Process for the purification of produced water deriving from hydrocarbon extraction wells

Abstract

Process for the purification of produced water deriving from hydrocarbon extraction wells comprising the following steps: (a) growing the microalga Dunaliella salina in the presence of water with a salinity ranging from 20 g / L to 200 g / L, said salinity being the same as the one of said produced water, so as to obtain an inoculum; (b) introducing the inoculum obtained in said step (a) in a tank for the treatment of said produced water so as to obtain: (b1) an aqueous suspension of algal biomass; (b2) purified produced water. Said process makes it possible to obtain produced water deriving from hydrocarbon extraction wells, in particular produced water deriving from hydrocarbon extraction wells having a lower content of total petroleum hydrocarbons (TPH), which can be directly discharged into the environment without the need for further treatment or used for other purposes [for example, re-injection into said wells for further hydrocarbon recovery known as "EOR"("Enhanced Oil Recovery")]. In addition, said process makes it possible to obtain an increased algal biomass, thanks to the use of hydrocarbons in the produced water, which can advantageously be used for the production of bio-oil.

Description

[0001] PROCESS FOR THE PURIFICATION OF PRODUCED WATER DERIVING FROM HYDROCARBON EXTRACTION WEEES

[0002] Cross-Reference to Related Applications

[0003] This Patent Application claims priority from Italian Patent Application No. 102022000023940 filed on November 21, 2022, the entire disclosure of which is incorporated herein by reference.

[0004] Technical Field

[0005] The present invention relates to a process for the purification of produced water deriving from hydrocarbon extraction wells.

[0006] More specifically, the present invention relates to a process for the purification of produced water deriving from hydrocarbon extraction wells in the presence of the microalga Dunaliella salina.

[0007] Said process makes it possible to obtain produced water deriving from hydrocarbon extraction wells, in particular produced water deriving from hydrocarbon extraction wells having a lower content of total petroleum hydrocarbons (TPH), which can be directly discharged into the environment without the need for further treatment or used for other purposes [for example, reinjection into said wells for further hydrocarbon recovery known as “EOR” (“Enhanced Oil Recovery”)]. In addition, said process makes it possible to obtain an increased algal biomass, thanks to the use of hydrocarbons in the produced water, which can advantageously be used for the production of bio-oil.

[0008] Background

[0009] It is well known that in hydrocarbon extraction wells there is always a presence of water, which, especially in oil wells, is very abundant and often higher than the amount of hydrocarbons present: this can have serious negative economic consequences. In fact, the excessive production of water leads to an increase in the costs associated with the management of the water itself (e.g., treatment and disposal), to the point of a possible premature abandonment of the well itself because it is no longer profitable to produce.

[0010] Produced water is derived either from the water naturally present in most hydrocarbon extraction wells, or from water or steam that is generally injected into the wells in order to push oil or gas to the surface, particularly when the pressure in the wells drops (known as reinjection water).

[0011] In the productive life of hydrocarbon extraction wells, the problems associated with produced water are becoming increasingly important as the watercut (i.e. the ratio between the water flow rate of a well and the total flow rate of water and hydrocarbons) increases.

[0012] Generally, produced water is water comprising various types of anions (e.g., chlorides, bicarbonates, carbonates, sulphates, bromides); dissolved gases (e.g., carbon dioxide, methane, nitrogen); sodium chloride at a concentration of up to 300 g / L.

[0013] Produced water may also comprise various types of metals (e.g., zinc, copper, boron), which are generally 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 PAH; aliphatic hydrocarbons (e.g., butane, propane, pentane, hexane).

[0014] In addition, organic additives used in order to facilitate the extraction of hydrocarbons may be present (e.g., de-emulsifiers, anti-foam agents, anti-fouling agents, biocides). In addition to said additives, glycols (e.g., diethylene glycol, triethylene glycol) or alcohols (e.g., methanol), that are generally used in case the formation of methane hydrates must be avoided in wells, in transfer pipelines from wells to the hydrocarbon treatment centre, or in transfer pipelines from clusters attached to that centre, are particularly important.

[0015] Depending on the legislation of the country where operations are carried out, subject to authorisation, said produced water, after undergoing chemical-physical treatments (e.g., by oxidation, precipitation, API oil-water separators and oil skimmers, corrugated plates, filtration) with the aim of fulfilling the required parameters depending on the final destination, may be re-injected into the well for the extraction of hydrocarbons via an injector well, or it may be discharged into a surface water body, or it may be sent for final disposal in a landfill or an open evaporation pond.

[0016] As the volume of produced water increases with the productive life of hydrocarbon extraction wells, the costs of treatment and / or management continuously increase due to the high charges and implications, including penalties, provided for by the legislation.

[0017] However, the produced water that is sent for disposal in open evaporation ponds, even after the above physico-chemical treatments, retains a hydrocarbon content that tends to accumulate within said open evaporation ponds.

[0018] In addition to the aforesaid chemical-physical treatments, processes for the phycoremediation of produced water are known in the art.

[0019] For example, Talebi A. F. et al., in “RSC Advances” (2016), “Enhanced algal-based treatment of petroleum produced water and biodiesel production”, Vol. 6, pg. 47001-47009, doi:10.1039 / c6ra06579a, report the growing of the microalga Dunaliella salina in various dilutions of produced water and seawater such as 1:1, 1:2 and 1:3. The results obtained show that the presence of produced water (1:1 dilution with seawater) makes it possible to obtain an increase in biomass production of approximately 120% and in the lipid content of approximately 65% compared to growing in seawater (control). In addition, the ability of the microalga Dunaliella salina to remove nitrogen (65%), phosphorous (40%) and heavy metals such as nickel (about 90%) and zinc (about 80%) when grown in produced water diluted with seawater is reported. Operating under the aforesaid conditions, the non-ability of Dunaliella salina to degrade total petroleum hydrocarbons (TPH) present in the produced water is also reported.

[0020] Alsarayreh M. et al. in “ Sustainability” (2022), “Biological-based Produced Water Treatment Using Microalgae: Challenges and Efficiency”, Vol. 14, 499, doi.org / 10.3390 / sul4010499, report on various produced water treatments including microalgae treatments. Among the various microalgae, Dunaliella salina is said to be able to remove nitrogen, phosphorous and heavy metals such as nickel and zinc.

[0021] Das B. et al., in "Scientific Reports” (2019), Vol. 9, 15232, doi.org / 10.1038 / s41598-019-51806-5, “A cost-effective and environmentally sustainable process for phycoremediation of oil field formation water for its safe disposal and reuse”, report the phycoremediation of produced water in the presence of the alga Chlorella vulgaris BS1 isolated from said produced water resulting in a 98.63% decrease in the content of total petroleum hydrocarbons (TPH).

[0022] Indian Patent Application IN 2019 / 31000192 relates to a process for the purification of produced water from oilfields by treatment with the algal strain Chlorella vulgaris BS1 native to oilfield produced water, comprising the following steps: i) sterilising the produced water at an appropriate temperature and for the desired time; ii) sending the sterilised produced water obtained from step (i) to an inoculation vessel, which is then inoculated with an inoculum of Chlorella vulgaris BS1 through an inoculation port; iii) incubating the inoculated produced water with Chlorella vulgaris BS1 obtained in step (ii) at an appropriate temperature with orbital stirring; iv) disposing of the treated effluent obtained in step (iii) safely in the ecosystem or alternatively reusing it as injection water in oil fields; said Chlorella vulgaris BS1 meeting its growth requirements for carbon and macro-nutrients from oil hydrocarbons and the ionic composition of the produced water, respectively. The aforesaid process is said to be able to remove 99.96% of the total petroleum hydrocarbons (TPH) present in the produced water.

[0023] However, the above-reported processes are carried out either in produced water diluted with seawater and thus, at a lower salinity than the one of produced water as such, or in the presence of a specific algal strain and a sterilisation step of said produced water. Furthermore, in the case of the microalga Dunaliella salina, as reported above, the non-ability thereof to remove total petroleum hydrocarbons (TPH) present in the produced water is highlighted.

[0024] Summary

[0025] The Applicant therefore faced the problem of finding a process for the purification of produced water deriving from hydrocarbon extraction wells in the presence of a microalga that is able to grow in the produced water as such and that does not require dilution with seawater and / or sterilisation steps of said produced water.

[0026] The Applicant surprisingly found a process for the purification of produced water from hydrocarbon extraction wells in the presence of the microalga Dunaliella salina. In particular, unlike what is known in the art, the Applicant has found that the use of the microalga Dunaliella salina makes it possible to obtain produced water deriving from hydrocarbon extraction wells, in particular produced water deriving from hydrocarbon extraction wells having a lower content of total petroleum hydrocarbons (TPH), which can be directly discharged into the environment without further treatment or used for other purposes [for example, re-injection into said wells for further hydrocarbon recovery known as “EOR” (“Enhanced Oil Recovery”)]. In addition, said process makes it possible to obtain an increased algal biomass, thanks to the use of hydrocarbons in the produced water, which can advantageously be used for the production of bio-oil.

[0027] It is therefore an object of the present invention to provide a process for purifying produced water deriving from hydrocarbon extraction wells comprising the following steps:

[0028] (a) growing the microalga Dunaliella salina in the presence of water with a salinity ranging from 20 g / L to 200 g / L, said salinity being the same as the one of said produced water, so as to obtain an inoculum;

[0029] (b) introducing the inoculum obtained in said step (a) in a tank for the treatment of said produced water so as to obtain:

[0030] (bi) an aqueous suspension of algal biomass;

[0031] (b2) purified produced water.

[0032] For the purpose of the present description and the following claims, the definitions of the numerical ranges always comprise the extreme values unless otherwise specified.

[0033] For the purpose of the present description and the following claims, the term "comprising" also includes the terms "which essentially consists of" or "which consists of".

[0034] In order to obtain said inoculum, the growing of the microalga Dunaliella salina can be conveniently carried out in growing systems known in the art such as, for example, laboratory cultures, open ponds (OP), photo -reactors (FR), photobioreactors (FBR), or combinations thereof, preferably in photo -bioreactors, in the presence of water with a salinity ranging from 20 g / E to 200 g / L. For the purpose of the present invention, in order to achieve a good growth of the algal biomass and a consequent decrease in hydrocarbons present in the tank for produced water treatment, the quantity of inoculum introduced into the tank must be such as to achieve immediate growth according to a maximum slop (i.e. according to Monod's laws of kinetics).

[0035] For the purpose of the present invention, the duration of said step (b) depends on both the characteristics of the produced water and the environmental conditions.

[0036] According to a preferred embodiment of the present invention, said step (b) may be carried out for a time sufficient to obtain the desired reduction of the content of total petroleum hydrocarbons (TPH).

[0037] According to a preferred embodiment of the present invention, said step (b) may be carried out at a temperature ranging from 4°C to 60°C, preferably ranging from 20°C to 30°C.

[0038] According to a preferred embodiment of the present invention, said step (b) may be carried out at a pH ranging from 7 to 10, preferably ranging from 7.5 to 9.5.

[0039] According to a preferred embodiment of the present invention, in said step (b) the produced water may have a salt content ranging from 20 g / L to 200 g / L.

[0040] For the purpose of the present invention, in said step (b), nutrients, in particular nitrogen (N)- and phosphorous (P)-based compounds, may be added.

[0041] According to a preferred embodiment of the present invention, nitrogen- and phosphorous-based compounds may be added in said step (b) in such a quantity that they do not become a growth-limiting factor.

[0042] As mentioned above, one of the advantages of the process that is the subject of the present invention is the possibility of obtaining an increased algal biomass, thanks to the use of hydrocarbons in the produced water, which can advantageously be used for the production of bio-oil.

[0043] For this purpose, said aqueous suspension of algal biomass (b i ) , after having been recovered from the tank for the treatment of produced water deriving from hydrocarbon extraction wells by techniques known in the art such as, for example, vacuum filtration, centrifugation, filter presses or belt presses, may be subjected to separation in order to obtain a concentrated aqueous suspension of algal biomass and an aqueous phase. Said separation may be carried out by various processes such as, for example: gravitational separation by sedimenters and / or thickeners; flocculation; coagulation; flotation; separation by hydrocyclones or spirals; centrifugation by disc-stack centrifuge, or by “Sedi canter®”; ultra- or micro-filtration through tangential ultra- or micro-filtration devices provided with ceramic membranes, or other types of filters; vacuum filtration; treatment by filter presses or belt presses; or combinations thereof.

[0044] The concentrated aqueous suspension of algal biomass obtained may be advantageously used in the production of bio-oil. Said bio-oil may be obtained, for example, by subjecting the concentrated aqueous suspension of algal biomass obtained to liquefaction treatments, or by subjecting said concentrated aqueous suspension of algal biomass, previously dried, to pyrolysis. Said bio-oil may be advantageously used in the production of biofuels that may be used as such, or in a mixture with other fuels, for automotive purposes. Alternatively, said bio-oil may be used as such (bio-fuel), or in a mixture with fossil fuels (fuel oil, lignite, etc.), to generate electricity or heat.

[0045] Brief Description of the Drawings

[0046] The present invention will now be shown by means of an illustrative form with reference to the accompanying Figures, wherein:

[0047] - Figure 1 is a diagram of the main steps of the invention process;

[0048] - Figure 2 shows the experimental results of some tests carried out according to the invention process.

[0049] Description of Embodiments

[0050] With reference to Figure 1, according to a typical embodiment of the process of the present invention, the microalga Dunaliella salina is grown in a photo- bioreactor to which water (H2O) (e.g., seawater) (line 1), carbon dioxide (CO2) (line 2) and nutrients (e.g., soluble nitrogen and phosphorus salts) (line 3) are sent.

[0051] From said photobioreactor, an inoculum of algal biomass (line 4) is obtained, which is fed to a growing tank for the treatment of produced water (line 5) to which produced water (line 6) and nutrients such as, for example, nitrogen (N) and phosphorous (P) compounds (line 7) are also fed, obtaining an aqueous suspension of algal biomass (line 8) and purified produced water (line 9).

[0052] Said aqueous suspension of algal biomass may be concentrated and subjected to the treatments described above (not shown in Figure 1). Said produced water may be directly discharged into the environment without further treatment or used for other purposes [for example, re-injection into said wells for further hydrocarbon recovery known as “EOR” (“Enhanced Oil Recovery”)].

[0053] In order to better understand the present invention and to put it into practice, some illustrative and non-limiting examples thereof are reported below.

[0054] EXAMPLE 1

[0055] An algal strain of Dunaniella salina, which normally grows in seawater, purchased from the Norwegian Institute for Water Research (NIVA) was used.

[0056] Below is the growing procedure adopted.

[0057] In order to test the growth of the microalga Dunaniella salina in the presence of hydrocarbons, an oil sample deriving from an Italian production site with the characteristics as shown in Table 1 was used.

[0058] Table 1

[0059] The following operations were also carried out: sterilisation of glassware and culture medium in an autoclave; algal culture preparation; preparation of 2 sets of samples containing culture medium, hydrocarbons and algae (the algal cell inoculum was taken from the growth column) with filling of the bottles with 400 ml of algal cells in J medium, consisting of water plus nutrients as listed in Table 2, and addition of crude oil (300 ppm or 500 ppm); preparation of the control sample in duplicate, one containing only culture medium and hydrocarbons and another containing only culture medium and algae; positioning of the bottles in the shaker. Table 2

[0060] The experimental conditions adopted were as follows: lighting: Neon h 24 / 24, 8 neon Extrastar T513W; - shaker: orbital h 24 / 24, speed: 90 rpm; culture medium: J medium (reported in Table 2); salinity: 160 g NaCl / kg solution (PSU); pH: 9; temperature: room temperature (25°C); - inoculum: 0.4 g / L; addition of the following nutrients NaNCh and KH2PO4:

[0061] NaNCh: 0.5 g / L; and

[0062] KH2PO4: 0.02 g / 1; volume of culture per flask: 400 ml; - creation of an Eracheck calibration line with different concentrations of crude oil in water 62.5 ppm, 125 ppm, 250 ppm, 500 ppm; periodic, non-repeatable measurements of the residual hydrocarbon content in the analysis and control flasks, using Eracheck: before measurement, 40 ml of cyclohexane was added to each flask in a 1:10 ratio to extract crude oil; positioning of the flasks in the shaker for a further 10 minutes; separation of the oil from the nutrients with a separating funnel; collection of the cyclohexane fraction containing hydrocarbons and analysis by Eracheck.

[0063] During the test, the contents of the flasks were analytically compared over time. The results obtained are reported in Figure 2 (“nutrients”), from which it is inferred the decrease in the hydrocarbon content (“crude oil removal percentage”) in the bottles containing the inoculum, at day 6, for both hydrocarbon concentrations (i.e. 300 ppm and 500 ppm) compared to the control. During the first 5 days, no difference in hydrocarbon content was found.

[0064] The decrease in hydrocarbons even in the control flasks is due to the fact that some light hydrocarbons are removed by evaporation.

[0065] The results obtained show that the microalga Dunaliella salina is able to survive both at high salinities and in the presence of hydrocarbons, producing a simultaneous abatement effect of the contents of the latter.

Claims

CLAIMS1. A process for the purification of produced water deriving from hydrocarbon extraction wells comprising the following steps:(a) growing the microalga Dunaliella salina in the presence of water with a salinity ranging from 20 g / L to 200 g / L, said salinity being the same as the one of said produced water, so as to obtain an inoculum;(b) introducing the inoculum obtained in said step (a) in a tank for the treatment of said produced water so as to obtain:(bi) an aqueous suspension of algal biomass;(b2) purified produced water.

2. The process for the purification of produced water deriving from hydrocarbon extraction wells according to claim 1, wherein said step (b) is carried out for an amount of time sufficient to obtain the desired reduction of the content of total petroleum hydrocarbons (TPH).

3. The process for the purification of produced water deriving from hydrocarbon extraction wells according to claim 1 or 2, wherein said step (b) is carried out at a temperature ranging from 4°C to 60°C, preferably ranging from 20°C to 30°C.

4. The process for the purification of produced water deriving from hydrocarbon extraction wells according to any one of the preceding claims, wherein said step (b) is carried out at a pH ranging from 7 to 10, preferably ranging from 7.5 to 9.5.

5. The process for the purification of produced water deriving from hydrocarbon extraction wells according to any one of the preceding claims, wherein in said step (b) the produced water has a content of salts ranging from 20 g / L to 200 g / L.

6. The process for the purification of produced water deriving from hydrocarbon extraction wells according to any one of the preceding claims, wherein in said step (b), nitrogen- and phosphorus-based compounds are added in a quantity such as not to become a growth limiting factor.

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