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Title:
WATER TREATMENT PROCESS FOR OIL AND GAS FIELDS PRODUCED WATER
Document Type and Number:
WIPO Patent Application WO/2011/121619
Kind Code:
A1
Abstract:
The invention concerns a continuous process for treating waste water and/or produced water from oil and/or gas fields, such that the treated water has a re-use quality for reinjecting it into the reservoir or for release as surface water. The process makes use of a suspension of activated silica and of an organic polymeric flocculation agent.

Inventors:
CIANFRUGLIA RAIMONDO (IT)
Application Number:
PCT/IT2010/000130
Publication Date:
October 06, 2011
Filing Date:
March 29, 2010
Export Citation:
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Assignee:
ABB SPA (IT)
CIANFRUGLIA RAIMONDO (IT)
International Classes:
C02F9/02; C02F9/04; C02F9/08; C02F101/32
Foreign References:
GB827586A1960-02-10
US7520993B12009-04-21
Other References:
FAKHRU'L-RAZI A ET AL: "Review of technologies for oil and gas produced water treatment", JOURNAL OF HAZARDOUS MATERIALS, ELSEVIER, AMSTERDAM, NL LNKD- DOI:10.1016/J.JHAZMAT.2009.05.044, vol. 170, no. 2-3, 30 October 2009 (2009-10-30), pages 530 - 551, XP026521167, ISSN: 0304-3894, [retrieved on 20090519]
K. SHAM ASHAGHI ET AL., THE OPEN ENVIRONMENTAL JOURNAL, vol. 1, 2007, pages 1 - 8
THE CHEMISTRY IS TO BE FOUND IN ILER, 1979, pages 231
K. R. LANGE; R. W. SPENSER, ENVIR. SCI. AND TECHNOLOGY, vol. 2, no. 3, 1968, pages 212 - 6
T. HASEKAWA ET AL., WATER SCIENCE AND TECHNOLOGY, vol. 23, 1991, pages 1713 - 1722
C. HENRY, J. AM. WATER WORKS ASS., vol. 30, no. 1, 1958, pages 61 - 71
Attorney, Agent or Firm:
ZANOLI, Enrico et al. (Via Melchiorre Gioia 64, Milano, IT)
Download PDF:
Claims:
CLAIMS

1. Process for treating waste water or produced water in oil or gas fields, comprising the steps of:

a) collecting the water to be treated in a pre-treatment and homogenizing tank (10) in which a first separation by gravity of suspended matters occurs, whereby oils and other hydrocarbons form an upper floating phase and solids sediment at the bottom of the tank, said upper floating phase and said solids being continuously removed from said pre-treatment tank;

b) transferring water from said pre-treatment tank (10) to a corrugated plate interceptor (CPI) unit (20) in which separation by gravity of oils and other hydrocarbons and solids is continued;

c) transferring the treated water from said CPI unit (20) into a flocculation tank (30) in which a mixing is carried out;

d) adding a flocculation agent to the water discharged from said flocculation tank (30) and transferring said water containing said flocculation agent to an air flotation tank (40) in which water saturated by air is injected from the bottom of said tank, whereby oils and other hydrocarbons are entrained by air bubbles moving upwards and caused to form an upper floating phase and solids sediment at the bottom of the tank;

e) subjecting the water coming from the flotation tank (40) to a filtration step through filtration means (60) comprising sand;

f) subjecting the water coming from the filtration step to an ultra-filtration step through filtration means (70) comprising active carbon.

characterized in that:

- in said step c) a suspension of activated silica is added to said water before or after said treatment in said CPI unit (20) and before it is transferred to said flocculation tank (30);

in said step e) a suspension of activated silica is also added to the water prior to passing through said filtration means (60).

2. Process according to claim 1, characterized in that in said step d) said flocculation agent is a polymeric cationic or amphoteric flocculant.

3. Process according to claim 2, characterized in that:

- in said step c) said suspension of activated silica is added to said water after said treatment in said CPI unit (20) in an amount of from 50 to 300 ppm of Si02; in said step e) a suspension of activated silica is also added to the water prior to passing through said filtration means (60) in an amount of from 20 to 100 ppm of Si02.

4. Process according to claim 2, characterized in that:

in said step c) said suspension of activated silica is added to said water after said treatment in said CPI unit (20) in an amount of from 80 to 200 ppm of Si02;

- in said step e) a suspension of activated silica is also added to the water prior to passing through said filtration means (60) in an amount of from 30 to 80 ppm of Si02.

5. Process according to claim 2, characterized in that:

in said step c) said suspension of activated silica is added to said water after said treatment in said CPI unit (20) in an amount of from 90 to 120 ppm of Si02;

in said step e) a suspension of activated silica is also added to the water prior to passing through said filtration means (60) in an amount of from 40 to 60 ppm of Si02

6. Process according to claim 1, characterized in that said polymeric cationic or amphoteric flocculant is added in an amount of from 2 to 20 ppm.

7. Process according to claim 1, characterized in that said polymeric cationic or amphoteric flocculant is added in an amount from 5 to 10 ppm.

8. Process according to claim 1, characterized in that said suspension of activated silica has a concentration of Si02 of from 0.1 to 10 % wt.

9. Process according to claim 1, characterized in that said suspension of activated silica has a concentration of Si02 of from 0.5 to 5 % wt.

10. Process according to claim 1, characterized in that said suspension of activated silica has a concentration of Si02 of from 0.8 to 1.2 % wt.

Description:
WATER TREATMENT PROCESS FOR OIL AND GAS FIELDS PRODUCED WATER

DESCRIPTION

Field of the invention

The present invention relates generally to waste water treatment for oil and gas fields, and, more specifically, to a process to remove suspended solids, oils and other hydrocarbons from produced water.

Background of the invention

Oil and gas reservoirs are sub-soil cavities containing crude oil and/or natural gas. Oil and gas reservoirs have a natural water layer, called formation water, that, being denser, lies under the hydrocarbons. Oil reservoirs frequently contain large volumes of water, while gas reservoirs tend to produce only small quantities. To achieve maximum hydrocarbons recovery, additional water is usually injected into the reservoirs to help force the oil/gas to the surface. Both formation and injected water are eventually produced along with the hydrocarbons and, as an oil field becomes depleted, the amount of produced water increases as the reservoir fills with injected water. Such water is called "produced water".

Produced water is typically removed from the oil, but still contains a number of undesirable components such as paraffins, oils, other hydrocarbons and organic substances, muds, salts, solids and the like. Oil may be present in different amounts, depending on the type of oilfield and other factors, from a few hundreds parts per million (ppm) to large amounts such as up to 5% by volume . The contaminated produced water must be disposed of in some manner. It can be treated and released as surface water or it can be injected back into the oil or gas reservoir. Recycling produced water by reinjecting it into the oil/gas reservoir is particularly interesting when the oil/gas field is onshore and in a desertic area. However, the processing required to render produced water fit for reinjection is costly.

After extraction from reservoirs hydrocarbons are also washed with water to remove or reduce the high content of salts. The amount of such water, called "washing water" may exceed the amount of produced water. Washing water contains hydrocarbons too, and must be disposed of in some manner or treated.

Description of the Prior Art

A number of technologies have been disclosed and/or tried to various extents to treat waste water in extraction installations of oil and gas reserves. Such technologies include membrane filtration technology, electrocoagulation with membrane technology, extraction with supercritical fluids (CN 10186350) and treatment with ozone (US 2007/0056913 Al ; CN 201276464), to name a few. K. Sham Ashaghi et al, The Open Environmental Journal, 2007, 1, 1-8, disclose ceramic membranes for ultra- and nano-filtration for oilfield produced water.

While certain of these technologies produce an acceptably clean product on a laboratory size scale, they have generally proved to be uneconomical when scaled up for use in commercial installations.

US 2007/0095761 Al discloses a method for preparing acidic solutions of activated silica and polyvalent metal salt for water treatment. Use of activated silica is disclosed in connection with sewage treatment plants located in urban areas, particularly to remove disinfection byproducts (DBP) and DBP precursors. Reported amounts of DBP in such water is in the order of hundreds of ppb (parts per billion).

US 6,077,439 discloses using activated silica to remove metals from industrial waste streams, in particular heavy metals toxic salts. There is no disclosure on removing oil or other hydrocarbons from water.

In light of these difficulties and of the increasing awareness about environmental issues, there is a need in the art for a water treatment process that makes produced water and/or washing water fit for reinjection into oil/gas reservoirs. More particularly, there is a need for a water treatment process that removes oil and other suspended hydrocarbons and solids from produced water and washing water to makes it fit for reinjection or possibly that makes it acceptable for surface discharge.

Preferably, such an improved process will be capable of cleaning the waste waters constantly and continuously during the oil/gas extraction operations.

A need also exists for such a process which produces an acceptable product at a cost which is more economical than the existing technologies presently in use in oil and gas fields.

Summary of the invention

The present invention is an improved process for treating waste water and/or produced water in oil or gas fields for rendering the water fit for reinjection or for surface release.

The method includes:

a) collecting the water to be treated in a pre-treatment and homogenizing tank in which a first separation by gravity of suspended matters occurs, whereby oils and other hydrocarbons form an upper floating phase and solids sediment at the bottom of the tank, said upper floating phase and said solids being continuously removed from said pre-treatment tank; b) transferring water from said pre-treatment tank to a corrugated plate interceptor (CPI) unit in which separation by gravity of oils and other hydrocarbons and solids is continued;

c) adding a suspension of activated silica to said water before or after said treatment in said CPI unit and transferring the treated water containing said activated silica into a flocculation tank in which a mixing is carried out;

d) adding a flocculation agent to the water discharged from said flocculation tank and transferring said water containing said flocculation agent to an air flotation tank in which water saturated by air is injected from the bottom of said tank, whereby oils and other hydrocarbons are entrained by air bubbles moving upwards and caused to form an upper floating phase and solids sediment at the bottom of the tank;

e) subjecting the water coming from the flotation tank to a filtration step through a filtration means that comprises sand, wherein a suspension of activated silica is also added to the water prior to passing through said filtration means;

f) subjecting the water coming from the filtration step to an ultra-filtration step through a filtration means that comprises active carbon.

The water coming from the ultrafiltration step is transferred to a storage tank and is ready for reinjection into the oil/gas reservoir. The solids and hydrocarbons phase separated from the produced water in the various steps of the process are also further treated and disposed of according to suitable methods.

The process according to the invention removes any suspended matters from the produced water, including oils, other hydrocarbons and solids. Typically the final content of oils and hydrocarbons does not exceed 40 ppm, or even 10 ppm.

Additional objects, features and advantages of the process according to the invention will be apparent in the written description which follows.

Brief description of the drawings

FIG. 1 is a schematic flow diagram of the process of the invention;

Detailed description of the invention

The invention comprises a continuous process for treating waste water and/or produced water from oil and/or gas fields, such that the treated water has a re-use quality for reinjecting it into the reservoir. Water treated according to the process of the invention can also be released as surface water, with or without additional treatments, depending on the quality of the water and of the applicable environmental laws. As used herein and in the appended claims, the term "produced water" means any waste water either from sub-soil or from other sources onshore or offshore associated with oil and/or gas fields that needs to be treated for further use or release in the environment. In particular, "produced water" includes also "washing water".

A particular task of the process according to the invention is that of essentially removing all the hydrocarbons contained in produced water, so that the residual hydrocarbons content after treatment is in the order of a few tenths of parts per million (ppm). Also, a task of the process according to the invention is that of essentially removing all the solid particles and suspended matters contained in produced water.

Produced water may contain different amounts of oil, depending on the type of oilfield and other factors. Also, it has typically an acidic pH, for example between 5 and 6. Oil content may vary from a few hundreds parts per million (ppm) such as 100-500 ppm, to large amounts such as up to 5% by volume or more. After separation from the extracted oil and/or gas and having undergone a separation of coarse solids such as stone, sand and the like, as known in the field, the produced water is treated according to the process of the invention. The process for treating produced water according to the invention combines essentially physical treatments such as those listed as a) and b) above, with chemical treatment steps such as those listed as c) and d) above.

FIG. 1 is a schematic flow diagram showing the main components of steps from a) to f) in the process of the invention.

In step a) the produced water, which has a flow rate that is not constant and regular, is fed to a tank 10 of sufficient size, so that the produced water is temporarily stored and homogenized. In a typical embodiment a produced water with 2% vol. of oil is collected in a 500 m 3 homogenizing tank 10 for a residence time of about 8 hours. Tank 10 allows also to carry out a pre-treatment of the produced water, consisting essentially in a physical separation of suspended matters, namely substances lighter and heavier than water which undergo a first separation by gravity. Lighter substances such as oils and other hydrocarbons form a floating phase 12 on the aqueous phase 14. Suspended matters such solid particles fall and deposit at the bottom of tank 10. Scraping blades 16 rotated by a motor Ml help discharging the deposited mud into a discharge conduit 1 1. The upper floating phase 12 is continuously removed from pre-treatment tank 10 by a pump 13, and pumped to an oil storage tank 15. In step b) the produced water is transferred from the pre-treatment tank 10 to a corrugated plate interceptor (CPI) unit 20, as known in the art, in which separation by gravity of oil and other hydrocarbons, solids and water is continued. Again an oil phase is formed in the upper part of the CPI unit 20, and is continuously removed by a deoiling device 13', such as a device commercially known as "Discoil", and pumped to an oil storage tank 15. Solid particles deposit at the bottom of tank 20 and are removed. Treated water overflows from tank 20 into a chamber 22 connected to a line 28 towards a further treatment step. According to the invention, a water suspension of activated silica is added via line 26 to the treated water in chamber 22, before water is transferred by gravity to a further treatment step.

Activated silica is a well-known material which may be described as a highly dispersed polymeric form of silica produced when dilute aqueous solutions of alkali metal silicates are reacted with mineral acids, or with multivalant metal ions such as calcium, iron or aluminum. Descriptions of the chemistry and method of preparation of activated silica are well covered in the literature. The chemistry is to be found in Her (1979, p. 231), J. G. Vail ("Soluble Silicates," Vol II New York: Reinhold, 1960), K. R. Lange and R. W. Spenser (Envir. Sci. and Technology 2: 3 (1968) 212-6), and T. Hasekawa et al. (Water Science and Technology 23 (1991) 1713)-1722). The manufacture of this material is described by C. Henry (J. Am. Water Works Ass. 30: 1 (1958) 61-71), and is disclosed in U.S. Pat. No. 3,963,640 (Jun. 15, 1976 to Anglian Water Authority, U.K. and No. 4,147,657 (to the PQ Corporation).

Activated silica may be manufactured and stored using specialized equipment, or can be prepared in situ. The material has been used for many years in the commercial purification of drinking water and has found application in the flocculation of alumina and silver bromide sols, and also as a retention/drainage aid in papermaking. However, no description was found of any special ability of activated silica to remove oils and/or hydrocarbons from water. Therefore it was surprisingly found in the course of the experimentation that led to the to the present invention that activated silica is very effective in the process to treat produced water, even produced water having a high hydrocarbons and salts content.

In the present description the term "suspension of activated silica" means a highly dispersed polymeric form of silica produced when dilute aqueous solutions of alkali metal silicates are reacted with mineral acids, as described above.

Turning to the addition of a water suspension of activated silica to the water that has been treated in chamber 22, it is disclosed that such suspension has a silica content expressed as percentage by weight of Si0 2 of from 0.1 to 10%, preferably from 0.5 to 5%, more preferably from 0,8 to 1.2 %. Such suspension is added to the treated water in an amount such that the silica content is from 50 to 300 ppm, preferably from 80 to 200 ppm, more preferably from 90 to 120 ppm. The separation treatment carried out according to step b) reduces the oil content to about 100- 200 ppm and the content of suspended matters to about 15-50 ppm.

In step c) the water withdrawn from chamber 22 of he CPI unit 20 via line 28 is introduced into a coagulation and flocculation tank 30, in which mixing is carried out by a stirrer 32 actuated by a motor M2. Average residence time in tank 30 is from 15 to 20 min. A flocculating agent is added via line 34 to the water stream leaving tank 30.

The flocculating agent used in step c) of the process according to the invention is a polyelectrolyte selected from linear or branched organic polymers that form complex ions in solution. More preferably the flocculating agent is a polymeric cationic or amphoteric flocculant, such as the flocculants commercialized under the trade name "Kurifix" and available from Kurita Water Industries Ltd. The flocculating agent is added to the water to be treated in an amount of from 2 to 20 ppm, preferably from 5 to 10 ppm.

In step d) water withdrawn from the coagulation and flocculation tank 30 via line 36 is mixed with a stream of water saturated with air under pressure released from tank 38 into line 37. The pressurized air-saturated water stream is introduced via line 39 into the bottom of an air flotation tank 40. In the air flotation tank 40 oils and other hydrocarbons are entrained by air bubbles moving upwards and caused to form an upper floating phase, while solids deposit at the bottom of the tank.

From a hydrodynamic viewpoint the air flotation tank 40 is divided into 3 zones:

A highly turbulent zone around the injection point of the pressurized water and air stream, in which contact between air bubbles and floes if promoted;

A laminar flow zone in the periphery of the tank, in proximity of the walls thereof;

An upper dead zone, in which foam accumulates.

Scraping blades 42 promote discharging mud from the conical bottom of tank 40 via conduit 46. Blades 42 are connected to a shaft rotated by a motor M3. Preferably the mud is discharged at predefined time intervals by means of an automatic discharge valve 43. An upper foam and floating floes discharge device 44 is also mounted in tank 40. Both device 44 and conduit 46 transfer the matter removed from the treated water to a mud thickening apparatus 48. Residence time of water in air flocculation tank 40 is of about 30 min, and is such to achieve a residual oil content after treatment of equal to or less than 10 ppm. After treatment the clarified water is discharged from tank 40 via line 49 to tank 50, from which it is partly fed to pressurizer tank 38 via line 47 and partly transferred to the subsequent process step e). In step e) water clarified in the previous process steps is withdrawn from temporary storage tank 50 and transferred to a filtration step. According to the invention, a suspension of activated silica is added to the water upstream the filtration step. Preferably activated silica is added in an amount of from 20 to 100 ppm, more preferably from 30 to 80 ppm, more preferably from 40 to 60 ppm. To ensure that any suspended material still present in the treated water has a particle size not greater than 5 microns, and ultra-filtration step f) is also included in the process.

The filtration step e) is carried out through a filtration means that comprises a double layer of quartz sand and hydroanthracite. In a preferred embodiment the clarified water stream is fed to three filters 60 in parallel, then to two filters 79 to carry out the ultra-filtration step f).

The ultrafiltration step f) is carried out through a filtration means that comprises active carbon.

The suspension of active silica is the same suspension that is used in step b) above. It has surprising been found that addition of active silica improves the performance of the filtration and ultra-filtration step.

The filters 60 and 70 are regularly cleaned by temporarily stopping the introduction of water to be treated in one of the filters and by feeding a counter-current stream of clean water. The finest solid particles deposited on the filters and traces of oil are thus removed.

The water discharged from filters 70 is clear and has an oil content typically not greater than

10 ppm. Although the treated, clean water has a pH that is still acidic, it is a water that is fit to be re-injected into the oil or gas reservoir or possibly released as surface water.

The oil and hydrocarbons phase separated from the water treated in the process of the invention and collected in the oil storage tank 15 is recycled in the oil extraction installation of the oilfield for further treatment and/or reuse. Muds and solids are sent to mud thickening apparatus 48 for further treatment and final disposal.

Example

In an exemplary embodiment, a produced water treatment process is carried out in a plant as illustrated in FIGS. 1-3, in which the pre-treatment tank 20 had a capacity of collecting 500 m 3 /day of produced water.

The water to be treated had an oil content of 2% vol. The flow rate of water delivered from pre-treatment tank 10 was of 100 mVhour.

A suspension of activated silica having a Si0 2 content of 1% by wt. was used. It was added to step b) to reach a concentration of Si0 2 in the water to be treated of 100 ppm, and in step e) of 50 ppm. An aqueous solution containing 0.1% by weight of the polyectrolyte flocculating agent commercialized under the trade name "Kurifix" and available from Kurita Water Industries Ltd was prepared. Such solution was added to the water to be treated in step c) in an amount of 8 ppm of polyectrolyte flocculating agent.

The clear and clean water after treatment had a oil content of less than 10 ppm, and no suspended matters having a size greater than 5 microns. The only chemicals used were activated silica and a polyectrolyte flocculating agent.

Comparative Example

The same water treatment process of Example 1 was performed without use of activated silica.

The water after treatment had a oil content of from 80 to 100 ppm. About 50% of the suspended matters had a size greater than 5 microns.

As it can be appreciated from the foregoing description and examples, the combined use of activated silica in steps b) and e) and of a polyelectrolyte as flocculating agent in step c) of the process of the invention, allows to achieve an almost complete removal of oil and hydrocarbon from produced water, as well as of any suspended matters. Use of flocculating agents only does not allow to achieve a similar result. The process proved to be efficient from an economic point of view.