WO2010108687A1

WO2010108687A1 - Process for producing a biocide aqueous composition from produced water deriving from oil or gas wells and biocide aqueous composition

Info

Publication number: WO2010108687A1
Application number: PCT/EP2010/001892
Authority: WO
Inventors: Claudio Ghiselli
Original assignee: Eni S.P.A.
Priority date: 2009-03-27
Filing date: 2010-03-19
Publication date: 2010-09-30
Also published as: ITMI20090486A1; IT1400219B1; WO2010108687A8

Abstract

Process for producing a biocide aqueous composition from produced water deriving from oil or gas wells comprising subjecting said produced water to electrolysis and thus obtaining a aqueous composition. The biocide aqueous composition thus obtained can be advantageously used as disinfectant, and/or sterilizer, and/or bactericide, and/or bacteriostatic agent, in numerous industrial and civil fields.

Description

PROCESS FOR PRODUCING A BIOCIDE AQUEOUS COMPOSITION FROM PRODUCED WATER DERIVING FROM OIL OR GAS WELLS AND BIOCIDE AQUEOUS COMPOSITION

The present invention relates to a process for producing a biocide aqueous composition from produced water deriving from oil or gas wells.

More specifically, the present invention relates to a process for producing a biocide aqueous composition from produced water deriving from oil or gas wells which comprises subjecting said produced water to electrolysis .

Furthermore, the present invention relates to a biocide aqueous composition obtained by means of the process described above. The biocide aqueous composition thus obtained can be advantageously used as disinfectant, and/or sterilizer, and/or bactericide, and/or bacteriostatic agent, in numerous industrial and civil fields.

The co-production of water (i.e. produced water) is a reality which relates, in different degrees and times, to all oil or gas wells and can have considerable negative economic repercussions. The excessive production of water, in fact, causes both an increase in costs linked to the management of the water itself (e.g., disposal), and a reduction in the earnings due to the reduced production of hydrocarbons.

Produced water derives from both the water naturally present in most of underground oil or gas wells (known as "formation water"), and from the water or vapour which is generally injected into wells to push the oil or gas towards the surface, in particular when the level of oil or gas in the well descends

(known as "injected water") .

In the production life of a reservoir, problems associated with produced water are becoming increasingly more important due to the increase of the water-cut (i.e. the ratio between the capacity of water of a well and the total capacity of water + hydrocarbons) . Furthermore, the exploration boundaries are moving towards offshore fields, in increasingly deep water, and in areas often characterized by severe legislative regulations.

Produced water is generally water which comprises various types of anions (e.g., chlorides, bicarbonates, carbonates, sulfates, bromides); dissolved gases (e.g., carbon dioxide, methane, nitrogen); sodium chloride at a concentration which can reach 300 g/1.

Produced water can also comprise various types of metals (e.g., zinc, copper, boron), said metals generally being present in the form of salts,- aromatic hydrocarbons (e.g., benzene, toluene, ethylbenzene, o-, m- and p-xylene, generally known as BTEX; polycyclic aromatic hydrocarbons, generally known as IPA,- aliphatic hydrocarbons (e.g., butane, propane, pentane, hexane) .

Produced water can also generally comprise organic, non-hydrocarbon substances. Among these, organic acids, organic structures obtained by molecular re-arrangement following biochemical and thermal degradation of organic matter originally deposited in sedimentary basins (e.g. humic structures, polyphenols), can be mentioned, for example.

Furthermore, organic additives used in the production of hydrocarbons can also be present (e.g., antifoam agents, antiscaling agents, antivegetative agents, biocides) . In addition to these additives, glycols (e.g., diethyleneglycol, triethyleneglycol) , or alcohols (e.g., methanol), generally used when the formation of methane hydrates in the transfer pipelines from the well to the hydrocarbon treatment centre, or in the transfer pipelines from the clusters relating to said centre, must be avoided, are of particular importance . In addition to the produced water described above, the water deriving from the regeneration of the above glycols, to which produced water can be optionally added, can also be subjected to the above process for the production of a biocide aqueous composition. In relation to the legislation of the country of use, after authorization, said produced water can be re-injected into the oil or gas well through an injection well, or it can be injected into a well specifically used for disposal.

In the case of offshore installations, the produced water is generally subjected to de-oiling treatment and discharged into the sea, in conformance with the limits established by International Conventions. When this is not possible, or not allowed, the produced water must be transferred to land and handled in the ways indicated above (e.g. re-injected into an oil or gas well through an injection well, or injected into a well specifically used for disposal) .

Due to an increase in the volume of produced water with the production life of the reservoir, the treatment and/or management costs are constantly increasing.

In Italy, after authorization, the produced water can normally be injected into a deep geological unit. Produced water which cannot be injected into a deep geological unit, on the other hand, must be sent for disposal. From an economic point of view, this has an important repercussion due to the high burdens and implications, also of a penal nature, provided for by law.

The Applicant has considered the problem of finding a process which allows the produced water deriving from oil or gas wells to be advantageously used.

The use of electrochemical processes for the production of hypochlorite starting from saline aqueous solutions, is known in the art. Hypochlorite can be useful in the sterilization of water for obtaining drinking water, to prevent the proliferation of bacteria in the water of swimming-pools, to prevent and/or remove the formation of biological masses and algae, for the oxidation of organic material and in the treatment of cooling water.

United States Patent US 4,172,773, for example, describes an electrolysis process, starting from an aqueous solution containing at least 25 g/1 of a halide of an alkaline metal (e.g. sodium chloride) . The halogenation of the water, which comprises passing a current through a permeable porous anode and a cathode which form an electrodic gap between which the water passes, requires the control of the hydrodynamic flow, in order to maintain a weight ratio between active halogen and halide in the water which leaves the electrodic gap at least equal to 2. The above process is particularly useful in the production of sodium hypochlorite .

United States Patent US 4,488,945 describes a process for the production of hypochlorite from seawater by electrolysis in an electrolytic cell equipped with anodes and cathodes forming inter- electrodic spaces, characterized by mixing the seawater, before the electrolysis, with a quantity of hypochlorite recycled from said electrolytic cell which is sufficient to substantially oxidize the bromine, iodine and sulfur ions, present in said water as impurities, to their elemental form and by maintaining the temperature of the water fed to the electrolytic cell at a value not lower than 9.6⁰C.

Electrolytic treatment for removing contaminants from aqueous mediums is also known.

United States Patent US 6,802,956, for example, describes an electrolytic process for removing contaminants from an aqueous medium, comprising:

- adding one or more catalytic enzymes to the aqueous medium;

- providing an electrolytic cell including an anode and a cathode;

- adding the aqueous medium to the electrolytic cell so that the anode and the cathode are substantially immersed in the aqueous medium;

- applying a current of at least 10 A through the electrodes.

Said aqueous medium can be freshwater, wastewater, produced water from an oil well. In particular, in the case of produced water from an oil well, at the end of the above process, thanks to the addition of lanthanum chloride heptahydrate , a lowering of the salinity is obtained (i.e. a lowering of the concentration of sodium chloride) . Furthermore, at the end of the above process, an abundant calcification is observed together with an accumulation of chloride salts at the electrodes. The subsequent use of this produced water at the end of the above process is not mentioned. The Applicant has now found that, by subjecting the produced water deriving from oil or gas wells to electrolysis, operating within a particular pH range (i.e. at a pH ranging from 2.5 to I)₁ a biocide aqueous composition can be obtained, which can be advantageously used as disinfectant, and/or sterilizer, and/or bactericide, and/or bacteriostatic agent, in numerous industrial and civil fields.

Said biocide aqueous composition, for example, can be advantageously used: - in the microbiological treatment of civil and/or industrial wastes before being discharged into a surface water body,- - in the disinfection of industrial surfaces from biological agents (e.g. mildew, fungi, bacteria); - in the reduction of the COD (chemical oxygen demand) of civil and/or industrial wastes. Said biocide aqueous composition is particularly effective in eliminating bacteria such as, for example, Escherichia coli, Enterococchi sp., Stafilococchi sp . , contained in waters deriving from civil wastes. Tests carried out on said waters, in fact, have demonstrated a particular efficacy even when operating with minimum dosages and with reduced contact times (e.g., 5 minutes) . It should also be pointed out that said biocide aqueous composition is capable of maintaining this efficacy even after storage. Furthermore, said biocide aqueous composition has a high redox potential (i.e. higher than or equal to +500 mV) and is therefore capable of oxidizing organic substances .

According to a first aspect, the present invention therefore relates to a process for producing a biocide aqueous composition from produced water deriving from oil or gas wells which comprises subjecting said produced water to electrolysis, said electrolysis being carried out at a pH ranging from 2.5 to 7, preferably from 3 to 5.

According to a second aspect, the present invention relates to a biocide aqueous composition obtained by subjecting the produced water deriving from oil or gas wells to electrolysis, said electrolysis being carried out at a pH ranging from 2.5 to 7, preferably from 3 to 5.

For the purpose of the present invention and of the following claims, the definitions of the numerical ranges always comprise the extremes unless otherwise specified.

For the purpose of the present invention and of the following claims, the meaning of some of the terms used herein is defined hereunder:

- the term "salinity" refers to the concentration of sodium chloride in both the produced water, and in the biocide aqueous composition; - the term "total dissolved salts" (TDS) refers to the total concentration of salts, such as, for example, bicarbonates, carbonates, sulfates, bromides, which can have, as counter-ion, cations such as, for example, sodium, calcium, magnesium, in both the produced water and in the biocide aqueous composition;

- the term "active chlorine species" refers to molecular chlorine (Cl₂) , hypochlorous acid (HClO) and hypochlorite ion (ClO^"); - the term "concentration of active chlorine" refers to the quantity of chlorine released by the acid reaction of the hypochlorous acid (HClO) , and optionally of the hypochlorite ion (ClO^") , present in the biocide aqueous composition; - the term "concentration of chlorides" refers to the concentration of chlorine ions (Cl^") in both the produced water and in the biocide aqueous composition. It should be pointed out that by operating within the pH range indicated above (i.e. pH ranging from 2.5 to 7) , the concentration of the active chlorine is substantially due to the presence of hypochlorous acid (HClO) in the biocide aqueous composition. The concentration of active chlorine represents the quantity of molecular chlorine (Cl₂) which would be produced if the hypochlorous acid reacts with hydrochloric acid (or with another acid such as, for example, glacial acetic acid) according to the following reaction:

HClO + HCl -> H₂O + Cl₂ during which molecular chlorine (Cl₂) is developed in an equi-stoichiometric concentration with respect to the hypochlorous acid.

For the purpose of the present invention, the electrolysis is continued until the desired concentration of active chlorine in the biocide aqueous composition is reached (i.e. a concentration of active chlorine lower than or equal to 150 g/1) . The maximum concentration of active chlorine which can be obtained is in relation to the concentration of chlorides in the produced water. For the purpose of the present invention, the electrolysis can be indifferently carried out in any electrolytic cell known in the art, with or without a membrane or diaphragm.

It should be pointed out that if the electrolysis is carried out in an electrolytic cell with a membrane or diaphragm, the biocide aqueous composition is the so-called "anolite" (i.e. the aqueous solution which is formed in the compartment comprising the anode) . If a cell is used with a membrane or diaphragm, the so- called "catolite" is also formed (i.e. the aqueous solution which is formed in the part comprising the cathode) , which can be used instead for other purposes (e.g. as detergent, as an agent for increasing the pH) .

For the purpose of the present invention, the electrolysis can be carried out using an electrolytic cell, or various electrolytic cells in series and/or in parallel.

According to a preferred embodiment of the present invention, said electrolysis can be carried out at a temperature ranging from 10⁰C to 45⁰C, preferably from 15°C to 30⁰C. According to a preferred embodiment of the present invention, said electrolysis can be carried out at a current density ranging from 0.3 kA/m² to 3 kA/m², preferably from 0.9 kA/m² to 1.5 kA/m².

During the electrolysis, the potential difference (ΔV) depends on the current density used, the temperature, the total dissolved salts (TDS) and the ion force of the produced water.

According to a preferred embodiment of the present invention, said electrolysis can be carried out at a potential difference (ΔV) ranging from 2 V to 10 V, preferably from 3 V to 8 V.

During the electrolysis, the flow of the produced water in the electrolytic cell depends on the residence time, said time in turn depending on the concentration of chlorides in the produced water, the operating conditions (i.e. pH, temperature, current density, potential difference) and the concentration of active chlorine to be obtained.

In order to eliminate the hydrocarbons, in particular the aromatic compounds and the polycyclic aromatic compounds present in the produced water described above, said produced water can be subjected, before electrolysis, to purification treatments.

According to a preferred embodiment of the present invention, the process of the present invention can comprise subjecting the produced water, before electrolysis, to at least one purification treatment.

Said purification treatment can be selected from those known in the art such as, for example:

- solid-liquid filtration-separation [for example, sand bed filtration (dual media filters) , filtration with cartridge filters, hydrocyclones] ; flotation (for example, Dissolved Air Flotation or DAF, Induced Gas Flotation or IGF) ;

- adsorption on suitable carriers such as, for example, activated carbons, organophilic bentonite, zeolites.

It should be noted that the organic substances which remain in the produced water, after the above purification treatment, are useful as they can contribute to maintaining the pH of the electrolysis within the above ranges (i.e. pH from 2.5 to 7, preferably from 3 to 5) . Before or during the electrolysis, if the pH does not fall within the ranges indicated above, it is possible to add mineral acids (e.g. hydrochloric acid, phosphoric acid), or glycols (e.g. diethylene glycol, triethylene glycol) to the produced water, in such a quantity as to bring the pH back to the above ranges and to maintain it at these values .

During the electrolysis, gases can be released

(e.g., hydrogen, chlorine, oxygen). These gases are generally treated in specific scrubbers in order to separate the chlorine from the other gases. The chlorine thus separated can be sent to the electrolytic cell in order to keep the concentration of active chlorine high in the biocide aqueous composition, whereas the hydrogen, oxygen and other optional gases present, can be sent for recovery or discharged into the atmosphere .

For the purpose of the present invention, the electrolysis can be carried out in continuous operating, for example, according to a "feed & bleed" type process. In this case, the produced water is fed in continuous to the electrolytic cell, whereas the biocide aqueous solution "within specification" (i.e. having the desired concentration of active chlorine) , is discharged in continuous downstream of the electrochemical cell.

Alternatively, the electrolysis can be carried out batchwise. In this case, the produced water is fed to the electrolytic cell in the desired volumes. The electrolysis is carried out under the operating conditions described above and continued until the desired concentration of active chlorine is reached. At this point, the electrolysis is stopped and the biocide aqueous composition is discharged from the electrochemical cell. When the discharging has been completed, a new electrolysis can be initiated.

As already mentioned above, the present invention also relates to a biocide aqueous composition.

According to a preferred embodiment, said biocide aqueous composition can have a concentration of active chlorine lower than or equal to 150 g/1, preferably ranging from 1 g/1 to 10 g/1. The concentration of active chlorine was measured according to the analytical method APAT IRSA-CNR 4080 Manual 29/03.

According to a preferred embodiment, said biocide aqueous composition can have a pH ranging from 2.5 to 7, preferably from 3 to 5.

The pH was measured operating according to the analytical method APAT IRSA 2060 Manual 29/2003. Said biocide aqueous composition was also subjected to further analyses and proved to have the following characteristics :

- concentration of chlorate ions (ClO₃ ^") lower than or equal to 3000 ppm, preferably ranging from 1000 ppm to 2000 ppm (measured according to the analytical method EPA 9056A/2000) ;

- concentration of chlorite ions (ClO₂ ^") lower than or equal to 10 ppm, preferably ranging from 2 ppm to 8 ppm (measured according to the analytical method Standard Methods 4110 D 21 st. ed/05) ; - concentration of calcium ions (Ca²⁺) lower than or equal to 7000 ppm, preferably ranging from 200 ppm to 5000 ppm (measured according to the analytical method EPA 3010/A + EPA 6010/C) ; concentration of molecular sulfur (S₂) lower than or equal to 100 ppm, preferably ranging from 20 ppm to 80 ppm (measured according to the analytical method EPA 3010/A + EPA 6010/C) ; - concentration of boron (B) lower than or equal to 150 ppm, preferably ranging from 10 ppm to 100 ppm (measured according to the analytical method EPA 3010/A + EPA 6010/C) ; salinity lower than or equal to 300 g/1, preferably ranging from 5 g/1 to 150 g/1 (measured according to the analytical method APAT IRSA 2070 Manual 29/2003) ; COD (chemical oxygen demand) lower than or equal to 1000 mg/1, preferably ranging from 100 mg/1 to 500 τng/1 (measured according to the analytical method APAT IRSA 5330 Manual 29/2003) . The above biocide aqueous composition can also comprise varying quantities of aluminium, calcium, magnesium, manganese, potassium, sodium, iodides, bromides, ammonium, in the form of salts. Said quantities depend on the concentration of total dissolved salts presents in the produced water used.

During the electrolysis, free radicals such as, for example, O₂ (oxygen radical) , OH^" (hydroxide radical) , O₂ ^" (superoxide radical) can also be formed, which have a high biocide ability, but which are generally not found in the final biocide aqueous composition due to the short lifetimes (i.e. in the order of hours or tens of hours) .

According to a preferred embodiment of the present invention, said biocide aqueous composition can have a redox potential higher than or equal to +500 mV, preferably ranging from +600 mV to +1100 mV.

Due to the high redox potential, the biocide aqueous composition is preferably stored in plastic containers (for example, in polyethylene, polypropylene containers), or in dark glass containers, preferably at a temperature lower than or equal to 30⁰C, more preferably lower than or equal to 1O⁰C, in order to prevent the acceleration of decomposition and decay- phenomena of the active chlorine, essentially due to the following reaction:

2Cl₂ + 2H₂O → 4HCl + O₂. Some illustrative and non-limiting examples are provided hereunder for a better understanding of the present invention and for its embodiment. EXAMPLE 1 Electrolysis of produced water The electrolysis of the produced water was carried out in a Pyrex glass electrolytic cell, comprising an anode and a cathode. The anode used was made of titanium coated with oxides of transition elements. The cathode used was made of titanium. A direct current generator was connected to the anode and cathode. Physico-chemical characterization

Before the electrolysis, the produced water was characterized from a physico-chemical point of view: the data obtained are indicated in Table 1.

TABLE 1

The produced water was also subjected to pH measurement according to the analytical method APAT IRSA 2060 Manual 29/2003 obtaining a value of 6.15.

The electrolysis was carried out under the following operating conditions: pH = 4; current density = 1 kA/m³; area of each electrode = 100 cm²; potential difference (ΔV) = 4.5 V; - temperature = 25⁰C; time = 25 minutes; volume = 2.5 1.

At the end of the electrolysis, the biocide aqueous composition obtained was subjected to analysis of the concentration of active chlorine operating according to the analytical method APAT IRSA 4080 Manual 29/03 obtaining a value of 1.2 g/1.

The biocide aqueous solution thus obtained was stored in a polypropylene container, at a temperature equal to 4⁰C, for 20 days. EXAMPLE 2 Microbiological evaluation

The aqueous solution obtained as described in Example 1, was subjected to microbiological evaluation in order to verify its biocidal efficacy.

In this respect, a sample of civil waste was preliminarily subjected to microbiological analysis for the quantification of the following parameters:

(A) Bacteria charge at 36⁰C (analytical method: APAT IRSA 7050/C Manual 29/2003) ;

(B) Positive Stafilococchi coagulasi (Istisan reports 2000) ;

(C) Escherichia coli (analytical method: APAT IRSA 7030/F Manual 29/2003)

(D) Enterococchi sp. (analytical method: APAT IRSA 7040/C Manual 29/2003)

The results obtained (the analyses were carried out three times) are indicated in Table 2.

TABLE 2

The biocide aqueous solution of Example 1, after storage as described above, was again subjected to analysis of the concentration of active chlorine, operating according to the analytical method APAT IRSA 4080 Manual 29/03 obtaining a value of 0.285 g/1.

In order to verify the biocidal efficacy of the above biocide aqueous composition, 0.05 ml of the above biocide aqueous solution (concentration of active chlorine equal to 14.25 μg/1) and 99.95 ml of sterile water were added to 100 ml of the civil waste indicated above .

The sample thus obtained was maintained, under stirring, at 25°C, for 5 minutes.

After 5 minutes, the sample was subjected to microbiological analysis, for the quantification of the following parameters:

(A) Bacteria charge at 36°C (analytical method: APAT IRSA 7050/C Manual 29/2003) ;

(B) Positive Stafilococchi cσagulasi (Istisan reports 2000) ;

(C) Escherichia coli (analytical method: APAT IRSA 7030/F Manual 29/2003) (D) Enterococchi sp. (analytical method: APAT IRSA 7040/C Manual 29/2003)

The results obtained (the analyses were carried out three times) are indicated in Table 3. TABLE 3

The results indicated in Table 3, compared with those indicated in Table 2, clearly show that the biocide aqueous composition object of the present invention, even at very low concentrations of active chlorine (i.e. 14.25 μg/1) , has a good biocidal activity with respect to the bacterial species present in the civil waste.

EXAMPLE 3

Toxicity evaluation

The biocide aqueous composition obtained as described in Example 1, was subjected to direct and indirect toxicity evaluation.

For this purpose, the toxicity test was carried out on Daphnia Magna, using a "IQ Toxicity Test Kit" of Sclavo Diagnostics International s.r.l. A sample of civil waste as such (Sample 1) was preliminarily subjected to this test: the result obtained is indicated in Table 4.

The test was subsequently carried out on a sample (Sample 2) which was prepared by adding 98 ml of sterile water and 2 ml of the biocide aqueous solution of Example 1, after storage as described above, to 100 ml of civil waste. In this way, the supplementary toxicity or indirect toxicity due to the addition of the biocide aqueous solution to the sample of civil waste was evaluated: the result obtained is indicated in Table 4.

Finally, the direct toxicity was evaluated, by adding 1 ml of biocide aqueous solution of Example 1, after storage as described above, to 99 ml of growth water of Daphnia Magna (Sample 3) : the result obtained is indicated in Table 4.

TABLE 4

The results indicated above clearly show that the biocide aqueous solution of the present invention has a very low toxicity both when added to civil waste and also when added to the growth water of Daphnia Magna.

Claims

1. Process for producing a biocide aqueous composition from produced water deriving from oil or gas wells comprising subjecting said produced water to electrolysis, said electrolysis being carried out at a pH ranging from 2.5 to 7.

2. Process according to claim 1, wherein said electrolysis is carried out at a pH ranging from 3 to 5.

3. Process according to claim 1 or 2 , wherein said electrolysis is carried out at a temperature ranging from 10⁰C to 45°C.

4. Process according to claim 3, wherein said electrolysis is carried out at a temperature ranging from 15°C to 30°C.

5. Process according to any one of the previous claims, wherein said electrolysis is carried out at a current density ranging from 0.3 kA/m² to 3 kA/m².

6. Process according to claim 5, wherein said electrolysis is carried out at a current density ranging from 0.9 kA/m² to 1.5 kA/m².

7. Process according to any one of the previous claims, wherein said electrolysis is carried out at a potential difference (ΔV) ranging from 2 V to 10 V.

8. Process according to claim 7, wherein said electrolysis is carried out at a potential difference

(ΔV) ranging from 3 V to 8 V.

9. Process according to any one of the previous claims, wherein the produced water, before the electrolysis, is subjected to at least one purification treatment.

10. Biocide aqueous composition obtained by subjecting the produced water deriving from oil or gas wells to electrolysis, said electrolysis being carried out at a pH ranging from 2.5 to 7.

11. Biocide aqueous composition according to claim 10, wherein said electrolysis is carried out according to any one of claims 2 to 9.

12. Biocide aqueous composition according to claim 10 or 11, wherein said composition has an active chlorine concentration lower than or equal to 150 g/1.

13. Biocide aqueous composition according to claim 12, wherein said composition has an active chlorine concentration ranging from 1 g/1 to 10 g/1.

14. Biocide aqueous composition according to any one of claims 10 to 13, wherein said composition has a pH ranging from 2.5 to 7.

15. Biocide aqueous composition according to claim 14, wherein said composition has a pH ranging from 3 to 5.

16. Biocide aqueous composition according to any one of claims 10 to 15, wherein said composition has a redox potential higher than or equal to +500 mV.

17. Biocide aqueous composition according to claim 16, wherein said composition has a redox potential ranging from +600 mV to +1100 mV.

18. Use of the biocide aqueous composition according to any one of claims 10 to 17, as disinfectant and/or sterilizer and/or bactericide, and/or bacteriostatic agent .