Abstract:
A method and an arrangement for separating oil and water by means of a two-phase liquid/liquid separator. The oil phase and the water phase respectively are separated over a portion of the length of the separator from an upstream end to a downstream end. The oil phase and the water phase respectively are extracted gradually over a length of the separator, and each of the phases is conducted out of the separator separately. A method and an arrangement for orienting the separator in a well are also described.

Description:
RELATED APPLICATION 
   This application is a National Phase entry in the United States of the International Application PCT/NO01/00156 filed Apr. 09, 2001 and claims the benefit of the Norwegian application number 2000 1954 filed Apr. 13, 2000. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention regards an outlet arrangement for use in separation of water and oil by means of a two-phase liquid/liquid separator in which the oil and water phases are respectively separated over at least part of the length of the separator from an upstream end to a downstream end of the separator. In particular, the invention regards an outlet arrangement for a horizontal gravity separator with a high length/diameter ratio. The invention especially regards a downhole separator for placing in an underground well. The invention also regards a method for inhibiting re-mixing of the oil and water phases in a separator where the separator includes a separator chamber with an upstream end and a downstream end and an oil outlet and a water outlet, the water outlet being situated in an upper part of the separator chamber and the oil outlet being situated in a lower part of the separator chamber. 
   The invention also regards a method for orienting an outlet arrangement in a horizontal downhole separator, as well as means of implementing this orientation. 
   2. Description of the Related Art 
   Separation of well fluids is usually carried out by use of a 3-phase gravity separator on a platform. Typically, the vessels have a diameter/length ratio in the range of 1:3 to 1:5. The separator is usually partially filled with gas, so as to provide two interfaces; a gas-liquid interface and an oil-water interface. Water and oil are extracted by means of a suitably elevated, vertically oriented pipe with a vortex breaker. The pipes are typically provided in the lower part of the vessel, at the opposite end from the fluid inlet. A weir plate is often used to isolate the water phase from the oil outlet. In this case, the oil-water interface is kept below the level of the weir plate, so that only the oil phase will flow over this. 
   In a horizontal tube or a vessel, the oil and water are separated by gravity due to the difference in density between the two phases, so as to form a lighter phase and a heavier phase. A stratified flow pattern must be maintained in order to be able to separate the liquid phases. The cross-sectional flow area is sized with respect to fluid velocity and water cut. If the residence time is sufficiently high to transfer droplets from the continuous phases, clean oil and water can be produced. The oil droplets will rise from the water phase, and if given sufficient residence time, they will reach the oil-water interface. Water droplets in the oil phase will settle and eventually reach the interface. Accumulated droplets of oil and water will form a dense layer in the interfacial region, in which coalescence takes place. If the residence time is sufficient, the dense layer will eventually be broken up. 
   Gas may be separated out from the well fluid by use of one or more cyclone separators. The water may also be cleaned by use of hydrocyclones. In a multi-stage separation system consisting of such separators, the main aim is to provide a clean oil phase. This is best achieved by employing a long, slim separator geometry in order to maximise the oil-water interface where the coalescence of the droplets takes place. Furthermore, the distance to the oil-water interface, across which distance the droplets have to travel, is reduced. 
   A downhole oil/water gravity separator in which the separation takes place in a section of the production tubing in a horizontal well, such as disclosed in WO 98/41304, is a separator with a very high length to diameter ratio. A separator of this type will typically have a length of approximately 100 m, while the diameter is the same as for the production tubing, for instance 9″, 10¾″ or 13″, i.e. a length-diameter ratio of 1:300 to 1:400. 
   A large oil-water interface and sufficient residence time alone is not enough to achieve clean oil and water phases at the outlet of the separator. It is crucial that the separation of oil and water take place without either of the phases becoming contaminated by the other upon extraction from the separator. 
   Since the diameter of the outlet is small, this will easily influence the oil-water interface and thereby result in water entering the oil phase and oil entering the water phase. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to ensure that each of the phases is distributed to the appropriate outlet without interfering with the oil-water interface and re-mixing the phases. As the phases exist in a separated state as much as several meters before the end of the separator, each of them should be extracted gradually over a certain distance before the end wall of the separator. Through such gradual extraction of separated fluids, the residence time for the remaining liquid in the separator will be extended, allowing the smaller oil and water droplets to migrate to the oil and water phases respectively. This is achieved by a two phase liquid/liquid separator in which the oil and water phases respectively are separated over at least part of the length of the separator from an upstream end to a downstream end of the separator wherein the oil and water phases are respectively are extracted gradually over a length of the separator, the length extending substantially all the way from the downstream separator and wherein the oil and water phases respectively are extracted substantially symmetrically respective to longitudinal axis of the separator and wherein each of the phases is extracted from the separator separately. This is further achieved by an arrangement at a separator for separating oil from water, the arrangement including a separator chamber with an upstream and a downstream end and an oil outlet and a water outlet, the water outlet being situated in an upper part of the separator chamber and the oil outlet being situated in a lower outlet of the separator chamber wherein the oil and water outlet are respectively distributed over a length of the separator at the downstream end of this and wherein the oil and water outlets, with respect to each other, are arranged substantially symmetrically relative to the longitudinal axis of the separator. 
   In the case of an outlet arrangement of the type with which the present invention is concerned, it is important for the separator to have the correct orientation, so that the oil outlets face upwards and the water outlets face downwards. According to the present invention, a method and an arrangement have therefore been provided in order to achieve this. The method is characterised by orienting an outlet arrangement in a horizontal downhole separator such that a guide device is inserted into a hole and fixed relative to the hole, inserting a measuring tool into the hole in order to register the orientation of first guide means on the guide device, setting second guide means on the separator, the second guide means corresponding to the first guide means, in accordance with the orientation of the first guide means, inserting the separator with the outlet arrangement into the hole, and rotating the separator to a predetermined lintation upon contact between the first and second guide means. When oil and water are extracted from the separator over a certain distance, the effect on the interface is lessened. It becomes easier to avoid extracting liquid from the area near the interface. Small, dispersed droplets of the opposite phase will often be present near the interface, e.g. water droplets in the oil phase and oil droplets in the water phase. These droplets will be given more time to coalesce and migrate to the phase to which they belong. 
   The outlet arrangement according to the present invention also aims to have a design which is such that oil and water are gradually extracted in a radial direction from the upper and the lower part of the separator housing, near the separator wall. 
   By the arrangement according to the invention, the oil and water outlets respectively are distributed over a certain axial distance at the downstream end of the separator. 
   By such an arrangement, clean oil and water may be collected as early as possible upon becoming accessible, and then be transported out of the separator via the outlet pipe. Any disturbance of the liquid phases will be minimal when oil and water are removed over an extended distance. By constructing outlet slots or holes with a decreasing cross sectional area in the downstream direction, the velocity of the liquid in the outlet pipe will increase in the downstream direction. The acceleration is preferably linear. When the clean oil and water are brought out at a higher velocity than the mean bulk velocity in the separator, the total residence time is increased, making it easier for smaller droplets to coalesce and migrate. 
   Heavier solids will have time to settle in the separator, while lighter particles will be carried along by the liquid phases. By the arrangement according to the present invention, it is also possible to avoid solids accumulating near the outlet and blocking this. By extracting water near the bottom of the separator, the liquid flow will also sweep away solids near the bottom and carry these out with the water. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention will be explained by means of an example of an embodiment of an arrangement and a separator in accordance with the present invention, with reference to the accompanying drawings, in which: 
       FIG. 1  is a perspective drawing showing an outlet arrangement in accordance with the present invention; 
       FIG. 2  is a longitudinal section of a part of a separator according to the present invention, including the outlet arrangement shown in  FIG. 1 ; 
       FIGS. 3   a -c  are sections along the lines A—A, B—B and C—C in  FIG. 2  respectively; 
       FIG. 4  shows a separator equipped with guide means for orienting the separator; 
       FIG. 5  shows a guide sleeve to be used when orienting the separator; and 
       FIG. 6  shows a detail of a gear rim that forms part of the guide means. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 2  and  3   a, b , and  c  show a longitudinal and transverse sections respectively through part of a liquid-liquid separator according to the present invention and including the outlet arrangement shown in FIG.  1 . The separator has a high length to diameter ratio. 
   The separator comprises a separator wall  1  tubularly enclosing a separator chamber  20 . An outlet arrangement  2  is provided inside the separator chamber  20 . The outlet arrangement  2  is positioned along the longitudinal axis of the separator chamber  20 , and extends over a length of e.g. 2-4 meters at the downstream end of the separator. 
   The outlet arrangement  2  has an outer end  21  facing the upstream direction and an inner end  22  that is connected to transport conduits outside the separator (not shown). The outlet arrangement further comprises an outer wall  23  that forms a pipe having approximately the same cross section along the length of the outlet arrangement  2 . An annulus  10  is defined between the separator wall  1  and the outer wall  23 . First inlet orifices  4   a  are formed in the outer wall  23 , on one side of the outer wall  23 , and second inlet orifices  4   b  on the opposite side of the outer wall. As shown for orifices  4   a  in  FIG. 1 , the orifices  4   a  and  4   b  have a decreasing cross section from the outer end  21  of the outlet arrangement  2  to the inner end  22 . In the embodiment shown, this has been accomplished by reducing the width of the inwardlying orifice  4   a ,  4   b  in relation to the outwardlying orifice  4   a ,  4   b.    
   The outlet arrangement  2  includes an inner wall  5  that is concentric with the outer wall  23 , which inner wall  5  also extends tubularly with a circular cross section along the longitudinal axis of the separator. An annulus  32  is defined between the outer wall  23  and the inner wall  5 . 
   As shown in  FIG. 3   a , four longitudinal walls  24 ,  25 ,  26  and  27  are provided between the outer wall  23  and the inner wall  5 , which walls extend along most of the length of the outlet arrangement  2 . A channel  3   a  is defined between walls  24  and  25 , the outer wall  23  and the inner wall  5 . A channel  7  is defined between walls  25  and  26 , the outer wall  23  and the inner wall  5 . A channel  3   b  is defined between walls  26  and  27 , the outer wall  23  and the inner wall  5 . A channel  6  is defined between walls  27  and  24 , the outer wall  23  and the inner wall  5 . 
   Near the outer end  21 , the walls  24 ,  25 ,  26  and  27  become a cone  28 . Near the inner end  22 , the walls  24 ,  25 ,  26  and  27  become two longitudinal walls  29  and  30  (see  FIG. 3   b ) extending between the outer wall  23  and the inner wall  5 . Above walls  29  and  30  and between the outer wall  23  and the inner wall  5 , there is defined a channel  12 . Below walls  29  and  30  and between the outer wall  23  and the inner wall  5 , there is defined a channel  11 . 
   The walls  24 ,  25 ,  26  and  27  and the walls  29  and  30  all have a point of intersection at the longitudinal axis of the separator. 
   Even closer to the inner end  22 , the walls  29  and  30  deflect upwards and converge to form a sloping wall  13 . The sloping wall  13  closes off the channel  12  above the walls  29  and  30 . 
   In the part of the outer wall  23  that is located outside of the channels  6  and  17 , a plurality of orifices  33  are formed so as to let the annulus  10  communicate freely with the channels  6  and  7 . The separator chamber available for performing separation outside of the outlet arrangement is therefore the combination of the annulus  10  and the channels  6  and  7 , between which there is fluid communication. In principle, the portions of the outer wall  23  located outside of the channels  6  and  7  have no other function than to ensure that the outlet arrangement achieves greater rigidity. 
   Immediately outside of the sloping wall  13 , a plurality of packings  8  have been provided in the annulus  10 , which packings  8  form a liquid tight end of the separator chamber  20 . 
   An opening  9  is formed in the outer wall  23  between the packings  8  and the sloping wall  13 . This opening  9  connects the channel  3   a  with an annulus  34  located inside the packings and between the separator wall  1  and the outer wall  23 . Beyond the sloping wall  13 , an annulus  32  is defined between the outer wall  23  and the inner wall  5 . 
   A channel  35  is defined within the tubular inner wall  5 . 
   The functioning of the separator and the outlet arrangement according to the invention will now be explained. 
   Water and oil flow in towards the outlet arrangement  2  from the left in FIG.  2 . The water and the oil are then already more or less completely separated. The oil indicated by arrows  36  will settle at the top of the separator and flow in through the orifices  4   a  in the upper side of the outer wall  23 . The water indicated by arrows  37  will settle at the bottom of the separator and flow in through the orifices  4   b  in the underside of the outer wall  23 . 
   A local drop in the velocity of the liquid phases will occur at the outer end  21  due to the cone  28  that causes a narrowing of the cross section. 
   The oil will flow on through channel  3   a  at a velocity that increases as the flow nears the inner end  22  of the outlet arrangement  2 . Most of the oil will flow into channel  3   a  through the orifices  4   a  closest to the outer end  21  of the outlet arrangement and to a lesser extent near the inner end  22 . The oil continues further through channel  3   a  and channel  12 , and out through the opening  9  and into the annulus  34 , from which it is transported to the surface. 
   Likewise, the water will flow on through channel  3   b  at a velocity that increases as the flow nears the inner end  22  of the outlet arrangement  2 . Most of the water will flow into channel  3   b  through the orifices  4   b  closest to the outer end  21  of the outlet arrangement and to a lesser extent near the inner end  22 . The water flows from channel  3   b  and into channel  11 . Immediately beyond the sloping wall  13 , the water is allowed to fill the entire annulus  32 . From here, the water may be transported to the surface, or it may be sent back via channel  35  by means of a pump (not shown), to be injected into the formation. If injection is not an issue, this pipe may be replaced by partitions. 
   In the case of an outlet arrangement of the above mentioned type, it is important that this is oriented correctly in the separator, so as to leave the orifices  4   a  facing upwards and the orifices  4   b  facing downwards. Even though the outlet arrangement can tolerate a certain amount of deviation from this, the performance of the outlet arrangement will deteriorate with any major deviation from this orientation. Below is described an example of an embodiment of the present invention, which embodiment ensures the correct orientation of the outlet arrangement. 
     FIGS. 4 and 5  show devices for orienting the outlet arrangement according to  FIGS. 1-3 .  FIG. 4  shows a separator according to the present invention. The separator wall  1  is shown furthest out. The outlet arrangement  2  is placed inside this. The end  22  of the outlet arrangement faces left in the Figure. Here, several openings  9  are shown. The channel  35  in the pipe  5  is also shown. 
   On the outside of the separator is provided lock nuts  40  and  41  for a packer (not shown). A gear rim  42  equipped with a key  43  is also provided. The key  43  is a wedge that may be moved along the periphery of the gear rim  42  and locked into this. This may for instance be implemented by placing the key  43  in one of several slots in the toothed ring  42 . A locking ring  56  is also provided in connection with the gear rim  42 , in order to lock the gear rim  42  to the separator. 
     FIG. 6  shows the gear rim  42  with wedge  43  and key slot  57  in detail. The key slots are spaced at angular interval of e.g. 5°. 
     FIG. 5  shows a guide sleeve  50 . A wedge belt  51  is provided about the periphery of the guide sleeve  50 . This wedge belt  51  may have a conventional construction, and is designed to fix the guide sleeve  50  in the casing (not shown). At one end  52 , the guide sleeve  50  has been bevelled so as to form a guide edge  53  and  54 . The guide edges  53  and  54  extend in towards a guideway  55  that extends along a length of the guide sleeve. 
   When the separator is to be placed in the well, the guide sleeve  50  is first inserted into the hole with the end  52  facing upwards, until it reaches a horizontal part of the well, in which part the separator is required to be placed. Upon reaching this site, the wedge belt  51  is activated so that the guide sleeve is immobilised against the casing. Then a monitoring tool of a type that is known per se is inserted into the well in order to register the direction in which the guideway  55  has been oriented. This measurement determines the positioning of the key  43  along the periphery of the gear rim  42 . 
   The key is then placed at the same angle relative to the inlet orifices  4   a  as that formed by the guideway  55  in relation to the upward direction. The separator is then inserted into the well. They key  43  contacts one of the guide edges  53 ,  54 . As the separator is inserted into the guide sleeve, the guide edge  53  or  54  causes the separator to rotate until the key  43  is flush with the guideway  55 . When the key  43  is inserted into the guideway  55 , the separator has the correct orientation. Finally, the separator is fixed by means (not shown) that are know per se, and the remaining required pipes are connected to it. 
   Instead of the key being located on the separator and the guide surfaces with the guideway being provided on a guide sleeve, it is also possible to provide the separator with guide surfaces and a guideway, and fixing a device comprising a key in the casing.