NO830543L - OIL / GAS SEPARATOR. - Google Patents

Description

The present invention relates to the separation of gas from

oil in a mixture thereof, in particular a method and a device for separating gas from oil at high pressure, for example such high pressures as exist in the wellhead in an oil well.

It is known that the gas content of the oil that comes from

an oil well limits the availability and usability of the oil in the well, and it is common practice to burn excess gas from an oil well in a so-called flare. This represents a loss of potentially usable energy. Gas-

one in the oil, however, constitutes a significant disadvantage from the point of view of the handling of the oil, because the gas produces irregular flow and pressure fluctuations that break the smooth functioning of the oil handling equipment. The primary resource in an oil well is naturally the oil that can be extracted from the well, and the most important factor in an oil well is therefore the achievement of a continuous oil flow. The present invention aims to provide a device for separating gas from oil in a mixture thereof at high pressure, in which device a continuous flow of oil can be achieved despite irregular flow of the mixture and pressure fluctuations of the type that exist in the wellhead, the device also offers the possibility of adapting the pressure drop to set requirements.

The present invention also aims to provide

a device for separating gas from oil, which device utilizes the energy in the outgoing mixture from the wellhead to operate the separation process.

According to one aspect of the present invention includes

a separator device for separating gas from oil a mixture thereof under high pressure, such as at a wellhead in an oil well, a guide channel for guiding a stream of the mixture under high pressure along a flow path where

part of the surface area is formed by a gas-permeable surface, the flow path being shaped in such a way that expansion of the gas content of the mixture drives this through the gas-permeable surface, where it is collected in a gas collection channel, which guide channel controls the oil fraction to an oil delivery channel.

In the separator device according to the present invention, the pressure in the gas is therefore used to drive the gas through the gas-permeable wall, and in one embodiment this is supported by increasing the potential contact area between the gas and the gas-permeable wall to the greatest possible extent by leading the mixture in a flow path which has a very flattened flow cross-section. The separation effect is also supported

of a venturi effect if the flattened part of the guide channel in practice has a smaller cross-sectional area than the channel that leads the gas/oil mixture to the separator device, and also smaller than the cross-sectional area of the oil delivery and gas collection channels that each lead the separated fractions away from the separator device.

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The separation effect of the device according to the present invention is increased if several such guide channels are provided which are formed by occupying a tightly fitted central core in a mainly cylindrical gas permeable sleeve in which several shallow, mainly axially running channels are formed. These shallow channels form the sections with a flattened cross-section, and an increase in the separation effect can be achieved if the shallow channels, instead of running purely axially, have a helical shape along the central core, as this causes a certain swirling of the gas and oil mixture when it passes through the device, which swirl supports the collection of gas at the radially outer part of the separator, while the oil tends to remain in a radially inner position relative to the gas.

It is assumed that during the controlled flow through a line, such as the line leading to the separator, the gas will tend to collect centrally in the line, surrounded by the flowing oil which nevertheless also contains gas bubbles and is therefore in a frothy state or a frothy state. The effect of the separator according to the invention is thus to cause the gas to react differently than the oil to the forces to which they are both exposed, after which the gas is allowed to pass through the gas-permeable wall, while the oil is kept inside the separator, at a distance from the gas-permeable wall. Part of the oil will, however, pass through the gas-permeable wall, especially if this, as in the preferred embodiment, consists of a perforated sleeve around a central core, and this entrapped oil can be separated under the influence of gravity at the relatively lower applied pressure by placing the separator with the flow axis running in the main case vertically.

According to another aspect, the present invention includes a separator device for separating gas from oil in a mixture thereof under high pressure, such as at a wellhead in an oil well, which separator device includes means for causing the mixture to follow a substantially helical flow path which partially is formed by a gas-permeable wall over which a separate pressure differential is maintained that the gas in the mixture is caused to pass through the wall and into a gas-collecting gallery or channel, while the oil is led by guiding means to an oil delivery channel. This helical flow or "swirling" of the mixture conveniently takes place inside the aforementioned, essentially cylindrical sleeve which forms the gas-permeable wall and which forms the radially outer surface of the essentially helical flow path for the mixture. In a preferred embodiment of the invention, the gas-permeable wall is provided with screw-line shaped rows of perforations that align with internal channels in such a way that the perforations only extend over part of the axial extent of the channels and leave a dense wall portion for retention of oil. These wall parts are preferably arranged with an axial distance. It is preferred that the elongate core has several helical channels which may be formed by projecting helical ridges on the core or may be formed by helical grooves in the core. The worm core is provided with ridges, so these are preferably close to the inner side of the gas-permeable wall, so that each single helical channel constitutes a completely separate helical flow path for the mixture.

Oil entrained in the gas passing through the aforementioned gas-permeable wall can be collected at the lower end of the separator device if this is arranged upright, with the flow path for the mixture extending upwards through the device, provided that the flow cross-section in the gas gallery is sufficiently large in relation to to the flow cross-section in the said helical flow path to provide a significant velocity reduction of the gas, so that entrapped oil particles are not carried along with the gas, but fall down towards the lower end of the gas gallery, where an oil collection chamber is placed.

The gas-permeable wall may have any suitable design, and may be of a porous material with pores of any suitable size. In order to avoid the danger of clogging, however, it is preferred that the gas-permeable wall is a perforated wall with a perforation pattern through which oil particles can also pass if they are entrapped in the gas then, with the gas gallery arrangement described above, a small amount of oil entrapped in the gas can separated afterwards, and this is preferred over the alternative which will require periodic cleaning, either by back-flushing or dismantling, when the wall is impermeable to oil.

In a preferred embodiment of the invention, the resistance to a gas flow through the aforementioned gas-permeable surface can be regulated between certain minimum and maximum values.

This can be achieved with an arrangement where the gas-permeable wall is a perforated wall or sleeve, whereby means are arranged for varying the effective opening value for the openings in the perforated wall or sleeve. Preferably, the opening value is varied by arranging a secondary perforated wall or sleeve closely adjacent to the first and displaceable relative to it, so that at least some of the openings in the secondary wall or sleeve can be moved flush with or out of flush with the openings in the said wall or sleeve.

If the secondary perforated sleeve is coaxial with the aforementioned porous sleeve, regulation can take place by means of a relative axial displacement, but preferably the secondary perforated sleeve is rotatable about its axis by means of a motor via a suitable drive transmission.

Advantageously, several separator devices can be used

with a flow control valve for controlling the mixture flow through a selected number of separator devices depending on the gas pressure, whereby the oil flow rate can be kept within certain limits.

Preferably, a pressure safety valve can also be arranged which opens if the pressure in the mixture in the device exceeds a critical threshold value. In a preferred embodiment, several separator devices are placed in a ring pattern with vertical axes around a central oil collection chamber which is connected to the individual oil collection devices which collect oil from the gas galleries. This oil collecting chamber is preferably also connected to the inlet of one or more of the separator devices, so that thereby the collected oil can be recycled through the associated separator device or devices and led to the aforementioned oil delivery channel. The pressure which drives the oil from the aforementioned oil collection chamber in a recycle loop through one or more of the separator devices is suitably taken from separated gas which is drawn from the gas collection channel that runs from another of the lined up separator devices.

The device according to the present invention can further include a safety valve which is placed in the drill string at such a level that it will be between the seabed and the surface when the final breakthrough in the oil field takes place. This safety valve is preferably in the form of a single gas separator device of the type defined above and is connected in series in the drill string below the water surface as mentioned above, the gas gallery in the separator device having one or more one-way pressure relief valves which are set to open when they are present in the gallery a pressure above a certain threshold value.

The gas-permeable wall that forms part of the mixture's flow path can have a specific fixed effective area and therefore also a fixed resistance to the gas flow through it,

but as the ratio between gas and oil in the mixture will vary considerably and large "gas bubbles" will often go to the wellhead with the oil and thereby cause rapid and strong fluctuations in pressure, it can be advantageous to be able to vary the effective area of the gas-permeable wall.

This can be achieved, for example, by varying the proportion of the wall area taken up by the gas openings.

One of the known measures to counteract the destructive effects of such pressure variations is to provide a very powerful valve at the wellhead as a blowout-preventing valve (placed in the so-called BOP stack), which valve closes quickly when sensors in the drill string and in well-

the head detects gas bubbles in the oil which will be able to cause

sudden and destructive fluctuations in pressure. However, in accordance with the principles of the present invention, the separator can be built in such a way that the resistance to the gas flow can be varied in order to thereby accommodate variations in the gas and oil pressures that occur at the wellhead.

In accordance with this aspect of the present invention, there is therefore provided a separator device for separating gas from oil in a mixture thereof under high pressure, such as at a wellhead in an oil well, which device includes a flow path for the mixture, a substantial part of the flow path surface area is formed by a gas-permeable surface, and further has means by which the resistance to gas flow through the surface can be regulated between certain minimum and maximum values. Such regulation of the resistance to flow through the gas-permeable surface can be achieved in several ways. For example, as mentioned above, the gas-permeable wall can be formed by a perforated wall or sleeve which surrounds a central core and limits the flow path of the mixture, as there can be arranged means for varying the effective opening value for the openings in the perforated wall or sleeve.

The opening value or the number of openings that are open can, for example, be varied by arranging a secondary perforated wall or sleeve closely adjacent to the first and displaceable in relation to it, so that at least some of the openings in the secondary wall or sleeve can be moved flush with or out of alignment with the openings in said wall or sleeve.

A secondary filtration system can also be arranged for the separation of residual oil caught in the gas passing through the perforated wall. Such filtration systems can include several passages and oil traps which enable an expansion of the gas through them, with collection of the oil for delivery to a suitable oil line.

In the preferred embodiment of the invention, the gas flow passage is formed by a core with streamlined grooves therein, closely surrounded by a perforated sleeve, the secondary sleeve being closely fitted around said perforated sleeve and capable of turning about a common axis to thereby bring the openings flush with or out of flush with the openings in said perforated sleeve. The turning movement of the secondary sleeve can occur automatically, via a suitable drive device and transmission depending on signals from sensors in the drill string and/or in the wellhead, which sensors can detect the arrival of gas bubbles or areas with strongly varying pressure, so that the effective gas passage area in the perforated sleeve can be significantly increased to thereby enable the absorption of the extra gas flow that occurs when such bubbles reach the wellhead.

In what follows, several embodiments of the present invention will be described in more detail with reference to the drawings, where: Fig. 1 shows a perspective view of a device with several

separator devices according to the invention,

fig. 2 shows an axial section of a first embodiment of

the separator in fig. 1,

fig. 3 shows a cross-section along the line III-III in fig. 2, fig. 4 shows an axial section of a drill string safety valve which is included in the separator device according to

the invention,

fig. 5 shows an axial section through a separator device

which constitutes a second embodiment of the present

invention,

fig. 6 is a diagram showing an arrangement of openings in the separator device of FIG. 5, for provision

of a variable flow resistance,

fig. 7 shows a further separator sleeve design, and fig. 8 shows a section of a part of that in fig. 7 showed sleeve.

In fig. 1 shows a device with a support frame 11 with

a support platform 12. Above this platform rise six separator devices, each of which is designated by 13. Below the platform there is an inlet connection 14 for the mixture of oil and gas to be separated, and a gas flow control valve arrangement 15.

Each gas/oil separator device 13 has two outlets 16, 17 at the upper end, the outlets 16 forming oil delivery channels and the outlets 17 forming gas collection channels leading to a common gas manifold 18 from which a gas delivery line 19 runs.

The internal structure of the gas/oil separator device

13 can be seen in fig. 2 and 3. Each separator column 13 has an outer cylindrical housing 20 with a flange at each end for connection to the platform 12 and a manifold unit 21 respectively. Inside the cylindrical housing 20 and coaxially with this, a cylindrical perforated sleeve 22 is placed which contains a core 23 with six helically extending, projecting ridges 24 which between each other form six shallow helical channels 27 (see Fig. 3). The diameter of the core 23 is such that the radially outer surfaces of the ridges 24

is closely fitted against the inner surface of the perforated sleeve 22, so that the helical channels 27 inside the sleeve 22 are separated from each other over the entire length of the core 23, which core extends axially over the entire length of the outer housing 20, from the lower flange of the housing and up to the upper one.

The perforations in the perforated sleeve 22 can be evenly distributed over the length of the sleeve or they can follow a spacing pattern designed in a manner best adapted to pressure variations in the separator's flow path. Each perforation is suitably circular and has a diameter of approx. 3 mm. The relative diameters of the outer housing and the perforated sleeve 22 are such that the flow cross-section in the annular space between them is substantially greater than the flow cross-section in the streamlined channels formed between the perforated sleeve 22 and the core 23. As shown, the helical channels have only one turn over the core's 2 3 length, but in practice the conditions may be different, and channels with several windings may be used. The number of windings can be varied depending on the conditions at the wellhead, in order to thereby achieve the best effect, and this can be achieved by replacing the cores with others that have a different pitch, or that the core is made with a variable pitch.

The gas/oil mixture supplied to the lower end of each separator 13 is fed to the separator through a respective mixing line 25 which enters from a control valve assembly 26

where gas and oil are received through a branch line 28 from an inlet connection 29.

From fig. 3 it appears that only four of the separators 13 receive the mixture directly from the control valve unit 26. These separators are given the letters a, b, d and e. The other two separators 13 identified with the letters c and f are connected so that they receive recycled oil on

a way to be described in more detail below.

A respective branch of the gas manifold unit 21 is connected

the upper flange of each of the outer cylindrical housings 20

in each separator 13. The gas manifold unit has several gas collection channels 19 which lead to a common gas delivery line 19a which, as shown in fig. 1, runs centrally from the upper end of the device.

Each perforated sleeve 22 in the separator 13 has a tight part

30 at the upper end, which portion continues as an oil delivery channel 31 leading to the oil delivery channels 16 in fig. 1. These latter can of course be brought together to a common oil delivery pipe somewhere on the downstream side of the separation device.

Centrally located within the lined up six separators is a recycling oil tank 32. This tank has an inlet 33 which is connected to lines 34 (see fig. 3) from the lower ends of the cylindrical outer housings 20 in each of the four separators 13a, 13b, 13d, 13e. Two outlets 35 from the lower end of the recirculation tank 32 lead via lines 36 to the separators 13c, 13f main inlet for the mixture. At the upper end of the oil recirculation tank 32 is a gas inlet. This gas inlet is supplied with gas through a line 38 with valve 39 from the gas gallery formed by the annulus between the outer housing 20 and the perforated sleeve 22 in one or more of the separators 13a, 13b, 13d, 13e.

The device described above works in the following way. The mixture of oil and gas, as it arrives at the wellhead in an oil well, is supplied to the inlet coupling 28, from where it passes through the channels 28 and to the control valve unit 26. This control valve unit is operated in dependence on the available pressure and the flow rate of the incoming material to provide connection to one or more of the supply channels 25 leading to the separators 13a, 13b, 13d and 13e. When the pressure and flow rate are high, all valves in unit 26 will be open, so that all four separator columns will be in operation. At lower pressure, one or more of the columns are closed, whereby the device is compensated for such a variation, so that the working pressure inside the device is maintained as much as possible, within certain limits.

When the mixture of gas and oil enters the separator column

13 from the supply channel 25, the mixture will be subjected to a progressive reduction with regard to the available flow path, and is directed into the helical channels 27. From fig. 2 it can be seen that the core 23 is pointed in the beginning part of the mixture's flow path in the separator, so that the change: >of the flow cross-section takes place progressively over an inlet section. With the dimensions shown, the individual channels 27 will have a flow cross-section which is mainly determined by the cylindrical inner wall of the perforated sleeve 22 and the curved surface of the core 23. Only a small part, less than 20%, of the total surface area in flow- ning cross-section is determined by the 24 radial walls of the spine. As a result, and because of the vortex effect provided by the mixture's streamlined flow path, the gas will tend to accumulate in the radially outer part of the channels 27, while the oil flows up into the radially inner part and in contact with the surface of the core 23 . The gas, which is under considerable pressure, passes through the openings in the perforated sleeve wall 22 and into the gas galleries formed by the annulus 27 between the outer housing 20 and the sleeve 22. Here the gas is exposed to a significant pressure drop as a result of the much larger cross-sectional area in this passage compared to the flow cross-section in the helical channels 27. The gas then exits through the manifold 21 and the delivery channels 19 and on to the gas distribution line 19a. The oil continues up the central passage in the separator 13 and into the oil delivery channel 31 and on to the oil delivery lines 16.

It is inevitable that some of the oil will pass through the sleeve

22 as it is captured by the gas when it expands. As a result of the increased flow cross-section, the gas velocity will decrease quickly and significantly, and entrapped oil particles can therefore no longer be held up in the gas flow and they will fall under the influence of gravity into an oil collection sump formed by a dense lower part of the sleeve 22, between this sleeve and the outer housing 20. Oil separated in this way passes through the recirculation line 34 and into the oil recirculation chamber 32 where the oil, under the influence of the gas pressure acting via the inlet 27 and the line 38, is driven to the inlet openings in the recirculation columns 13c, 13f, through the lines 36. These columns necessarily work at a lower pressure than the other four columns because the propellant gas is taken from somewhere on the downstream side of the first four separator columns and the gas pressure is therefore lower than the pressure at the device's inlet 28.

The operation of these recirculation separators can be intermittent if the amount of entrapped oil that collects in the sumps in the four main separators is insufficient to maintain a continuous flow. For this purpose, a valve 39 is arranged in the line 38. Although this is shown as a manually operated sluice valve, such a valve can of course be operated automatically or by remote control.

When the valve is closed, oil can collect in the sumps at the lower end of the separators 13 and can, under the influence of gravity, pass into the recirculation tank 32 until a sufficient amount is collected, after which the recirculation ion separators 13c and/or 13f can be opened.

In this way, the device's operation can be controlled so that

can cover a wide fluctuation range for inlet pressure and flow rate.

The device can be affected further by replacing the cores 23 with cores with other pitch ratios. It will be appropriate to use cores with pitches of 0.6, 1.2, 1.8 and 2.4 m, and these cores are used independently of each other in the device, so that the number of turns in the helical structure can thereby be increased or reduced course over the length of the separator, so that fluctuations in pressure and flow rate can be covered as the well conditions change.

In fig. 4 shows an underwater safety valve which is connected in series in the drill string so that it is in an underwater position when the final penetration into the oil field takes place. As shown in fig. 4, the safety valve 40 has an outer housing 41 which tapers from an upper end 42 towards a

lower end or inlet end 43.

Within the conical outer housing 41 there is an essentially cylindrical inner sleeve which forms a parallel-sided, cylindrical gas-permeable membrane 44 which can have a construction similar to that of the perforated sleeve 22 in the gas/oil separator device 13 in fig. 2 and 3. In the same way, the cylindrical sleeve 44 occupies the core 45 with several helical projecting ridges which between each other form shallow helical channels 47.

Around the wider end of the housing 41 are located six one-way pressure relief valves 48 which communicate with the gas gallery in the chamber 49, between the outer surface of the perforated sleeve 44 and the housing 41. Each one-way valve 48 is set at a specific pressure above the normal working pressure in the drill string , so that if such a higher pressure occurs, especially when breaking through the last rock layer and into the oil field, the drilling rig will be relieved of the sudden current, because this will be relieved through the valves 48. At normal pressures, the valves 48 will remain closed and the safety device ning will then only act as part of the hollow drill string which enables the passage of casting compound, mud and gas without special changes.

In the area between the safety valve 40 and the surface, sensors 50 can be placed which sense the relief of gas bubbles through the valves 48, so that a pressure increase which causes such a pressure relief can be sensed before it reaches the rig, whereby suitable measures can be taken.

The alternative separator device shown in fig. 5 and

6, has an essentially cylindrical housing 51 mounted on a platform 5 2 which also carries a foundation 53.

The foundation 5 3 supports a cylindrical sleeve 5 4 which is placed around one central core 55 with several helical grooves. The core 55 is tapered at its lower end and enters the opening in a venturi block 56 which is attached to the platform 52 and forms an inlet connection to the separator, for connecting the upper end of a delivery line from a blowout preventer valve in the wellhead. The flow passage in the venturi block 56 is given a uniformly curved constriction so that the flow path from the blowout preventer is split up at the inlet end of the central core 55 with the helical grooves without creating turbulence.

The sleeve 54 surrounds the core 55 and has several openings 65 over the main part of its length. A secondary sleeve 57 is placed around the sleeve 54, which also has several openings. Of-

however, the position between these openings is different than between the openings in the sleeve 54. The outer sleeve 57 has a sliding fit over the inner sleeve 54 and rests on a ring carrier 58 which is in turn carried by a carrier 59. The carrier 59 has a gear ring 60 which has gear engagement with a drive 61 on a shaft 62 from a motor 63. The motor 63 can be a stepper motor.

Around the outer sleeve 57 there is a cylindrical chamber 64

which accommodates a secondary oil separation filter of a suitable type.

In the embodiment shown, the filter is a tubular spiral

filter with high-tensile nickel-titanium tubes arranged in six peripheral adjacent semi-cylindrical groups. Around this filter there is a further group of separator cartridges in the form of composite plates which form a wax cake structure. The spiral tube filter groups 64 and the outer wax cake structure

74 are both arranged to capture the oil carried by gas passing through the openings in the sleeves 54, 57. Sonr shown in fig. 6, the openings in the sleeve 54, which openings are denoted by 65, have a mutual distance d from each other, and the openings are arranged in a regular pattern in both the circumference

direction as axial direction. The openings in the sleeve 57, on the other hand, do indeed have a regular internal distance d in the axial direction, but in the circumferential direction they have an irregular distance, some having the distance d, some having the distance d/2 and some having the distance a/3. The pattern shown in fig. 6 is repeated around the circumference of the sleeve 57. In this simple embodiment, the sleeve 57 can thus be placed in a first position in which all openings 65 in the inner sleeve 54 align with a corresponding opening 66. By displacing the sleeve 57 to the left in fig . 6 in relation to the sleeve 56, however, the opening 66 can be shifted so that it no longer aligns with the opening 65, while an opening 67 aligns with one of the four openings 65 in the sleeve 56. In this margin, only 1/4 of the openings 65 will thus be open, the rest being closed with the adjacent sleeve 57. A further movement of the sleeve 57 until openings 68 align with two of the openings 65 allows half of the openings 65 to be open, while the rest is closed. By appropriate arrangement of the opening pattern in the sleeve 57, four positions can be achieved, in which positions respectively one, two, three or four openings 65 in each set of four can be selectively opened by bringing an opening in the sleeve 57 flush with the respective opening. In this way, the separator can be given four different resistances to flow. Typically, the sleeve 57 will be set with two or one opening in each set of four open, and the sleeve can be rotated to open three or four openings as needed, to cover a pressure increase when gas bubbles reach the wellhead.

Fig. 7 and 8 show a perforated sleeve which is suitable for replacement with the perforated sleeve 56 in the embodiment in Fig.

5 or fig. 2. This sleeve, only a part of which is shown in fig. 7, is given the reference number 71. The sleeve has a helical row of openings 72 with a limited axial extent, i.e. across the length of the helical row. Two further rows 73, 74 are also arranged, so that a triple helix line is formed. The wall parts of the sleeve 71 between adjacent rows of openings 72, 73 or 73, 74 are sealed.

Fig. 8 shows the cross-sectional shape of a channel 75 formed in a core 76 which is tightly fitted into the sleeve 71 in the same way that the core 55 fits into the sleeve 56 in the embodiment in fig. 5.

The spiral groove 75 has a triangular cross-section which is limited by a mainly radial wall 77 and an inclined wall 78 which forms an angle of approx. 45° with the diametral plane of the core which is parallel to the generatrix of the wall 77. In fig. 8

if you see two of the openings in row 72, indicated by 72a,'72b,

and these openings are, as shown in fig. 7, elliptical, with the main axis perpendicular to the sleeve's 71 longitudinal extent. As shown in fig. 8, the row of openings 72 is arranged flush with the part of the groove 76 which is located at the apex of the triangle and farthest from the side 77. Thus, when the oil and gas mixture flows in the helical path formed by the channel, the forces that the gas and oil will be exposed to to cause the gas to occupy the space parallel to the side 78, while the oil will collect in the space formed at the apex of the triangle opposite the side 78. The gas thus communicates with the openings 72a, 72b, while the oil is retained in the channel as the sleeve is close to this part of the canal. The oil is therefore carried on to the end of the helical channel and goes from there to the oil collection passages.

With renewed reference to fig. 5 it should be mentioned that the cleaning of the gas plate separator device can be programmed to take place at regular intervals, after a certain amount of current has passed. This can be determined using ultra-sound sensors 75, 76 which are placed on the outer housing wall 77 and on the cylindrical main housing 51.

Claims (23)

1. Separator device for separating gas from oil in a mixture thereof under high pressure, such as at a wellhead in an oil well, characterized in that it includes a guide channel for guiding a flow of the mixture under high pressure along a flow path where part of the surface area is formed of a gas-permeable surface, the flow path being shaped in such a way that expansion of the gas content of the mixture drives this through the gas-permeable surface, where it is collected in a gas collection channel, which guide channel directs the oil fraction to an oil delivery channel. 2. Separator device according to claim 1, characterized in that a major proportion of the surface area of the flow path is made up of two opposite surfaces in the guide channel, which surfaces make up at least 80% of the guide channel's surface area. 3. Separator device according to claim 1 or 2, characterized in that several guide channels are designed which are limited by a mainly cylindrical gas permeable sleeve which occupies a tightly fitted central core which has several shallow, mainly axially running channels therein. 4. Separator device according to claim 3, characterized in that the aforementioned shallow, mainly axially extending channels in the central core extend helically along the core to thereby cause the gas and oil mixture to swirl as it passes through the device. 5. Separator device for separating gas from oil in a mixture thereof under high pressure, such as at a wellhead in an oil well, characterized by means which cause the mixture to follow an essentially helical flow path which is partially limited by a gas-permeable wall, through which wall the a pressure differential is maintained so that the gas in the mixture is forced to pass through the wall and into a gas collection gallery, while the oil is led by means of guide means to an oil delivery channel. 6. Separator device according to one of the preceding claims, characterized by means for collecting oil that is caught in the gas passing through the aforementioned gas-permeable wall. 7. Separator device according to claim 5 or 6, characterized in that the gas-permeable wall is a mainly cylindrical sleeve within which the mainly helical flow path for the mixture lies. 8. Separator device according to one of claims 5-7, characterized in that the mixture's flow path is further limited by an elongated core inside the cylindrical sleeve, which core has at least one helical channel to cause the mixture to follow the aforementioned essentially helical flow path. 9. Separator device according to claim 8, characterized in that said oblong core has several such helical channels which are limited by projecting helical continuous ridges on the core, which ridges fit closely against the inner surface of the gas permeable wall. 10. Separator device according to one of the preceding claims, characterized in that the gas-permeable wall is perforated. 11. Separator device according to one of the preceding claims, characterized in that the resistance to the gas flow through the gas-permeable surface can be regulated between certain minimum and maximum values. 12. Separator device according to claim 2 or 7, or any claim dependent thereon, characterized by means for varying the effective opening value for the openings in the perforated wall or sleeve. 13. Separator device according to claim 12, characterized in that the opening value is varied in that a secondary perforated wall or sleeve is provided closely adjacent to the first and displaceable in relation to it, so that at least some of the openings in the secondary wall or sleeve can be moved to flush with or out of flush with the openings in said wall or sleeve. 14. Separator device according to claim 13, characterized in that the secondary perforated sleeve is coaxial with the aforementioned perforated sleeve and can be rotated about its axis by means of a motor via a suitable drive transmission. 15. Device for separating gas from oil in a mixture thereof under high pressure, such as at a wellhead in an oil well, characterized in that it includes several separator devices according to one of the preceding claims and a flow control valve unit for controlling the mixture flow through a selected number of separator devices in dependence on the gas pressure, thereby keeping the oil flow rate within certain values. 16. Device according to claim 15, characterized by an overpressure safety valve which opens if the pressure in the mixture in the device exceeds a critical threshold value. 17. Device according to claim 15 or 16, characterized in that an oil collecting chamber for collecting oil which carries through the gas permeable wall is trapped by the gas, which chamber is placed in a central position surrounded by lined up gas separator devices. 18. Device according to claim 17, characterized in that the oil collecting chamber is also connected to the inlet of one or more of the separator devices to thereby recycle the oil collected therein through the separator device or devices for delivery to the aforementioned oil delivery channel. 19. Device according to claim 17 or 18, characterized in that the pressure to drive the oil from the aforementioned oil collection chamber for recycling through one or more of the separator devices is taken from separated gas that is drawn from the gas collection channel leading from another of the lined up separator devices. 20. Device according to one of claims 15-19, characterized by the safety valve located in the drill string at such a level that it will be between the seabed and the surface when the final breakthrough in the oil field takes place, which safety valve is a separator device according to one of claims 1- 14 and is connected in series in the drill string, as the gas gallery of the separator device has one or more one-way pressure relief valves which are set so that they open when the pressure in the gallery exceeds a certain threshold value. 21. Separator device according to claim 1, characterized in that the flow path is essentially helical and on the radially outer side is limited by an essentially cylindrical wall which has several openings arranged in a helical pattern, the axial dimension of the pattern being smaller than that of the aforementioned flow path axial dimension. 22. Separator device according to claim 21, characterized in that the aforementioned flow path has a triangular cross-section. 23. Separator device according to claim 22, character i-.s characterized in that the said triangular cross-section for the flow path includes a side that is inclined at a certain angle in relation to the said cylindrical limiting wall, and that the said opening pattern is located adjacent to the pointed angle. 1. Separator device for separating gas from oil in a mixture thereof under high pressure, such as a wellhead in an oil well, characterized by means which cause the mixture to follow an essentially helical flow path which is partially limited by a gas-permeable wall, is in the form of a essentially cylindrical sleeve within which lies the essentially helical flow path of the mixture, and through which wall a pressure differential is maintained so that the gas in the mixture is forced to pass through the wall and into a gas-seal gallery located radially outside the wall, while the oil by means of guide means is fed to an oil delivery channel. 2. Separator device according to claim 1, characterized by means for separate collection of oil which is caught in the gas passing through the aforementioned gas-permeable wall. 3. Separator device according to claim 1 or 2, characterized in that the mixture's flow path is further limited by an elongated core inside the cylindrical sleeve, which core has at least one helical channel to cause the mixture to follow the aforementioned essentially helical flow path. 4. Separator device according to claim 3, characterized in that said oblong core has several such helical channels which are limited by projecting helical continuous ridges on the core, which ridges fit closely against the inner surface of the gas permeable wall. 5. Separator device according to one of the preceding claims, characterized in that the gas-permeable wall is perforated and that the resistance to the gas flow through the gas-permeable surface can be regulated between certain minimum and maximum values. 6. Separator device according to claim 5, characterized in that the resistance to the gas flow through the gas-permeable surface is provided by varying the effective opening value for the openings in the perforated wall or sleeve. 7. Separator device according to claim 6, characterized in that the opening value is varied in that a secondary perforated wall or sleeve is provided closely adjacent to the first and displaceable in relation to it, so that at least some of the openings in the secondary wall or sleeve can be moved flush with or out of flush with the openings in said wall or sleeve, the secondary perforated sleeve being coaxial with said perforated sleeve and can be rotated about its axis using a motor via a suitable drive transmission. 8. Device for separating gas from oil in a mixture thereof under high pressure, such as at a wellhead in an oil well, characterized in that it includes several separator devices according to one of the preceding claims and a flow control valve unit for controlling the mixture flow through a selected number of separator devices depending on the gas pressure, thereby keeping the oil flow rate within certain values. 9. Device according to claim 8, characterized in that an oil chamber for collecting oil which carries through the gas-permeable wall with the gas, which chamber is placed in a central position surrounded by lined up gas separator devices, which oil collection chamber is also for connected with the inlet to one or more of the separator devices in order to thereby recycle the oil collected therein through the separator device or devices for delivery to the aforementioned oil delivery channel. 10. Device according to claim 9, characterized in that the pressure to drive the oil from the aforementioned oil collection tank: for recycling through one or more of the separator devices is taken from separated gas that is drawn from the gas collection channel leading from another of the stacked seaprators devices.