WO2002001044A1

WO2002001044A1 - Inclined separator for separating well fluids

Info

Publication number: WO2002001044A1
Application number: PCT/NO2001/000253
Authority: WO
Inventors: Otto Skovholt; Dag ØKLAND
Original assignee: Den Norske Stats Oljeselskap A.S
Priority date: 2000-06-22
Filing date: 2001-06-15
Publication date: 2002-01-03
Also published as: NO20003291D0; AU2001264436A1

Abstract

Inclined separator for separating well fluids, in particular water and hydrocarbons, and possibly gas, the separator copmrising at least on inlet hich comprises insert tubes for well fluids, and at least two outlets for discharge of the separated liquids. The separator comprises insert tubes which from one of the separator ends extends to an inlet position centrally in the longitudinal direction of the separator, a first outlet at the upper end of the separator, for discharge of the light-weight fraction of the well fluids, and a second outlet located at the bottom of the separator for discharge of the heavy fraction of the separator.

Description

Inclined separator for separating well fluids.

This invention relates to an inclined separator for separating well fluids, in particular water and hydrocarbons, and possibly gas, which comprises an insert tube for well fluids, and at least two outlets for discharge of the separated fluids .

In most oil wells a mixture of oil and water is produced, and in some cases gas, which at a later stage will have to be separated. At present, separation typically takes place after pressure reductions in large tanks located on oil installations, at a pressure in the same order of magnitude as the atmospheric pressure. As there is a certain pressure is kept on the liquid to be separated, such a separation involves reducing the pressure before separating, with corresponding loss of pressure energy. For reinjection of water or conveying of hydrocarbons in pipelines, then further application of pressure is required. For this reason, it is desirable to carry out the separation at high pressure. The high pressure also has the advantage that dissolved gas will not be released, and that the properties of the oil for this and other reasons are different from the properties of the oil at atmospheric pressure. Separation of hydrocarbons and water generally proceeds more easily at high than at low pressure. By maintaining a high pressure in the separator, the various phases may easily be conveyed out of the separator in the desired amounts by means of valve control .

When arranging a separator close to the seabed (in the case of offshore wells) or downhole (in the well) , a higher pressure is obtained than by arrangement, e.g., on a platform. As water then will be separated at high pressure, the pressure energy from the reservoir can be utilized more efficiently, as transport completely to the receiver location is required only for the hydrocarbon phase. In order to reduce the amount of water in the oil, a number of solutions, such as separation of oil and water in the oil well itself as described in GB 2.326.985 and EP 977.621. However, disturbances in the well itself means a

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be used for further transportation, and moreover leads to increased mixture of oil/gas/water. Thus, an object of the present invention is to provide a separator which can be located on or at the seabed and which does not result in the separator tank itself, but which can use strict requirements for existing equipment for the purpose.

Thus, an object of the invention is to provide a separator which can be easily provided and which facilitates optimal separation conditions. The present invention thus relates to a separator as described above and which is characterized as stated in claim 1.

According to the invention, a separator is obtained which can be formed of a suitable wellbore in which the dimensions may be adapted to the task, without taking the production conditions into account, so that it can be designed with optimal flow conditions. The suitable wide well can preferably be dedicated for arrangement of separator as the upper part of a production well. Alternatively, the separator according the invention is made of available piping systems which is sufficiently pressure resistant for withstanding the separator pressure without requiring more expensive supplementary solutions .

The invention will be described below with reference to the drawings, which illustrates the invention by means of an example .

Figure 1 which shows a longitudinal section of a preferred embodiment of the separator according to the invention. Figure 2 which shows a corresponding longitudinal section with indication of the separation zones. Figure 3 which shows a longitudinal section of an embodiment according to the invention in which it is connected to means for controlling the separation process.

Figure 4 which shows a longitudinal section of an alternative embodiment of the separator as a part of a production well. Figure 5A og 5B illustrates the connection between production rate and angle in a separator according to the invention. In figure 1, the separator 11 is shown shaped as an inclined tube or well in which the inlet is made of an insert tube 12 extending from the top of the well to an inlet location approximately in the middle of the well . The location centrally in the separator is not an absolute requirement, but the insert tube preferably extends along at least 20% of the separator, but preferably between 40 and 50% of the same.

The insert tube itself can be designed in a number of different ways in order to obtain favourable separation conditions. It can be provided with perforations 18 in various places along its length in order to obtain a slow and an advantageously small mixture of the fluid supplied • with the fluid already present in the separator. In this manner, the separation process will be distributed along a larger length of the separator, so that turbulence in the separation zone is reduced. The perforations are preferably located on the lower and the upper portions of the tube, but may also be located on the complete circumference of the tube.

In figure 2 the separator is divided into three zones with regard to illustration, in which the separation itself takes place in zone 1. In this zone, the flow conditions usually are turbulent when the water and oil are separated and move in opposite directions. In order to reduce this turbulence, the insert tube as mentioned above is provided with perforations so that the fluids can be gradually separated already in the insert tube. The oil will for this reason leak from the insert tube and be moved upwardly to an earlier stage and reduce the momentum of fluids which otherwise would be washed out of the end of the insert tube. In the separation zone 1, the transition between oil- containing water and water-containing oil will be unclear and be dependent on various conditions such as for example flow rates. The light-weight fluids, usually oil, will move upwardly and follow the upper edge of the separator, while the heavy-weight, ususally water, will sink toward the lower portion of the separator. As the separator is tilted, the fluids will move out from the separation zone. Should the velocity of the fluids be too large, the fluid flow in both directions will draw along droplets and particles of undesired types, so that oil flows will draw some droplets and particles of water. In order to avoid this and let the separation become as complete as possible, the fluid flow from the separation zone should for this reason be slow. For this reason, there will be a consideration between production rate and purity of the extracted fluids.

In the two remaining zones 2 and 3 in the separator, the separation has essentially been undertaken, and extraction of the fluids can be made. The length and the dimensions in these zones are less critical and can be adapted to the situation. However, they should be of a certain length and have steady flow so that any possible remains of unseparated fluid can be separated and flow in the opposite direction to the main flow, so that for example water droplets in the oil flow fall down towards the bottom of the separator and form a flow down to the separator zone 1. If the flow velocity in these zones is too high, the water droplets will be driven along with the flow, so that the separation will become poorer. This leads to a consideration between the desired production rate and the degree of separation.

The perforations 18 are arranged in the separation zone 1 and are not located higher than the level at which the well flow already has generally started to separate. Moreover, the perforations should not extend further down than the length of the separation zone. As mentioned above, the perforations have the purpose of dampening the flow velocity and the turbulence in the separation zone. From an ideal point of view, the fluids which flow from the insert tube in the lower end, or possibly from the upper end if the insert tube guides fluids in from below, should not have an essentially greater velocity than the separated fluids which have the same movement direction as the fluid supplied. w ) M to μ> H

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surface installation. The control unit is connected with two sensors 20,21 for measuring the fluid content below and above the separation zone 19. If the sensor upper 21 measures a water content which exceeds a predetermined limit, an indication is given to the valves 16,17 which regulate the flow rates out of the separator. The separation zone 19 can be lowered by reducing the flow rate through the upper valve 16 and/or by increasing the flow rate through the lower valve 17. The choices here will be determined by production conditions and the amount of flow rate which can be accepted through the separator in order to obtain sufficiently purity of the fluids being separated.

The control unit can be designed as a very simple battery operated mechanism which can be arranged at the seabed independent on continuous contact with other parts of the pipe systems. By using a separator as an integrated part of a pipe system, the control unit and the separator can provide a passive purification of the fluid flow within certain variations in the content of the fluid flow. Under certain conditions, for example if the separator is arranged in a well, the lower valve 17, which then will be difficult of access, can be a robust valve with relatively coarse settings, while the upper valve 16 takes care of the fine adjustments of the flow rates. For additional control of the flow conditions in the separator in the separator, guide means or similar may be provided along at least parts of the longitudinal direction of the separator.

If the separator is constituted by a well, the lower outlet 10 can be a drainage directly into a formation which is suitable for receiving water. Such formations may be of different kinds, such as water-filled reservoirs or water- or hydrocarbon reservoirs which has been drained in part or completely. If the surrounding formation is not suitable for receiving the heavy fraction, or if it for other reasons is difficult to discharge the heavy fraction at the lower end of the separator, regardless of its location, a particular tube may be used which is connected to the lower end of the separator for retrieving the heavy fraction through the upper end. This tube may extend upwardly within the separator or be arranged on the outside of the same.

For obtaining favourable flow conditions, the separator volume preferably has a diameter which exceeds 12", and should generally be as large as possible, but within the practical limits which is set by the arrangement of the separator by means of for example a drilling vessel. This involves diameters in the area of 20 to 30", but as a rule, the diameter should be as large as possible. The length may vary, but will be essentially larger than the diameter, typically larger than 20m and possibly preferably as long as that the lower end gives direct discharge into a deeposit area, such as described above, for discharge of produced water. This involves separator lengths of up to 1000m or more. The separator may further have an inclined angle in the area of 1-15° in relation to the horizontal. The angle will depend on the length, as the most important in this connection is to have a difference in altitude between the lower and the upper end of the separator, so that sufficiently well separation is obtained.

Figures 5A og 5B illustrate the production rate R for a separator according to the invention in relation to the angle V relative to the horisontal plane. As appears, the production rate increases strongly at smaller angles for a maximum of at least 10°. However, here it should be noted that the measurements in these diagrams have been made on separator tubes having a length of 6 metres, which is far less than that which will typically be in use. It is considered likely that a longer separator having the same difference in altitude between the upper and lower valve will have calmer flow conditions due to the smaller angle, and thus also improve the separation, and possibly the production rate.

Figure 5A further illustrates the separation differences depending on the properties of the insert tube, in which the graph T3 shows the separation using an insert tube having 4 apertures on the top side with a diameter of ca. 1/3 of the thickness of the insert tube. T6 shows the effect of apertures both on the top side and the underside of the insert tube having the same diameter.

Figure 5B further illustrates the difference of the rate in relation to the water cut in the mixture to be separated. As appears, the separation is poorest (see W₅₀) when the water cut is 50%, while water cuts of 20% and 80% give better separation rate. Maximum separation in figure 5B is achieved at an angle of approximately 15°. As mentioned above, this top would probably be shifted to lower angles by using longer separators. The dimensions and the angle of the separator will depend on the local conditions, such as water fraction, flow conditions and similar. For example, the cross section of the insert tube may be as large as the annulus outside. In addition, cases are likely in which the angle varies throughout the separator, for example if it forms a part of a well, such as in figure 4, or a part of a tube which follows the seabed.

In other respects, the different parts of the separator can be formed of known elements per se, so that valve types and tube types beyond those described above are not essential for the invention and may vary depending on use and availability.

This could be made in a simple manner with conventional drilling operations, and thus in a very simple manner provide a separator volume which is sufficiently large for facilitating favourable flow conditions.

Figure 4 shows an embodiment of the invention in which the separator 1 is formed of the annulus around a production tube 40 in a well. The perforated area 18, which length is strongly understated for illustration reasons, enables a separation of the light-weight fluids from the top of the production tube. These fluids flows further upwardly and can, according to a chosen length which among other things will depend on the desired degree of separation, be transported back to the production tube through another perforation and be further conveyed up to the surface. In the figure, the annulus is delimited by packings 44 for defining a finite separation volume and preventing intrusion of other fluids along the production tube. The separated water in figure 4 is carried from the annular space down into a well branch 45 which for example can lead to a suitable formation via an outlet tube 46. The outlet tube 46 is in the figure connected to the production tube, but is sealed with a packing 43 for preventing penetration of the produced fluids. Under the of the packing 43, the tube 46 is perforated in order to permit entry of separated water and further is provided with a pump or valve 41 for pumping or permitting entry of the separated water down into the predetermined geological formation.

As appears above, the separator according to the invention can be embodied by wells which is drilled into the earth, and by tubes or equivalent mounted on or at the seabed. It is clear that the solution also could be used aboard vessels, drilling platforms and similar. For some installations of such types, e.g., offshore platforms, the support structure may extend deep into the sea, in some cases to the sea bed, wich enables installation of very long tubes in connection with such. For such uses, tubes suitable for use at high pressure could be used, e.g., of the same type which is used for gas transfer at the sea bed.

Another embodiment of the invention may comprise a number of separators arranged in parallel, so that the total separation rate is increased, or possibly so that a gradual separation can be obtained through a series of separators. This can for example be a solution of interest in cases wherein the separators form an integrated part of a pipeline from a subsea installation to an onshore installaton.

Claims

C l a i m s

1. Inclined separator for separation of well fluids, in particular water and hydrocarbons, possibly incluiding gas, which separator comprises at least one inlet comprising an insert tube for the well fluids, an at least two outlets for discharge of the separated fluids, c h a r a c t e r i z e d in that the separator comprises an insert tube which from one of the ends of the separator extends to an inlet position centrally in the longitudinal direction of the separator, a first outlet at the top end of the separator for discharge of the light-weight fraction of the well fluids, and a second outlet located at the bottom of the separator, for discharge of the heavy fraction in the separator.

2. Separator according to claim 1, c h a r a c t e r i z e d in that the outer diameter of the separator is formed of a designated well drilled to a chosen depth in the ground and that the upper end of the separator is located near the earth surface.

3. Separator according to claim 2 , c h a r a c t e r i z e d in that the lower part of the separator has an outlet of the heavy fraction directly to a suitable geological formation.

4. Separator according to claim 1, c h a r a c t e r i z e d in that the insert tube extends along at least 20% of the longitudinal direction of the separator.

5. Separator according to claim 1, c h a r a c t e r i z e d in that at least one of the inlets and outlets comprises a valve for guiding the fluid flow.

6. Separator according to claim 1, c h a r a c t e r i z e d in that the insert tube comprises more than one opening to the separator for obtaining a distributed supply of well fluid to the separator.

7. Separator according to claim 1, c h a r a c t e r i z e d in that the insert tube extends from the upper end of the separator and is perforated on top for extraction of a portion of the light-weight fraction.

8. Separator according to claim 1, c h a r a c t e r i z e d in that the insert tube extends along the complete length of the separator and is perforated at least on top for extracting a portion of the light-weight fraction.

9. Separator according to claim 1, c h a r a c t e r i z e d in that the top side of the insert tube is oil wetting and that its underside is water wetting.

10. Separator according to claim 1, c h a r a c t e r i z e d in that the insert tube comprises a tube having a gradually increasing diameter towards the inlet area.

11. Separator according to claim 1, c h a r a c t e r i z e d in that it has an inner diameter which exceeds 12", and preferably 20" og and a length which exceeds 100 metres.

12. Separator according to claim 1, c h a r a c t e r i z e d in that the insert tube extends from the lower end of the separator, and that the lower end of the insert tube is directly connected to an oil well.

13. Separator according to claim 1, c h a r a c t e r i z e d in that the outer diameter of the separator is constituted by a tilted tube.

14. Separator according to claim 1, c h a r a c t e r i z e d in that the separator has a small angle, preferably between 1° and 15°, relative to the horisontal .