In an oil installation where oil is extracted from one or more wells in an oil field, oil will usually be extracted together with water and gas. The water has to be removed from the oil and this is mainly done by means of settling tanks (separators) in which the water is permitted to settle under the action of gravity.

However, complex oil-water emulsions may develop during the production of the oil. For example, the removal of gas from the oil-water emulsion by means of gas-liquid cyclones might contribute to a more complex emulsion that will be difficult to separate only by means of settling.

It is common practice to use so-called electrostatic coalescers for the purpose of destabilizing water-in-oil emulsions, i. e. to achieve a water droplet enlargement or coalescence of water in the oil. An electrostatic coalescer can be employed to speed up the separation of any dispersion where the continuous phase is an electrical insulator and the dispersed phase is an electrical conductor. In an electrostatic coalescer, the dispersion is subjected to an alternating current field or to a continuous or pulsed direct current field. Electrostatic coalescers are for instance disclosed in WO 01/85297 A1 and US 6,136, 174 A.

It has been shown that the use of a coalescer under suitable operating conditions can speed up the separation of an emulsion, such as a water-in-oil emulsion, thereby making it possible to reduce the required size of the settling tank or separator employed in the separating operation. There are large

gains, e. g. in operating capacity, space requirement and cost reduction, that can be made by optimizing the efficiency of a coalescer employed in a separation plant. However, the effects of electrostatic coalescing on an emulsion and its separating process are rather complex and difficult to predict in each specific case.

In a publication by O. Urdahl et al, entitled"Electrostatic destabilisation of water-in-oil emulsions under conditions of turbulent flow", Trans IchemE, vol. 74, Part A, March 1996, p.

158-165, there is disclosed a testing apparatus and a method for studying the electrostatic destabilization of emulsions under conditions of turbulent flow. The testing apparatus disclosed in said publication comprised a coalescing device and a power supply adapted to apply an alternating current of adjustable voltage to the electrodes of the coalescing device. Said coalescing device comprised an electrocoalescer duct comprising seven modules arranged in series, each module being provided with a pair of electrodes. The testing apparatus was designed to drive the emulsion sample through the coalescing device at a constant flow rate and at a constant temperature, while subjecting different emulsion samples to different field strengths by varying the applied voltage level. The residence time, i. e. the time spent in the electric field of the coalescing device by the emulsion samples, was adjusted by varying the number of electrode pairs energized for each sample. The effect of the electric field on the emulsion samples was determined by measuring the change in droplet size distribution from one end to the other of the electrocoalescer duct. This testing apparatus does not directly indicate how the separation process for a specific emulsion is affected by the electrostatic coalescing. Furthermore, the design of the testing apparatus only allows a limited number of parameters affecting the coalescing process to be varied. Consequently, the disclosed testing apparatus has a very limited use as a tool for

assisting in the evaluation and optimization of a coalescer and a separation plant.

SUMMARY OF THE INVENTION The object of the present invention is to provide an improved testing apparatus that is suitable for use in determining the effects of electrostatic coalescing on a dispersion, particularly in the form of an emulsion, and that is suitable as a tool for determining the effects of electrostatic coalescing of a dispersion on the settling behaviour of the dispersion.

According to the invention, this object is achieved by a testing apparatus having the features of claim 1. By allowing the dispersion, after it has been subjected to the electric field of the coalescing device, to settle in a settling receptacle and registering the settling process, the effects of electrostatic coalescing on a specific dispersion under different conditions can be determined in a very simple and reliable manner without requiring any complex measuring devices.

According to a preferred embodiment of the invention, the electrocoalescer duct of the coalescing device is helically wound so as to form a coil, the coalescing device further comprising an inner field element arranged inside the coil and an outer field element arranged outside the coil, at least one of said field elements being connected to the power supply so as to allow an electric field to be generated between the field elements. When the coalescing device is adapted to generate an A. C. field, the inner and outer field elements are both connected to the power supply. A coalescer duct of a given length will occupy a considerably smaller space when arranged in coil-shape as compared to a straight-line arrangement. Consequently, the coil- shaped design of the coalescer duct makes it possible to achieve a very compact coalescing device, and thereby to reduce the overall size of the testing apparatus. The coil-shaped

design of the coalescer duct will make it possible to design a compact and efficient portable testing apparatus.

According to a further preferred embodiment of the invention, the testing apparatus is provided with means for adjusting the flow rate of the dispersion flowing through the coalescing device. In this way, it will be possible to vary the residence time, thereby making it possible to use the testing apparatus as a tool in the dimensioning of the electrocoalescer duct of a full-scale coalescer, since the residence time depends inter alia on the electrocoalescer duct volume. By adjusting the flow rate, it will also be possible to vary flow conditions, such as the Reynolds number. It is for instance possible to accomplish a turbulent flow or a laminar flow in the coalescing device of the testing apparatus by a suitable adjustment of the flow rate. The flow rate of the testing apparatus can for instance be adjusted so as to simulate the real flow conditions at a separation plant.

According to a further preferred embodiment of the invention, the means for driving the dispersion through the coalescing device comprises a source of pressurized medium, preferably gas, wherein the flow rate adjustment means comprises a regulating member adapted to regulate the pressure of the medium supplied from the source. In this way, the flow rate can be adjusted in a very simple and reliable manner.

According to a further preferred embodiment of the invention, the testing apparatus is provided with an arrangement for controlling the temperature of the dispersion, the temperature controlling arrangement being adjustable so as to allow the temperature of the dispersion to be adjusted. In this way, it will be possible to determine the influence of the temperature on the coalescing and separation of a specific dispersion and to adjust the testing temperature so as to, for instance, simulate the real temperature conditions at a separation plant.

The temperature controlling arrangement suitably comprises means for circulating a heating medium through the testing apparatus and means for controlling the temperature of said medium. In this way, the temperature of the testing apparatus can be controlled and adjusted in a simple and reliable manner.

According to a further preferred embodiment of the invention, the temperature controlling arrangement included in the testing apparatus comprises a container filled with heating medium, the settling receptacle being arranged in said container so as to allow the temperature of dispersion received in the receptacle to be controlled by the heating medium in the container. In this way, the temperature of the dispersion received in the settling receptacle can be controlled in a simple and reliable manner.

According to a further preferred embodiment of the invention, the temperature controlling arrangement comprises means for circulating the heating medium through the coalescing device so as to allow the temperature of dispersion flowing through the coalescing device to be controlled by the heating medium. In this way, the temperature of the dispersion flowing through the coalescing device can be controlled in a simple and reliable manner.

According to a further preferred embodiment of the invention, the testing apparatus is adapted to apply an alternating current of adjustable frequency to the coalescing device. In this way, it will be possible to determine the influence of the A. C. frequency on the coalescing and separation of a specific dispersion, thereby making it possible to use the testing apparatus as a tool in the designing of a suitable power supply for a coalescer.

The testing apparatus according to the invention is preferably designed as a portable apparatus, so that it can be moved to any desired location. The portable testing apparatus can for instance be brought to an oil extraction or production installation, where water is removed from the extracted oil by

means of a separation plant comprising a coalescer, in order to diagnose or optimize said installed coalescer by performing tests on real water-in-oil emulsions and under real operating conditions.

The invention also relates to a method for studying the effects of electrostatic coalescing on a dispersion having the features of claim 16.

Further advantages as well as advantageous features of the in- vention will appear from the following description and the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, a specific description of preferred embodiments of the invention cited as examples follows below.

In the drawings: Fig 1 is a very simplified illustration of a testing apparatus according to the invention, Fig 2 is a schematic illustration of a testing apparatus according a preferred embodiment of the present invention, Fig 3 is a schematic illustration of a testing apparatus according another preferred embodiment of the present invention, Fig 4 is a flow chart illustrating a test procedure for determining a suitable electrical field strength to be applied to a coalescer, and

Fig 5 is a diagram comprising settling curves illustrating a possible result of a test procedure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION In Fig 1 a testing apparatus 1 according to the invention is illustrated in a very simplified manner. The inventive testing apparatus comprises an electrostatic coalescing device, schematically indicated at 2, through which a dispersion sample comprising at least two immiscible phases is to flow in order to subject the dispersion sample to an electric field generated between at least one pair of electrodes. The electrodes of the coalescing device 2 are not shown in Fig 1. A power supply, schematically indicated at 3, is arranged for applying a current of adjustable voltage to the coalescing device 2. If a D. C. electric field is used for the coalescing of the dispersion sample, one electrode of each electrode pair is connected to the power supply 3, whereas the associated electrode is grounded. If an A. C. electric field is used for the coalescing of the dispersion sample, both electrodes of an electrode pair are connected to the power supply 3. In the latter case, the power supply 3 is preferably adapted to apply an alternating current of adjustable frequency as well as adjustable voltage to the coalescing device.

The coalescing device 2 is preferably vertically arranged or inclined with an inlet at an upper part of the coalescing device and an outlet at a lower part thereof, which implies that a dispersion sample is to flow downwards through the coalescing device.

The testing apparatus 1 further comprises means for introducing a dispersion sample in the coalescing device. In the testing apparatus illustrated in Fig 1, said means comprises a container 4 for keeping a dispersion sample before it is introduced in the

coalescing device 2. The container 4 preferably comprises a mixing chamber provided with a mixing device for achieving a desired mixing of the dispersion sample that is to be introduced in the coalescing device. Such a mixing chamber will be more closely described hereinafter with reference to Figs 2 and 3. The container 4 is connected to the upstream end of the coalescing device 2 via suitable connection means, schematically indicated at 5, comprising e. g. a conduit and a drain valve.

The testing apparatus 1 further comprises a receptacle 7, such as a settling bottle or the like, which is adapted to receive dispersion that has passed through the coalescing device 2. The receptacle 7 is designed to allow the received dispersion to settle and to allow the settling process to be registered. The receptacle 7 is connected to the downstream end of the coalescing device 2 via suitable connection means, schematically indicated at 8, comprising e. g. a conduit and a drain valve.

The dispersion that is to be tested is driven out of the container 4, into the coalescing device 2 and through the coalescing device 2 by suitable means, schematically indicated at 6, comprising e. g. a source of pressurized medium, preferably pressurized gas, connected to the container 4 via a suitable conduit. Said means 6 may also comprise a conduit for feeding pressurized medium from an external source to the container 4.

According to a further alternative, said means 6 comprises a mechanical driving member, such as a piston or the like, displaceably arranged in the container 4 for forcing a dispersion sample received in the container 4 out of the container 4 and into the coalescing device 2. Such a mechanical driving member is preferably used together with the driving pressure from pressurized gas.

The testing apparatus 1 is preferably provided with means, schematically indicated at 9 in Fig 1, for adjusting the flow rate

of the dispersion flowing through the coalescing device 2. When the means 6 for driving the dispersion through the coalescing device comprises a source of pressurized medium or a conduit for supplying pressurized medium to the coalescing device 2 from an external source, the flow rate adjustment means preferably comprise a regulating member adapted to regulate the pressure of the medium supplied from the source.

The testing apparatus 1 is preferably also provided with an arrangement, not shown in Fig 1, for controlling the temperature of the dispersion samples so as to secure that the temperature of a dispersion sample can be kept at a pre-set level during testing. The temperature controlling arrangement is preferably adjustable so as to allow the temperature of the dispersion to be adjusted. An example of a suitable temperature controlling arrangement will be described hereinafter with reference to Figs 2 and 3.

Preferred embodiments of the inventive testing apparatus will be described hereinafter with reference to Figs 2 and 3. In Figs 1-3 corresponding details have been given the same reference signs.

The testing apparatuses illustrated in Figs 2 and 3 comprise a mixing chamber 4 into which a dispersion sample or the liquids that are to be mixed so as to form a dispersion sample, e. g. in the form of an emulsion, are introduceable via an opening provided with a removable lid 10 or other closure. In the embodiment illustrated in Fig 2, the mixing chamber 4 is provided with a mixing device comprising a rotary mixing member 11 arranged inside the mixing chamber. In the embodiment illustrated in Fig 3, the mixing device comprises a mixing pump 20 connected to the mixing chamber 4.

The mixing chamber 4 is connected to the upstream end of coalescing device 2 via a conduit 12 provided with a drain valve

13. The mixing chamber 4 is also connected to a source of pressurized medium via a conduit 15. In the embodiment illustrated in Fig 2, the mixing chamber 4 is connected to a pressure vessel 14 containing a pressurized drive gas, preferably pressurized nitrogen gas. In the embodiment illustrated in Fig 3, the mixing chamber 4 is intended to be connected to an external source of pressurized medium. The pressure of the medium supplied to the mixing chamber 4, and thereby the flow rate of the dispersion, is controlled by a regulating member 16. The downstream end of the coalescing device 2 is connected to a settling receptacle 7 via a conduit 17 provided with a drain valve 18. In the embodiments illustrated in Figs 2 and 3, said receptacle 7 is a settling bottle 7. This settling bottle 7 is here cone-shaped, but may of course also have any other suitable shape. The receptacle 7 may also be a conventional PVT cell (Pressure-Volume-Temperature cell).

The testing apparatuses 1 illustrated in Figs 2 and 3 are provided with a temperature controlling arrangement 21 comprises means 22 for circulating a heating medium through the testing apparatus and means 23,24 for controlling the temperature of said medium. The heating medium is here circulated by means of one or several heating medium pumps 22. The temperature controlling arrangement 21 preferably comprises a container 25 filled with heating medium, the settling receptacle 7 being arranged in said container 25 so as to allow the temperature of dispersion received in the settling receptacle 7 to be controlled by the heating medium in the container 25.

The temperature of the heating medium is preferably controlled by a heating member 24 arranged in the container 25, which is adapted to generate heating energy for heating the heating medium. The temperature of the heating member, and thereby the temperature of the heating medium, is adjusted by means of a temperature controller 23. The heating member 24 preferably comprises one or several electrically energized heating coils. In the embodiments illustrated in Figs 2 and 3 the temperature

controlling arrangement comprises means for circulating the heating medium via the mixing chamber 4, for instance via channels 31 arranged in the walls of the mixing chamber 4, so as to allow the temperature of dispersion contained in the mixing chamber 4 to be controlled by the heating medium, and also via the coalescing device 2 so as to allow the temperature of dispersion flowing through the coalescing device to be controlled by the heating medium. Said circulating means comprise suitable conduits 26-28 and valves 29,30. In the illustrated embodiments, the heating medium is circulated by the pump 22 from the container 25 to the mixing chamber 4, from the mixing chamber 4 to the coalescing device 2 and from the coalescing device 2 back to the container 25. The heating medium can of course also be arranged to circulate through the testing apparatus in any other suitable manner.

The testing apparatus according to the invention can also be provided with any other suitable type of arrangement for controlling the temperature of the dispersion samples, e. g. an arrangement for direct electrical heating of said samples in addition to or as an alternative to the temperature controlling arrangement described above. Another way of controlling the temperature of the dispersion samples is the placement of the coalescing device 2, the mixing chamber 4 and the receptacle 7 in an industrial oven.

A particularly preferred embodiment of a coalescing device 2 for use in a testing apparatus according to the invention is illustrated in Figs 2 and 3. This coalescing device 2 comprises a duct 40 in which the dispersion is to flow, said duct 40 being helically wound so as to form a coil, the coalescing device further comprising an inner field element 41 arranged inside the coil and an outer field element 42 arranged outside the coil.

Said inner and outer field elements constitute the electrodes of the coalescing device. Consequently, at least one of said field elements 41,42 is connected to the power supply 3 so as to

allow an electric field to be generated between the field elements. In the embodiments illustrated in Figs 2 and 3, the power supply 3 is a high voltage A. C. power source connected to both of the field elements 41,42 and capable of applying an alternating current of adjustable voltage and frequency to said field elements. The coalescing device is preferably provided with a heating medium tube 43 arranged inside the coil for allowing heating medium to be circulated through the coalescing device 2. The inner field element is suitably arranged to surround the heating medium tube 43, as illustrated in Figs 2 and 3. For safety reasons, the heating medium tube 43 and the coil 40 are preferably connected to ground, as illustrated in Figs 2 and 3.

The inner field element 41 is suitably cast within a stable dielectric substance, and the outer field element 42, which provides the system shielding, is suitably mounted within an insulating thermal protection tube. Grid elements to induce turbulence can be arranged in the duct 40, if so desired.

If so desired, the duct 40 of the coalescing device 2 could also be arranged in a straight-lined manner instead of being helically wound. Also other arrangements of the coalescer duct are feasible.

The power supply 3 is preferably adapted to deliver frequencies ranging from D. C. to 10 kHz, preferably from 50 Hz to 1 kHz, and to achieve field strengths up to 4 kV/cm.

The testing apparatus according to the invention is preferably designed as a portable apparatus.

In order to study the effects of electrostatic coalescing on a dispersion comprising at least two immiscible phases, such as water and oil, a sample of the dispersion is introduced in the electrostatic coalescing device 2 of the testing apparatus 1 under pre-set testing conditions in terms of temperature, electrical field strength, flow geometry and residence time. The

term flow geometry here refers to the dimensions'of the coalescer duct 40. After having passed through the coalescing device, i. e. through the coalescing duct 40, the dispersion is made to flow into the settling receptacle 7. The dispersion is then allowed to settle in the settling receptacle 7, and the settling process of the dispersion in the settling receptacle 7 is registered in order to determine the effects of the electrostatic coalescing of the dispersion under said pre-set testing conditions on the settling process of the dispersion. In order to determine the effect of a specific operating condition, several test cycles are performed on similar dispersion samples, while varying the operating condition in question between every test cycle. The operating conditions that can be varied are e. g. the temperature, the electrical field strength, the frequency of the electrical power, the flow geometry and the residence time. The flow geometry can for instance be varied by connecting a varying number of coalescing ducts 40 in series. The residence time is preferably controlled by adjusting the flow rate of the dispersion flowing through the coalescing device 2, e. g. by adjustment of the pressure applied to the dispersion sample in the mixing chamber 4. The flow rate can also be adjusted so as to secure that the dispersion is made to flow through the coalescing device under turbulent or laminar flow conditions.

The term test cycle here refers to an individual test cycle comprising the steps of introducing a dispersion sample in the coalescing device, passing the dispersion sample through the coalescing device and into the settling receptacle, and allowing the dispersion sample to settle in the settling receptacle.

It is of course also possible to alter the characteristics of the dispersion samples so as to determine how different dispersions react under pre-set operating conditions. The dispersion sample can be pre-mixed before being introduced in the mixing chamber 4, without any further mixing being carried out before the dispersion sample is introduced in the coalescing device 2. It is

also possible to subject the dispersion sample to a desired mixing action in the mixing chamber 4 so as to achieve desired characteristics of the dispersion sample, e. g. in terms of droplet size distribution. The two phases included in the desired dispersion sample can also be introduced in unmixed condition into the mixing chamber 4, and thereafter be subjected to a desired mixing action therein so as to form the desired dispersion sample.

If so desired, the testing apparatus 1 according to the invention may be provided with conventional measuring devices for determining the droplet size distribution of the dispersion sample when entering the coalescing device 2 and/or when leaving the coalescing device 2.

The testing apparatus illustrated in Fig 3 is suitable for use at the location of an oil production installation, where water is separated from the oil by means of a separation plant comprising a separator and a coalescer, in order to diagnose or optimize said installed coalescer by performing tests on real water-in-oil emulsions and under real operating conditions. In this case, the mixing chamber 4 is preferably connected to the oil line from a separator via the conduit 15 so as to supply a real oil from the separator to the mixing chamber 4 under the pressure in the separator oil line. In the embodiment illustrated in Fig 3, the mixing chamber 4 is also connected to a water source (for example on an oil production facility either real or synthetic formation water),, via the conduit 45, thereby allowing the oil to be mixed with a desired content of water. The pump loop 44 ensures a uniform water/oil content in the water-in-oil emulsion introduced in the coalescing device 2.

A possible test procedure for determining a suitable electrical field strength to be applied to a coalescer is schematically illustrated by the flow chart in Fig 4. In a first step 50 the testing apparatus 1 is started up and checked. In a second step 51 the

testing conditions are set as desired by a proper adjustment of temperature, drive pressure, voltage level, current frequency etc. In a third step 52 the dispersion sample is introduced into the mixing chamber 4 and mixed to a desired degree. In a fourth step 53 the dispersion sample is driven through the coalescing device 2 and down into the settling receptacle 7. In a fifth step 54 the dispersion sample is allowed to settle in the settling receptacle 7 and the settling process is observed and recorded.

The settling process is preferably recorded as a function of time by means of so called settling curves (see Fig 5). In case of a water-in-oil emulsion, the settling process is suitably recorded by recording the free-water level, the free-oil level and the dispersion band thickness, i. e. the thickness of the region between free water and free oil, at fixed time intervals. In a sixth step 55 it is determined whether or not the settling process resulted in a sufficient separation of the phases of the dispersion sample. If this is not the case the voltage level is set to a higher value in a seventh step 56 and a new test cycle is carried out, preferably restarting with the above-indicated third step 52. These procedures are repeated until a test cycle resulting in a desired degree of separation is carried out. In this way, the lowest field strength causing the desired degree of separation on the given dispersion will be established. By simple calculations, the result of the performed test can then be used to establish a suitable voltage level to be applied to an installed coalescer. Other operating conditions can be evaluated in the corresponding manner.

A dispersion sample to be tested is advantageously pre-heated in a suitable oven so as to attain the desired testing temperature before being introduced into the mixing chamber 4.

The degree of liquid phase separation observed in the settling receptacle 7 is a measure of the efficiency of the coalescing on the settling process of the dispersion sample.

Fig 5 shows a diagram comprising settling curves illustrating a possible result of a test procedure carried out on a water-in-oil emulsion in accordance with the flow chart illustrated in Fig 4.

The lower curves 1a, 2a, 3a indicate the free water level in the settling receptacle as a function of time for three different test cycles. The upper curves 1b, 2b, 3b indicate the free oil level in the settling receptacle as a function of time for the same three test cycles. Curves associated to the same test cycle are indicated with the same type of lines. A lack of convergence between two curves associated with the same test cycle indicates an unsuitable combination of testing conditions. The lines 1a, 1b, 2a, 2b relate to test cycles where the field strength was not high enough to achieve a coalescing of the emulsion sample resulting in a complete separation. The lines 3a, 3b, on the contrary, relate to a test cycle where the field strength was high enough to achieve a coalescing of the emulsion sample resulting in a complete separation.

The testing apparatus according to the invention can be used as a tool for experimental determination of design parameters or suitable operating conditions for a full-scale coalescer. The testing apparatus and the testing method according to the invention can for instance be used for determining a suitable field strength and/or temperature and/or residence time to be applied to a given dispersion flowing through an electrostatic coalescer, and/or suitable dimensions of an electrostatic coalescer adapted for treatment of a given dispersion, and/or a suitable frequency of the current applied to an electrostatic coalescer adapted for treatment of a given dispersion. The testing apparatus and the testing method according to the invention can with advantage be used for finding optimal combinations of the above-indicated parameters. Many other possible uses of the inventive testing apparatus will be apparent to a man with ordinary skill in the art.

The testing apparatus and the method according to the invention can be used to treat any dispersion comprising an electric insulator as continuous phase and an electric conductor as dispersed phase.

The invention is of course not in any way restricted to the pre- ferred embodiments described above, but many possibilities to modifications thereof will be apparent to a man with ordinary skill in the art without departing from the basic idea of the in- vention such as defined in the appended claims.

For example, the pitch of the helically wound duct 40 of the coalescing device 2, as shown by way of example in Figs 2 and 3, in which the dispersion is to flow, can vary e. g. depending on whether the coalescer device 2 is vertically arranged or inclined.

Other geometrical shapes of the extension of the duct 40, such as in a zig-zag pattern, or the shape of the duct itself as well as different geometrical shapes of the field elements 41,42 are also within the inventive concept.