Method of removing hydrocarbons from produced water

Abstract

The present invention relates to a method for removing hydrocarbons from produced water, comprising: (i) separating produced water from a hydrocarbon and water mixture extracted from a subterranean formation: (ii) contacting said produced water with multivalent metal cations to produce a mixture of produced water and multivalent metal cations; and (iii) removing hydrocarbons from said mixture in a hydrocyclone and/or a compact flotation unit to give treated produced water, wherein the concentration of hydrocarbons in said produced water is less than 10% wt.

Claims

1. A method for removing hydrocarbons from produced water, comprising: (i) separating produced water from a hydrocarbon and water mixture extracted from a subterranean formation; (ii) contacting said produced water with multivalent metal cations to produce a mixture of produced water and multivalent metal cations; and (iii) removing hydrocarbons from said mixture in a hydrocyclone and/or a compact flotation unit to give treated produced water, wherein a concentration of hydrocarbons in said produced water is less than 10% wt; and wherein an amount of precipitate produced during the method is less than 0.5% by weight.

2. A method as claimed in claim 1, wherein the concentration of hydrocarbons in said produced water is less than 5% wt.

3. A method as claimed in claim 1, wherein said produced water comprises a polymer.

4. A method as claimed in claim 3, wherein a concentration of said polymer in said produced water is 50-5000 ppm.

5. A method as claimed in claim 3, wherein said polymer is a water- soluble polymer.

6. A method as claimed in claim 3, wherein said polymer is an anionic polymer.

7. A method as claimed in claim 6, wherein said polymer comprises monomers of acryl amide and acrylic acid.

8. A method as claimed in claim 7, wherein said polymer is a hydrolysed polyacrylamide (HPAM).

9. A method as claimed in claim 8, wherein said polymer further comprises monomers of acrylamide tertiary butyl sulfonate.

10. A method as claimed in claim 9, wherein said polymer is a sulfonated hydrolysed polyacrylamide.

11. A method as claimed in claim 1, wherein a concentration of multivalent metal cations in said mixture is 5-500 ppm.

12. A method as claimed in claim 1, wherein said multivalent metal cations comprise trivalent metal cations.

13. A method as claimed in claim 1, wherein said multivalent metal cations are trivalent aluminium cations or trivalent iron cations.

14. A method as claimed in claim 1, wherein said multivalent metal cations are provided in the form of a metal salt or a metal complex.

15. A method as claimed in claim 14, wherein said metal salt is selected from FeCl.sub.3, Fe.sub.2(SO.sub.4).sub.3, AlCl.sub.3, Al.sub.2(SO.sub.4).sub.3, and FeClSO.sub.4.

16. A method as claimed in claim 14, wherein said metal complex is aluminium citrate.

17. A method as claimed in claim 1, wherein in step (iii) the hydrocarbons are removed from said mixture in a hydrocyclone and at least one compact flotation unit connected in series.

18. A method as claimed in claim 1, wherein an efficiency of the hydrocarbon removal in step (iii) is at least 60%.

19. A method as claimed in claim 1, wherein the concentration of hydrocarbons in said treated produced water is 0-100 ppm.

20. A method of recovering hydrocarbons from a hydrocarbon-containing formation, said method comprising: (i) providing water, to a hydrocarbon-containing formation; (ii) allowing said water to contact at least a proportion of the hydrocarbons in said formation; (iii) recovering from said formation a mixture comprising hydrocarbons and water; (iv) separating said mixture into recovered hydrocarbons and produced water; and (v) removing hydrocarbons from said produced water according to the method of claim 1 to give treated produced water.

21. A method as claimed in claim 20, wherein said treated produced water is reinjected into a formation in a recovery operation.

22. A method as claimed in claim 3, wherein said multivalent cations are not added in excess in relation to said polymer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described with reference to the following non-limiting Figures and examples wherein:

(2) FIG. 1 is a schematic of a large scale test rig used in the examples described herein;

(3) FIG. 2 is a plot of viscosity (cP) versus iron concentration (ppm) of simulated produced water comprising different concentrations and types of polymer;

(4) FIG. 3 is a plot of hydrocarbon removal (de-oiling) efficiency for a CFU versus Fe.sup.3+ cation concentration from produced water comprising different concentrations and types of polymer;

(5) FIG. 4 is a plot of hydrocarbon removal (de-oiling) efficiency for a hydrocyclone and for a hydrocyclone and CFU connected in series as well as overall hydrocarbon removal efficiency versus Fe.sup.3+ cation concentration from produced water comprising different concentrations and types of polymer;

(6) FIG. 5 is a plot of viscosity (cP) versus shear rate (s.sup.−1) for samples of simulated treated produced water taken from the CFU water outlet (after having been treated with differing concentrations of Fe.sup.3+ cations) and having further polymer introduced therein; and

(7) FIG. 6 is a plot of the amount of precipitation versus concentration of Fe.sup.3+ cations in simulated produced water.

EXAMPLES

(8) Materials

(9) HPAM-AMPS polymer, i.e. sulfonated hydrolysed polyacrylamide comprising monomers of acryl amide, acrylic acid and acrylamide tertiary butyl sulfonate.
The polymers used were purchased commercially.
General Method: Description of Test Rig

(10) A large-scale test rig as shown in FIG. 1 was used to generate the data contained in the Examples described herein. The test rig comprised a polymer rig connected to a water rig. The water rig comprised a full-scale hydrocyclone (HC) liner and two down-scaled, downstream compact flotation unit (CFU) tanks. The running conditions of the test rig were as follows: fluid flow rate: 3 m.sup.3/h; temperature: 50° C.; and pressure: in the range of 20 to 30 barg.

(11) The polymer rig comprised two separate polymer tanks, with each tank having a capacity for 4.5 m.sup.3 of pre-mixed polymer solution. The polymer was pumped to a pressure of 60 barg and sheared by applying various pressure drops over the shear valves to simulate shear degraded breakthrough polymer.

(12) After mixing the polymer solution with hydrocarbons from an oil tank in the water rig, the mixture was added to seawater. A pressure drop of typically 5 to 15 bar was then applied over the oil choke to generate oil droplets having a size of 10-20 μm to simulate produced water.

(13) Multivalent metal cations, e.g. Fe.sup.3+ or Al.sup.3+ cations, were then introduced to the simulated produced water at various locations upstream of the hydrocyclone.

(14) The test rig was equipped with on-line oil-in-water analysis, such that any changes to the efficiency of the hydrocarbon removal method could be detected immediately. Droplet size and viscosity was determined after sampling from the test rig. Samples were taken from sampling points upstream of the hydrocyclone, downstream of the hydrocyclone and downstream of the CFUs.

Example 1

Effect of the Addition of Multivalent Metal Ions on the Viscosity of Produced Water

(15) A test rig as described above was set up. The polymers employed in the polymer tanks were HPAM-AMPS polymers, i.e. sulfonated hydrolysed polyacrylamides comprising monomers of acryl amide, acrylic acid and acrylamide tertiary butyl sulfonate. The concentration of polymer in the simulated produced water was set at either 100 ppm or 500 ppm.

(16) Different concentrations of Fe.sup.3+ cations were then introduced to the simulated produced water upstream of the hydrocyclone. Samples were taken from the test rig and the viscosity of the resultant mixture was tested at 20° C., and at a shear rate of 25 s.sup.−1, using a laboratory rheometer. The results are shown in FIG. 2.

(17) FIG. 2 shows a graph of viscosity versus Fe.sup.3+ cation concentration. The horizontal dotted line indicates the baseline viscosity for seawater, which does not contain any polymer. The viscosity of seawater is unaffected by an increasing concentration of Fe.sup.3+ cations. In contrast, when the concentration of Fe.sup.3+ is zero, the viscosity of produced water containing polymer is higher than the viscosity of seawater.

(18) The results show that as the concentration of Fe.sup.3+ cations increases, the viscosity of the produced water containing polymers approaches the viscosity of a solution that does not contain any amount of polymer (i.e. the baseline viscosity for seawater). This is thought to be due to the Fe.sup.3+ cations binding to the anionic sites of the polymer molecules and thereby minimising the ability of the polymer to increase the viscosity of the water phase.

(19) This corroborated by the fact that the observed effect is more pronounced when a greater concentration of polymer is present in the simulated produced water. The initial viscosity (i.e. when the Fe.sup.3+ concentration is zero) for simulated produced water containing 500 ppm polymer (plots of red circles or orange squares) is much higher than for simulated produced water containing 100 ppm polymer (plots of light blue triangles and dark blue diamonds). This means that a higher concentration of Fe3+ cations is required to ensure the viscosity of produced water containing a larger amount of polymer approaches the baseline viscosity.

(20) Reducing the viscosity of the simulated produced water improves the efficiency of the hydrocarbon removal methods in the hydrocyclone and CFUs.

Example 2

Analysis of the Efficiency of a Hydrocarbon Removal Method Using CFUs

(21) A test rig as described above was set up. In this example, however, the hydrocyclone was bypassed such that the simulated produced water entered the first CFU directly. The polymers employed in the polymer tanks were HPAM-AMPS polymers. The concentration of polymer in the simulated produced water was set at either 100 ppm or 500 ppm. Different concentrations of Fe.sup.3+ cations were then introduced to the simulated produced water upstream of the first CFU. The results are shown in FIG. 3.

(22) FIG. 3 shows a graph of hydrocarbon removal (de-oiling) efficiency versus Fe.sup.3+ cation concentration. The results show that as the concentration of Fe.sup.3+ cations increases, the efficiency of the hydrocarbon removal method in the CFUs approaches that of the baseline efficiency, i.e. the efficiency when a polymer is not present in the produced water. This effect occurs at both polymer concentrations, e.g. at 100 ppm and at 500 ppm. It is also observed that a higher concentration of Fe.sup.3+ cations is required to ensure the hydrocarbon removal efficiency for produced water containing a larger amount of polymer approaches the baseline efficiency.

(23) FIG. 3 also shows that at certain concentrations of Fe.sup.3+ cations, the hydrocarbon removal efficiency increases above the baseline efficiency. This may be due to interactions between the hydrocarbon droplets, Fe.sup.3+ cations and polymer molecules.

Example 3

Analysis of the Efficiency of a Hydrocarbon Removal Method Using a Hydrocyclone and a CFU in Series

(24) A test rig as described above was set up. The polymers employed in the polymer tanks were HPAM-AMPS polymers. The concentration of polymer in the simulated produced water was set at either 100 ppm or 500 ppm. Fe.sup.3+ cations were then introduced to the simulated produced water upstream of the hydrocyclone. The results are shown in FIG. 4.

(25) FIG. 4 shows a graph of hydrocarbon removal (de-oiling) efficiency versus Fe.sup.3+ cation concentration. Separate plots for the efficiency of the hydrocyclone and of the CFUs are shown, in addition to a total plot for the efficiency of the overall method.

(26) The results show that as the concentration of Fe.sup.3+ cations increases, the overall hydrocarbon removal efficiency approaches that of the baseline, i.e. the efficiency when a polymer is not present in the produced water, for all of the tested combinations, i.e. the hydrocyclone alone, the CFU alone, and the hydrocyclone and CFU in series.

(27) The results also show that the negative effects of the presence of a polymer in the simulated produced water on the efficiency of the hydrocarbon removal method increases with increasing polymer concentration. Thus, it is necessary to increase the concentration of Fe.sup.3+ cations when the concentration of polymer also increases.

Example 4

Recycling of Treated Produced Water

(28) The treated produced water produced in the methods of the present invention can be recycled and used in a further polymer flooding cycle. Investigations into the whether the presence of Fe.sup.3+ cations in the treated produced water would affect the viscosity in a new polymer flooding cycle when polymer is added were therefore conducted.

(29) A test rig as described above was set up. The polymers employed in the polymer tanks were HPAM-AMPS polymers. The concentration of polymer in the simulated produced water was set at either 100 ppm or 500 ppm. Fe.sup.3+ cations were then introduced to the simulated produced water upstream of the hydrocyclone.

(30) Samples of simulated treated produced water were taken from the CFU water outlet after having been treated with differing concentrations of Fe.sup.3+ cations (25, 50 and 100 ppm). An additional 2000 ppm of a HPAM-AMPS polymer was added to each of samples to simulate a second polymer flood. The viscosity of each sample was then measured as a function of shear rate and compared to the viscosity of sea water also containing 2000 ppm HPAM-AMPS polymer but without the presence of Fe.sup.3+ cations from the treatment following the first polymer flood operation. The results are shown in FIG. 5.

(31) FIG. 5 shows a graph of viscosity versus shear rate for each of the tested samples. The results show that the presence of low concentrations of Fe.sup.3+ cations did not have a significant effect on the viscosity of the polymer-containing water and thus would not be expected to have a negative effect on a new polymer flooding cycle. The red plot (labelled P100/Fe25) shows the result for simulated treated produced water containing 100 ppm polymer and treated with 25 ppm Fe.sup.3+ cations. After adding an additional 2000 ppm of polymer a negative effect on the viscosity was not observed due to the presence of Fe.sup.3+ cations (based on a comparison to the seawater baseline test).

(32) The green and purple plots (labelled P500/Fe25 and P500/Fe50, respectively) show the results for simulated treated produced water containing 500 ppm polymer and either 25 or 50 ppm Fe.sup.3+ cations. After adding an additional 2000 ppm of polymer, the viscosity of each of these samples was found to be only slightly higher than the baseline viscosity. It is thought that this effect could be overcome by increasing the amount of Fe.sup.3+ cations added.

(33) The light blue plot (labelled P500/Fe100) shows the results for treated produced water containing 500 ppm polymer and 100 ppm Fe.sup.3+ cations. After adding an additional 2000 ppm of polymer, the viscosity of this sample was found to be lower than the baseline viscosity. This is thought to be due to the concentration of Fe.sup.3+ cations being high enough to also influence the additional polymer added into the treated produced water.

(34) These results show that it is possible to use the treated produced water of the methods of the present invention in a further polymer flooding cycle.

Example 5

Evaluation of Precipitation of Metal-Rich Compounds

(35) Multivalent metal cations such as Fe.sup.3+ cations may precipitate with hydroxides and/or carbonates present in the produced water. It was therefore decided to evaluate the extent of precipitation as the concentration of multivalent metal cation increases.

(36) A test rig as described above was set up. The polymers employed in the polymer tanks were HPAM-AMPS polymers. The concentration of polymer in the simulated produced water was set at either 100 ppm or 500 ppm. Fe.sup.3+ cations were then introduced to the simulated produced water upstream of the hydrocyclone.

(37) Samples of treated produced water were collected from the water outlet of the hydrocyclone (dark blue and red plots) or from the water outlet of the second CFU (light blue and orange plots). The samples were then centrifuged and evaluated for the presence of any precipitates. The results are shown in FIG. 6.

(38) FIG. 6 shows a graph of the amount of precipitation versus concentration of Fe.sup.3+ cations. Baseline tests were performed using seawater, which did not contain any polymer (green plot is hydrocyclone baseline and purple plot is CFU baselines). In these tests, a large amount of precipitate formed which increased as the concentration of Fe.sup.3+ cations increased. The formation of precipitates is thought to be due to the presence of carbonates and/or hydroxides in the sea water. Tests conducted on simulated treated produced water containing a polymer resulted in the formation of considerably less precipitate.

(39) These results suggest that the Fe.sup.3+ cations bind the polymer molecules as stable cation-polymer complexes. This is because the Fe.sup.3+ cations have a higher affinity for the anionic polymer molecules than the hydroxides and/or carbonates also present and so do not form the precipitates observed during the baseline tests.

Example 6

Evaluation of Metal Accumulation

(40) It was decided to evaluate whether either iron or polymer were accumulating in a particular phase e.g. in the water or in the oil phase.

(41) A test rig as described above was set up. The polymers employed in the polymer tanks were HPAM-AMPS polymers. The concentration of polymer in the simulated produced water was set at either 100 ppm or 500 ppm. Fe.sup.3+ cations were then introduced to the simulated produced water upstream of the hydrocyclone.

(42) Samples of treated produced water were collected from the hydrocyclone water inlet (Fe.sub.HC-In), from the hydrocyclone water outlet (Fe.sub.HC-Out), from the hydrocyclone reject line (Fe.sub.HC-Reject), from the CFU reject line (Fe.sub.CFU-Reject) and from the CFU water outlet (Fe.sub.CFU-Out). The results are shown in the table below. The iron concentration in the water phase was assessed by Inductive Coupled Plasma Optical Emission Spectroscopy (ICP-EOS).

(43) TABLE-US-00001 Polymer Fe HCl Fe.sub.HC-in Fe.sub.HC-Out Fe.sub.HC-Reject Fe.sub.CFU-Out Fe.sub.CFU-Reject Mean St.dev. ppm ppm Y/N ppm ppm ppm ppm ppm ppm ppm — 25 Y 17 20 57 19 16 26 18 — 100 Y 74 81 87 48 67 71 15 100 25 N 19 17 14 18 12 16 3 Y 19 18 21 12 18 18 3 500 50 N 12 27 36 24 36 27 10 Y 34 38 36 37 35 36 2

(44) Two compositions from the baseline tests (i.e. seawater containing no polymer) and two compositions from the polymer tests were selected, and some samples were further treated with hydrochloric acid to dissolve any iron-containing precipitates. Even though some results showed relatively high standard deviation, and some data fell outside it (marked in red and orange to indicate high or low standard deviation, respectively), there was no clear trend of any iron enrichment. It can therefore be concluded that iron was not enriched in any of the water phases during the hydrocarbon removal method.

(45) The table below shows the analyses of iron concentration in the oil phase of the hydrocyclone reject line by ICP-EOS, followed by microwave digestion.

(46) TABLE-US-00002 Polymer Fe Oil Fe in oil ppm ppm ppm ppm — 25 300 1660 — 100 300 1660 100 25 300 210 600 25 300 30 600 50 300 900

(47) The results indicate an enrichment of iron occurs in the oil phase of the hydrocyclone reject line. The baseline samples (i.e. seawater not containing polymer) were found to contain a very high concentration of iron in the oil phase of the hydrocyclone reject line (e.g. 1560 ppm). Conversely, when polymer was present in sample, the iron enrichment in the oil phase was much lower. This is thought to be due to the high affinity between the polymer and Fe.sup.3+ cations, which renders the iron less available for interactions with the oil phase.

(48) The table below shows the analyses of polymer concentration in the water phase by Size-Exclusive Chromatography (SEC-UV).

(49) TABLE-US-00003 Polymer Fe HCl Fe.sub.HC-In Fe.sub.HC-Out Fe.sub.HC-Reject Fe.sub.CFU-Out Fe.sub.CFU-Reject Mean St. dev. ppm ppm Y/N ppm ppm ppm ppm ppm ppm ppm 100 25 N 167 121 131 91 135 129 27 500 25 N 601 147 593 229 571 428 221 500 50 N 152 161 149 207 136 161 27

(50) Although the presence of iron in the samples made the determination of polymer concentration difficult such that some measurements fell outside the standard deviation of the test results, the data in the table is in general agreement that the polymer was not enriched in any of the water phases tested.