RU2820792C1 - Method for complex control of characteristics of water-oil mixture in dynamic state - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: invention relates to the field of oil and gas industry and can be used for conducting the process mode of intra-field pumping of water-oil mixture. Method for complex control of characteristics of water-oil mixture in dynamic state consists in determination of characteristics of water-oil mixture in dynamic state, at which onset of inversion phenomenon is possible. Method is characterized by the fact that it includes an algorithm for calculating the characteristics of a water-oil flow in a dynamic state, according to which the structure of the water-oil mixture is successively determined, type of water-oil mixture, its density, viscosity of mixture depending on flow structure, average flow rate, hydrodynamic flow mode, characterized by Reynolds criterion, coefficient of hydraulic resistance depending on value of Reynolds criterion. Obtaining these values of the characteristics of the water-oil mixture is carried out, according to the control scheme, using a measuring installation at the cluster site with a multiphase gamma-meter flow meter installed on the liquid line, and with in-line rotary viscometer at the inlet or outlet of the measuring apparatus. After that, in the information processing device, based on the received data and the performed calculations, the moment of the onset of the phase inversion phenomenon is determined, which is characterized by a certain range of water content, viscosity and hydrodynamic mode, in which there is a maximum jump in the coefficient of hydraulic resistance. Taking into account the data on the onset of said moment, the water cut of the mixed flows of the water-oil mixture coming from the production wells to the cluster site is adjusted by redirecting the flows for selective mixing, at which the value of the final water cut of the mixed flow will be outside the calculated range of water cut, at which the inversion phenomenon occurs, which ensures stable maintenance of the pumping process mode.

EFFECT: increased stability of the process inside the field transfer.

1 cl, 6 dwg

Description

The invention relates to the field of oil and gas industry and can be used to maintain the technological regime of in-field pumping of an oil-water mixture.

Technological processes of oil production enterprises, from production, in-field pumping and preparation of oil, to oil storage, are associated with the formation of highly viscous oil-water emulsions. At the stage of production and in-field pumping, highly viscous emulsions formed in the flow in the pipeline lead to increased energy costs for pumping and loads on equipment due to uneven flow, as a result of which vibrations along the pipeline and the phenomenon of cavitation occur. There is also a complication of preparation processes - the inclusion of additional equipment in the technological process, an increase in the cost of treatment with demulsifiers. As a result of the formation of intermediate layers represented by multiple emulsions, inaccurate readings of flow meters, density meters and moisture meters are observed in the preparation apparatus. Once in oil storage tanks, intermediate layers accumulate at the interface between the oil and water phases, which leads to an increase in oil-containing waste in the form of tank-type oil sludge.

At the beginning of the process of pumping an oil-water mixture from wells through an in-field pipeline to treatment facilities, a redistribution of the water and oil phases occurs in the flow. This is due to pipeline pressure and temperature, as well as flow rate. Since the flow comes from the wells and then mixes at the well pads, and only after that it goes through the main pipelines to the oil treatment plants, then at the moment of mixing the flows at the well pad, sharp jumps in the water cut of the flow occur. Thus, the water cut of wells from one pad can vary on average up to 70-80%.

Water-oil emulsions, like all dispersed systems, have the ability to undergo phase transition. As a result, an emulsion from the “water in oil” type can transform into the “oil in water” type at a certain ratio of dispersed phases. This transition is called the inversion phenomenon - phase reversal. Each type of emulsion is characterized by differences in viscosity, since it depends on the content of the dispersed phase, i.e. the phase in which the other is dispersed. For field pumping, the best option is the direct type of emulsion - oil in water, since such a flow is transported with lower energy loads.

Due to the phase inversion phenomenon, a sharp increase in the viscosity of the flow occurs, and then after a short time it decreases, as a result of which vibrations in the pipeline and the phenomenon of cavitation can be observed, reducing the service life of the pipeline and the quality of the final product. In this regard, there is a need to develop a comprehensive methodology for monitoring the state of the oil-water mixture in the flow in order to timely prevent the phenomenon of phase inversion.

A method of transporting a high-viscosity oil-water emulsion is known from the prior art, including the introduction of a demulsifier reagent into the oil, degassing and partial separation of the emulsion into oil and water, followed by pumping oil and water through one pipeline, carried out to prevent the formation of high-viscosity emulsions in the pipeline and reduce the coefficient of hydraulic resistance (USSR Author's Certificate No. 855335 A1, priority date 02/05/1979, publication date 08/15/1981, authors: Rosenzweig A.K. et al., RU).

The disadvantages of this known method are the complication of the technological scheme for transporting and pumping oil, which consists in implementing step-by-step separation before the oil-water mixture arrives at the treatment plants. In addition, the characteristics of the oil-water mixture in a dynamic state are not taken into account.

There is a known method for determining the characteristics of emulsions, including probing the dispersed medium under study with a beam of monochromatic light radiation, photographic recording of images of dispersed elements of the studied medium, analysis of these images and determination of the concentration and fractional composition of dispersed elements. Multiple photographic recording of images of colored drops makes it possible to determine the speed and trajectories of discrete drops in the studied environment from the displacement of image coordinates (USSR Author's Certificate No. 1578588 A1, priority date 08/06/1987, publication date 07/15/1990, authors: Martynov Yu.V. et al., RU).

However, this method does not provide complete information about the characteristics of the oil-water mixture in a dynamic state, such as viscosity and hydrodynamic regime of fluid flow. In addition, this method involves the introduction of a photochromic substance into the stream, which remains in it, which is unacceptable for the production of commercial oil.

As a prototype, a method for on-line quality control of oil and petroleum products is adopted, according to which, in a sample placed in a constant magnetic field, proton magnetic resonance spin-echo signals are excited by a series of radio frequency pulses, spin-echo amplitudes are recorded in the reference and measured samples, and then determined the concentration of water and oil, according to the corresponding mathematical expressions, in addition, the integral parameters of the dispersed distribution of water droplets from the times of spin-lattice relaxation of water are determined according to a certain formula (RF Patent No. 2519496 C1, priority date 12/24/2012, publication date 06/10/2014, authors : Kashaev R.S. et al., RU, prototype).

The disadvantage of the prototype is that this method does not determine the full range of characteristics of the oil-water mixture flow, namely, only the specified amount of water and oil, and dispersion parameters are determined.

The technical problem solved by the invention is to prevent the occurrence of the phenomenon of phase inversion of the oil-water emulsion in the flow in the pipeline during in-field pumping, which can be implemented through the development of a dynamic method for comprehensive monitoring of the characteristics of the oil-water mixture in the flow, the calculation algorithm of which is integrated into a complex control scheme with measuring installation and information processing device.

To solve a technical problem, a method is proposed for comprehensive monitoring of the characteristics of an oil-water mixture in a dynamic state, which consists in determining the characteristics of an oil-water mixture in a dynamic state, in which the onset of an inversion phenomenon is possible, and the method is characterized by the fact that it includes an algorithm for calculating the characteristics of an oil-water flow in a dynamic state, according to which is successively determined by the structure of the oil-water mixture, the type of oil-water mixture, its density, the viscosity of the mixture depending on the flow structure, the average flow rate, the hydrodynamic flow regime, characterized by the Reynolds criterion, the coefficient of hydraulic resistance depending on the value of the Reynolds criterion. Obtaining these values of the characteristics of the oil-water mixture is carried out according to the control scheme, as shown in Fig. 1, using a measuring installation on a well pad with a multiphase gamma meter flowmeter installed on the liquid line, and with a flow rotational viscometer at the inlet or outlet of the measuring installation, after which in the information processing device based on the received data and calculations made using the corresponding mathematical formulas determine the moment of onset of the phase inversion phenomenon, which is characterized by a certain range of water cut, viscosity and hydrodynamic regime in which the maximum jump in the hydraulic resistance coefficient is observed. Taking into account the data on the occurrence of this moment, the water cut of mixed flows of the oil-water mixture coming from production wells to the well pad is regulated by redirecting the flows for selective mixing, in which the value of the final water cut of the mixed flow will be outside the calculated water cut range, at which the inversion phenomenon occurs, which allows ensure stable operation of the pumping process.

The algorithm for calculating the characteristics of an oil-water mixture in a dynamic state is as follows.

In the fields, to calculate the characteristics of oil-water emulsions, the type and structure of the oil-water mixture in the flow is initially determined (Mishchenko I.T. Calculations for oil and gas production - M.: Publishing house "OIL and GAZ" Russian State University of Oil and Gas named after I.M. Gubkina. 2008. - 296 pp.).

Before determining the structure of the oil-water mixture and its type, it is necessary to calculate the following characteristics of the mixture:

– volume fraction of the dispersed phase in the emulsion:

, , (1)

where G _n – volumetric oil flow rate in m ³ /s;

G _in – volumetric flow rate of water in m ³ /;

Volumetric flow of water and oil:

, (2)

(3)

where b _n and b _in are, respectively, the volumetric coefficients of water and oil at given p and T.

For water take b _in =1, for oil b _n – calculated.

In our case, we take b _n from calculations of the technological regime of the oil reserve of the field.

1) There are two flow structures of the oil-water mixture: droplet and emulsion. Their area of existence is estimated by the critical speed of the mixture w _{cm cr} (in m/s):

(4)

Where D _G – hydraulic diameter of the channel, m, determined by the formula:

(5)

where P is the wetted perimeter of the channel cross-section, m;

F – cross-sectional area of the channel, ^m2 .

For a round pipe D _g - the hydraulic diameter is equal to the internal diameter of the pipe.

The wetted perimeter is that part of the perimeter of the live section along which the liquid comes into contact with the walls of the pipeline. For a circular pipe completely filled with liquid, the wetted perimeter is equal to the circumference:

, (6)

where D _in is the internal diameter of the pipe, m.

Formula for calculating the cross-sectional area of a pipe:

(7)

Where D _V – internal diameter of the pipe, m.

It is also necessary to know the reduced speed in m/s of the water-oil mixture in a given section of the channel:

(8)

where G _n – volumetric oil flow rate in m ³ /s;

G _in – volumetric flow rate of water in m ³ /s;

If , then the oil-water flow has a droplet structure: the internal dispersed phase in the form of droplets with a diameter of 0.5 - 2 cm is distributed in the external continuous phase.

If , then the water-oil flow has an emulsion structure: the dispersed internal phase is represented by spherical drops with a diameter of 10 ^-3 -10 ^-5 cm.

2) Determination of the type of oil-water mixture.

For a droplet flow structure, the type of mixture is determined by the volumetric water content:

– if β < 0.5, then the mixture will be of the water in oil (W/O) type – the dispersed internal phase is water, and the continuous external phase is oil;

– if β > 0.5, then the mixture will be of the oil-in-water (O/W) type – the discrete internal phase is oil, and the continuous external phase is water.

For an emulsion flow structure, the type of mixture is determined not only by the value of β, but also by the critical velocity of the emulsion w _{e cr} , calculated as follows:

, (9)

– if β < 0.5 and w _{cm cr} > w _screen - emulsions of the W/N type;

– if β < 0.5 and w _{cm cr} w _screen or β > 0.5 - N/V type emulsion.

3) Calculation of density and viscosity.

A) Drop structure:

The surface tension of oil at the interface with water is calculated

where σ _ng , σ _вг – surface tension at the “oil-gas” and “water-gas” boundaries, mN/m.

Surface tension σ _вг – is determined by the formula:

, (10)

where ρ is the current pressure (in the reservoir, in the pipeline, in the line), MPa.

, (eleven)

The true volume fractions of the phases in the flow are calculated. For a W/H mixture, the true volume fraction of water is:

, (12)

, (13)

where w _{in pr} – reduced water velocity, m/s;

ρ _in , ρ _n - respectively, the density of water and oil at given p and T, kg/m ³ .

True volume fraction of external phase (oil):

, (14)

where φ _in is the true volume fraction of water,

_φн – true volume fraction of oil.

For an N/W mixture, the true volume fraction of oil is:

, (15)

, (16)

where w _{n pr} – superficial oil velocity, m/s.

True volume fraction of external phase (water):

, (17)

The density of the oil-water mixture ρ _int is calculated:

, (18)

The dynamic viscosity of the oil-water mixture of the droplet structure is assumed to be equal to the dynamic viscosity of the external phase:

– for a mixture of B/N μ _in = μ _n ;

– for a mixture of H/V μ _in = μ _in ; (19)

Where μ _n , μ _in – respectively, the viscosity of oil and water at given p and T, mPa⋅s.

B) Emulsion structure.

The true volume fractions of the phases in the emulsion are calculated. Taking into account that emulsions are characterized by a high degree of phase dispersion, the relative movement between them is not taken into account, and the true volume fractions are taken equal to the volumetric consumption contents:

, (20)

, (21)

The density of the oil-water emulsion is determined:

, (22)

, (23)

Apparent emulsion viscosity:

Using the well-known formula [Medvedev V.F. Collection and preparation of unstable emulsions in the fields. Moscow. 1987. 144 p.], for the viscosity of VNE in [Tarasov M.Yu., Portnyagina S.S. / Technological calculation of oil and gas separation equipment using correlation dependencies of the viscosity of oil-water emulsions on oil density / Article in the electronic journal Prooneft. 2022. No. 2.] clarified it depending on the temperature:

, (24)

Also, determination of viscosity is possible using the formulas:

- emulsion type W/N:

, (25)

- type of emulsion N/V:

, (26)

- for emulsion water content in the range from 50-70%:

(27)

4) The basis for calculating the process of the onset of phase inversion in an oil-water flow is taken from the calculation of pipelines according to A.I. Guzhev. and Medvedev V.F. [Medvedev V.F. Collection and preparation of unstable emulsions in the fields. Moscow. 1987. 144 p.]:

The average emulsion flow rate is calculated:

, (28)

The Reynolds criterion is calculated:

, (29)

The coefficient of hydrodynamic resistance is calculated, depending on the value of the Reynolds criterion:

- Reynolds criterion from 0 to 2000 laminar fluid flow:

(thirty)

- Reynolds criterion from 2000 to 4000 transient fluid flow:

, (31)

- Reynolds criterion from 4000 to 10,000 (or more) turbulent fluid flow:

, (32)

After calculating the characteristics of the oil-water mixture flowing from the wells into the reservoir, it is determined in what range of viscosity and water cut the phase inversion occurs. This is evidenced by the maximum value of the hydraulic resistance coefficient, after which a sharp drop in viscosity is observed. It can be seen by graphically analyzing the dependencies, as shown in Fig. 2.

Taking this fact into account, the water cut of mixed flows of the oil-water mixture coming from production wells to the well pad is regulated by redirecting the flows for selective mixing, in which the value of the final water cut of the mixed flow will be outside the calculated water cut range, at which the inversion phenomenon occurs, which allows for stable production technological pumping mode and avoid pipeline vibrations, gusts and cavitation phenomena.

Also, differences in water cut when mixing flows can be adjusted by the speed and operating parameters of the pumping equipment.

The claimed method for comprehensive control of the characteristics of an oil-water mixture in a dynamic state allows taking into account all the flow characteristics necessary to prevent the phenomenon of phase inversion, thereby allowing the technological process of in-field pumping to be carried out stably, without pressure drops in the pipeline and the formation of multiple emulsions in the turbulent core of the flow, which complicate technological processes of oil preparation and storage in the future.

Example. The characteristics of the produced oil were taken from an oil and gas field in central Siberia.

The constructive system of the measuring installation is a set of instruments for measuring the mass of liquid and gas, density, temperature, pressure and volumetric water content in the oil-water mixture.

An in-line rotational viscometer is installed at the entrance to the measuring installation to determine the viscosity of the oil-water flow, as well as the viscosity of water and oil.

The mixed water-oil flow of well production is supplied to a measuring installation at the well pad, where it is separated into gas and liquid using a separator.

A multiphase gamma flow meter is installed on the liquid line in the measuring installation, which, using gamma radiation, provides data on the phase state of flows in the emulsion, and a Coriolis flow meter is installed in the gas line.

Next, after separation in the separator, the flows are sent to the liquid and gas lines. Quantitative characteristics are measured in each line: mass of liquid and gas, volume fraction of water (using flow meters, moisture meters).

The measurement results are transmitted to the information processing device.

In the information processing device the following are calculated:

- liquid and gas consumption in kg/s;

- gas volume m ³ /day;

- density of oil and water kg/ ^m3 ;

- volume fraction of phases in the emulsion t/h;

- emulsion density kg/ ^m3 ;

- coefficient of hydraulic resistance;

- critical range of water cut and flow viscosity values for the onset of the phase inversion phenomenon.

Next, according to the algorithm for calculating the characteristics of the water-oil mixture, the ranges of water cut of the flow and viscosity at which phase inversion occurs are determined.

Calculation of the pad showed that the structure of the oil-water mixture is emulsion, the dispersed phase is oil - the average volume fraction of water is βw = 0.36, the average volume fraction of oil is βn = 0.64, the type of oil-water mixture is “water in oil.”

According to figure 3, when calculating the coefficient of hydraulic resistance and flow viscosity, it was found that the dependence of these quantities is high, since the approximation reliability coefficient is equal to R2=0.70, the correlation coefficient is equal to A=0.99.

Also, correlation analysis showed that the dependence of the hydraulic resistance coefficient on the viscosity of the emulsion is higher than the dependence on the average flow velocity of the emulsion, since the approximation reliability coefficient is equal to R2 = 0.49, the correlation coefficient is equal to A = 0.70.

The calculations performed show Fig. 4 that the degree of correspondence of the selected trend model to the original data is high and the theoretical distribution describes the real distribution of values.

This confirms the existence of a dependence of the hydraulic resistance coefficient on the viscosity and water cut of the oil-water flow.

Therefore, further calculations were carried out to determine the dependences of the distribution of the hydraulic resistance coefficient on changes in the volume fraction of water and flow viscosity. The results are presented in Fig. 5.

A graphical analysis of the dependence of the hydraulic resistance coefficient on changes in the volume fraction of water was also carried out.

According to the graph, in Fig. 5 in the range of the volume fraction of water from 0.63-0.75 there is a jump in the value of viscosity and coefficient of hydraulic resistance, which confirms the presence of the phenomenon of phase inversion in the water-oil flow.

The production of production wells under wellhead pressure (Rcalc - 6.3 MPa, Pmin - 0.5 MPa, Prab - 4.0 MPa) enters the oil-gathering reservoir into the PT system.

According to the graphical dependence of the pressure drop in the pipeline on the viscosity of the flow (Fig. 6), the pressure drop in the pipeline section from well pad 216 to the field pipeline averages 0.4 MPa, however, at the moment the viscosity increases to 12.09 cP (phase inversion point ) a pressure drop of 1.2 MPa is observed.

As a result of calculating the characteristics of the flow of an oil-water mixture at a well pad from an oil and gas field according to the developed method, it was established that the flow has an emulsion structure, with a viscosity value of 12.09 cP and a water cut of the mixed flow of 73%, the presence of a phase inversion phenomenon is confirmed, as a result of which a pressure drop is observed in the pipeline 1.2 MPa (Fig. 6).

This information is displayed to shift personnel on the control panel. Next, the operating personnel makes decisions on regulating the flow using valves on the pipeline at the well pad, or regulating the operation of pumping equipment.

In operating oil fields, regulation of flow mixing can be achieved by constructing an additional pipeline connecting flows coming from wells to selectively mix them with each other.

At newly commissioned oil fields, it is necessary to build a pipeline to well pads in such a way that all wells are connected to each other and it is possible to mix flows taking into account their water content and regulate the direction of flows using electric valves.

The technical result achieved by the invention is to prevent the phenomenon of phase inversion and ensure stable operation of the technological process of in-field pumping.

Claims (1)

A method for complex control of the characteristics of an oil-water mixture in a dynamic state, which consists in determining the characteristics of an oil-water mixture in a dynamic state, in which the onset of an inversion phenomenon is possible, and the method is characterized by the fact that it includes an algorithm for calculating the characteristics of an oil-water flow in a dynamic state, which is used to consistently determine the structure of the oil-water mixture mixture, type of oil-water mixture, its density, viscosity of the mixture depending on the flow structure, average flow rate, hydrodynamic flow regime characterized by the Reynolds criterion, coefficient of hydraulic resistance depending on the value of the Reynolds criterion, obtaining these values of the characteristics of the oil-water mixture is carried out according to the scheme control, using a measuring installation at the well pad with a multiphase gamma meter flow meter installed on the liquid line, and with an in-line rotational viscometer at the inlet or outlet of the measuring installation, after which in the information processing device based on the received data and calculations made using the corresponding mathematical formulas determine the moment of onset of the phase inversion phenomenon, which is characterized by a certain range of water cut, viscosity and hydrodynamic regime in which the maximum jump in the hydraulic resistance coefficient is observed, taking into account the data on the onset of this moment, the water cut of mixed flows of the oil-water mixture going from production wells to the well pad is regulated, by redirecting flows for selective mixing, in which the value of the final water cut of the mixed flow will be outside the calculated water cut range, at which the inversion phenomenon occurs, which allows for stable maintenance of the pumping process mode.