OA21285A - Method and system for the temporal determination of a phase interface level of a multiphase fluid present in a vertical pipe. - Google Patents

Abstract

Method and system for the temporal determination of a phase interface level of a multiphase fluid present in a vertical pipe. The invention relates to a method and a system for the temporal determination of a phase interface level of a multiphase fluid present in a vertical pipe (2), comprising the installation of a distributed optical fiber sensor comprising an optical fiber cf.ble (8) spirally wound around the pipe and optically coupled to a DAS interrogator (10), from the data acquired by the DAS interrogator, determining the power spectral density over a duration predetermined and for each point of a discretized length of the optical fiber cable, integrating the power spectral density over a predefined frequency band for each point of the discretized length of the optical fiber cable, and putting it into matrix form results of the integration of the power spectral density in order to determine at least one interface level of the multiphase fluid.

Description

Description . . Title of the invention: Method and system for the temporal determination of an ïfc- ^: phase interface level of a multiphase fluid present in a vertical pipe

Technical Field • The invention relates to the general field of detection of phase interfaces. . of a multiphase fluid circulating in vertical pipes, in particular in - gravity separators or rising pipes (i.e. risers) catenary / io used in the field of hydrocarbon production, for example oil and gas.

Prior art

The extraction of underwater hydrocarbon production wells generates a multiphase mixture (water, oil, gas and sand) which must be treated to recover only what is going to be used, namely the oil and gas. .

- Generally, this polyphasic mixture is brought back on board an FPSO (for _; “Floating Production Storage Offloading” in English, namely a floating storage, production and unloading support) to be treated there with a view to separating the oil strictly speaking, water, gas, and possible solid components.

The oil, once separated, is then stored on board, the gas is washed, then sent to the gas turbines for the production of electricity and heat necessary for; on board, then the surplus is reinjected into the oil field reservoir so as to put it back under pressure. The water, after having been released from the solid particles in ^: suspension, is either discharged into the sea after extraction of any particles. of oil, is also reinjected into the tank. Finally, solid particles? extracted, which. it represents only minimal quantities, is partly reprocessed and recycled on site to be discharged at sea or in special basins, and

Y/ ·; partly sent to land for treatment and storage and/or reinjected into the subsoil through the well.

One of the known methods of separating the water and the oil contained in the multiphase mixture extracted from the production well consists of using a tank of very large volume, generally of cylindrical shape: the oil enters at one end of the tank and runs along it to allow the different 5 phases of the mixture to separate naturally by gravity and reach the other end of the tank. This type of separator, hereinafter referred to as a "gravity separator", is generally used for crude oil also containing gas, the gas then being recovered in the upper part of the reservoir, the water and sand in the lower part, and the oil ( oil) in the intermediate part.

For this purpose, it is known to use underwater gravity separators installed at the bottom of the sea. We thus know from document WO 2015/114247 an underwater gravity separator comprising in particular a plurality of cylindrical pipes forming reservoirs in which circulates the multiphase mixture extracted from the production well. During this circulation, the different phases of the mixture separate naturally by gravity: the water rests at the bottom of the tank, the oil is above the water and the gas is above the oil. At the outlet of the pipes, the water is typically recovered for treatment before being reinjected into the well by water injection pumps, while the oil and gas phases are conveyed to the surface to the FPSO.

When the different phases of the multiphase mixture present in the gravity separator tank have separated, it is important to accurately measure the phase interface level in the tank, i.e. the level in the tank between two superimposed phases (water/oil interface and oil/gas interface). The result of this measurement ensures perfect regulation of the flow rate of the water injection pumps, which improves the operation of the separator.

Measuring the phase interface level in the tank of a gravity 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 uses radioactive radiation sources which are harmful to the underwater environment as well as in terms of health and safety for personnel. In addition, the detectors associated with these sensors are complex components with high cost and low reliability according to operators, which makes the measurement results imprecise.

The measurement of the phase interface level can also be carried out using an ultrasonic sensor as described in particular in the publications WO

2018/065128 and WO 2009/063194. In these publications, a sensor emits ultrasonic pulses that are reflected by the external surface of the tank. The travel time of the reflected ultrasonic signal is directly proportional! to: distance traveled. If the shape of the reservoir is known, we can then deduce its interface levels.

ίο We still know the publication US 6,943,566 which describes the principle of level measurement which is based on the variation in capacitance of a capacitor. With this type of measurement, the probe and the tank wall form a capacitor whose capacity depends on the quantity of fluid present in the tank.

We also know the publication US 9,052,230 which describes a method for imaging the interior volume of a container associated with an industrial process and detecting the physical and chemical characteristics of a medium present in the container, on which the industrial process acts. An example area of application is the coking process during which a number of undesirable conditions can arise in the vessel. In practice, the method detects an interface by qualitatively identifying greater spectral energy (by visual comparison of the spectra). Above and below said interface, the energies of the spectra are lower and have nothing remarkable to distinguish them. The detection of the position of the interface therefore lacks precision.

Presentation of the invention

The object of the present invention is to propose a method making it possible to monitor in real time the phase interface level of a multiphase fluid present in a. vertical pipe which does not present the disadvantages of the methods of the prior art.

In accordance with the invention, this aim is achieved thanks to a method for the temporal determination of a phase interface level of a multiphase fluid present in a vertical pipe, comprising

- the installation of a distributed fiber optic sensor comprising a fiber optic cable wound spirally around the pipe and optically coupled to a DAS interrogator;

- from the data acquired by the DAS interrogator, determining the power spectral density over a predetermined duration and for each point of a discretized length of the optical fiber cable;

- the integration of the power spectral density over a predefined frequency band for each point of the discretized length of the optical fiber cable; And

- putting into matrix form the results of the integration of the power spectral density in order to determine at least one interface level of the polyphase fluid.

The method according to the invention is remarkable in that it makes it possible to determine in real time and continuously from a distributed optical fiber sensor wound around the vertical pipe the phase interface level of the polyphase fluid present in the driving. In addition, this process has the advantages of being non-intrusive to the fluid and easy to install on the vertical pipe. No maintenance is necessary. In addition, the distributed fiber optic sensor that is used can also be used to monitor other parameters of the fluid flow, such as pressure, vibrations, possible leaks, composition, turbulence intensity, etc. .).

The method according to the invention is also remarkable in that it is the vertical variation of the energy, calculated from spectral analyzes and quantified between the fluids, which makes it possible to identify the presence of one or more interfaces. In other words, for each position “in z” in the pipe (given by the winding pitch of the fiber) and over the time t, the method according to the invention makes it possible to obtain a value which characterizes the energetic 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.

On the other hand, spectral analyzes reveal a greater energy density in dense fluids and thus give a remarkable and quantifiable character to what happens above and below the interfaces. The precision of these analyzes 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 precisely quantify the position(s) of one or more interfaces and the thickness of one. .··. emulsion, if present. We somehow obtain a precise image of the distribution of fluids.

The process according to the invention thus has numerous advantages. It allows real-time and multi-parameter monitoring from a distributed sensor, of the optical fiber type, which is wound around a gravity separator. It also makes it possible to obtain real-time images of the interfaces and the distribution of fluids and emulsions in a gravity 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 quantity of gases in the liquids via an analysis of the speed of sound within each fluid (this in order to to give elements of performance of the separator over time). It also makes it possible to ensure parallel monitoring of the pressure all along the separator with the possibility of following the evolution of the hydrostatic pressure as a function of the altitude in the separator.

According to one application of the process, the vertical pipe is a separator type pressure vessel.

According to another application of the method, the vertical pipe is a catenary rising pipe (i.e. a riser).

The integration of the power spectral density can be carried out over a frequency band between 10 and 1000 Hz. As for the measurement of the power spectral density, it can be carried out over a duration of the order of ls.

The fiber optic cable can be spirally wound around the pipe forming contiguous turns, which provides great precision in determining the phase interface level. Alternatively, the optical fiber cable can be wound in a spiral around the pipe, forming turns spaced from each other with the same non-zero pitch.

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

Correlatively, the invention also relates to a system for the temporal determination of a phase interface level of a multiphase fluid present in a vertical pipe, comprising:

10. - a distributed fiber optic sensor comprising a fiber optic cable intended to be spirally wound around the pipe and a DAS interrogator optically coupled to the fiber optic cable;

- means for determining, from the data acquired from the DAS interrogator, the power spectral density over a predetermined duration and for each point of a discretized length of the optical fiber cable;

- means for integrating the power spectral density over a predefined frequency band for each point of the discretized length of the optical fiber cable; And

- means for putting the results of the integration of the power spectral density into matrix form in order to determine at least one interface level of the polyphase system.

Brief description of the drawings

[Fig. 1] Figure 1 is a schematic view of an example of a vertical pipe equipped with a system according to the invention for the temporal determination of a phase interface level of a multiphase fluid flowing in the pipe .

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

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

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

Description of embodiments

The invention relates to a method and a system for the temporal determination of a phase interface level of a multiphase fluid present in a vertical pipe.

By “temporal determination”, we mean here that the phase interface level is determined as a function of time so as to be able to follow its evolution over time.

îo By “multiphase fluid” is meant here any polyphasic system comprising different phases separated into several superimposed strata of an initially polyphasic mixture (in particular water, gas and oil).

By “vertical pipe”, we mean here any portion of vertical pipe in which the multiphase fluid stagnates or flows. For example, the vertical pipe 15 may be a gravity separator pipe or a catenary rising pipe (also called a riser) used in the underwater production of hydrocarbons.

The method according to the invention provides for using DAS technology (for “Distributed Acoustic Sensing” or “Distributed Acoustic Sensing” in French) 20 to determine the interface levels of a multiphase fluid present in such a vertical pipe.

Fiber optic distributed acoustic sensing (DAS) is a known type of sensing in which an optical fiber is deployed as a 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 which may be contiguous.

In each discrete detection portion, mechanical disturbances of the optical fiber, for example deformations due to incident acoustic waves, cause a variation in the properties of the radiation which is backscattered from this detection portion. This variation can be detected and analyzed and used to give a measurement of the disturbance of the fiber at this detection portion.

Figure 1 schematically represents an example of application of the method according to the invention to a vertical pipe 2 inside which a multiphase fluid flows from top to bottom.

In this example of application, pipe 2 is closed at its lower end by. a plug provided with a drain outlet 4 for the multiphase fluid. The multiphase fluid enters from the top of the pipe via a pipe 6. Taps (not shown) make it possible to control the flow rates of fluid entering and leaving pipe 2.

An optical fiber cable 8 is wound and glued in a spiral around the pipe 2 from the bottom of the latter to a height h of approximately 85cm. The optical fiber is wound into contiguous turns (the pitch between adjacent turns is zero) and is optically coupled to a DAS 10 interrogator.

Of course, depending on the measurement precision desired for the phase interface level, it is possible to wind the optical fiber cable so that it forms turns spaced from each other of the same not non-zero. The larger the step, the less the measurement precision will be.

Likewise, the precision of the measurement also depends on the spatial discretization chosen for the optical fiber. In the example illustrated, a spatial discretization of the optical fiber of 1 m is chosen, which corresponds to a vertical spatial resolution along the pipe of approximately 3 mm (the optical fiber cable here having a diameter of 0 .9mm).

Furthermore, for this application we choose a reference of 0% of the height of the optical fiber for the bottom of pipe 2, a reference of 50% of the height of the optical fiber for the middle of the pipe, and a reference of 100% of the height of the optical fiber for the top of the pipe.

The multiphase fluid which is circulated in line 2 from top to bottom is here a liquid/gas mixture consisting of water and air.

The method according to the invention provides, 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.

The raw data acquired by the DAS interrogator are the temporal variations: of deformation of the optical fiber. These data make it possible to calculate the power spectral density over a predetermined duration d (typically of the order of a second) as shown in Figure 2A. This calculation is carried out all along the . optical fiber for each discretized point i thereof. In known manner, the power spectral density is obtained by calculating the square of the module of the Fourier transform of the deformation of the optical fiber at point i, multiplied by the integration time d.

The next step of the method according to the invention consists of integrating the power spectral density thus calculated over a predefined band of frequencies fl, f2 (typically between 10 and 1000 Hz) for each point i of the discretized length of the fiber cable optical. This integration is represented by the curve in Figure 2B.

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.

The method according to the invention can then provide for the construction of an image representative of this matrix of the results of the integration of the power spectral densities in order to be able to visually determine your interface levels of the multiphase fluid.

A free graphical representation of the matrix of integration results: power spectral densities is illustrated in Figure 2C. .

In this figure, the abscissa axis represents the 1st time (here in seconds from t=0s to t=30s) and the ordinate axis characterizes the height h (here in cm from 0cm to 85cm) on the vertical pipe of the coiled optical fiber. Color is associated with the intensity of the power spectral density (according to an algorithmic scale).

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This figure thus makes it possible to easily visualize the temporal evolution of the water/air phase interface of the polyphase mixture flowing in the vertical pipe of Figure 1. Indeed, this phase interface is materialized in this Figure 2Ç by the limit between the two colors (here it is approximately 30cm high at t=Os to reach approximately 56cm at t=30s).

We see here that the phase interface level varies as a function of time. It could of course be substantially constant over time. : ·.

Furthermore, the temporal determination of the phase interface level was obtained here from a graphical representation of the matrix of the results of the integration of the power spectral densities.

Alternatively, it is possible to provide that an algorithm makes it possible, from the matrix, to directly determine and follow the evolution of the phase interface levels as a function of time.

Claims (9)

[Claim 1] Method for the temporal determination of a phase interface level of a multiphase fluid present in a pipe 5 vertical (2), comprising: - the installation of a distributed optical fiber sensor comprising an optical fiber cable (8) wound in a spiral around the pipe and coupled: n-. optically to a DAS interrogator (10); . < - . - from the data acquired by the DAS interrogator, determination of the 10 power spectral density over a predetermined duration (d) and for each point (i) of a discretized length of the optical fiber cable; - the integration of the power spectral density over a predefined frequency band (fl, f2) for each point of the discretized length of the optical fiber cable; And 15 - putting into matrix form the results of the integration of the power spectral density in order to determine at least one interface level of the polyphase fluid. [Claim 2] A method according to claim 1, wherein the vertical pipe is a separator type pressure vessel. 20 [Claim 3] A method according to claim 1, wherein the vertical pipe is a catenary riser pipe. [Claim 4] Method according to any one of claims 1 to 3, in which the integration of the power spectral density is carried out over a frequency band between 10 and 1000 Hz. 25 [Claim 5] Method according to any one of claims 1 to 4, in which the measurement of the power spectral density is carried out over a duration of the order of 1s. [Claim 6] Method according to any one of claims 1 to 3, in which the optical fiber cable is wound spirally around the conduit forming contiguous turns. [Claim 7] Method according to any one of claims 1 to 3, in which the optical fiber cable is wound spirally around the pipe forming turns spaced apart from each other with the same non-zero pitch. [Claim 8] Method according to any one of claims 1 to 7, further comprising constructing an image representative of the matrix of the results of the integration of the power spectral density in order to. visually determine at least one interface level of the polyphase fluid ^f . [Claim 9] System for the temporal determination of a phase interface level of a multiphase fluid present in a vertical pipe (2), comprising: - a distributed fiber optic sensor comprising a fiber optic cable (8) intended to be spirally wound around the pipe and a DAS interrogator (10) optically coupled to the fiber optic cable; - means for determining, from the 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 optical fiber cable; - means for integrating the power spectral density over a predefined frequency band (fl, f2) for each point of the discretized length of the optical fiber cable; And - means for putting the results of the integration of the power spectral density into matrix form in order to determine at least one interface level of the polyphase system.