METHOD AND SYSTEM FOR DETERMINING OVER TIME THE LEVEL OF PHASE INTERFACE OF MULTIPHASE FLUID PRESENT IN A VERTICAL TUBE

Description

1 / 11 METHOD AND SYSTEM FOR DETERMINING OVER TIME THE LEVEL OF PHASE INTERFACE OF MULTIPHASE FLUID PRESENT IN A VERTICAL TUBE Technical Field

[001] The invention relates to the general field of detecting phase interfaces of a multiphase fluid circulating in vertical pipes, in particular in density separators or catenary risers used in the field of hydrocarbon production, for example, oil and gas. Previous Technique

[002] Extraction from subsea hydrocarbon production wells generates a multiphase mixture (water, oil, gas and sand) that needs to be treated to recover only what will be used, namely oil and gas.

[003] Generally, this multiphase mixture is embedded in a FPSO (Floating Production Storage and Offloading Unit) to be treated there in order to separate the oil itself from water, gas and any solid components.

[004] Once separated, the oil is then stored on board, the gas is then sent to the gas turbines for the production of the electricity and heat needed on board, and the surplus is then re-injected into the oil field reservoir in order to re-compress it. The water, after being freed from the suspended solid particles, is discharged into the sea after the extraction of any oil particles or re-injected into the reservoir. Finally, the extracted solid particles, which represent only minimal quantities, are partially re-treated and recycled on-site to be discharged into the sea or to special basins, and partially sent ashore for treatment and storage and / or re-injected into the reservoir. Petition 870260047617, dated 05 / 19 / 2026, page 8 / 42 2 / 11 underground by the well.

[005] One of the known methods for separating water and oil contained in the multiphase mixture extracted from the production well consists of using a very large volume reservoir, usually cylindrical in shape: the oil enters at one end of the reservoir and advances along it to allow the different phases of the mixture to separate naturally by gravity and reach the other end of the reservoir. This type of separator, hereinafter referred to in this document as a densimetric separator, is generally used for crude oil that also contains gas, with the gas then being recovered at the top of the reservoir, the water and sand at the bottom, and the oil in the middle.

[006] For this purpose, the use of subsea density separators installed on the seabed is known. Thus, the document in WO 2015 / 114247 discloses a subsea density separator comprising, in particular, a plurality of cylindrical tubes that form reservoirs in which the multiphase mixture extracted from the production well circulates. During this circulation, the different phases of the mixture naturally separate by gravity: the water rests at the bottom of the reservoir, the oil is above the water, and the gas is above the oil. At the outlet of the tubes, the water is typically recovered for treatment before being re-injected into the well by water injection pumps, while the oil and gas phases are transported over the surface to the FPSO.

[007] When the different phases of the multiphase mixture present in the reservoir of the densimetric separator have separated, it is important to accurately measure the phase interface level in the reservoir, that is, the level in the reservoir between two overlapping phases (water / oil interface and oil / gas interface). The result of this measurement makes Petition 870260047617, dated 05 / 19 / 2026, p. 9 / 42 3 / 11 It is possible to ensure perfect regulation of the flow rate of the water injection pumps, which improves the operation of the separator.

[008] The measurement of the phase interface level in the reservoir of a densimetric separator is generally carried out by gamma ray level sensors, as described, in particular, in publications EP 1,314,006 and EP 2,329,234. However, this type of sensor implements radioactive radiation sources that are harmful to the underwater environment, as well as in terms of health and safety for employees. Additionally, the detectors associated with these sensors are complex components with a high cost and low reliability according to operators, which makes the measurement results inaccurate.

[009] Phase interface level measurement can also be performed using an ultrasonic sensor, as described, in particular, in publications WO 2018 / 065128 and WO 2009 / 063194. In these publications, a sensor emits ultrasonic pulses that are reflected by the outer surface of the reservoir. The travel time of the reflected ultrasonic signal is directly proportional to the distance traveled. If the shape of the reservoir is known, the interface levels can then be deduced.

[0010] Publication in US 6,943,566 is also known, which describes the principle of level measurement based on the variation in capacitance of a capacitor. With this type of measurement, the probe and the reservoir wall form a capacitor whose capacitance depends on the amount of fluid present in the reservoir.

[0011] Publication in US 9,052,230 is also known, which describes a method for imaging the interior volume of a vessel associated with an industrial process and detecting the physical and chemical characteristics of a medium present in the vessel, on which the industrial process acts. An example of a field of application is the Petition 870260047617, dated 05 / 19 / 2026, p. 10 / 42 4 / 11 of the coking process during which a series of undesirable conditions can occur in the vessel. In practice, the method detects an interface by qualitatively identifying a higher spectral energy (by visual comparison of the spectra). Above and below said interface, the energies of the spectra are weaker and have nothing notable to distinguish them. Thus, the detection of the interface position lacks precision. Disclosure of the Invention

[0012] The object of the present invention is to propose a method that makes it possible to monitor in real time the phase interface level of a multiphase fluid present in a vertical pipe that does not have the disadvantages of prior art methods.

[0013] According to the invention, this objective is achieved by means of a method for determining over time the phase interface level of a multiphase fluid present in a vertical tube, comprising: - to place a distributed fiber optic sensor comprising a fiber optic cable coiled in a spiral around the tube and optically coupled to a DAS interrogator; - to determine, from the data acquired by the DAS interrogator, the power spectral density over a predetermined duration and for each point of a discretized length of the optical fiber cable; - integrate the power spectral density into a predefined frequency band for each point along the discretized length of the fiber optic cable; and - To configure the results of the power spectral density integration in matrix form in order to determine at least one interface level of the multiphase fluid.

[0014] The method according to the invention is remarkable in the sense that Petition 870260047617, dated 05 / 19 / 2026, page 11 / 42 5 / 11 which makes it possible to determine, in real time and continuously, from a distributed fiber optic sensor wrapped around the vertical pipe, the phase interface level of the multiphase fluid present in the pipe. Furthermore, this method has the advantages of being non-intrusive to the fluid and easy to install on the vertical pipe. No maintenance is required. Additionally, the distributed fiber optic sensor used can also be used to monitor other fluid flow parameters, such as pressure, vibrations, potential leaks, composition, turbulence intensity, etc.

[0015] The method according to the invention is also noteworthy in that it is the vertical variation of energy, calculated from spectral analyses and quantified between the fluids, that makes it possible to identify the presence of one or more interfaces. In other words, for each position z in the tube (indicated by the fiber winding pitch) and over time t, the method according to the invention makes it possible to obtain a value that characterizes the energy behavior of the fluid. Thus, it is possible to identify the level of the interface(s) by the presence of slope break(s) in the vertical energy profile.

[0016] On the other hand, spectral analyses reveal a higher energy density in dense fluids and thus provide a remarkable and quantifiable nature to what happens above and below the interfaces. The precision of these analyses makes it possible to see the influence of hydrostatic pressure with an increase in energy due to the weight of the fluid column. Thus, it is possible to accurately quantify the position(s) of one or more interfaces and the thickness of an emulsion, if present. An accurate picture of the fluid distribution is obtained.

[0017] The method according to the invention thus has numerous advantages. It allows real-time, multi-parameter monitoring from a distributed, fiber optic-type sensor that is wrapped around a densimetric separator. It also makes it possible to obtain Petition 870260047617, dated 05 / 19 / 2026, page 12 / 42 6 / 11 Real-time images of the interfaces and distribution of fluids and emulsions in a density separator. It also makes it possible to characterize the evolution of the physical properties of fluids (such as water, oils, gases and critical gases) and, in particular, the amount of gas in liquids through an analysis of the speed of sound within each fluid (this in order to provide separator performance data over time). It also makes it possible to ensure parallel pressure monitoring throughout the separator with the possibility of monitoring the evolution of hydrostatic pressure as a function of altitude in the separator.

[0018] According to one application of the method, the vertical tube is a separator-type pressure device.

[0019] According to another application of the method, the vertical pipe is a catenary riser.

[0020] Power spectral density integration can be performed in a frequency band between 10 and 1,000 Hz. When measuring power spectral density, it can be performed over a period on the order of 1 s.

[0021] The fiber optic cable can be spirally wound around the tube forming contiguous turns, which provides high accuracy in determining the phase interface level. Alternatively, the fiber optic cable can be spirally wound around the tube forming turns spaced from each other by the same non-zero pitch.

[0022] The method may further comprise constructing a representative image of the matrix of power spectral density integration results in order to visually determine at least one multiphase fluid interface level.

[0023] Correspondingly, the object of the invention is also a system for determining an interface level over time. Petition 870260047617, dated 05 / 19 / 2026, page 13 / 42 7 / 11 of the phases of a multiphase fluid present in a vertical tube, comprising: - a distributed fiber optic sensor comprising a fiber optic cable intended to be coiled around the tube and a DAS interrogator optically coupled to the fiber optic cable; - means to determine, from data acquired from the DAS interrogator, the power spectral density over a predetermined duration and for each point of a discretized length of the fiber optic cable; - means to integrate the power spectral density into a predefined frequency band for each point along the discretized length of the fiber optic cable; and - means to configure the results of power spectral density integration in matrix form in order to determine at least one interface level of the multiphase system. Brief Description of the Drawings

[0024] Figure 1 is a schematic view of an example of a vertical tube equipped with a system according to the invention for determining over time a phase interface level of a multiphase fluid flowing in the tube.

[0025] Figure 2A shows an example of the implementation of a step of the method according to the invention.

[0026] Figure 2B shows an example of the implementation of another step of the method according to the invention.

[0027] Figure 2C shows an example of the implementation of yet another step of the method according to the invention. Description of the Modalities

[0028] The invention relates to a method and a system for determining over time a phase interface level of Petition 870260047617, dated 05 / 19 / 2026, p. 14 / 42 8 / 11 a multiphase fluid present in a vertical tube.

[0029] By determination over time it is understood here that the phase interface level is determined as a function of time in order to be able to monitor its evolution over time.

[0030] Multiphase fluid here means any multiphase system comprising different phases separated into several overlapping layers of an initially multiphase mixture (in particular water, gas and oil).

[0031] Vertical tube here means any portion of the vertical tube in which the multiphase fluid stagnates or flows. For example, the vertical tube may be a tube of a density separator or a catenary riser used in subsea hydrocarbon production.

[0032] The method according to the invention provides the use of DAS (Distributed Acoustic Sensing) technology to determine the interface levels of a multiphase fluid present in such a vertical tube.

[0033] Fiber optic distributed acoustic sensing (DAS) is a type of sensing known in which an optical fiber is implemented as the sensing fiber to provide detection of acoustic activity along its entire length. Typically, one or more laser pulses are sent through the optical fiber and, by analyzing the backscattered radiation, the fiber can be divided into a plurality of discrete sensing portions that may be contiguous.

[0034] In each discrete sensing portion, mechanical interferences of the optical fiber, for example, deformations due to incident acoustic waves, cause a variation in the properties of the radiation that is backscattered from that sensing portion. This va Petition 870260047617, dated 05 / 19 / 2026, p. 15 / 42 9 / 11 The interaction can be detected and analyzed and used to provide a measurement of fiber interference at the level of that detection portion.

[0035] Figure 1 schematically represents an example of applying the method according to the invention to a vertical tube 2 within which a multiphase fluid flows from top to bottom.

[0036] In this application example, tube 2 is closed at its lower end by a plug equipped with a flow outlet 4 for the multiphase fluid. The multiphase fluid enters the tube from the top through a tube 6. Taps (not shown) make it possible to control the fluid flows entering and exiting tube 2.

[0037] An optical fiber cable 8 is spirally wound and glued around tube 2 from its bottom to a height h of approximately 85 cm. The optical fiber is wound in contiguous turns (the pitch between adjacent turns is zero) and is optically coupled to a DAS interrogator 10.

[0038] It is evident that, depending on the desired measurement accuracy for the phase interface level, it is possible to wind the fiber optic cable to form turns spaced from each other by the same non-zero pitch. The larger the pitch, the lower the measurement accuracy.

[0039] Similarly, the accuracy of the measurement also depends on the spatial discretization chosen for the optical fiber. In the example illustrated, a spatial discretization of 1 m is chosen for the optical fiber, which corresponds to a vertical spatial resolution along the tube of about 3 mm (with the optical fiber cable here having a diameter of 0.9 mm).

[0040] Furthermore, a reference of 0% of the optical fiber height for the bottom of tube 2, a reference of 50% of the optical fiber height for the middle of the tube, and a reference of 100% of the optical fiber height for the top of the tube are chosen for this application. Petition 870260047617, dated 05 / 19 / 2026, p. 16 / 42 10 / 11

[0041] The multiphase fluid that is circulated in tube 2 from top to bottom is here a liquid / gas mixture consisting of water and air.

[0042] The method according to the invention allows, from the raw data acquired by the interrogator 10 coupled to the optical fiber, to determine the power spectral density over a predetermined duration and for each point of the discretized length of the optical fiber cable.

[0043] The raw data acquired by the DAS interrogator are the time variations of the optical fiber strain. This data makes it possible to calculate the power spectral density over a predetermined duration d (typically on the order of a second) as shown in Figure 2A. This calculation is performed along the entire optical fiber for each discretized point i of the fiber. As is known, the power spectral density is obtained by calculating the square of the Fourier transform magnitude of the optical fiber strain at point i, multiplied by the integration time d.

[0044] The next step of the method according to the invention consists of integrating the power spectral density thus calculated into a predefined frequency band f1, f2 (typically between 10 and 1,000 Hz) for each point of the discretized length of the optical fiber cable. This integration is represented by the curve in Figure 2B.

[0045] The results of integrating the power spectral densities over the entire length of the optical fiber are then stored in the form of the same 2D matrix in order to be visualized.

[0046] The method according to the invention can then provide the construction of a representative image of this matrix of the results of the integration of the power spectral densities in order to be able to visually determine the interface levels of the multiphase fluid. Petition 870260047617, dated 05 / 19 / 2026, page 17 / 42 11 / 11

[0047] An example of a graphical representation of the matrix of results from the integration of power spectral densities is illustrated in Figure 2C.

[0048] In this figure, the abscissa axis represents time (here in seconds from t=0 to at=30 s) and the ordinate axis characterizes the height h (here in cm from 0 cm to 85 cm) on the vertical tube of the coiled optical fiber. The color is associated with the intensity of the power spectral density (according to a scaling algorithm).

[0049] This figure thus makes it possible to easily visualize the evolution over time of the water / air phase interface of the multiphase mixture flowing in the vertical tube of Figure 1. In fact, this phase interface is materialized in this Figure 2C by the boundary between the two colors (here it is about 30 cm high at at=0 s to reach about 56 cm at at=30 s).

[0050] It can be seen here that the phase interface level varies as a function of time. It is evident that it can be substantially constant over time.

[0051] Furthermore, the determination of the phase interface level over time was obtained here from a graphical representation of the matrix of the power spectral density integration results.

[0052] Alternatively, it is possible to provide an algorithm that makes it possible, from the matrix, to directly determine and monitor the evolution of phase interface levels as a function of time. Petition 870260047617, dated 05 / 19 / 2026, page 18 / 42

Claims

1 / 2 CLAIMS 1. Method for the time-dependent determination of a phase interface level of a multiphase fluid present in a vertical pipe (2) characterized in that it comprises: - placing a distributed fiber optic sensor comprising a fiber optic cable (8) spirally wound around the pipe and optically coupled to a DAS interrogator (10); - determining, from the data acquired by the DAS interrogator, the power spectral density over a predetermined duration (d) and for each point (i) of a discretized length of the fiber optic cable; - integrating the power spectral density in a predefined frequency band (f1, f2) for each point of the discretized length of the fiber optic cable; and - configuring the results of the power spectral density integration in matrix form in order to determine at least one multiphase fluid interface level.

2. Method according to claim 1, characterized in that the vertical tube is a separator-type pressure apparatus.

3. Method according to claim 1, characterized in that the vertical pipe is a catenary riser.

4. A method, according to any one of claims 1 to 3, characterized in that the power spectral density integration is performed in a frequency band between 10 and 1,000 Hz.

5. Method, according to any one of claims 1 to 4, characterized in that the measurement of the power spectral density is carried out over a period on the order of 1 s.

6. Method, according to any of the claims Petition 870260047617, dated 05 / 19 / 2026, page 19 / 42 2 / 2 1 to 3, characterized in that the fiber optic cable is wound in a spiral around the tube forming contiguous turns.

7. A method, according to any one of claims 1 to 3, characterized in that the optical fiber cable is spirally wound around the tube by forming turns spaced from each other by the same non-zero pitch.

8. A method, according to any one of claims 1 to 7, characterized in that it further comprises constructing a representative image of the matrix of power spectral density integration results in order to visually determine at least one multiphase fluid interface level.

9. System for determining over time a phase interface level of a multiphase fluid present in a vertical tube (2) characterized in that it comprises: - a distributed fiber optic sensor comprising a fiber optic cable (8) intended to be spirally wound around the tube and a DAS interrogator (10) optically coupled to the fiber optic cable; - means for determining, from data acquired from the DAS interrogator, the power spectral density over a predetermined duration (d) and for each point (i) of a discretized length of the fiber optic cable; - means for integrating the power spectral density in a predefined frequency band (f1, f2) for each point of the discretized length of the fiber optic cable; and - means for configuring in matrix form the results of the power spectral density integration in order to determine at least one interface level of the multiphase system.Petition 870260047617, dated 05 / 19 / 2026, p. 20 / 42.

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