WO2002004364A2

WO2002004364A2 - Process for biodegrading crude oil

Info

Publication number: WO2002004364A2
Application number: PCT/GB2001/002995
Authority: WO
Inventors: Christopher George Osborne; Richard Owen Snell; Frank Michael Sweeney; Ian Vance
Original assignee: Bp Exploration Operating Company Limited
Priority date: 2000-07-07
Filing date: 2001-07-04
Publication date: 2002-01-17
Also published as: GB0016633D0; WO2002004364A3; AU2001267733A1

Abstract

A process for purifying water contaminated with crude oil. The process comprises: (a) feeding water contaminated with crude oil into a flexible storage tank; (b) biodegrading at least a portion of the crude oil contaminant; and (c) withdrawing from the flexible storage tank a water stream which is reduced in crude oil contaminant.

Description

PROCESS FOR BIODEGRADING CRUDE OIL

The present invention relates to a process for purifying crude oil-contaminated water.

There are increasingly stringent emission targets being imposed upon the oil industry. For example, concerns have been expressed over the release of crude oil into the environment, in particular, when production water or other waters which come into contact with crude oil are discharged into the sea. Production water comprises formation water and any water (e.g. seawater) which is injected into an oil well to increase its production rate (i.e. in a water flood treatment). The production water is separated from the crude oil in a separator. However, the separated production water will inevitably be contaminated by trace amounts of crude oil. It would therefore be desirable to reduce the amount of crude oil contaminating the production water before discharging the production water into the environment.

GB 1 440 800 relates to a flexible marine transport tank for transport afloat, comprising at least two flexible elements of rotation longitudinally connected by flexible linking means, and means for maintaining an internal pressurization within the elements during the various phases of utilization thereof in order to maintain the geometric shape of the elements. The tank may be provided with a settling arrangement which makes it possible to separate immiscible liquid phases of different densities during a filling operation. Such separating means are useful for separating liquid having its origin in a pollution of water, especially by crude oil. However, trace amounts of crude oil will remain in the water which is separated from the crude oil.

It has now been found that a reduction in the amount of crude oil which contaminates, for example, production water can be achieved by biodegrading the crude oil contaminant on a large scale in a flexible storage tank.

According to the present invention, there is provided a process for purifying water contaminated with crude oil comprising:

(a) feeding water contaminated with crude oil into a flexible storage tank; (b) biodegrading at least a portion of the crude oil contaminant; and

(c) withdrawing from the flexible storage tank a water stream which is reduced in crude oil contaminant. The water which is contaminated with the crude oil may be formation water, seawater used in the production of the crude oil, "wash" water, water from a production platform drainage system and aquifer water (hereinafter referred to as "crude oil- contaminated water").

Preferably, the crude oil-contaminated water is continuously fed into the flexible storage tank and the water stream which is reduced in crude oil contaminant is continuously withdrawn from the flexible storage tank. Suitably, the water stream which is reduced in crude oil contaminant is released into the environment.

Suitably, the crude oil contaminant is biodegraded by introducing biomatter into the flexible storage tank. Thus, in a first embodiment of the present invention there is provided a process for purifying water contaminated with crude oil comprising:

(a) feeding into the flexible storage tank (i) water contaminated with crude oil and (ii) biomatter capable of biodegrading the crude oil contaminant;

(b) maintaining aerobic conditions within the flexible storage tank such that at least a portion of the crude oil contaminant is biodegraded by the biomatter;

(c) withdrawing from the flexible storage tank a gaseous purge stream comprising a gaseous metabolite of the crude oil; (d) withdrawing from the flexible storage tank a water stream which is reduced in crude oil contaminant. The biomatter which is fed into the flexible storage tank may be endogenous to the crude oil-contaminated water. Where necessary, exogenous biomatter may be introduced into the flexible storage tank. Suitably, the exogenous biomatter may be introduced into the crude oil-contaminated water before feeding the crude-oil contaminated water to the flexible storage tank. Alternatively, the exogenous biomatter and crude oil-contaminated water may be fed separately into the flexible storage tank. It is also envisaged that the internal surfaces of the flexible storage tank may be coated with a coating material which comprises biomatter immobilised on a suitable substrate (for example, a clay or resin). Thus, in a second embodiment of the present invention there is provided a process for purifying water contaminated with crude oil comprising: (a) coating the internal surfaces of a flexible storage tank with a coating material comprising a substrate on which is immobilised biomatter capable of biodegrading the crude oil contaminant;

(b) feeding into the flexible storage tank water contaminated with crude oil;

(c) maintaining aerobic conditions within the flexible storage tank such that at least a portion of the crude oil contaminant is biodegraded by the biomatter;

(d) withdrawing from the flexible storage tank a gaseous purge stream comprising a gaseous metabolite of the crude oil;

(e) withdrawing from the flexible storage tank a water stream which is reduced in crude oil contaminant. The biomatter which biodegrades the crude oil contaminant may be a microorganism such as a bacterial strain or a yeast strain. Preferably, the strain is non- pathogenic, non-toxic and non-haemolytic. Suitable bacterial strains include comanonas acidovorans, megateria, acinetobacter, arthrobacter, rhodococcus, pseudomonas aeruginosa and pseudomonas putida. Suitable yeast strains include Candida maltosa.

Preferably, one or more mineral additives are introduced into the oil- contaminated water. Suitable mineral additives include nitrogen, phosphorus or potassium containing nutrients. Without wishing to be bound by any theory, it is believed that the nutrients support and may even increase the rate of the biodegradation process within the flexible storage tank.

The biodegradation process may convert the crude oil into gaseous metabolites thereof, such as, carbon dioxide. It is also envisaged that the crude oil may be converted into lighter liquid products such as biosurfactants. A further advantage of the purification process of the present invention is that other water contaminants, such as short chain carboxylic acids, may be biodegraded together with the crude oil contaminant.

The flexible storage tank can be located on land. Alternatively, the flexible storage tank may be submerged in or float on a body of water. Preferably, the flexible storage tank is submerged to a depth where the flexible storage tank is not subjected to wave and current forces. Suitably, the flexible storage tank is submerged to a depth at which it may be anchored. Suitably, the flexible storage tank is submerged by means of a ballast system. The flexible storage tank may be positioned on the sea bed. Alternatively, the flexible storage tank may be anchored in such a manner that it is capable of moving between a first (lower) submerged depth (which may be the sea bed) and a second (higher) submerged depth. Suitably, the flexible storage tank may be anchored using at least one anchor such as a "drag in" or a pile based anchor. It is envisaged that the flexible storage tank may be provided with lines which are attached to the anchors. Alternatively, the anchors may be attached to lines or netting positioned over the flexible storage tank.

The flexible storage tank is generally not totally filled with crude oil- contaminated water during the process of the present invention so that above a certain level of crude oil-contaminated water a gas cap is present in the flexible storage tank. Suitably, the volume of the gas cap is not more than 20%, preferably not more than 10%, for example, not more than 5% of the volume of the flexible storage tank. Suitably, the gaseous purge stream is withdrawn from the gas cap.

The gaseous purge stream is taken from the flexible storage tank to prevent the accumulation of gaseous metabolites, for example, carbon dioxide, in the flexible storage tank. The gaseous purge stream may be withdrawn from the flexible storage tank either intermittently or continuously, preferably intermittently. Suitably, the gaseous purge stream is withdrawn from the flexible storage tank via a pressure relief valve which opens above a predetermined pressure.

Aerobic conditions are maintained within the flexible storage tank by feeding an oxygen containing gas, preferably air, into the flexible storage tank. Where the flexible storage tank is submerged in a body of water, the pressure of the oxygen containing gas is suitably greater than the external hydrostatic pressure acting on the flexible storage tank.

The oxygen containing gas may be introduced into the flexible storage tank either continuously or intermittently, preferably intermittently. Thus, when the % volume of oxygen in the purge stream falls below a predetermined level (for example, less than 5 % volume), fresh oxygen containing gas may be fed to the flexible storage tank. Preferably, the oil-contaminated water which is to be purified using the process of the present invention remains in the flexible storage tank for a sufficient period of time for the amount of oil contaminant to be reduced to less than 5 ppm of oil, preferably less than 2.5 ppm, for example, less than 1 ppm of oil. Generally, the oil- contaminated water remains in the.flexible storage tank for up to 3 months, preferably, less than 1 month, for example 1 to 3 weeks. The extent of biodegradation of the crude oil contaminant may be monitored by analysing the level of gaseous metabolite(s) (for example, carbon dioxide) in the gaseous purge stream. Preferably, the crude-oil contaminated water remains in the flexible storage tank until the % volume of carbon dioxide in the gaseous purge stream falls below a predetermined value (such a 1% volume, preferably, 0.5% volume). The water stream which is reduced in oil contaminant (hereinafter referred to as "purified water stream") may then be discharged into the body of water (for example, the sea).

Suitably, the flexible storage tank has a volume of at least 100,000 m³, preferably, at least 250,000 m³, more preferably, at least 500,000 m³, for example, at least 600,000 m³. Generally, the volume of the flexible storage tank is in the range 500,000 to 1000,000 m³, preferably, 500,000 to 750,000 m³.

Preferably, the flexible storage tank is dome shaped or is elongated in shape (including elongated shapes of circular, square or rectangular cross-section). Preferably, an elongated flexible storage tank has a length of at least 150 metres, more preferably, at least 250 metres, most preferably, at least 500 metres, for example, at least 750 metres. Preferably, the flexible storage tank has a breadth and a depth of at least 25 metres.

Preferably, the flexible storage tank has an inlet in communication with a supply line and an outlet in communication with a discharge line. The supply line may be used to introduce the oil-contaminated water or the oxygen containing gas into the flexible storage tank. The discharge line may be used to withdraw the gaseous purge stream or the purified water stream from the flexible storage tank. Preferably, at least one closure valve is positioned in the supply line and in the discharge line. The discharge line and supply line may also be provided with a pressure relief valve.

It is also envisaged that the flexible storage tank may be provided with a first inlet in communication with a first supply line for the oil contaminated water and a second inlet in communication with a second supply line for the oxygen containing gas. The flexible storage tank may also be provided with a first outlet in communication with a first discharge line for the gaseous purge stream and a second outlet in communication with a second discharge line for the purified water stream. Preferably, closure valves are positioned in supply lines and discharge lines. The discharge lines and supply lines may also be provided with pressure relief valves.

Where the flexible storage tank is located on land or is floating, it is envisaged that the gaseous purge stream may be discharged directly into the atmosphere (i.e. there is no requirement for a discharge line). Where the flexible storage tank is submerged and the hydrostatic pressure above the flexible storage tank is less than the pressure of the gaseous purge stream, it is envisaged that the gaseous purge stream may be discharged directly into the body of water (i.e. the discharge line for the gaseous purge stream may be omitted).

The walls of the flexible storage tank may be made of an impermeable resilient material, for example, a fabric coated with elastomer. The walls of the flexible storage tank may be formed from a single layer of impermeable resilient material. It is also envisaged that the walls may be multi-layered so as to minimise the risk of spillage of the oil-contaminated water.

Preferably, the flexible storage tank has a plurality of compartments formed by watertight bulk heads. Preferably, each compartment has an inlet and an outlet. The watertight bulk heads may be made from an impermeable resilient material which may be single layered or multi-layered. The use of a flexible storage tank having a plurality of compartments minimises the risk of discharge of oil-contaminated water into the environment, in the event of accidental rupture of the flexible storage tank. However, any oil-contaminated water released in the event of accidental rupture of the storage tank would meet existing legal specifications. Preferably, the flexible storage tank has at least 25 compartments, more preferably a least 50 compartments, for example 100 to 750 compartments, preferably 250 to 600 compartments.

The compartments are reservoirs intended to receive the oil-contaminated water which may fill or partially fill the compartment to any level. Where a compartment is completely filled with the oil-contaminated water, the compartment may be in communication with a gaseous metabolite collection chamber. The gaseous metabolite collection chamber is provided with an outlet for the gaseous purge stream. It is envisaged that each compartment may be in communication with a dedicated gaseous metabolite collection chamber or that the compartments may be in communication with a common gaseous metabolite collection chamber.

Suitably, each of the inlets of the compartments of the flexible storage tank may be in communication with a supply line via a duct means. The duct means may comprise a main duct having branch ducts leading to the inlets of the compartments. Each branch duct may have a valve which is independently operable, when the flexible storage tank is in use, so that the oil-contaminated water (or oxygen containing gas) may be introduced into one or more of the compartments. It is also envisaged that each of the compartments of the flexible storage tank maybe provided with a first inlet and a second inlet, the first inlets being in communication with a first supply line via a first duct means and the second inlets being in communication with a second supply line via a second duct means. The first duct means may comprise a main duct having branch ducts leading to the first inlets of the compartments. Similarly, the second duct means may comprise a main duct having branch ducts leading to the second inlets of the compartments. Each branch duct may have a valve which is independently operable. As discussed above, the first and second supply lines may be used to introduce the oil- contaminated water and the oxygen containing gas respectively into the flexible storage tank.

Suitably, each of the outlets of the compartments of the flexible storage tank may be in communication with a discharge line via a duct means. The duct means may comprise a main duct having branch ducts leading to the outlets of the compartments. Each branch duct may have a valve which is independently operable, when the system is in use, so that the purified water stream may be withdrawn from one or more of the compartments. Where the compartments are partially filled with oil-contaminated water, each compartment may be provided with a first outlet for the gaseous purge stream and a second outlet for the purified water stream, the first outlets being in communication with a first discharge line via a third duct means and the second outlets being in communication with a second discharge line via a fourth duct means. The third duct means may comprise a main duct having branch ducts leading to the first outlets of the compartments. Similarly, the fourth duct means may comprise a main duct having branch ducts leading to the second outlets of the compartments. As discussed above, the first and second discharge lines may be used to discharge the gaseous purge stream and the purified water stream respectively from the flexible storage tank. Each of the branch ducts may have valves which are independently operable. The valves in the branch ducts of the first duct means are excess pressure valves which open when the pressure in the gas caps of any of the compartments rises above a predetermined value. Where the compartments have dedicated gaseous metabolite collection chambers, these chambers have outlets which may be in communication with the first discharge line via a duct means. This duct means may comprise a main ducts and branch ducts leading to the outlets of the dedicated gaseous metabolite collection chambers. Where the compartments have a common gaseous metabolite collection chamber, this chamber has an outlet for the gaseous purge stream which is in communication with the first discharge line. It is envisaged that purified water may be withdrawn from the compartments via a duct means and a second discharge line as described above.

It is also envisaged that, where the process of the present invention is operated continuously, the compartments of the flexible storage tanks may be connected in series such that crude oil-contaminated water is introduced into the first compartment in the series via the first supply line and is subsequently passed to the next and subsequent compartments in the series. The purified water stream which is reduced in crude oil contaminant is therefore removed from the last compartment in the series (via the second discharge line). Suitably, the oxygen containing gas is introduced into the first compartment in the series via the second supply line. It is also envisaged that oxygen containing gas may be introduced into each of the compartments in the series via a duct means, as described above. Preferably, each of the compartments in the series are in communication with either a dedicated or common gaseous metabolite collection chamber. Suitably, the gaseous purge stream is removed from the dedicated or common gaseous metabolite collection chambers as described above.

The supply line(s) and discharge line(s) may be in communication with a dedicated pumping means disposed remotely from the flexible storage tank, for example, where the flexible storage tank is floating or submerged, the pumping means may be located on a vessel or platform floating on the surface of the body of water.

A first pumping means may be provided for introducing crude oil-contaminated water into the flexible storage tank and a second pumping means for withdrawing the purified water stream from the flexible storage tank. A compressor may be provided for introducing, for example, compressed air into the flexible storage tank (in order to maintain aerobic conditions therein) while a vacuum pump may be provided for withdrawing the gaseous purge stream from the flexible storage tank.

For conveying to the storage site, the flexible storage tank may be empty, deflated and folded to a minimum volume which is advantageous for maintenance and transport. Shipping the flexible storage tank empty and folded may also assist in it being released into a body of water at the location of utilisation.

In a further embodiment of the present invention there is provided a flexible storage tank having: a) a volume of at least 250,000 m³; b) a first inlet in communication with a first supply line which leads to a first pumping means; and c) a first outlet in communication with a first discharge line which leads to a second pumping means. Where the flexible storage tank is to be submerged in a body of water, it is preferred that the flexible storage tank is provided with a ballast system.

Preferred features of the storage tank of the present invention are as described above.

Claims

Claims:

1. A process for purifying water contaminated with crude oil comprising:

(a) feeding water contaminated with crude oil into a flexible storage tank;

(b) biodegrading at least a portion of the crude oil contaminant; and

(c) withdrawing from the flexible storage tank a water stream which is reduced in crude oil contaminant.

2. A process as claimed in claim 1 which comprises:

(a) feeding into the flexible storage tank (i) water contaminated with crude oil and (ii) endogenous and/or exogenous biomatter capable of biodegrading the crude oil contaminant; (b) maintaining aerobic conditions within the flexible storage tank such that at least a portion of the crude oil contaminant is biodegraded by the biomatter;

(c) withdrawing from the flexible storage tank a gaseous purge stream comprising a gaseous metabolite of the crude oil;

(d) withdrawing from the flexible storage tank a water stream which is reduced in crude oil contaminant.

3. A process as claimed in claims 1 or 2 which comprises:

(a) coating the internal surfaces of a flexible storage tank with a coating material comprising a substrate on which is immobilised exogenous biomatter which is capable of biodegrading the crude oil contaminant; (b) feeding into the flexible storage tank water contaminated with crude oil;

(c) maintaining aerobic conditions within the flexible storage tank such that at least a portion of the crude oil contaminant is biodegraded by the exogenous biomatter; (d) withdrawing from the flexible storage tank a gaseous purge stream comprising a gaseous metabolite of the crude oil;

(e) withdrawing from the flexible storage tank a water stream which is reduced in crude oil contaminant.

4. A process as claimed in any one of the preceding claims wherein the biomatter which biodegrades the crude oil contaminant is a bacterial strain selected from the group consisting of comanonas acidovorans, megateria, acinetobacter, arthrobacter, rhodococcus, pseudomonas aeruginosa and pseudomonas putida.

5. A process as claimed in any one of the preceding claims wherein at least one mineral additive selected from the group consisting of nitrogen, phosphorus and potassium containing nutrients is introduced into the water contaminated with crude oil.

6. A process as claimed in any one of the preceding claims wherein a gas cap is present in the flexible storage tank and the gaseous purge stream is withdrawn from the gas cap.

7. A process as claimed in claim 6 wherein the gas cap is not more than 5% of the volume of the flexible storage tank.

8. A process as claimed in any one of the preceding claims wherein aerobic conditions are maintained within the flexible storage tank by feeding an oxygen containing gas into the flexible storage tank.

9. A process as claimed in any one of the preceding claims wherein the water contaminated with crude oil remains in the flexible storage tank for up to 1 month.

10. A process as claimed in any one of the preceding claims wherein the flexible storage tank is located on land, is floated on the surface of a body of water or is submerged in a body of water.

11. A process as claimed in claim 10 wherein the flexible storage tank is submerged in a body of water and is capable of moving between a first submerged depth and a second submerged depth. ' c

12. A process as claimed in any one of the preceding claims wherein the flexible storage tank has a volume of at least 250,000 m³.

13. A process as claimed in any one of the preceding claims wherein the flexible storage tank has a length of at least 150 metres, a breadth of at least 25 metres and a depth of at least 25 metres.

14. A process as claimed in any one of the preceding claims wherein the flexible storage tank is provided with a first inlet which is in communication with a first supply line for the oil contaminated water, a second inlet which is in communication with a second supply line for the oxygen containing gas, a first outlet which is in communication with a first discharge line for the gaseous purge stream and a second outlet which is in communication with a second discharge line for the purified water stream.

15. A process as claimed in any one of the preceding claims wherein the walls of the flexible storage tank are made of an impermeable resilient material.

16. A process as claimed in any one of the preceding claims wherein the flexible storage tank has a plurality of compartments for the water contaminated with crude oil, formed by watertight bulk heads.

17. A process as claimed in claim 16 wherein the compartments are connected in series and the water contaminated with crude oil is continuously introduced into the first compartment in the series and is passed through the next and subsequent compartments in the series such that the water stream which is reduced in crude oil contaminant is continuously removed from the last compartment in the series.

18. A process as claimed in any one of claims 14 to 17 wherein the supply lines and discharge lines are each in communication with dedicated pumping means disposed remotely from the flexible storage tank.

19. A flexible storage tank having: a) a volume of at least 250,000 m³; b) a first inlet in communication with a first supply line which leads to a first pumping means; and c) a first outlet in communication with a first discharge line which leads to a second pumping means.

20. A flexible storage tank as claimed in claim 22 wherein the flexible storage tank has a length of at least 150 metres, a breadth of at least 25 metres and a depth of at least 25 metres.

21. A flexible storage tank as claimed in claims 22 or 24 wherein the flexible storage tank is provided with a second inlet in communication with a second supply line leading to a third pumping means, and a second outlet in communication with a second discharge line leading to a fourth pumping means.

22. A flexible storage tank as claimed in any one of claims 19 to 21 wherein the walls of the flexible storage tank are made of an impermeable resilient material.

23. A flexible storage tank as claimed in any one of claims 19 to 22 wherein the flexible storage tank has a plurality of compartments connected in series.

24. A flexible storage tank as claims in any one of claims 19 to 23 wherein the flexible storage tank is provided with an anchoring means.

25. A flexible storage tank as claimed in any one of claims 19 to 24 wherein the flexible storage tank is provided with a ballast system.