WO1998017941A1

WO1998017941A1 - A process for treating a non-stabilized crude oil

Info

Publication number: WO1998017941A1
Application number: PCT/NO1997/000277
Authority: WO
Inventors: Geir B. Lorentzen; Otto Skovholt; Carsten SØRLIE
Original assignee: Den Norske Stats Oljeselskap A.S
Priority date: 1996-10-22
Filing date: 1997-10-20
Publication date: 1998-04-30
Also published as: NO964489L; NO964489D0; AU4794897A; NO304564B1

Abstract

A process for treating a raw material, consisting of a well stream comprising crude oil and natural gas, or of a stream of non-stabilised crude oil, for storage/transportation of a stabilised crude oil and a stabilized gas product in separate tanks. Gaseous and volatile components are separated from the raw material and are contacted with water under hydrate-forming pressure and temperature conditions in a hydration zone to produce a hydrate mass. Said mass is cooled down to a final temperature lower than the freezing point of water, to form a liquid gas hydrate-containing hydrocarbon product comprising particles of gas hydrate suspended in a hydrocarbon-containing liquid supplied during the production or cooling of the hydrate mass.

Description

A process for treating a non-stabilized crude oil

The present invention relates to a process for treating a hydrocarbon mixture comprising volatile components which may cause problems during handling and transportation of the mixture, especially a well stream comprising crude oil and natural gas, or a stream of a non-stabilized crude oil, for storage/transportation of a stabilized crude oil and a stabilized gas product in separate tanks.

When producing crude oil from an oil field, a separation of the well stream into oil, water and gas is effected as a first step. This step is required in particular on offshore oil fields to obtain a stabilized crude oil suitable for transpor- tation by tankers. Natural gas accompanying the produced crude oil in the well stream must be handled in one way or another after its separation from the crude oil. This is often effected by burning the gas or by reinjecting it into the oil field. However, burning is not a satisfactory solution, since burning represents a waste of valuable hydrocarbon resources, and also is a source of air pollution. Reinjection, which adds costs to the crude oil production, will often be unacceptable due to such added costs and due to possible interference with the crude oil production on the field. Therefore, increasing efforts are made to recover the hydrocarbon gas contained in crude oil based well streams, either by conditioning and stabilizing the gas for storage and transportation ashore, or by converting it into more easily transportable products on the oil field.

While traditional transportation of the separated gas from the field in a condensed form has entailed inconveniences connected with the high pressures and/or low temperatures required for such transportation, conversion of the gas to other, more easily transportable products has often been restricted by shortage of space, in particular on offshore oil fields.

In the later years, propositions have been made to transport natural gas in the form of gas hydrates. The gas hydrates in question are obtained by contacting the natural gas with water under suitable temperature and pressure conditions, e.g. at temperatures somewhat above 0 °C, and at pressures of the order of 60 bars. In particular, the following gas components are hydratable, given in order of increasing reactor pressure: isobutane, propane, ethane, C0₂, methane and nitrogen. N-butane is also hydratable, when present in mixture with hydrocarbons having 1 to 3 carbon atoms. Heavier hydrocarbon components do not form hydrates, or only to a small extent, because there is no room for the large gas molecules in the voids of the hydrate grid. In hydrate form, the natural gas can reach a packing density of up to 180 Sm³ of gas per m³ of gas hydrate (calculated for methane gas). This high packing density and the possiblity of transporting the gas hydrate at pressures near the atmospheric pressure and at low temperatures makes transportation of natural gas in hydrate form interesting as an alternative to transportation of the natural gas after liquefaction at high pressure and/or low temperature.

For further details regarding the forming of gas hydrates, reference may be had e.g. to WO 93/01153, and WO 94/00713, as well as to US Patent No. 2,356,407; PCT/NO97/00111, and PCT/NO97/00112. However, these earlier publications do not disclose any practical and economical method of stabilizing by gas hydrate formation the hydratable gaseous and volatile components of a well stream on crude oil basis.

Processes are now proposed in which a well stream comprising crude oil and natural gas can be treated to produce a stabi- lized crude oil, while at the same time recovering in a convenient and economical way the natural gas contained in the well stream.

According to a first aspect of the present invention, such object is achieved by vaporizing from the well stream gaseous and volatile components which are then at least in part subjected to a hydration to form a gas hydrate mass, which hydrate mass is withdrawn in a cooled state, suspended in a hydrocarbon-containing liquid. The obtained gas hydrate- containing slurry will be very suitable for transportation at low pressure and moderate temperature in tanks separate from the crude oil tanks in a tanker.

Thus, according to this first aspect of the invention, a process is provided for treating a raw material, consisting of a well stream comprising crude oil and natural gas, or of a stream of non-stabilized crude oil, for storage/transportation of a stabilized crude oil and a stabilized gas product in se- parate tanks, wherein gaseous and volatile components and any water are separated from said raw material, to obtain a crude oil which is stable at a selected crude oil storage and/or transportation pressure. The new process is characterized by the following steps: a substantial part of the gaseous and volatile components which are separated from the raw material are contacted with water under hydrate-forming pressure and temperature conditions in a hydration zone to produce a hydrate mass comprising hydrates of hydratable components in the mixture of gaseous and volatile components, the obtained hydrate mass is cooled down in a cooling zone to an average final temperature/storage temperature which is lower than the freezing point of water, at which temperature it is stable at a selected storage or transportation pressure, so as to form a liquid gas hydrate-containing hydrocarbon product comprising particles of gas hydrate suspended in a hydrocarbon-containing liquid and being storable and transportable as a stable product, said hydrocarbon-containing liquid being supplied during the production or cooling of the hydrate mass, and gaseous and/or volatile compounds which have not converted to gas hydrates are optionally subjected to further treatment .

in a preferred embodiment of this process, the heat generated during the formation of hydrate in the hydration zone is absorbed by said hydrocarbon-containing liquid, which in this case is a cold light oil supplied from an external source, and/or is constituted by such condensed components of the supplied component mixture which have not formed hydrates. Thus, a liquid gas hydrate-containing hydrocarbon product is obtained, containing gas hydrate particles suspended in a carrier liquid constituted by the light oil, which product is then further cooled - optionally after previous replacement of its light oil carrier liquid with fresh light oil - to a temperature lower than the freezing point of the water.

The components subjected to hydration may be constituted by the entire mixture of volatile components separated from the raw material, or they may be constituted by such mixture after previous separation therefrom of a fraction containing no substantial amounts of hydrate-forming components. It may be advantageous that only such gaseous components and volatile components which would exist in a gaseous form at the prevailing pressure and temperature conditions in the hydration zone are introduced into that zone.

The hydrate-forming pressure and temperature conditions uti- lized in the process are usually pressures in the range of 10 to 150 bars, more often from 30 to 100 bars, and temperatures in the range of 0 °C to 10 °C, preferably in the range of 0 °C to 4 °C.

Said light oil which is useful as a cooling and carrier liquid is constituted essentially by hydrocarbons having a number of carbon atoms in the C₅-C₁₀ range and being highly liquid at the temperatures prevailing in the hydration zone and in the appurtenant cooling units, especially in the cooling units for cooling and stabilizing the hydrate product. The light oil may be separated from the raw material, i.e. from the crude oil- containing well stream, or it may be supplied from an external source. However, the light oil may also, wholly or partly, be condensed out from the supplied gaseous phase during the hydration process and thus be formed in situ. Under given preconditions, even other hydrocarbon-containing liquid may be useful as a cooling and carrier liquid in the process, such as liquid propane. The cooled, liquid gas hydrate-containing hydrocarbon product obtained by the process, comprising gas hydrate particles suspended in a carrier liquid consisting preferably of a light oil, can be stored and handled by means of traditional storage s and transportation equipment for liquids and slurries. The term "liquid", as used herein in connection with suspensions and slurries, is meant to include states in which the substance in question can be liquefied, e.g. by fluidization of a deposit of gas hydrate particles in a carrier liquid for the o particles.

In connection with the final cooling of the suspended gas hydrate to the desired storage and/or transportation temperature, it may in certain instances be desirable to substitute a s "fresh" light oil for the light oil utilized as a cooling and carrier liquid during the hydration process proper. The reason for that is that the light oil, as a consequence of its being used in the hydration process, may acquire an undesireably high content of solved, non-hydratable gas components which 0 might create problems during the final cooling, because this cooling is carried out at a lower pressure than the hydration process. A "fresh" light oil is here meant to be a light oil which does not contain undesired amounts of such solved gas components, for instance a light oil which has previously been 5 utilized in the hydration process but which has been degased.

According to a second aspect of the present invention, there is vaporized from the well stream a fraction constituted by gaseous and volatile components, said fraction including in o addition to light hydrocarbon components certain heavier hydrocarbon components which in a traditional well stream separation would be included in the crude oil fraction. A light gas phase and a somewhat heavier condensate phase are then produced from the separated vaporized fraction. The light 5 gas phase is then converted to a gas hydrate, which during the process will be suspended in the condensate phase and cooled. The obtained gas hydrate-containing suspension or slurry will be of the same nature as the one obtained according to the first aspect of the invention. Thus, according to said second aspect of the invention, a process is provided for treating a raw material, consisting of a well stream comprising crude oil and natural gas, or of a stream of non-stabilized crude oil, for storage/transportation of a stabilized crude oil and a stabilized gas product in separate tanks, wherein gaseous and volatile components and any water are separated from said raw material, to obtain a crude oil which is stable at a selected crude oil storage and/or transportation pressure. The process is characterized by the following steps:

(a) the gaseous and volatile components separated from the raw material are separated in a light gas phase containing a main part of the hydrate-forming gas components in the mixture, and a condensate phase formed by volatile but somewhat heavier components,

(b) the separated light gas phase, or a substantial part thereof, is contacted with water under hydrate-forming pressure and temperature conditions in a hydration zone to achieve hydrate formation, (c) heat generated during the hydrate formation is absorbed by a cold condensate phase separated in step ( a ) , which condensate phase is supplied to the hydration zone, whereby a liquid hydrocarbon product is obtained comprising gas hydrate particles suspended in a carrier liquid consti- tuted by the supplied condensate phase and any condensed components of the supplied gas phase which have not formed hydrate, the obtained liquid, gas hydrate-containing hydrocarbon product is further cooled - optionally after previous re- placement of its carrier liquid with fresh condensate phase - to a temperature lower than the freezing point of the water, at which temperature it is stable at a selected storage or transportation pressure, and volatile components which have not been converted to gas hydrates are optionally subjected to further treatment.

Regarding the condensate phase to be used as a cooling and carrier liquid, what has been said above about the light oil also applies to the condensate phase: it is constituted essen- tially by hydrocarbons having a number of carbon atoms in the C₅-C₁₀ range and being highly liquid at the temperatures prevailing in the hydration zone and in the appurtenant cooling units, especially in the cooling units for cooling and stabi- lizing the hydrate product. The meaning of a "fresh" condensate phase corresponds to the abokve given meaning of a "fresh" light oil.

The gas phase and the condensate phase which are utilized in the process may be produced by a separation process, wherein the raw material is separated in several serially connected crude oil separators at successively decreasing pressure, with separation of a gas phase from the crude oil in each separator. The gas phases from the crude oil separators are then separated in the desired light gas phase and the desired condensate phase.

If desirable or required, the condensate phase to be used as a cooling and carrier liquid in the process may be supplemented with a light oil supplied from an external source.

The process of the invention is described in more detail below with reference to the drawings, wherein:

Fig. 1 shows schematically an embodiment of a plant for carrying out a process according to the invention, and

Fig. 2 shows schematically an embodiment of a hydration unit 90 shown in Fig. 1, and equipment for a subsequent cooling of liquid gas hydrate-containing hydrocarbon product.

In the Figures, identical reference numerals have been utilized for equivalent parts of the plant. All pressures given in the detailed specification below are meant to be overpressures .

in the plant shown in Fig. 1, a well stream containing crude oil and natural gas, produced from a production well on an offshore oil field is passed via a pipeline 1 to a first crude oil separator 5. In this crude oil separator, operated at a pressure of e.g. 70-90 bars, the well stream is separated into a water stream which is withdrawn through an outlet 8, a light gas stream which is withdrawn via a line 6, and a crude oil stream which is conveyed to a second crude oil separator 10. In the latter separator, which may be operated at a pressure 5 of about 20 bars, water and a gas are again separated from the crude oil. Said gas is withdrawn via a line 11 and may contain hydrocarbon components up to e.g. C₇-C₈. The crude oil from the crude oil separator 10 is then expanded to e.g. 1-2 bars and is introduced into a third crude oil separator 15, wherein it o is separated into a water stream, a crude oil stream and a gas stream, which latter is withdrawn via a line 16. The water stream and the crude oil stream are combined and passed via a line 17 to a coalescer 20. The water is withdrawn from the coalescer via a line 22, while the crude oil is withdrawn via s a line 21, cooled in a heat exchanger 23 to 15 °C to 20 °C, and passed as a stabilized crude oil to storage tanks for transportation from the field.

The three gas streams withdrawn from the three crude oil sepa- o rators 5, 10 and 15 via lines 6, 11 and 16, respectively, are treated to produce a light gas phase having a high content of hydratable hydrocarbon components, and a condensate phase consisting of a light oil. The gas which is withdrawn from the third crude oil condenser via line 16, and which has a pres- 5 sure of 1-2 bars, is cooled in a first heat-exchanger 30 to about 30 °C and is introduced into a gas/liquid separation means 33, wherein the pressure may be e.g. 0,1 bar. Liquid phase which is separated out therein, and which will contain the heavier volatile components of the raw material, may be o returned via a line 35 to the crude oil separator 10, while the gas phase is fed to a compressor 40, in which it is compressed to e.g. 6 bars. The compressed gas phase is then introduced into a second heat exchanger 45, in which it is cooled to about 30 °C, and is then introduced into a second 5 gas/liquid separator means 50. A liquid phase is withdrawn therefrom via a line 52. A gas phase is also withdrawn therefrom and is introduced into a second compressor 60, in which it is compressed to a pressure at the same level as the pressure in the gas line 11 from the second crude oil separator 10, for instance to about 18 bars. The gas phase from compressor 60 is then combined with the gas stream withdrawn via line 11 from crude oil separator 10, and the combined gas stream is passed via a line 61 to a third heat exchanger 65, in which it 5 is cooled to about 30 °C. The gas stream is then introduced into a third gas/liquid separation means 70. A liquid phase is withdrawn therefrom via a line 72, and a gas phase is also withdrawn, and is compressed in the third compressor 75.

o The compressed gas phase from compressor 75 is withdrawn via a line 76 and is combined with the gas stream withdrawn from the first crude oil separator 5 through line 6. The combined gas stream is passed via a line 77 to a heat exchanger 80, in which it is cooled to about 30 °C at a pressure of about 70 s bars. The stream is then introduced into a gas/liquid separation means 85, in which it is separated in a liquid phase which is withdrawn via a line 86, and a light gas phase which is withdrawn via a line 87. The light gas phase, or at least a substantial part thereof, is passed via a line 88 to a hydra- o tion reactor 95 in a hydration unit 90, in which it is contacted under hydrate-forming pressure and temperature conditions with water supplied via a line 96 ending in nozzles, and with a cold condensate phase supplied via a line 180 from a cooling unit 91 in the hydration unit 90. Said condensate 5 phase is supplied to the cooling unit 91 via a line 73 and is constituted by the liquid phase from the gas/liquid separation means 85 and the two liquid phases withdrawn via lines 52 and 72 from the second gas/liquid separation means 50 and the third gas/liquid separation means 70, respectively. 0

The production of gas hydrate will be described in more detail further below, with reference to Fig. 2. The produced gas hydrate, suspended in condensate phase, is withdrawn through a line 105 and is cooled to a temperature lower than 0 ° C (see 5 description with reference to Fig. 2), whereupon it is passed to thermally insulated storage tanks, while unreacted gas comprising unreacted light hydrocarbons and inert gases is withdrawn through a line 110 from the top of the hydration reactor 95. If the gas phase withdrawn from crude oil separator 5 via line 6 contains only small amounts of condensable liquid, the gas/- liquid separation means 85 may be omitted and the gas phases in lines 6 and 76 may be introduced directly into the hydra- s tion unit 90.

Optionally, a partial stream which is not to be subjected to gas hydrate formation may be split off through line 89 from the light gas phase withdrawn from the gas/liquid separation o means 85, or from some other convenient point in the process if such separation means is not to be used. Said partial stream may be stabilized for transportation in conventional manner, e.g. by drying it with glycol and compressing it to a pressure lower than 200 bars for separate storage and trans- s portation at a temperature of about 30 °C.

The production of gas hydrate in the hydration unit 90 shall now be described in more detail with reference to Fig. 2. A light gas phase from the gas/liquid separation means 85 is 0 passed via lines 87 and 88 to a hydration reactor 95, consisting for instance of an elongated, vertical container. In the hydration reactor 95, the light gas phase is contacted with water supplied through a line 96 equipped with one or more nozzles, under conditions creating intimate contact between 5 liquid and vapour, and under hydrate-forming pressure and temperature conditions. Soft water or sea water may be used as hydration water. During the hydration reaction which takes place between the water and hydrate-forming components of the light gas phase in the hydration reactor 95, the water mole0 cules form grid structures having voids in which gas molecules are entrapped. When the supplied water, atomized to fine droplets through nozzles, is introduced into the reactor 95, the hydrate is formed as small snow flake-looking crystalline particles which are sinking slowly down through the reactor. 5

The hydration reaction taking place between the hydratable compounds in the gas phase and the water is an exothermal reaction and it is therefore necessary to remove the heat generated during the reaction so as to maintain the desired temperature conditions during the hydration. The hydration- forming temperature conditions are maintained by effecting the hydration reaction in the presence of a highly liquid hydrocarbon-containing cooling and carrier liquid which is supplied to the hydration reactor 95 in a cold state, i.e. with a temperature lower than the selected operating temperature for the hydration reaction. Said cooling and carrier liquid is constituted by a light oil, which in the embodiment here described is a condensate phase derived from the treated well stream and supplied to the hydration unit 90 via line 73. If desirable or required, a light oil supplied from an external source may also be used, i.e. a light oil which has been transported to the production field or loading place. The condensate phase is preferably supplied to the gas volume in the reactor in the form of fine droplets. The condensate phase is circulated in a pumping circuit comprising the reactor 95, a separator container 100, a heat exchanger 106, and a pump 107.

The condensate phase (the light oil) utilized as a cooling and carrier liquid for the produced gas hydrate must be highly liquid at the hydration and storage temperatures given below. It is also essential that the condensate phase should not contain, or should only contain insignificant amounts of, components which would separate out as a wax or other solid or viscous substance at the lowest temperatures in the process. Any light oil supplied from an external source should be non- expensive and easily available for use in processes of this kind within the petroleum industry.

Gas hydrate which is formed in the reactor 95, and which is suspended in condensate phase utilized as a cooling liquid for removal of heat generated during the hydration reaction, is withdrawn at the bottom of reactor 95 via a line 97 and is introduced into a separator container 100. Because the gas hydrate has a higher density than the condensate phase in which it is suspended, the gas hydrate will have a tendency to sink down through the condensate phase, which results in the formation in container 100 of a lower zone 101 consisting of gas hydrate slurry, and a zone 102, above said zone 101, con- sisting of condensate phase containing no substantial amounts of gas hydrate. The condensate phase in zone 102 is passed via a line 104 to the heat exchanger 106, wherein it is cooled by indirect heat exchange with a selected external cooling medium to the desired temperature for sustaining the hydration reaction, and is returned from said heat exchanger via a pump 107 to the reactor 95, more specifically to the top section thereof. If from a thermodynamical point of view the hydrate formation has not been finished in the reactor 95, a certain amount of reaction between free or dissolved gas and remaining amounts of water may still occur in the container 100.

Any heavier hydrocarbon components of the light gas phase which have been condensed but not hydrated under the existing hydration conditions will circulate in the pumping circuit (95, 100, 106, 107) together with the condensate phase and will thus be included as a part of the cooling liquid.

The temperature in the reactor 95 must be sufficiently low to allow formation of gas hydrate from water and hydrate-forming components of the light gas phase, i.e. lower than the equilibrium temperature for formation/dissociation of gas hydrate at the actual operating pressure, but not sufficiently low to allow water in the reactor to form ice instead of participa- ting in hydrate formation together with the hydrate-forming components of the light gas phase. The presence of free, unconverted water in the gas hydrate or in the cooling liquid will reduce the energy content and create problems in the handling of the gas hydrate mass, and such presence may create problems when the gas hydrate mass is cooled down to temperatures lower than 0 °C, because the free water will freeze to ice, which may result in a sintering of the gas hydrate mass and clogging of lines and ducts, and lead to the formation of an unmanageable, hard or lumpy mass. Care should therefore be taken that water be not supplied in such amounts, relative to other material and energy streams to and from the reactor 95, which would lead to the formation of a gas hydrate product containing free water ( in the form of water or ice ) in more than insignificant amounts. The hydration reaction in the hydration reactor 95 is usually carried out at pressures in the range of 10 to 150 bars, more often from 30 to 100 bars, and at temperatures in the range of 0 °C to 10 °C, preferably in the range of 0 °C to 4 °C. At an operating pressure of about 60 bars in the hydration reactor 95, a temperature of 6 °C to 8 °C will be sufficiently low to achieve gas hydrate formation in the reactor. However, the hydration temperature should preferably be lower than that and preferably down towards 0 °C. Nonetheless, the temperature should not be lower than the freezing point of the water.

When a predetermined amount of hydrate slurry has been collected in the separator container 100, the slurry is withdrawn from the container via a line 105, a gate 115 and a line 116, and is passed to a cooling tank 120. The gate 115 is equipped with lines, control means (not shown) and valves for starting and shutting off fluid circulation between a high pressure zone represented by the hydration reactor 95, the separator container 100 and the circulation system between them, and a low pressure zone constituted by the cooling tank 120 and the equipment connected therewith, so as to secure batchwise withdrawal of slurry from the high pressure zone to the low pressure zone. The design and functioning of the gate 115 may be for instance as described in the PCT/N097/00112 referred to above. The hydrate slurry is cooled in the cooling tank 120 to a desired storage temperature which is preferably lower than -10 °C, more preferably lower than -15 °C, by introduction and circulation of a cooling medium. The cooling medium is preferably constituted by a light oil (e.g. a condensate phase as described above) having a vapour pressure lower than the ambient pressure (about 1 ata) at the subsequent storage and transportation of the hydrate slurry. The cooling medium may be constituted by the carrier liquid in the hydrate slurry which is received in the cooling tank 120, or it may be a separate cooling medium of a similar type which replaces the carrier liquid in the received hydrate slurry after decanting of the latter carrier liquid. The cooling medium is cooled in a cooling plant 125 connected to the cooling tank 120 via lines 126 and 127. The cooled gas hydrate slurry is withdrawn from the bottom of the cooling tank 120 via a line 121 and is passed to one or more separate, thermally insulated storage tanks for hydrate slurry. As already mentioned, the gas hydrate slurry obtained after the final cooling generally has a s temperature lower than -10 °C, preferably lower than -15 °C.

Gas components which are not converted to gas hydrate under the actual hydration conditions, such as for instance excess nitrogen, oxygen, noble gases, hydrogen, any unreacted hydro- o carbons and similar, are withdrawn at the top of the reactor 95 via a line 110, and optionally at the top of the container 100 via a line 111. This withdrawn non-hydrated gas, which will contain a certain amount of unreacted/non-hydrated hydrocarbons, may be flared or reinjected into the field, or, more 5 preferably, employed as a fuel so as to utilize the energy potential thereof and limit the venting of hydrocarbons to the atmosphere.

As mentioned above, heavier components which do not form hyd0 rates may be condensed from the supplied gaseous and volatile components during the hydration process, which heavier components may be included in, or may even entirely constitute, the condensate phase/light oil needed as a cooling and carrier liquid in the hydration process. Condensed components which 5 are not needed for this purpose may be stored in separate tanks, or - if they are not too volatile - together with the stabilized crude oil. A larger or smaller part thereof may also be used as a fuel at the location.

0 The plant for carrying out the process of the invention must to the extent required be equipped with means for controlling the process. This will be obvious to a man skilled in the art, and such means, e.g. valves, have therefore not been shown in the Figures or accounted for in the description. 5

For further details, reference is made to the above-mentioned PCT/N097/00111 and PCT/N097/00112, which disclose similar embodiments and other embodiments of the hydration reactor 95 and the hydration process. The obtained gas hydrate slurry may be stored and handled by means of conventional storage and transportation equipment for liquids and suspensions. In order to convey the gas hydrate slurry from the hydration unit 90 to the storage tanks, pumps s may be used (not shown). A slurry temperature lower than -10 °C, preferably lower than -15 °C, is considered sufficient for the gas hydrate slurry to be sufficiently stable to be stored adiabatically at atmospheric pressure in the thermally insulated storage tanks. The temperature of the gas hydrate o slurry in the storage tanks may optionally be controlled by performing a decantation of light oil from the gas hydrate slurry from the top of the storage tanks, cooling the light oil utilized as carrier oil in a heat exchanger and returning the light oil to a point near the bottom of the storage tanks. 5

The finished gas hydrate slurry stored in the storage tanks will advantageously have the lowest possible content of light oil/condensate phase which is consistent with the pumpability requirements, so as to obtain a maximum concentration of the 0 hydrated light gas phase in the gas hydrate slurry.

The storage tanks may be protected by means of overpressure and vacuum valves, or they may be vented to the atmosphere. Some form of agitation in the tanks may be contemplated. It 5 may also be useful to have agitators arranged at the outlets of the tanks.

The process of the invention for treating a well stream comprising crude oil and natural gas is useful not only in the 0 production of oil and gas, but also in well testing operations, especially in such operations involving associated gas. In a well testing operation, the well stream is passed to a separation plant, which may comprise one or more separators 5, 10, 15. After having subjected the gas and liquid fractions 5 obtained in the separation plant to the analyses required by the test, said gas and liquid fractions are treated by the process of the invention. Thus, the process of the invention allows recovery of hydratable and volatile components of the well stream which would otherwise have been flared. Such recovery results in less hydrocarbons and C0₂ being released to the atmosphere, and may also result in improved economy, not the least by avoiding governmental C0₂ taxation.

Claims

Patent claims

1. A process for treating a raw material, consisting of a well stream comprising crude oil and natural gas, or of a stream of non-stabilized crude oil, for storage/transportation of a stabilized crude oil and a stabilized gas product in separate tanks, wherein gaseous and volatile components and any water are separated from said raw material, to obtain a crude oil which is stable at a selected crude oil storage and/or transportation pressure, characterized by the following steps: a substantial part of the gaseous and volatile components which are separated from the raw material are contacted with water under hydrate-forming pressure and temperature con- ditions in a hydration zone to produce a hydrate mass comprising hydrates of hydratable components in the mixture of gaseous and volatile components, the obtained hydrate mass is cooled down in a cooling zone to an average final temperature/storage temperature which is lower than the freezing point of water, at which temperature it is stable at a selected storage or transportation pressure, so as to form a liquid gas hydrate-containing hydrocarbon product comprising particles of gas hydrate suspended in a hydrocarbon-containing liquid and being storable and transportable as a stable product, said hydrocarbon-containing liquid being supplied during the production or cooling of the hydrate mass, and gaseous and/or volatile compounds which have not converted to gas hydrates are optionally subjected to further treatment.

2. A process according to claim 1, characterized in that: heat generated during the formation of hydrate in the hydration zone is absorbed by said hydrocarbon-containing liquid, consisting of a cold light oil supplied from an external source and/or constituted by condensed components of the supplied component mixture which have not formed hydrates, whereby there is obtained a liquid gas hydrate-containing hydrocarbon product containing gas hydrate particles suspended in a carrier liquid constituted by the light oil, and the obtained liquid gas hydrate-containing hydrocarbon product is then further cooled - optionally after previous s replacement of its light oil carrier liquid with fresh light oil - to a temperature lower than the freezing point of the water.

3. A process according to claim 2, characterized in that o the entire mixture of gaseous and volatile components separated from the raw material is introduced into the hydration zone.

4. A process according to claim 1 or 2, characterized in s that a fraction containing no substantial amounts of hydratable components is separated from the mixture of gaseous and volatile components separated from the raw material, before the remaining part of the mixture is introduced into the hydration zone. 0

5. A process according to claim 1, 2 or 4, characterized in that only such components which are present in gaseous form at the prevailing pressure and temperature conditions in the hydration zone are introduced into said hydration zone. 5

6. A process according to any of claims 2-5, characterized in that at least a part of the light oil utilized as cooling and carrier liquid has been condensed from the supplied component mixture during the hydrate formation. 0

7. A process according to any of claims 2-6, characterized in that the light oil utilized as cooling and carrier liquid is a light oil at least part of which has been separated from the raw material, which light oil is supplied 5 to the hydration zone.

8. A process according to any of claims 2-7, characterized in that the further cooling of the obtained liquid gas hydrate-containing hydrocarbon product to a tempera- ture lower than the freezing point of the water is effected by circulating the carrier liquid of the hydrocarbon product through a cooling plant.

s 9. A process according to any of claims 2-8, characterized in that the light oil is constituted essentially by hydrocarbons having a number of carbon atoms in the C₅-C₁₀ range and being highly liquid at the temperatures prevailing in the hydration zone and in the appurtenant cooling units, 0 especially in the cooling units for cooling and stabilizing the hydrate product.

10. A process for treating a raw material, consisting of a well stream comprising crude oil and natural gas, or of a 5 stream of non-stabilized crude oil, for storage/transportation of a stabilized crude oil and a stabilized gas product in separate tanks, wherein gaseous and volatile components and any water are separated from said raw material, to obtain a crude oil which is stable at a selected crude oil storage and/or 0 transportation pressure, characterized by the following steps:

(a) the gaseous and volatile components separated from the raw material are separated in a light gas phase containing a main part of the hydrate-forming gas components in the mixture, and a condensate phase formed by volatile but 5 somewhat heavier components,

(b) the separated light gas phase, or a substantial part thereof, is contacted with water under hydrate-forming pressure and temperature conditions in a hydration zone to achieve hydrate formation, 0 (c) heat generated during the hydrate formation is absorbed by a cold condensate phase separated in step ( a ) , which condensate phase is supplied to the hydration zone, whereby a liquid hydrocarbon product is obtained comprising gas hydrate particles suspended in a carrier liquid consti- 5 tuted by the supplied condensate phase and any condensed components of the supplied gas phase which have not formed hydrate, the obtained liquid, gas hydrate-containing hydrocarbon product is further cooled - optionally after previous re- placement of its carrier liquid with fresh condensate phase - to a temperature lower than the freezing point of the water, at which temperature it is stable at a selected storage or transportation pressure, and 5 volatile components which have not been converted to gas hydrates are optionally subjected to further treatment.

11. A process according to claim 10, characterized in that the condensate phase utilized as a cooling and carrier ιo liquid is supplemented with a light oil supplied from an external source.

12. A process according to claim 10 or 11, characterized in that the further cooling of the obtained liquid gas is hydrate-containing hydrocarbon product to a temperature lower than the freezing point of the water is effected by circulating the carrier liquid of the hydrocarbon product through a cooling plant.

20 13. A process according to claim 11 or 12, characterized in that the raw material is separated in several serially connected crude oil separators at successively decreasing pressure, with a gas phase being separated from the crude oil in each separator; and that the gas phases from said crude oil

25 separators are separated into said light gas phase and said condensate phase.

14. A process according to claim 13, characterized in that: so the raw material ( 1 ) is separated in three serially connected crude oil separators (5, 10, 15, resp. ) at successively decreasing pressure, with separation of a gas phase (6, 11, 16, resp. ) in each separator, the gas phase (16) from the third crude oil separator

35 (15) is separated into a condensate phase (73) and a light gas phase (76) in a series of treatment units, each comprising a cooling means (30, 45, 65, resp.), a gas/liquid separation means (33, 50, 70, resp.), and a compressor (40, 60, 75, resp. ) , the gas phase (11) from the second crude oil separator (10) is introduced at a suitable point in the series of treatment units for treating the gas phase (16) from the third crude oil separator (15), s the compressed light gas phase ( 76 ) obtained in the last compressor (75) is mixed with untreated gas phase (6) from the first crude oil separator ( 5 ) , and the obtained mixture is cooled in a cooling means (80) and is separated in a separation means (85) into a liquid phase (86) and a light gas o phase (87), which light gas phase, or at least part of it, is contacted with water (96) and with cold condensate phase (73) to form the liquid gas hydrate-containing hydrocarbon product in a hydration zone in a hydration reactor (95), said condensate phase ( 73 ) comprising the liquid s phase (72) from the third gas/liquid separation means (70), and the liquid phase ( 52 ) from the second gas/liquid separation means ( 50 ) in the series of treatment units for treating the gas phase (16) from the third crude oil separator (15), as well as the liquid phase (86) from the separation means (85). 0

15. A process according to claim 14, characterized in that the gas phase (11) from the second crude oil separator (10) is introduced into the process step for separating the gas phase (16) from the third crude oil separator (15), at a 5 point between the compressor (60) and the cooling means (65).

16. A process according to claim 14, characterized in that the liquid phase (35) from the first gas/liquid separation means ( 33 ) is returned to the second crude oil sepa- 0 rator ( 10 ) .

17. A process according to any of claims 10-16, characterized in that the light oil is constituted essentially by hydrocarbons having a number of carbon atoms in the C₅-C₁₀ 5 range, and being highly liquid at the temperatures prevailing in the hydration zone and in the appurtenant cooling units, especially in the cooling units for cooling and stabilizing the hydrate product.

18. A process according to any of claims 1-17, for use in a well testing operation.