitive to extremely slight changes in the refractive index of metallic surfaces. Leveraging its unique optocoupler mechanism, it enables high-precision measurement of trace crude oil samples without intrusion and with minimal loss. Microstructured optical fiber (MOF) allows for flexible control of light transmission behavior by introducing periodic or aperiodic air hole structures within the fiber. Its unique waveguide characteristics, such as high design freedom, tunable optical performance, low transmission loss, and cutoff-free single-mode properties, make MOF an ideal platform for constructing high-performance SPR sensors. Utilizing the flexible optical field control capabilities of MOF, sensors can be optimized for the refractive index-viscosity correlation characteristics of crude oil samples, achieving rapid, highly sensitive, and label-free detection of trace crude oil viscosity. This technology not only continues the high precision and real-time advantages of the original SPR method but also enhances its compatibility with the complex media environment of crude oil through the structural tunability of microstructured optical fiber, providing a solution for viscosity monitoring in heavy oil extraction and crude oil transportation.

[0004] Current viscosity measurement technologies for complex crude oils (especially high-viscosity heavy oils) face three main limitations: First, traditional methods have limited applicability. Capillary methods are slow to respond and prone to clogging; falling ball methods have a narrow measurement range, making it difficult to cover the high viscosity range of heavy oils; while rotational viscometers have wide fluid adaptability, their accuracy decreases at high temperatures, and they require large sample volumes, failing to meet the requirements for micro-volume, online monitoring. Second, existing SPR technologies lack sufficient temperature adaptability. For example, the scheme proposed by Matthew's team in 2015 only distinguishes crude oil types at room temperature, failing to effectively sense viscosity by incorporating temperature variables, thus limiting its application value under real-world conditions. Third, the integration effect of MOF-SPR technology is poor; current SPR viscosity detection based on microstructured optical fibers has many defects. Summary of the Invention

[0005] To address the problems existing in the background technology, this invention provides a plasma optical sensing probe for online measurement of crude oil viscosity. This sensing probe overcomes the shortcomings of existing technologies in terms of high-temperature adaptability, sample volume, measurement range, and real-time performance. Based on the MOF-SPR principle, the probe is a crude oil viscosity sensing probe with a wide measurement range, high temperature resistance, small sample volume, and the ability to achieve online monitoring.

[0006] The technical solution adopted in this invention is as follows: a plasma optical sensing probe for online measurement of crude oil viscosity (MOF-SPR sensing probe based on SPR technology, MOF-SPR being an abbreviation for microstructured fiber-surface plasmon resonance). The plasma optical sensing probe is composed of microstructured optical fiber and gold nanowires. The cladding of the microstructured optical fiber has outer and inner air holes, with 12 inner air holes arranged uniformly in a regular hexagonal pattern and 18 outer air holes arranged uniformly in a regular hexagonal pattern. There are two gold nanowires, which are embedded in the inner walls of the two inner air holes near the fiber core on the Y-axis. The center distance between two adjacent outer air holes and two adjacent inner air holes is equal, and the center distance between two adjacent inner air holes and two adjacent outer air holes parallel to the Y-axis is equal. The inner and outer air holes are arranged alternately periodically along the sides of the regular hexagons.

[0007] The radius of the microstructured optical fiber is 18-22 micrometers; the radius of both the outer and inner air holes is 2.3-2.5 micrometers; the radius of the gold nanowire is 200-700 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.3-5.7 micrometers.

[0008] Furthermore, the radius of the microstructured optical fiber is 20 micrometers; the radius of both the outer and inner air holes is 2.4 micrometers; the radius of the gold nanowire is 500 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.3 micrometers.

[0009] Furthermore, the radius of the microstructured optical fiber is 18 micrometers; the radius of both the outer and inner air holes is 2.3 micrometers; the radius of the gold nanowire is 200 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.5 micrometers.

[0010] Furthermore, the radius of the microstructured optical fiber is 22 micrometers; the radius of both the outer and inner air holes is 2.5 micrometers; the radius of the gold nanowire is 700 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.7 micrometers.

[0011] Furthermore, the microstructured optical fiber is provided with a perfect matching layer on its exterior.

[0012] Furthermore, the background material for the microstructured optical fiber is silicon dioxide.

[0013] Furthermore, the microstructured optical fiber should be refractive index guided.

[0014] Furthermore, the core of the microstructured optical fiber is the center of an array of hexagonal air holes.

[0015] The beneficial effects of this invention are as follows: It provides a plasma optical sensing probe for online measurement of crude oil viscosity. This probe requires only a small amount of sample to achieve online measurement under high-temperature conditions, while extending the measurement range to 4~20000dB. It has low transmission loss and high response sensitivity. The microstructured optical fiber has air holes embedded with gold nanowires, enabling micro-scale, non-invasive, and non-destructive viscosity measurement of precious crude oil samples. By directly introducing SPR technology into the field of online crude oil viscosity detection, combined with its high sensitivity and periodic optical field modulation capability, the transmission loss of traditional single-mode optical fiber is significantly reduced, achieving wide-range, high-precision real-time monitoring of various crude oil viscosities. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the cross-section of a plasma optical sensing probe.

[0017] Figure 2 This is a schematic diagram of the arrangement of the inner and outer air holes in a microstructured optical fiber.

[0018] Figure 3 Block diagram of crude oil pretreatment system for crude oil sample.

[0019] Figure 4This is a block diagram of a detection system that uses a plasma optical sensing probe to measure the viscosity of crude oil online.

[0020] Figure 5 This is a graph showing the relationship between the limitation loss and the resonant wavelength at different temperatures when a plasma optical sensing probe detects a sample.

[0021] Figure 6 It is the refractive index fitting curve of the plasma optical sensing probe when it resonates at different temperatures when detecting the sample.

[0022] Figure 7 This is a graph showing the relationship between the refractive index of crude oil and temperature when a plasma optical sensing probe detects crude oil samples C1, C3, and C5.

[0023] Figure 8 It is a graph showing the relationship between viscosity and temperature of C1, C3, and C5 crude oils, plotted according to the TM-4 equation.

[0024] Figure 9 This is a graph showing the relationship between limiting loss and resonant wavelength at different viscosities when a plasma optical sensing probe detects C1 crude oil.

[0025] Figure 10 This is a graph showing the relationship between the resonant wavelength and the refractive index when a plasma optical sensing probe detects C1 crude oil.

[0026] Figure 11 This is a graph showing the relationship between limiting loss and resonant wavelength at different viscosities when a plasma optical sensing probe detects C3 crude oil.

[0027] Figure 12 This is a graph showing the relationship between the resonant wavelength and the refractive index when a plasma optical sensing probe detects C3 crude oil.

[0028] Figure 13 This is a graph showing the relationship between limiting loss and resonant wavelength at different viscosities when a plasma optical sensing probe detects C5 crude oil.

[0029] Figure 14 This is a graph showing the relationship between the resonant wavelength and the refractive index when a plasma optical sensing probe detects C5 crude oil.

[0030] Figure 15 This is a graph showing the spatial relationship between viscosity, temperature, and refractive index of three crude oils, C1, C3, and C5, when a plasma optical sensing probe is used to detect the crude oil being tested.

[0031] Figure reference numerals: 1. Microstructured optical fiber; 2. Gold nanowire; 3. Outer layer air pores; 4. Inner layer air pores; 5. Perfect matching layer. Detailed Implementation

[0032] Example 1

[0033] like Figure 1 and Figure 2 As shown, a plasma optical sensing probe for online measurement of crude oil viscosity is disclosed. The probe consists of a microstructured optical fiber 1 and gold nanowires 2. The cladding of the microstructured optical fiber contains outer air holes 3 and inner air holes 4. There are 12 inner air holes arranged in a uniform hexagonal pattern, and 18 outer air holes arranged in a uniform hexagonal pattern. Two gold nanowires are embedded in the inner walls of the two inner air holes near the fiber core on the Y-axis. The center distance between two adjacent outer air holes and two adjacent inner air holes is equal, and the center distance between two adjacent inner air holes and two adjacent outer air holes parallel to the Y-axis is equal. The inner and outer air holes are arranged alternately periodically along the sides of the hexagons.

[0034] The radius of the microstructured optical fiber is 20 micrometers; the radius of both the outer and inner air holes is 2.4 micrometers; the radius of the gold nanowire is 500 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.3 micrometers.

[0035] The microstructured optical fiber has a perfect matching layer 5 on its exterior. The perfect matching layer is the absorption domain outside the simulation domain, which eliminates boundary reflection and accurately simulates the SPR sensing characteristics of the optical fiber. The background material of the microstructured optical fiber is silicon dioxide.

[0036] Example 2

[0037] The microstructure fiber of the plasma optical sensing probe for online measurement of crude oil viscosity has a radius of 18 micrometers; the radius of both the outer and inner air holes is 2.3 micrometers; the radius of the gold nanowire is 200 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.5 micrometers.

[0038] Example 3

[0039] The microstructure fiber of the plasma optical sensing probe for online measurement of crude oil viscosity has a radius of 22 micrometers; the radius of both the outer and inner air holes is 2.5 micrometers; the radius of the gold nanowire is 700 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.7 micrometers.

[0040] In this plasma optical sensing probe, 30 air holes are arranged, with 4 on the Y-axis, 2 on the X-axis, and the remaining 24 arranged in a symmetrical hexagonal pattern around the fiber core. Two gold nanowires are embedded in the walls of the two inner air holes on the Y-axis side closest to the fiber core, forming the sensing unit. This probe optimizes the light transmission mode by precisely controlling the MOF air hole structure and achieves viscosity detection based on the surface plasmon resonance (SPR) mechanism. When an evanescent wave of a specific wavelength achieves phase matching with the surface plasmon resonance (SPP) wave excited by the gold nanowires, the light field energy couples and generates resonant loss. By monitoring the shift of the resonant wavelength in the loss spectrum, the viscosity of crude oil can be retrieved in real time and accurately. Compared to traditional measurement methods, this probe has advantages such as being non-invasive and requiring very little sample. The sensitivity for detecting the viscosity of a single type of crude oil can reach up to 36063 cP / RIU, with a wide measurement range of 4–20000 cP, covering a wide range of types from light oils to heavy oils. It provides key technical support for high-precision and high-reliability viscosity monitoring during oil extraction and transportation.

[0041] Example 4

[0042] like Figure 3 and Figure 4 As shown, the detection system for online measurement of crude oil viscosity using a plasma optical sensing probe mainly consists of a crude oil pretreatment system, a light source, a polarizer, a constant-temperature sample cell, a spectrometer, and a computer. The crude oil pretreatment system comprises a crude oil tank, a heater, a centrifuge, a filter dehydration and desalination device, and a waste liquid tank.

[0043] First, the oil sample undergoes pretreatment. The sample is obtained from the storage tank and its viscosity is reduced and its fluidity improved by a heater. Then, the sample enters a centrifuge, where centrifugal force separates large particles and some of the aqueous phase, completing the initial purification. The centrifuged sample is then filtered to further remove fine particles. Finally, it undergoes dehydration and desalination treatment to deeply remove residual water and salt from the crude oil. This significantly reduces the corrosiveness and contamination of the crude oil and improves the overall quality of the sample.

[0044] After pretreatment, the crude oil samples enter the detection stage. Based on the microstructural characteristics of this fiber optic probe, only a trace sample is required for measurement. Furthermore, leveraging the label-free and non-invasive advantages of SPR technology, the entire detection process does not alter the chemical composition of the crude oil. This characteristic is particularly important when analyzing precious or trace crude oil samples. In the crude oil viscosity detection system, the light source module uses a laser source. The emitted light is filtered by a polarizer, retaining only P-polarized light to optimize the excitation and detection effects of the SPR resonance condition. This P-polarized light enters the MOF-SPR sensing probe via a transmission fiber. The probe is placed in a constant-temperature sample cell, contacting different types of pretreated crude oil samples at different temperatures. At the same temperature, different types of oil samples are identified as C1 (T=70℃, U=8.8cP, where U is the dynamic viscosity of crude oil, measured in centipoise, symbol cP), C3 (T=70℃, U=47cP), and C5 (T=70℃, U=1786cP). The same type of oil sample at different temperatures is designated as C5 oil sample. The parameters of the four C5 oil samples are as follows: T=40℃, U=21056cP; T=70℃, U=1786cP; T=100℃, U=332cP; and T=130℃, U=118cP.

[0045] Under suitable optical and interface conditions, the probe excites the SPR resonance phenomenon, causing characteristic changes in the optical signal. This change is received in real time by the fiber optic spectrometer in the detection module, converted into an electrical signal, and transmitted to the data processing module (computer). Finally, the signal is analyzed and interpreted by software, and the results are displayed in real time on the system interface, realizing online detection of crude oil viscosity.

[0046] The MOF-SPR sensing probe uses gold nanowires embedded in air holes as the core sensing medium, and its operation is based on the phase-matched coupling mechanism of evanescent waves and surface plasmon radiation (SPP). Under the constraint of the periodically arranged air hole structure, the distribution of the evanescent wave optical field transmitted in the MOF is effectively optimized, thereby enhancing the interaction between the evanescent field and the gold nanowires. When the evanescent wave propagates along the fiber axis in a specific mode, its wave vector matches the wave vector of the surface plasmon radiation on the gold nanowire at a specific wavelength. The evanescent field energy is efficiently coupled to the surface of the gold nanowire, causing significant energy loss in the transmitted optical field. By real-time monitoring of the energy extrema corresponding to the resonant wavelength in the loss spectrum, highly sensitive detection of changes in the refractive index of the medium can be achieved, constructing a wavelength modulation sensing response mechanism.

[0047] Temperature detection performance based on MOF-SPR sensing probe, such as Figure 5 and Figure 6 As shown, experimental results indicate that the refractive index of crude oil increases with increasing temperature, demonstrating a clear temperature response characteristic of the probe. Figure 5As shown, when the temperature is gradually increased in 10°C intervals, phenomena such as a redshift of the resonance wavelength, an increase in the resonance peak value, and a decrease in the half-maximum width at half maximum (WWHM) can be observed. These changes indicate that the coupling strength between the fiber core guiding mode and the plasma mode is continuously enhanced, reflecting the probe's high sensitivity to temperature changes, thus providing a reliable performance basis for achieving high-sensitivity crude oil viscosity detection.

[0048] Based on the above research on the temperature sensitivity and refractive index change of crude oil, temperature-refractive index relationship curves for three types of crude oil were plotted, as follows: Figure 7 As shown in the figure. The results indicate that, at the same temperature, different types of crude oil exhibit differences in refractive index, and their refractive index variation curves with temperature are also significantly different. By establishing a corresponding crude oil temperature-refractive index mathematical model, the change in the SPR signal can be transformed into a response to the crude oil viscosity index, thus providing data support for subsequent research.

[0049] In crude oil viscosity testing applications, the TM-4 equation (proposed by Khalifa Bouresli) is used. Calculation, where and All parameters are empirical, μ is crude oil viscosity, and T is degree Celsius. In this embodiment, the empirical parameters were obtained using linear fitting with Python functions. Substituting these parameters into the TM-4 equation yielded the following graph: Figure 8 The temperature-viscosity relationship curves of the three crude oils shown can be used to further calculate the dynamic viscosity and empirical parameters of the three crude oils at different temperatures under ideal conditions.

[0050] like Figure 9 As shown, based on the characteristic that the dynamic viscosity of crude oil is significantly affected by temperature, the dynamic viscosity of crude oil can be measured by the spectral loss value. This is because as the temperature increases, the solubility of crude oil increases, promoting the dissociation of the micro-associative structure and the dissolution of the wax crystal network, directly affecting the refractive index of crude oil, which is then reflected in the spectral loss value. The effectiveness of this detection method has been verified over a wider viscosity range. Figure 11 and Figure 13 As shown, the plasma optical probe exhibits clear and effective detection capabilities in both high-viscosity and low-viscosity crude oils. This strongly confirms the outstanding advantage of this SPR probe in detecting the dynamic viscosity of crude oil over a wide viscosity range, making it suitable for monitoring the needs of different grades of crude oil. Figure 10 , Figure 12 and Figure 14The quantitative correlation curves between the refractive index and resonance wavelength of different crude oil samples are presented, along with corresponding function fitting equations and goodness-of-fit. This combination of intuitive graphical trends and quantitative mathematical models verifies the reliability and repeatability of the relationship between refractive index and resonance wavelength, demonstrating the high degree of certainty of the relationship and providing empirical evidence for the accuracy of SPR-based crude oil identification methods.

[0051] Experimental results show that the maximum sensitivity for detecting C1 crude oil with a viscosity range of 4-24 dB is 39.98 cP / RIU. For C3 crude oil with a viscosity range of 20-250 dB, the maximum sensitivity is 4647 cP / RIU; and for C5 crude oil with a viscosity range of 75-20000 dB, the maximum sensitivity is 36063 cP / RIU. Figure 15 The spatial distribution relationship of crude oil viscosity, temperature, and refractive index shown in the diagram intuitively and quantitatively demonstrates a highly defined indirect coupling relationship between crude oil viscosity and refractive index, with temperature being the key controlling variable. This three-dimensional relationship diagram not only reveals the intrinsic law governing the changes in crude oil physical properties with temperature, but also provides reliable data model support for subsequent temperature-calibrated viscosity detection and crude oil identification methods based on MOF-SPR probes.

[0052] The research results show that this probe possesses excellent viscosity adaptability, covering a detection range from 4 cP for light oil to 20,000 cP for heavy oil, demonstrating broad applicability in complex crude oil systems. More notably, it exhibits a sensitivity of up to 36,063 cP / RIU in crude oil viscosity detection, combining the advantages of wide measurement range and high precision. Its structural design and functional performance are highly optimized, providing a reliable sensing solution for online viscosity monitoring in the petroleum industry.

Claims

1. A plasma optical sensing probe for online measurement of crude oil viscosity, characterized in that: The plasma optical sensing probe is composed of a microstructured optical fiber and gold nanowires. The cladding of the microstructured optical fiber contains outer and inner air holes, with 12 inner air holes arranged uniformly in a regular hexagonal pattern, and 18 outer air holes also arranged uniformly in a regular hexagonal pattern. Two gold nanowires are embedded in the inner walls of the two inner air holes near the fiber core along the Y-axis. The center-to-center distance between two adjacent outer air holes and two adjacent inner air holes is equal, as is the center-to-center distance between two adjacent inner air holes and two adjacent outer air holes parallel to the Y-axis. The inner and outer air holes are arranged alternately along the sides of the regular hexagons. The radius of the microstructured optical fiber is 18-22 micrometers; the radius of the outer and inner air holes is 2.3-2.5 micrometers; the radius of the gold nanowire is 200-700 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.3-5.7 micrometers.

2. The plasma optical sensing probe for online measurement of crude oil viscosity according to claim 1, characterized in that: The radius of the microstructured optical fiber is 20 micrometers; the radius of the outer air hole and the inner air hole are both 2.4 micrometers; the radius of the gold nanowire is 500 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.3 micrometers.

3. The plasma optical sensing probe for online measurement of crude oil viscosity according to claim 1, characterized in that: The radius of the microstructured optical fiber is 18 micrometers; the radius of the outer air hole and the inner air hole are both 2.3 micrometers; the radius of the gold nanowire is 200 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.5 micrometers.

4. The plasma optical sensing probe for online measurement of crude oil viscosity according to claim 1, characterized in that: The radius of the microstructured optical fiber is 22 micrometers; the radius of the outer air hole and the inner air hole are both 2.5 micrometers; the radius of the gold nanowire is 700 nanometers; and the center distance d between two adjacent outer air holes and two adjacent inner air holes is 5.7 micrometers.

5. The plasma optical sensing probe for online measurement of crude oil viscosity according to any one of claims 1-4, characterized in that: The microstructured optical fiber has a perfect matching layer on its exterior.

6. The plasma optical sensing probe for online measurement of crude oil viscosity according to any one of claims 1-4, characterized in that: The background material of the microstructured optical fiber is silicon dioxide.

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