WO2001059257A1

WO2001059257A1 - Method of reducing the specific gravity of a crude oil, a hydrocarbon liquid therefor and use of a hydrocarbon liquid

Info

Publication number: WO2001059257A1
Application number: PCT/NO2001/000042
Authority: WO
Inventors: Jon Grepstad
Original assignee: Jon Grepstad
Priority date: 2000-02-08
Filing date: 2001-02-07
Publication date: 2001-08-16
Also published as: AU2001234260A1; NO20000637L; NO20000637D0; NO311103B1

Abstract

The invention regards a method and a hydrocarbon liquid for use in application of the method for reducing the specific gravity of a crude oil by a total of more than 2 % upstream of a second stage separator (28), and preferably upstream of a first stage separator (26), in an associated primary separation plant (24 or 24'). According to the invention, a hydrocarbon liquid that is lighter than produced crude oil, such as condensate, is injected at one or more mixing points (62, 64, 66, 68, 70, 72 or 74) downstream of a crude oil producing reservoir (2) or reservoirs. The hydrocarbon liquid is made up of hydrocarbon components in the methane series with carbon numbers from C2 to C16 inclusive, the hydrocarbon liquid preferably consisting of a mixture of such hydrocarbon components. The invention will among other things result in an increased well lift, an increase in the efficiency of the downstream separation of oil, water and possibly formation particles, and a significant reduction in the related costs of production and processing.

Description

Method of reducing the specific gravity of a crude oil, a hydrocarbon Liquid therefor and use of a hydrocarbon liquid.

The invention regards a method of improved separation of crude oil and/or improved separation of gas, oil and water in the crude oil, and a hydrocarbon fluid for use in the application of the method.

The background to the invention is the drawbacks that often occur in connection with production and/or processing of crude oils, in particular heavier crude oils, and where the drawbacks are essentially attributable to the high specific gravity of the oil.

The specific gravity of a crude oil may vary considerably from one oil reservoir to the next, as one oil reservoir may be made up of a very heavy crude oil with a specific gravity of e.g. 1.03 kg/dm³, while another oil reservoir may be made up of very light crude oil with a specific gravity of e.g. 0.74 kg/dm³. In comparison, the specific gravity of pure water is 1.00 kg/dm³, while the specific gravity of formation water normally is. somewhat higher due to the presence of dissolved salts in the formation water. When compared with lighter and more volatile crude oils, heavier crude oils contain little in the way of dissolved gas, i.e. only small quantities of lighter hydrocarbons such as methane, butane, propane and ethane. Likewise, heavier crude oils contain comparatively larger quantities of the heavy hydrocarbon components, which results in the specific gravity of heavy crude oils being close to or in some cases higher than the specific gravity of produced formation water.

In order to increase the capacity of a well for lifting crude oil and any other associated formation fluids, henceforth simplified and referred to as crude oil, along with any injected water from the reservoir to the surface, use is normally made of a gas injected at reservoir level into a crude oil stream in the production tubing in the well. Thus both the specific gravity of the crude oil and the hydrostatic pressure of the crude oil liquid column are reduced. Following injection, the gas will expand and drive the crude oil up through the production tubing to the surface. The gas that is used typically contains a relatively large fraction of methane gas, which may be taken directly from the gas processing plant at the surface. Methane gas will only to a small degree dissolve in the crude oil if the oil is saturated with dissolved gas. And so the lift gas will flow more or less as a free gas up through the production tubing and contribute to an increased well lift, and thereby to an increased oil production. This is normally referred to as "gas lift".

When produced crude oil arrives at the surface, it flows through at least one pressure reducing valve in order to thereby lower the pressure of the crude oil to the appropriate pressure(s) for further treatment of the crude oil. According to conventioneil separation technology, such further treatment is carried out in separation vessels, henceforth termed separators, in a primary separation plant. Such a separator is called a horizontal or a vertical separator, depending on whether it is positioned horizontally or vertically in the separation plant, horizontal separators being the most common separators in a separation plant. Alternatively, a separator may, based on it mode of operation or its principle of separation, be termed either a cyclonic separator or a centrifugal separator. In the primary separation plant, the crude oil is separated in such separators, generally through at least two in the downstream direction following separation stages, with crude oil separation at one separation stage taking place at one separator pressure, and crude oil separation at another stage taking place at another separator pressure. Crude oil separation at the first downstream separation stage (first stage separation) is normally carried out at the highest separator pressure (the first pressure stage) in the primary separation plant, while crude oil separation at the following downstream separation stage (second stage separation) is normally carried out at the second highest separator pressure (the second pressure stage) in the primary separation plant, the crude oil pressure being reduced through at least one associated pressure reducing valve located between the separation stages. In the first stage of separation, most of the gas phase of the crude oil is normally separated from the liquid phase of the crude oil, while water is separated roughly out from the liquid phase of the crude oil. By making use of gravity, one ensures that most of the water, which has a higher specific gravity than the crude oil, separates out and settles at the bottom of the separator, while gas separates out from or is driven out of the liquid phase of the crude oil and rises above the liquid surface, collecting in the upper part of the first stage separator, which causes oil to collect in an intermediate layer in the separator. Then the gas and liquid phases of the crude oil may be led away from the separator through separate branch streams . The oil branch stream, which now constitutes crude oil containing a considerably smaller fraction of gas and water, is normally transported further through at least one pressure reducing valve to second stage separation in the next downstream separator. In this separator, the remaining water in the crude oil is separated out while most of the remaining gas is driven out of the crude oil due to the lower separator pressure. In order to achieve a satisfactory quality of export petroleum products, it may in some cases be necessary to separate the crude oil further by use of additional downstream separation stages, where the separator pressure is lowered for each subsequent separation stage.

In order to achieve good separation of crude oil and water, the separator (s) must be large enough to give the crude oil flowing through it a long enough residence time, so as to allow the oil and water to separate to a satisfactory degree. For heavy crude oil, e.g. with a specific gravity of 0.92 kg/dm³, it is typical for a separator to be designed to allow residence times of e.g. 10 minutes or more. Correspondingly, the residence time for a light crude oil may for instance be 3-5 minutes.

In order to break most types of emulsions between crude oil and water, the separator is often operated at high temperatures. Moreover, it is advantageous to maintain a high degree of water circulation through the separator stages, while at the same time adding large quantities of emulsion breaking chemicals.

The combination of higher specific gravity, and thereby a greater hydrostatic pressure in the liquid column in the well, together with fewer volatile and soluble gas components in the crude oil will for instance make it more difficult to lift a heavy crude oil than a light crude oil from the reservoir and up to the surface. This has an unfavourable effect on the production rate of a well.

By using conventional separation methods that utilise gravity or centrifugal forces in order to separate materials or liquids with different specific gravities, the separation efficiency is higher the greater the difference in specific gravity between the crude oil and water. A small difference in specific gravity between a heavy crude oil and water will often cause problems when it comes to separating the oil and water efficiently in the separator(s ) , which may necessitate the crude oil having to be separated across more pressure stages than in the case of a large difference in specific gravity between crude oil and water. Among other things, this is time consuming, requires a lot of equipment, and is therefore costly.

At given pressure and temperature conditions, a heavy crude oil is usually more viscous than a light crude oil. In hydraulic terminology, this is expressed by saying that a heavy crude oil has a higher viscosity than a light crude oil. When crude oil flows through pipes and valves, larger quantities of oil/water emulsions are formed e.g. in a heavy crude oil with a high viscosity than in a similar crude oil stream consisting of a light and comparatively less viscous crude oil. Upon downstream separation of the crude oil, such emulsions can prove difficult to dissolve, and as such it is advantageous to reduce the formation of oil/water emulsions as much as possible. When crude oil flows through pipes and valves, a high viscosity in the crude oil will lead to a greater dynamic hydrostatic pressure loss than those dynamic hydrostatic pressure losses that occur in a similar crude oil stream consisting of a less viscous and normally lighter crude oil. Such pressure losses cause turbulent flow and thereby oil/water emulsions, and the formation of such emulsions intensifies as a function of the increase in hydrostatic pressure loss. Experience shows that such emulsions are difficult to break in the subsequent crude oil separation, resulting among other things in higher equipment and processing costs.

A heavy, and therefore normally high-viscosity crude oil will attach to the formation particles with a greater bonding strength than a light, and therefore normally low-viscosity crude oil. In a reservoir-producing fluid flow, a high- viscosity crude oil will therefore to a greater extent than a low-viscosity oil pull formation particles into the production tubing, where the fluid flow will then bring the particles up to the surface for further treatment. This is particularly a problem in those cases where the reservoir rock is unconsolidated and therefore prone to being broken off by the reservoir-producing fluid flow. The production of large quantities of formation particles may cause major production related problems, e.g. by formation particles having to be separated from the produced liquid phase and then disposed of in a suitable location. Separating production particles from e.g. a heavy crude oil is complicated by the bonding strength between the crude oil and the formation partici.es being greater than that which would have existed in a liqht, and comparatively less viscous crude oil. This also results in the cost of equipment and processing increasing as a function of the increased crude oil viscosity.

At a set separation capacity, the relatively long separator residence time required by a heavy crude oil will require the separator to be constructed with a larger volume than a separator for light crude oil. In order to achieve the same separation capacity, a separator for heavy crude oil may then for instance have to be twice the size of a separator for light crude oil, the equipment costs increasing at the same rate as the separator size.

In some cases, using a high separator temperature in order to break oil/water emulsion will require the addition of extra heat energy in order to achieve the optimum operating temperature. In addition, emulsion breaking chemicals may be used, which will also lead to an increase in processing costs along with possible environmental problems related to the handling of such chemicals.

Putting a high flow of water through the separation stages is a method that is used to facilitate the separation of produced water from the crude oil, achieved through heating the produced water and then circulating it back to the first stage separator. This requires extra pumping capacity, thus leading to higher equipment costs. Further, the increase in water throughput will also give a reduction in separation capacity. Gas lift through injection of free gas into the crude oil will lead to the gas expanding to an increasing degree downstream, thus causing the formation of gas bubbles. Gas bubbles cause turbulence in the crude oil stream, in turn causing the formation of more oil/water emulsions, which often results in foaming of the crude oil stream. This makes it more difficult to separate the water from the crude oil during the downstream separation, than in those cases where such lift gas is not employed.

The object of the present invention is to provide a method and a suitable hydrocarbon liquid for use in application of a method that is implemented at a downstream position relative to a crude oil producing reservoir, or possibly reservoirs, with the aim of reducing the specific gravity of the crude oil to an advantageous level, which may effect an increase in the production of crude oil and/or improved separation of gas, oil and water in the crude oil.

As appears from the characteristics of the present independent claims, the object is achieved by injecting a o sufficient amount of a hydrocarbon liquid that is lighter relative to the crude oil, such as condensate, into the crude oil stream. The hydrocarbon liquid is injected at one or more mixing points downstream of the crude oil producing reservoir or reservoirs, but upstream of one or more associated 5 separators wherein second stage separation, and preferably upstream of one or more associated separators wherein first stage separation of the crude oil is carried out in a primary separation plant allocated to the reservoir, but where the hydrocarbon liquid is preferably not injected directly into o any of the separators in the primary separation plant. In this connection, sufficient quantities of the hydrocarbon liquid are injected to reduce the specific gravity of the crude oil by a total of more than 2% upstream of the second stage separator(s) , and preferably before the first stage separator(s) in said primary separation plant.

According to the invention, a hydrocarbon liquid that is lighter relative to the crude oil, such as condensate, is injected in sufficient quantities into the crude oil stream upstream of the second stage separator(s) , and preferably upstream of the first stage separator(s ) , in the primary separation plant. This is linked up with the object of the invention being, among other things, to achieve an improved separation of gas, oil and water in the crude oil. In this connection, it is possible to provide above primary separation plant with e.g. one additional separator located between the first stage separator and the second stage separator. The hydrocarbon liquid is then injected at at least one mixing point located between the additional separator and the first stage separator, as the hydrocarbon liquid is injected into the crude oil stream at a pressure that is equal to or higher than that which exists in the first stage separator, and where this pressure may be supplied by at least one pump connected to the crude oil stream upstream of the relevant mixing point(s). Injection at such a pressure is necessary in order to ensure that the lightest and most volatile hydrocarbon components of the hydrocarbon liquid are not separated out but remain dissolved in the liquid phase in the crude oil stream, thereby reducing the specific gravity of the crude oil. Such a reduction in specific gravity would thereby lead to improved separation of preferably oil and water in the oil branch stream from the first stage separator being achieved in the additional separator. Then the downstream oil branch stream from the additional separator is sent to a second stage separator, if any.

The method of injecting hydrocarbon liquid into a crude oil stream at one or more mixing points between an additional separator and an upstream separator may in addition be taken further and used at any downstream separation stages in the primary separation plant, however this would increase the cost of such equipment while only giving marginal improvement of the crude oil separation. Sending the oil stream on to further additional separators in the primary separation plant is therefore not very cost effective and will in most cases not be implemented.

When processing crude oil in accordance with known methods, condensate is normally considered a problematic by-product that one attempts to dispose of by among other things injecting relatively small quantities of condensate into the crude oil stream. Such condensate is normally injected at one or more positions upstream of each separator associated with one particular separation stage in a primary separation plant, and where the injection pressure depends on and is determined by the pressures at which the various separators operate. In this connection, it is essential for the condensate to be added to the crude oil stream in small doses and in such a way that the condensate is present in low concentrations in the crude oil stream. By so doing, the hydrocarbon composition of the crude oil, and thereby its specific gravity, is not affected to any appreciable extent. In this connection it is usual for condensate to be mixed in in such small quantities that the overall reduction of the specific gravity of the crude oil is not more than 0.5% before the crude oil is separated at any pressure stage in the primary separation plant.

In this manner, an insignificant contamination consisting of condensate components is introduced into the finished and export-ready petroleum products that emerge from the processing, such as dry gas and various grades of crude oil having a higher specific gravity than condensate. By that means, a by-product is disposed of without the hydrocarbon composition of the crude oil being altered significantly, so that the composition of the export-ready petroleum products continues to conform to acceptable standards. On the other hand, the introduction of condensate in quantities sufficient to reduce the specific gravity of the crude oil by more than a total of 0.5% prior to separation at any pressure stage in the primary separation plant is considered unfavourable, as the hydrocarbon composition of the crude oil is thereby altered significantly and to such an extent that the quality of the export-ready petroleum products is reduced to an unacceptable standard. This object is fundamentally different from the object of the present invention, the object of the invention being to alter the hydrocarbon composition of the crude oil significantly, thereby to reduce the specific gravity of the crude oil by a total of more than 2% upstream of the second stage separator(s) , and preferably before the first stage separator(s) , in the primary separation plant, which would normally cause the quality of the export-ready petroleum products to deteriorate severely. Such deterioration normally consists in one or more of the properties of the products being altered more than that which is desirable, e.g. by a gas product being given a calorific value and/or a hydrocarbon dew point above the desired level, or by e.g. an oil product being given a vapour pressure above the desired level, such elevated vapour pressure being caused by the presence of a greater than desired content of light condensate components in the oil. In this connection, it would therefore be surprising if such a substantial addition of e.g. condensate to the crude oil stream were to have a positive effect on the export-ready petroleum products, which is exactly what the invention has. As mentioned above, the invention will among other things effect an improved separation of produced oil and water, which increases the quality of the export-ready petroleum products, for one, and results in improved separation of the gas, oil and water of the crude oil in comparison with those cases where the invention is not applied.

The specific gravity and hydrocarbon composition of a crude oil will vary in accordance with the amount of hydrocarbon liquid injected, and so the injection of large quantities of hydrocarbon liquid will cause a major change in the specific gravity and hydrocarbon composition of the crude oil, while the injection of small quantities of hydrocarbon liquid will cause a minor change in the specific gravity and hydrocarbon composition of the crude oil.

When injecting e.g. condensate with the aim of disposing of a problematic by-product, previous experience has shown that the specific gravity of the crude oil should not be reduced by more than a total of 0.5% if it is required that the export-ready petroleum products maintain a desired quality in accordance with the existing conditions. In order to compensate for a certain amount of uncertainty regarding what the total reduction in crude oil specific gravity should be, and in order to separate the present invention from injection of e.g. condensate for the purpose of disposing of a problematic by-product, the present independent claims state that the specific gravity of the crude oil is to be reduced by a total of more than 2% upstream of the second stage separator(s) , and preferably before the first stage separator(s) , in said primary separation plant. This will normally be regarded as inadvisable by persons skilled in the art.

Hydrocarbon liquid to be injected into a crude oil stream according to the invention consists of hydrocarbon components in the methane series with carbon numbers from C2 to C16 inclusive, with the hydrocarbon liquid preferably consisting of a mixture of such hydrocarbon components.

One condition of achieving the object of the invention is that the hydrocarbon liquid must be soluble in produced crude oil. The hydrocarbon components of this liquid must be at an equilibrium in order to ensure that they remain in the liquid phase under the temperature and pressure conditions that exist at the mixing point(s), and that they remain dissolved in the crude oil after injection. When injecting the hydrocarbon liquid into the crude oil at a mixing point, the blending together of these must take place relatively quickly and in quantities adjusted to the temperature and pressure conditions that exist in the crude oil stream. In connection with this, subsequent downstream crude oil separation must occur at a pressure and a temperature that will leave most of the hydrocarbon liquid dissolved in liquid produced crude oil, and neither the crude oil nor the hydrocarbon liquid must be miscible with water.

It is also assumed that sufficient quantities of the specified hydrocarbon components may be obtained, e.g. as transported liquid from other nearby production. Alternatively, and as an example, the specified hydrocarbon liquid may be provided in the form of condensate from the primary separation plant in question, which plant treats the crude oil and in which condensate accumulates and is then circulated, or even recirculated into the relevant crude oil stream. Recirculation of condensate may be carried out by letting the condensate flow onwards downstream from the first stage separator, wherein the condensate is present as a liquid phase, together with produced crude oil to the subsequent separation stage(s) in the processing plant. At each stage of the separation, the condensate will then, together with the remaining dissolved gas in the crude oil from the previous pressure stage, evaporate and separate out of, or be driven out of, the liquid phase of the crude oil and rise above the liquid surface and collect in the upper part of the associated separator. The driven-out (stripped) gas is then carried onwards to the respective gas compressor for each pressure stage, where the gas is cooled in a gas cooler both before and after gas compression, so that the gas is condensed to a gas condensate after each pressure stage in the processing plant. The purpose of the cooling is to provide optimum operating conditions for the gas compressor, and also to ensure that the quality of the export-ready petroleum product is satisfactory. Following cooling and gas compression, condensate from each pressure stage is preferably brought together in one condensate stream, which is then injected into the crude oil stream at the desired mixing point(s) upstream of the first stage separator. The injected condensate is then separated out again from the crude oil in accordance with the preceding description, and recirculated. By increasing the gas stripping and cooling/compressing the gas in this manner, it becomes possible to provide sufficient quantities of condensate to allow the continuous injection of condensate into a producing crude oil stream. At the same time, the quality of the petroleum products is maintained, while the separation of gas, oil and water in the crude oil is improved.

The injection of the hydrocarbon liquid into a crude oil stream has the effect of reducing the specific gravity of the crude oil, and thereby normally also the viscosity of the crude oil. A reduced crude oil viscosity causes the dynamic hydrostatic pressure loss in the crude oil stream to be reduced, and also causes fewer oil/water emulsions to be formed.

The method and use of the hydrocarbon liquid in accordance with this invention will therefore, among other things, result in a production well gaining an improved ability to lift crude oil from the reservoir or reservoirs up to the surface, so as to give an increased production rate.

The method and use of the hydrocarbon liquid also allows more efficient and cost saving downstream processing and separation of oil and water or if required, separation of oil and produced solids. As a result of the method, it will be possible to use smaller separators, or if so desired, increase the capacity of the existing separators.

The method and use of the hydrocarbon liquid also provides the possibility of reducing or eliminating the use of emulsion breaking, heat input and/or chemicals, while also making it possible to reduce any water flow through the separation stages, which in practice is implemented by reducing the pumping capacity.

The method and use of the hydrocarbon liquid may if desired be used in conjunction with gas lift in production wells, cf. the above mention of gas lift. The most advantageous thing to do, however, will be to replace or reduce the use of such lift gas, as experience has shown the use of such lift gas to make the separation of water from crude oil more difficult than is the case when such lift gas is not used.

The method and use of the hydrocarbon liquid in accordance with the invention may furthermore advantageously be used in connection with mobile processing plants, e.g. on a production ship or a in shore-based plant, as the hydrocarbon composition of the crude oil, and thereby the specific gravity of the crude oil, in accordance with the invention is regulated to the specific gravity or specific gravities, or possibly range of specific gravities, for which the plant in question has been designed. Normally, such mobile plants or units are equipped with production and processing equipment that is dimensioned and designed in a standard manner according to the existing conditions in e.g. one geographical area, in order to process the most commonly produced crude oil type(s) of this area. For such crude oil types, such a plant effects an optimum or near optimum separation of gas, oil and water. On the other hand, the same plant may effect considerably poorer separation of gas, oil and water for other types of crude oil, in which case it may be necessary to take steps to increase the separation efficiency of the plant, e.g. by adding emulsion breaking chemicals to the crude oil stream. By injecting hydrocarbon liquid into a crude oil stream, where the hydrocarbon liquid is wholly or in part made up of e.g. separated condensate from the crude oil primary separation plant, it will be possible to avoid having to mix relatively low concentrations of condensate into the crude oil stream in order to dispose of this, such condensate normally being considered a problematic by-product. On the contrary, such condensate may according to the invention be used as a hydrocarbon liquid to be injected into a crude oil stream, where the aim is to reduce the specific gravity of the crude oil by a total of more than 2% upstream of the second stage separator(s) , and preferably the first stage separator (s ) , of said primary separation plant.

The following part of the description will, with reference to the figures 1-4, refer to different and non-limiting examples of the invention, where one particular reference number relates to the same detail in all drawings in which this detail is shown, and where:

Figure 1 shows a schematic flow diagram showing essential stages in a crude oil production process from a reservoir and through a primary separation plant on the surface, which plant in the drawing constitutes a first stage separator and a second stage separator, and where examples of points for injection of the hydrocarbon liquid into a crude oil stream are indicated;

Figure 2 shows a schematic vertical section through a lower portion of a crude oil producing well, where the hydrocarbon liquid is injected into produced crude oil; Figure 3 shows a schematic section of a flow diagram of a crude oil production process, where some examples of points for injection of the hydrocarbon liquid into the crude oil stream are indicated and where preseparation of the crude oil is carried out;

Figure 4 in relation to figure 3 shows a schematic section of a similar flow diagram of a crude oil production process, which shows parts of a plant for further processing of crude oil, but where no preseparation of the crude oil is carried out;

Figure 5 in relation to figure 1 shows a schematic and alternative primary separation plant, in which an additional separator is shown positioned between the first stage separator and the second stage separator, and where the additional separator, unlike the second stage separator, operates at a pressure that is equal to or higher than the pressure of the first stage separator.

Well equipment and/or facilities that have no direct bearing on the invention but in all other respects are prerequisites of carrying out the invention, have not been specified or described in the following examples of embodiments. Such equipment may for instance include wellheads, pipes, couplings, valves and control equipment.

Crude oil is extracted from an underground reservoir 2 with overlying ground formation(s) 3, where the crude oil flows into a production well 4 through perforations 6 in the casing 8 of the well 4. The directions of flow for the crude oil, injected hydrocarbon liquid and separated fluids from the separation process are indicated by arrows in the drawing. The crude oil flows towards the surface through a production tubing 10 placed in the casing 8. In an annulus 14 between the casing 8 and the production tubing 10, at least one packing 12 has been positioned at a suitable location downstream of the perforations 6 in order to prevent the crude oil from flowing up through the annulus 14, allowing the crude oil to be conducted through the production tubing 10. As an initial step in the further processing on the surface, the crude oil first flows through a valve 16 normally termed a choke valve, the main function of the valve 16 being to reduce the pressure of the crude oil to a suitable level for the subsequent separation process. Then the crude oil flows into a production manifold 18 in which crude oil streams from several production wells are united, if applicable. The unified stream then continues to e.g. a cyclone separator 20 in which rough separation of formation water from the crude oil may take place, or possibly separation of formation particles from the crude oil. The separated crude oil then passes through a valve 22 that is intended for emergency shut-off of the liquid feed to a primary separation plant 24 or 24'. In the primary separation plant 24 or 24' indicated, the crude oil is separated through two pressure stages, one high pressure stage (first stage separation) and one low pressure stage (second stage separation), in horizontal separators 26 and 28. Separation at the high pressure stage is carried out in a first stage separator 26 often termed the primary separator, while separation at the low pressure stage is carried out in a second stage separator 28, cf. figure 1. The separation leads to gas, oil and water being separated to a great extent, and concentrations of each of these may then be drained away in branch stream, where one or more of the branch streams may be processed further. According to the invention, a hydrocarbon liquid that is lighter relative to the crude oil is injected into the crude oil stream at one or more mixing points upstream of the second stage separator 28, and preferably upstream of the first stage separator 26, but the hydrocarbon liquid is preferably not injected directly into any of the separators 26, 28 or 30 in the primary separation plant 24 or 24'. Examples of such mixing points are indicated in the drawings as mixing points 62, 64, 66, 68, 70, 72 or 74.

The first example of an embodiment regards downhole injection of the hydrocarbon liquid into the crude oil stream at mixing point 62, cf. figure 1. The hydrocarbon liquid is pumped down into the well 4 through a feed pipe 32 positioned in the annulus 14 between the casing 8 and the production tubing 10, cf. figure 2. The feed pipe 32 is connected to the production tubing 10 immediately above the packing 12, so that the hydrocarbon liquid can be injected into the production tubing 10 and mixed with the crude oil stream. This is shown in a highly simplified manner in figure 2, as the feed pipe 32 will in practice have to be connected to the production tubing 10 by means of a coupling device incorporating one or more valves that make it possible to inject the hydrocarbon liquid into the crude oil stream while preventing the crude oil from flowing into the feed pipe 32. Such injection may also be accomplished without a feed pipe 32, by pumping the hydrocarbon liquid down through the annulus 14 and providing the production tubing 10 with a valve (not shown in the drawing) that allows injection of the hydrocarbon liquid into the crude oil stream while preventing the crude oil from flowing into the annulus 14. Alternatively, the hydrocarbon liquid may be injected for the purpose of improving the processing characteristics of the crude oil, so as to achieve a more efficient separation of gas, oil and water and possibly formation particles. In this connection, the injection is preferably carried out after the crude oil has been lifted to a surface position and upstream of the second stage separator 28, preferably upstream of the first stage separator 26, but the hydrocarbon liquid is preferably not injected directly into any of the separators 26, 28 or 30 of the primary separation plant 24 or 24'. The following example of an embodiment describes this.

The second example of an embodiment deals with injection of the hydrocarbon liquid at mixing point 64, where the hydrocarbon liquid is injected into the crude oil stream through a feed pipe 34 upstream of and immediately before valve 16. Mixing in the hydrocarbon liquid reduces the viscosity of the crude oil, which in the case of flow through valves and pipes reduces turbulence in the crude oil stream, thereby ensuring that fewer problematic oil/water emulsions are formed in the crude oil.

In some cases, it is favourable to let the crude oil go through a preseparation of oil and water, or possibly a separation of formation particles and crude oil, before letting the oil continue into the primary separation plant 24 or 24'. Preseparation may for instance take place in a conventional separator, normally in the first stage separator 26, or in a cyclone separator 20, cf. figures 1 and 3. In the cyclone separator shown here, a rough separation of oil and water takes place, whereupon water is drained off through an outlet pipe 44 and oil through an outlet pipe 46. Use of the cyclone separator subjects the crude oil to a rotational force that separates liquids having different specific gravities. This rotation is indicated by a partly circular arrow on the cyclone separator 20 in figures 1 and 3.

The third example of an embodiment regards injection of the hydrocarbon liquid at one or more mixing points 66, 68, 70 or 72 located between the choke valve 16 and the first stage separator 26. In this connection, figures 3 and 4 are in principle identical, however figure 3 also includes a cyclone separator 20 located between the production manifold 18 and the valve 22. The hydrocarbon liquid is injected into the crude oil stream through one or more of feed pipes 36, 38, 40 or 42. Injection of the hydrocarbon liquid causes the difference in specific gravity between oil and water in the crude oil to increase, which results in more efficient separation of oil and water in the primary separation plant 24 or 24', and if applicable, also in the cyclone separator.

Figure 4 shows the principle of the progression of a crude oil through the first stage separator 26 following injection of the hydrocarbon liquid upstream of this. In the first stage separator 26, the crude oil mixture is separated into three layers. The gas layer 54 is shown uppermost in the first stage separator 26, the gas being removed through an outlet pipe 48; the oil layer 56 is shown in an intermediate layer in the first stage separator 26, the oil being drained off through an outlet pipe 50; and the water layer is shown at the bottom of the first stage separator 26, the water being drained off through outlet pipe 52. The oil in the outlet pipe 50 may continue on to be separated further at a different and lower pressure in the second stage separator 28, cf. figure 1. The fourth example of an embodiment, cf. figure 5, shows an alternative primary separation plant 24' in which is placed a first stage separator 26 and a second stage separator 28, similarly to figure 1, but in which is also placed an additional separator 30 between the first stage separator 26 and the second stage separator 28, and in which all the separators 26, 28 and 30 are connected to the same initial crude oil stream. The hydrocarbon liquid in question is then injected through at least one mixing point, which in this example of an embodiment is given as mixing point 74, located between the additional separator 30 and the first stage separator 26, the hydrocarbon liquid being injected at a pressure, e.g. 20 barg (bar overpressure), that is equal to or higher than the operating pressure of the first stage separator 26, e.g. 10 barg. The operating pressure used for crude oil separation in the additional separator 30 is in this example of an embodiment supplied to the crude oil stream by a pump 60 situated upstream of the additional separator 30 and the mixing point 74. Injection at this kind of pressure, e.g. 20 barg, is necessary to ensure that the lightest and most volatile hydrocarbon components do not separate out from the crude oil stream in the form of gas, but remain dissolved in the liquid phase of the crude oil so as to reduce the specific gravity of this before separation in the additional separator 30. Such a reduction in specific gravity will thereby make it possible to achieve improved separation of preferably oil and water in the additional separator 30. Then the oil branch stream from the additional separator is conducted further in the downstream direction to a possible second stage separator 28 that operates at a pressure of e.g. 0.5 barg, which pressure is lower than the operating pressure used in the first stage separator 26.

Claims

C l a i s

1. A method of reducing the specific gravity of a crude oil in a crude oil producing liquid stream by a total of more than 2% upstream of at least one associated second stage separator (28), and preferably upstream of at least one associated first stage separator (26), between which separators (26 and 28) the crude oil producing liquid stream may be connected up to at least one additional separator (30) in which separation of oil and water is carried out at a pressure that is equal to or higher than the separation pressure employed in the first stage separator (s) (26), and where the second stage separator(s) (28) employ a lower separation pressure than the first stage separator(s) (26) and if applicable, the additional separator (30), the specific gravity of the crude oil being reduced by altering the hydrocarbon composition of the crude oil, where the crude oil is produced from an underground reservoir (2) or reservoirs, the crude oil flowing into one or more wells (4) provided with the required casing (8), production tubing (10) and other well equipment, as well as the required connections to the surface, and where a choke valve (16) and an emergency shut-off valve (22) are provided at the surface in order to reduce and control the pressure of the crude oil stream respectively, along with a production manifold (18), a primary separation plant (24 or 24') incorporating one or more separators (26, 28 or 30) and possibly one or more pumps (60) and pressure reducing valves (61), possibly a cyclone separator (20) or similar equipment situated upstream of the primary separation plant (24 or 24'), in which equipment is performed, as an example, preseparation of crude oil and/or separation of formation particles, and where such surface equipment and components in addition are provided with the associated pipe connections, fittings, valves and other necessary equipment, c h a r a c t e r i z e d i n that sufficient quantities of a hydrocarbon liquid that is lighter relative to the crude oil are injected into the crude oil at one or more mixing points (62, 64, 66, 68, 79, 72 or 74) downstream of the crude oil producing reservoir (2) or reservoirs, and upstream of the second stage separator(s) (28), preferably the first stage separator(s) (26).

2. A method according to Claim 1, c h a r a c t e r i z e d i n that the hydrocarbon liquid is conducted to the mixing point(s) in the well (4) through at least one feed pipe (32).

3. A method according to Claim 1, c h a r a c t e r i z e d i n that the hydrocarbon liquid is conducted to the mixing position(s) in the o well (4) through an annulus (14) between the casing (8) and the production tubing (10).

4. A hydrocarbon liquid for reducing the specific gravity of a crude oil in a crude oil producing liquid stream according to one or more of the preceding claims, 5 c h a r a c t e r i z e d i n that the hydrocarbon liquid is made up of hydrocarbon components in the methane series with carbon numbers from C2 to C16 inclusive, and where the hydrocarbon liquid preferably consists of a mixture of such hydrocarbons.

5. A hydrocarbon liquid according to Claim 4, c h a r a c t e r i z e d i n that the hydrocarbon liquid is such that it dissolves in the crude oil.

6. A hydrocarbon liquid according to the claims 4 or 5, c h a r a c t e r i z e d i n that the mixture of crude oil and hydrocarbon liquid is not such that it dissolves in water.

7. A hydrocarbon liquid according to one or more of the claims 4-6, c h a r a c t e r i z e d i n that the hydrocarbon components of the crude oil stream are at an equilibrium at the existing pressure and temperature conditions.

8. A hydrocarbon liquid according to one or more of the claims 4-7, c h a r a c t e r i z e d i n that the hydrocarbon liquid is dissolved in the liquid crude oil from the mixing point(s) (62, 64, 66, 68, 79, 72 or 74).

9. The use of a hydrocarbon liquid as stated in one or more of the preceding claims, as an injection liquid in a crude oil producing liquid stream for the purpose of reducing the specific gravity of the crude oil by a total of more than 2% upstream of the second stage separator(s) (28), and preferably upstream of the first stage separator(s) (26).