WO2000039031A1

WO2000039031A1 - Advanced treatment for produced water

Info

Publication number: WO2000039031A1
Application number: PCT/US1999/030376
Authority: WO
Inventors: Robert T. Ehrlich
Original assignee: Amerada Hess Corporation
Priority date: 1998-12-23
Filing date: 1999-12-20
Publication date: 2000-07-06
Also published as: AU2482400A; WO2000039031A9

Abstract

Produced water, the aqueous fraction of the produced fluids generated by an oil or gas well poses a significant water treatment and disposal problem. The instant invention discloses methods and apparatus for the advanced treatment, particularly biological treatment, of produced water in an off-shore facility in order to reduce the amount of pollutants discharged with the produced water and to avoid the cost and other disadvantages of deep well injection disposal of produced water. Offshore biological treatment is disclosed in vessels designed for deployment on the sea floor, in the water collumn, and at the sea surface.

Description

ADVANCED TREATMENT FORPRODUCED WATER

SPECIFICATION

BACKGROUND OF THE INVENTION

Petroleum and natural gas production is generally accompanied by the generation of produced water, which is the mixture of water and impurities including but not limited to, hydrocarbons, aromatics, polycyclics, water soluble organics (hereinafter "WSO"), oil and grease (hereinafter O&G), other organic compounds, dissolved solids, and other materials separated from the produced petroleum or natural gas. This produced water derives from the natural presence of water in petroleum and natural gas reservoirs, the use of water and steam for injection subsurface to enhance oil recovery, and other uses of water in fossil fuel production. Fossil fuel production as used herein includes production of subsurface deposits of petroleum, natural gas and mixtures thereof. The large volume of produced water generated during fossil fuel production poses a substantial storage, treatment and disposal problem. Indeed the volume of produced water per unit of production tends to increase with the maturity of an oil field thereby posing an ongoing and growing disposal problem.

Produced water is characterized by various impurities, some or all of which may be present in any given sample of produced water. Characteristics of impurities that may be found in produced water, either in solution, colloid or separate phase, typically include high total dissolved solids, suspended solids, mineral content, i.e., hardness, hydrocarbons, including aromatics and cyclic hydrocarbons, oil, WSO, O&G, sulfur compounds such as H₂S, mercaptans and other organo-sulfur compounds, alkalinity, barium, boron, and other metals. The total measured amount of hydrocarbon and organic compounds in the produced water is conventionally referred to as the oil and grease (O&G) concentration. Dissolved in the water phase are the Water Soluble Organics (WSO) which are part of the total O&G. Produced water may also be hot, e.g., 65 °C to 82°C, due to the elevated temperatures of the petroleum reservoir or the use of steam injection to facilitate petroleum recovery. Characteristics of produced water vary greatly from site to site and can also vary during the lifetime of an oil well.

The volume of produced water may be a small fraction of, but is commonly comparable to, or even substantially in excess of, the volume of produced petroleum. Thus, capacity for high volume treatment and disposal of produced water is often essential in oil and gas production.

Treatment of produced water typically involves some form of "primary treatment" which relies usually on physical separation techniques. Primary treatment techniques which are well understood in the art include, but are not limited to, gravity separation, flotation, filtration, hydrocyclones, centrifuges, and cross-flow membrane filtration. The Produced Water Treatment Design Manual edited by Abbas Zaidi and Thomas Constable, incorporated herein by reference, describes the state of the art in treatment of produced water. The state of the art in treatment of offshore produced water is described in the Development Document for Final Effluent Limitations

Guidelines and Standards for the Coastal Subcategory of the Oil and Gas Extraction Point Source Category, Office of Water, U.S. Environmental Protection Agency, EPA-821-R-96-023 (Oct. 1996), incorporated herein by reference. Additional treatment may include hardness and solid reduction; acid extraction; removal of total dissolved solids by evaporation, electrodialysis, or reverse osmosis; and biological or chemical treatment. For example WSO containing cyclic, partially saturated organic acids such as naphthenic acids are removed from produced water through acid extraction by adding phosphoric acid based formulations which reduce the pH, protonate the acids, and cause the protonated acids to partition preferentially in the oil phase which is than separated from the produced water by other primary treatment separation methods. See, e.g., Robert F. Hickey et al., Treatment of Off-Shore Produced Water Using Biological and Coupled Bio-Ozone Processing, presented at the 15^th International Petroleum Environmental Conference (October 20-23, 1998). These additional treatments can be expensive and can require large volume capacity in order to retain, for sufficiently long periods of time, the large quantities of produced water generated during petroleum production. These additional treatments may also generate additional solid waste which must be managed.

Produced water following primary treatment typically still contains high degrees of dissolved and suspended impurities including aromatics, WSO, O&G and hydrocarbons. These levels of contamination produce deleterious environmental effects, even after primary treatment, due to their toxicity to local flora and fauna, drinking water contamination, irrigation water contamination, adverse health effects (e.g., from vaporous discharge) and adverse aesthetic effects from oil sheen on the water surface and oil accumulation when the produced water is discharged. Biologic treatment of water, sometimes referred to as "secondary treatment" is used in treatment of industrial wastewater in refineries and other facilities for the digestion of organic compounds, particularly oil, grease and other hydrocarbons, including aromatics, whether in solution, colloid or separate phase, through metabolism, or other action, by microorganisms which may be but are not necessarily part of an activated sludge process. Conventional methods of water treatment incorporating biological treatment are described in, e.g., George Tchobanoglous & Franklin L. Burton, Metcalf & Eddy, Inc.. Wastewater Engineering: Treatment. Disposal, and Reuse. (3^rd ed. 1991). Additional treatment beyond primary and secondary treatment, also called tertiary treatment may include chemical treatments or other methods tailored to the impurities in the water to be treated. Advanced treatment as used herein refers to all water treatment beyond primary treatment. Thus advanced treatment includes but is not limited to biological treatment and secondary treatment in general. Biological treatment, typically relying upon the use of microorganisms, sometimes in an activated sludge process, and retention times sufficient for the microorganisms to consume by metabolism some of the impurities, is commonly used in treatment of municipal sewage and wastewater.

However, biological treatment of produced water from oil production has been rarely used because the high salinity of produced water renders the produced water unsuitable for surface discharge into fresh water systems despite the reduction of WSO, O&G, hydrocarbons, and other impurities, apart from salinity, in the produced water. While offshore production poses no problems for disposal of high salinity water into seawater, offshore platforms have insufficient space for economical or feasible practice of secondary treatment of produced water on the platform.

Laboratory pilot-testing of advanced treatment of produced water using biological treatment alone or biological treatment following oxidation by ozone treatment to yield lower molecular weight organic compounds has demonstrated the use of biological treatment of produced water in the laboratory but not in an offshore facility. See, e.g., Robert F. Hickey et al., Treatment of Off-Shore Produced Water Using Biological and Coupled Bio-Ozone Processing, presented at the 15^th International Petroleum Environmental conference (October 20-23, 1998). Due to the limited space available on offshore drilling platforms and the prohibitive costs associated with allocation of adequate space offshore to the apparatus needed for secondary treatment, including biological treatment of produced water, the treatment of produced water generated by offshore fossil fuel production has heretofore been limited to primary treatment, typically gravity separation and cyclonic separation. The long residence time needed for successful biological treatment to reduce impurities and the high rate of generation of produced water together render a large volume capacity for containment of produced water for secondary treatment essential. This capacity need increases over the course of the useful life of an oil well because petroleum production from mature oil fields often generates substantially increased levels of produced water as the field matures.

Disposal of produced water in some cases is possible by reinjection into the drill hole or another well-hole. However, in offshore production, down-hole disposal of produced water can be undesirable either due to the cost of drilling an additional hole or because it is counterproductive to return the produced water to the reservoir. In either case, down-hole disposal of produced water can add dramatically to the cost of offshore oil production. Transport of produced water by pipeline or other vessel for further treatment onshore can also add significantly to the cost of operation of an offshore oil production facility. Consequently, ocean disposal of produced water following primary treatment, such as cyclonic separation, while conventional, is environmentally undesirable. Hence, the present practice of ocean disposal, often with deleterious environmental consequences, is only expected to increase due to the continuing and increasing global reliance on offshore oil - production and the associated increasing generation of produced water offshore. Demand for means of improved treatment of produced water is increasing due to the growing amount of produced water and the increasing sensitivity to the environmental consequences of ocean discharge of produced water subjected only to primary treatment. See Appert, Olivier and Burger, Jacques, Offshore Technology: The Next 40 Years. World Expo pp. 53-54.

SUMMARY OF THE INVENTION

The invention provides a method of treating produced water using biological and/or other secondary or advanced treatment techniques offshore prior to discharge of the treated, produced water that is generated by fossil fuel production.

The produced water, following an optional primary treatment, is delivered to a treatment vessel situated on, in, or below the water surface, wherein the produced water is exposed to a suitable advanced treatment process, e.g., biological treatment, for sufficient time and under suitable conditions that the impurities, including, e.g., hydrocarbons, WSO, and O&G are substantially reduced in amount before discharge into the receiving body of water or elsewhere.

It is a further object of this invention to locate the treatment vessel beneath the surface of the body of water (hereinafter "sea") and more specifically on the bottom of the body of water (hereinafter "seafloor") or buoyantly suspended above the bottom while anchored to the bottom.

It is a further object of this invention to locate the vessel in the water column, i.e. below the surface of the body of water and above the bottom of the body of water. It is an object of this invention to provide the vessel on or near the surface of the water.

It is an object of this invention to provide a method of producing fossil fuels, i.e., oil and/or gas, while treating the produced water by biological treatment or other advance of treatment offshore in order to reduce the amount of pollution produced in connection with the fossil fuel production.

It is an object of this invention to provide a vessel constructed at least in part of complaint material. It is an object of this invention to provide a vessel which is a sea-going vessel e.g. a boat, ship, barge, or tanker.

DESCRIPTION OF THE DRAWINGS

Figure 1 : Schematic diagram of Soft Submersible Produced Water Treatment Unit for Offshore Facilities showing production platform, gravity and mechanical treatment unit including hydrocyclone and air flotation units, discharge conduit from production platform to Vessel of Soft Tank Produced Water Treatment Cells, and conduit for return of skimmed oil to platform.

Figure 2: Schematic diagram of Vessel or Soft Tank Produced Water Treatment Cell including Produced Water and Air Line from Platform, Air Sparge mixing apparatus inside vessel, Biological Reaction Zone, Clarification Zone, oil trap, discharge control outlet, skimmed oil return conduit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is disclosed hereinbelow without any limitation implied by the description herein of the methods and apparatus used therein for biological treatment or other advanced treatment of produced water in a vessel on, in, or below the sea surface in conjunction with offshore, onshore, or near-shore oil and/or gas production.

Prior to performing the secondary treatment of produced water according to this invention, the produced water may be, and preferably is, subjected to primary treatment to substantially remove the suspended solids. Such primary treatment is preferably done on offshore platforms by gravity separation but may be accomplished by any means known in the art including those methods described in Zaidi, Abbas and Constable, Thomas, eds., Produced Water Treatment Design Manual (Wastewater Technology Centre 1994).

The produced water to be treated according to the invention is delivered via conduits, e.g. pipes, hoses, or other suitable means of fluid conveyance to the vessel for treatment with or without prior primary treatment. The produced water may be pumped or not pumped if there is sufficient pressure to deliver the produced water to the vessel. The produced water may be generated from one or more wells and may be delivered via pipes, hoses, or other conveyance means from adjacent or even distant wells. The vessel may be located on or below the surface and is preferably submerged to insulate the vessel from hazards associated with wind, wave action, and ship traffic. The vessel may be located on the seafloor, in the water column, on the sea surface or allowed to move between these positions. The vessel may be positively, negatively or neutrally buoyant and the buoyancy of the vessel may be selected or varied to control the position, shape or other characteristics of the vessel. The vessel is preferably buoyantly deployed at the water surface, more preferably fixed in location within the water column or most preferably anchored in a position on or near the seafloor. The vessel may be secured to the ocean floor, the offshore drilling platform or other suitable objects including but not limited to ships, barges and on-shore objects or fixtures. Depending upon the buoyancy of the vessel, the vessel may be secured from above, from below or both. The vessel may be secured anywhere in the water column and preferably is secured on the seafloor or is buoyantly suspended above the seafloor while anchored to the seafloor and/or the platform. The selection of the position of the vessel in the water column may be guided by variation in water temperature with water depth.

The anchoring means used to secure the vessel may include any conventional anchoring devices or methods including but not limited to the use of negatively buoyant objects such as pilings, concrete, sea anchors, bricks, rocks, soil, boulders, dredged material from the seafloor, or other material embedded in or resting on the seafloor. Additionally, the anchoring means may secure the vessel directly to the negatively buoyant objects in the anchoring means or may secure the vessel through connection with cables, lines, hoses, pipes, rope or like connecting means for connecting the vessel to the anchoring means. The anchoring means may also include a boat, an offshore drilling platform, or an onshore fixture which itself may be secured by an anchoring means. The anchoring means preferably limits or prevents the movement of the vessel with respect to the seafloor and/or the drilling platform. The anchoring means may also include means of securing by cable or other rigid or nonrigid means the vessel to the drilling platform so as to restrict the motion of the vessel with respect to the platform. The anchoring means preferably cooperates with means for manipulating the position, size, shape and deployment of the vessel including a means of retrieving the vessel.

The vessel may be located anywhere with respect to the drilling platform and is preferably located in such a fashion so as to protect the vessel and all associated apparatus including, but not limited to, anchoring means, cables, lines, pipes, hoses, and control and monitoring devices from interference or damage caused by sea traffic, ship anchors or by movement of the vessel with respect to the seafloor or the drilling platform. Ship traffic and anchorage will preferably be excluded from the immediate vicinity of the vessel and associated apparatus.

The primary treatment of the produced water by hydrocyclone or gravity separation will preferably supply sufficient pressure to convey the produced water by pipe or hose to the treatment vessel. Alternatively, pumps and other means for controlling the flow of the produced water to the vessel, including the pressure of the produced water, as delivered, may be used.

In operation the vessel may be positively, negatively, or neutrally buoyant, preferably positively buoyant. Buoyancy is controlled by the density of the vessel material, floats, buoys, closed cell marine foam, gas bladders or other buoyant material connected to the vessel, and the presence of gas in addition to water inside the vessel and the use of other buoyancy controlling mechanisms. Buoyancy may be varied by inflation or deflation of gas bladders or by controlling inflow and outflow of gases within the vessel. The vessel's position may be controlled by cables, tethers, or other lines manipulated by winches or similar means to pull release or firmly hold the cables, tethers or lines. The winch means may pull the cable means securely attached to the vessel directly or through pulley means attached to the drilling platform or otherwise attached to the seafloor. The winch means may be used to raise and lower the position of the vessel.

The vessel may be constructed of rigid or compliant material, or combinations thereof, and may optionally contain internal structure which may influence the shape or operation of the vessel. The vessel may be constructed of any material including but not limited to a compliant plastic such as high density polyethylene, metal foil, such as aluminum foil, a composite material or any combination of these or other materials. However, the vessel is preferably constructed of a compliant material and more preferably constructed of a flexible plastic. In one preferred embodiment the vessel is constructed of a flexible plastic of the kind used for containing aviation fuel in pouches typically used for air-drops in delivery of aviation fuel supplies to remote locations. The thickness of the material should be selected to adequately protect the integrity of the vessel against pressure differentials, abrasion against the seafloor or drilling platform, stresses applied to the vessel by subsurface sea currents, and other stresses experienced by the vessel when deployed and in operation. In a vessel constructed of high density polyethylene (HDPE) material the thickness of the vessel walls would preferably be in the range of approximately 10 to approximately 60 Mils where 1 mil = one thousandth of an inch. The minimum volume of the vessel is preferably determined according to the following formula: volume equals the maximum rate of delivery of produced water to the vessel anticipated over the life of the well or wells supplying produced water to the vessel multiplied by the residence time, or retention time of the produced water in the vessel sufficient to achieve the desired secondary treatment. Preferably the volume of the vessel will be greater than the above calculated minimum volume so as to allow for recirculation of water from inside the vessel and dilution of incoming produced water with seawater or other water while nonetheless maintaining the necessary minimum residence time for the desired secondary treatment. Even more preferably the bioreactor region of the vessel, exclusive of other parts of the vessel, will have a volume of at least the above-calculated minimum volume. Necessary residence times may exceed several days for biological treatment and are preferably determined according to the rate of treatment desired at the temperature and pressure prevailing in the vessel, the level of impurities, in the produced water, e.g. WSO, OBG, and hydrocarbons, and the type of resident microorganisms used. Preferably, and on average, a minimum residence time of 1 Vi to 2 days will be used. The vessel must contain means to effect biological treatment or other advanced treatment of the produced water. Preferably the vessel contains microorganisms capable of metabolically consuming at least some of the impurities. More preferably, the vessel contains microorganisms capable of oxidatively consuming at least hydrocarbon, WSO, or O&G impurities and thereby predominantly producing carbon dioxide, water and detritus. The detritus may be, at least in part, further decomposed to carbon dioxide water and other simple organic and inorganic compounds by resident microorganisms. Even more preferably the vessel is inoculated with a colony of specially selected microorganisms with a substantial capacity to metabolically consume at least hydrocarbons, WSO, or O&G. A preferred embodiment uses microorganisms contained naturally in sediments dredged from the vicinity or approximate depth of the vessel in order to acquire microorganisms acclimated to the prevailing pressure, and possibly other conditions such as four where the vessel will be located. Types of microorganisms which may be used include, without being limited to, the following: indigenous microorganisms gathered amidst dredged sediment in the vicinity of the site where the vessel will be located, preferably from within a one mile radius of the site where the vessel will be located; microorganisms collected from sediments from elsewhere on the seafloor or from other bodies of water; microorganisms collected, optionally within sludge, from treatment facilities for wastewater, including, e.g. petroleum refinery wastewater; specific microorganisms such as Pseudomonus spp. cultures and commercially available preparations thereof, e.g. MUNOX® (manufactured by Osprey Biotechnics); Arthrobacter spp. cultures and commercially available preparations thereof, e.g. BR (manufactured by Enviro-zyme International, Inc.); and other commercially available mixtures of microorganisms suitable for advanced treatment of waste water such as BIOPETRO® (manufactured by BioEnviroTech). The choice of microorganisms used in the vessel may depend on the nature and concentration of hydrocarbons, WSO, O&G, and other impurities in the produced water, the temperature, salinity, and chemical composition of the produced water the pressure and temperature of the seawater surrounding the vessel; and other criteria. The selected microorganisms may be either aerobic organisms, anaerobic organisms or a combination of both. Anaerobic microorganisms may be introduced into or allowed to accumulate in the zone or zones of the vessel dedicated to clarification and decomposition of detritus and biomolecules accumulated through recirculation of water within the vessel.

The vessel may be constructed without internal structural elements or it may contain internal structure which provides distinct compartments or other features which may influence the reaction of produced water impurities with resident microorganisms in the vessel or may influence the flow of produced water, seawater, and recirculated partially treated water through the vessel. Where internal structure is utilized it preferably comprises internal baffling which provides at least a first compartment containing the bioreactor charged with microorganisms wherein secondary treatment of produced water occurs and a second compartment where partially treated water and waste microorganisms accumulate as the treated water circulates from the bioreactor to the second compartment by convection, sparging, or other passive or active means of controlling flow of material inside the vessel and are subjected to further decomposition wherein the water continues to circulate from the second compartment eventually back to the first compartment.

The vessel may be fabricated in any suitable shape and may be so constructed of flexible or compliant material such that the shape of the vessel may change in response to changes in water pressure, ocean currents, depth, temperature, inflow, outflow, recirculation, aeration, sparging, or other changes in condition. Preferably, the vessel is conical or dome shaped with its apex or top pointing approximately upward relative to the seabed. Preferably the vessel has an outlet port in or near the top of the vessel as deployed or preferably at or near the apex of a convex upper surface of the vessel. The vessel should itself, or in conjunction with adjacent objects, structures, or fixtures, such as the seafloor, drilling platform, ship, or barge, define a sufficiently closed space such that produced water introduced into such sufficiently closed space in the vessel resides within the vessel for at least sufficient time for the resident microorganisms to reduce the amount of impurities to the degree desired by secondary treatment or for other advanced treatment to occur, for example, preferably for approximately 1 V2 to approximately 2 days, more preferably approximately 1.5 days. If the vessel is located on the seafloor the vessel may define a sufficiently closed space which is bounded on the bottom by the seafloor and above by the vessel, with an optional liner resting atop the seafloor and beneath the vessel. Preferably, the vessel comprises a closed or mostly closed bag or pouch with one or more inlet and outlet ports.

Means for delivery of air or other gases to the vessel may be provided through pipes or hoses from pumps, compressors, or tanks of compressed gas. The gases supplied can supply oxygen to meet the metabolic needs of the aerobic microorganisms in the bioreactor. Additionally gases can be used for sparging to agitate, stir, circulate microorganisms, sludge, detritus, other solids and water within the bioreactor and also, optionally, to promote recirculation from the bioreactor to the outer clarification and flocculation compartment and back to the bioreactor compartment. The gas may be delivered by one or preferably an array of separately controlled nozzles permitting control of the rate of gas flow, the distribution of nozzles delivering gas and size range of gas bubbles. The delivery of gas can also be used to inflate gas bladders for flotation and for control of water flow. As stated above, the vessel is preferably shaped like a dome or cone with flexible walls and an outflow opening of variable size that can be remotely controlled at the top of the cone. The outflow control preferably includes a substantially torroidally-shaped gas containing device which can grow in size by inflation with the inflow of gas and shrink in size by deflation with the outflow of gas thereby increasing and decreasing, respectively, the size of the outflow opening in the vessel. By controlling the size of the outflow opening, the balance between outflow and recirculation of the up welling water from the bioreactor compartment is controlled.

The shape of the vessel may be supported in whole or in part by an internal or external frame or skeleton of sufficient rigidity to hold the vessel's shape or cause the vessel to revert to its original shape following deformation by currents or other forces. The shape of the vessel may also be maintained in whole or in part by the maintenance of a positive pressure differential between the interior of the vessel and the surrounding seawater. By supplying the incoming produced water at a sufficient pressure while restricting outflow from the vessel a positive back pressure can be preferably maintained such that the vessel tends to retain its shape and tends to return to its shape after exposure to temporary deforming forces from currents or other sources. The shape of the vessel may additionally be maintained by use of positively buoyant materials including but not limited to floats and gas bladders which maintain an upward tension on the top portion of the vessel while the base of the vessel is secured below by anchoring means to the seafloor or by negatively buoyant materials. Circulation of water, gas, and solid material within the vessel is influenced largely by convection because produced water is appreciably warmer than seawater. For example produced water may be approximately 120°F to 190°F upon leaving the drill hole while seawater near the sea floor may be as cool as 35° to 40°F. Circulation may additionally be controlled by sparging and remote controlled valves or ports between compartments in the vessel or between the vessel and areas outside the vessel such as the outflow opening control device described above. Internal walls or baffles inside the vessel may be used to control or direct the flow of water and material within the vessel.

The vessel preferably includes means, either passive or active, to control or sustain environmental conditions inside the vessel or portions thereof sufficient for the continued efficient functioning of the resident microorganisms in providing secondary treatment of the produced water. Environmental conditions to be controlled include but are not limited to temperature, salinity, hydrocarbon, WSO or O&G concentration, chemical composition, pressure, dissolved oxygen levels, and essential nutrients. Passive means include all means not involving the use of pumps, heaters, agitators or other devices requiring externally supplied energy to operate. Preferably, passive means are used which may include, but are not limited to, temperature control based upon the shape, configuration and amount of and placement of insulation in the vessel and also by mixing seawater with the produced water. If the elevated temperature of the produced water is insufficient to sustain an adequate temperature for biological secondary treatment then active supplementary means to sustain the temperature is preferred. Means to control dissolved oxygen levels may include use of a separate means for introduction of oxygen or air and aeration of the contents of the vessel.

Preferably the apparatus includes a means for monitoring criteria essential to the control of and proper functioning of secondary treatment in the vessel. Such means include but are not limited to means for monitoring hydrocarbon, WSO, and O&G levels in produced water flowing in and out of the vessel, the temperature, salinity, and chemical composition of the produced water in the vessel, and the viability of the resident colony of microorganisms. Remote monitoring may be additionally performed with cameras and video cameras to track the mechanical condition and operation of the vessel.

Preferably any additives or chemicals used in the production of the field are screened for toxicity to the microorganisms used in the vessel. The additives or other chemicals used in production are preferably selected such that there is little or no toxicity due to the additive concentration resulting in the produced water, or more preferably, chemicals and additives used in the production of the field are selected such that, in the concentrations found in the produced water, they are susceptible to degradation in the vessel under ordinary operating conditions. The tolerance of the population of microorganisms resident in the treatment vessel can be enhanced by the operation of natural selection which causes an increase in the fraction of the population which more readily tolerates the additives to which it is exposed. This enrichment of the population of microorganisms with varieties more tolerant of the additives present in the vessel increases over time with prolonged exposure of the population to the relevant additives or other chemicals. The temperature of the interior of the vessel, more preferably the portion of the vessel containing the resident microorganisms, is approximately the same as or above the ambient temperature of the surrounding seawater. Preferably the temperature is within the range of 50 °F to 100°F and more preferably between 70 °F and 80 °F and most preferably within a temperature range optimized for the levels of and types of impurities in the produced water and the types of resident microorganisms. Temperature levels may also be adjusted to warmer or cooter levels to adjust the rate of metabolism of impurities in the produced water.

Frequently the influent produced water will have a temperature in excess of 100°F due to the elevated temperatures of subsurface reservoirs in a production field. The temperature of water in the vessel may be controlled by corrective mixing with sea water within the vessel, the degree of recirculation of water in the vessel, and mixing seawater with the inflow of produced water.

The salinity as measured by, for example, the total dissolved solids, of the water inside the vessel may be permitted to be any level consistent with the functioning of the resident microorganisms in metabolically reducing the amount of impurities in the produced water. Preferably the total dissolved solids is less than approximately 5% and more preferably approximately 3.8%, i.e., the ordinary concentration of total dissolved solids in seawater. Salinity levels in the produced water and the vessel are preferably monitored to avoid abrupt changes in the salinity of the water in the vessel. Salinity levels may be adjusted towards the preferable salinity of seawater by the addition of seawater to the inflow of produced water.

The pH of the water inside the vessel should be within a range compatible with the viability of the resident microorganisms and, preferably, between a pH of approximately 6 and approximately 9. If the pH requires adjustment, mineral acid or base, as appropriate, can be added to the produced water to adjust the pH of the water in the vessel. Sufficient buffering capacity may be supplied by the carbonate produced by the biologic activity of the resident microorganisms.

The petroleum, hydrocarbon, WSO, and O&G impurity concentration of the water inside the vessel is preferably maintained between approximately 50 and 100 ppm. The amount of impurities can be adjusted by adjusting the balance between recirculation of water and outflow from the vessel. If the petroleum, hydrocarbon, WSO, and O&G concentration is very low in the incoming produced water the population of the microorganisms will likely decline. The degree of aeration can be lowered to slow the rate of metabolism in the vessel if the petroleum, hydrocarbon, WSO, and O&G concentration becomes much lower than 50 ppm inside the vessel. If essential nutrients, including but not limited to fixed nitrogen and phosphorous, are not present in sufficient quantities in the water, microorganisms and sludge inside the vessel, small amounts of fertilizer, e.g., NH₄PO₄, or other materials containing essential nutrients, may be added to the inflowing produced water. Preferably only trace amounts, on the order of pounds per day for a 10 million pound reactor, would be added as supplements because recycling of essential nutrients would be expected inside the vessel.

The vessel provides a means for discharge of produced water following treatment including, optionally, ports discharging in the vicinity of the vessel, conveyances for open- water discharge from an outfall some distance from the vessel, or hoses or other conduits to bring the treated produced water to the surface. Preferably the vessel is configured for sub-sea surface discharge without additional pumping.

The vessel, when exposed to forces from currents, waves or pulses, preferably undulates, conforms to or deflects said forces. Such behavior is preferably sufficient to promote sloughing off of accreting marine life growing on the surfaces, whether internal or external, of the vessel. Additionally the vessel is preferably designed such that intended levels of buoyancy are not adversely affected by accretion of marine organisms on the vessel surfaces

Claims

1. A method of treating produced water containing impurities, which produced water is generated during fossil fuel production, comprising the steps of: (a) delivering the produced water to a vessel offshore;

(b) subjecting the produced water to advanced treatment within the vessel for sufficient time to reduce the amount of impurities in the produced water; and

(c) discharging the treated water from the vessel.

2. The method of claim 1 wherein the vessel is located substantially on the sea surface.

3. The method of claim 1 wherein the offshore vessel is located below the sea surface and above the sea floor.

4. The method of claim 1 wherein the vessel is located partially below the sea surface.

5. The method of Claim 1 wherein the vessel is located substantially on the sea floor.

6. The method of claim 1 wherein the vessel is substantially formed from a compliant material.

7. The method of claim 1 wherein the advanced treatment is at least one biological treatment performed by exposing the produced water to microorganisms capable of reducing the impurities in the produced water.

8. The method of claim 7 wherein the microorganisms are selected from the group consisting of microorganisms collected from wastewater treatment facilities, microorganisms collected from the seafloor, mutant microorganisms with enhanced capacity to reduce impurities in produced water, preparations of microorganisms containing varieties specially selected for their capacity to reduce inpurities in the produced water, Pseudomonas spp., and Arthrobacter spp.

9. The method of claim 7 wherein the microorganisms are obtained from sediments collected from the seafloor.

10. The method of claim 1 wherein the produced water containing impurities is treated using at least one primary treatment technique.

11. The method of claim 1 wherein the produced water is from offshore fossil fuel production.

12. An apparatus for performing biological treatment of a desired amount of produced water generated from fossil fuel production wherein a desired reduction in impurities in the produced water is achieved comprising:

(a) a vessel to contain the produced water during the biological treatment and designed for deployment offshore, wherein the vessel when deployed is formed with a substantially closed space which space having a volume sufficient to contain an amount of produced water desired to be treated. (b) a first port in the vessel for delivery of the produced water to the substantially closed space.

(c) a second port in the vessel for discharging the produced water after treatment.

(d) a third port for the introduction of gases into the vessel. (e) an aerator for release of gases inside the vessel; and

(f) a conduit connecting the third port to the aerator.

13. The apparatus of claim 12 further comprising means to control the outflow from the vessel and recirculation of water within the vessel.

14. The apparatus of claim 12 wherein the vessel substantially comprises a compliant structure.

15. The apparatus of claim 14 wherein the compliant structure is comprised of at least one sheet of high density polyethylene.

16. The apparatus according to claim 12, further comprising means to anchor the vessel offshore.

17. The apparatus according to claim 12 wherein the vessel is generally dome shaped.

18. A method of producing fossil fuels from a well wherein the method results in lowered pollutant emissions comprising the steps of

(a) producing fossil fuel, from a well, wherein produced water is also recovered, (b) separating the produced water from the fossil fuels produced.

(c) delivering the produced water to a vessel offshore;

(d) subjecting the produced water to advanced treatment within the vessel for sufficient time to reduce the amount of impurities in the produced water; and

(d) discharging treated water from the vessel.

19. The method of Claim 18 wherein the advanced treatment is biological treatment.

20. The method of Claim 18 wherein the produced water is generated offshore.

21. The method of Claim 18 wherein the vessel is located substantially on the sea surface.

22. The method of Claim 18 wherein the vessel is located substantially on the sea floor.

23. The method of Claim 18 wherein the vessel is located below the sea surface and above the sea floor.

24. The method of Claim 18 wherein the vessel is formed substantially from a compliant material.

25. The method of Claim 19 wherein the biological treatment is performed by exposing the produced water to microorganisms obtained from sediments dredged from the sea floor.

26. The method of Claim 18 wherein the produced water containing impurities is initially treated using at least one primary treatment technique.