Method of assessing chemicals in produced fluids

Abstract

A method of monitoring water-soluble treatment chemicals in a fluid that is immiscible with water and which may or may not contain some aqueous fluid, the method using at least one reagent that produces an optically detectable product, the detection step can take place without separation of the aqueous phase containing the treatment chemicals from the fluid immiscible with water.

Claims

1. A method of monitoring water-soluble treatment chemicals in a fluid that is immiscible with water and which may or may not contain some aqueous fluid, where said water-soluble treatment chemical is a hydrate inhibitor, the method comprising: a. obtaining a sample of said fluid; b. adding a first reagent to the fluid sample, wherein the first reagent is an oxidising agent being reactive with the treatment chemical to produce a first product, wherein the first product is an aldehyde; c. adding a second reagent to the fluid sample, wherein the second reagent is an aldehyde reactive reagent being reactive with the first product in step (b) to produce an optically detectable second product wherein the second reagent is Fluoral P; and d. detecting an optically detectable product; wherein the detection step takes place without separation of the aqueous phase containing the treatment chemicals from the fluid immiscible with water.

2. The method of claim 1, wherein the first and second reagent are added simultaneously.

3. The method of claim 1, wherein the first and second reagent are added sequentially.

4. The method according to claim 3, wherein the fluid immiscible in water is heated and/or mixed, rotated, vortexed, shaken, sonicated, inverted, filtered, or centrifuged after addition of the first reagent or a combination of these actions carried out simultaneously.

5. The method according to claim 1, wherein the second reagent is fluorogenic, chromogenic, luminescent, IR-active or Raman-active in the presence of the first product.

6. The method according to claim 1, wherein water is added to the fluid immiscible with water prior to step b.

7. The method according to claim 6, wherein the water added to the fluid immiscible in water in step (b) contains the first and second reagents.

8. The method according to claim 1, wherein water is added before or after reagents to make up the fluid immiscible in water to a specific volume.

9. The method according to claim 1, further comprising the step of pre-marking a container into which the fluid immiscible in water will be placed, in order to indicate desired volume.

10. The method according to claim 1, wherein the hydrate inhibitor comprises methanol, monoethylene glycol, or ethanol.

11. The method according to claim 1, wherein the first product is one of formaldehyde or acetaldehyde.

12. The method according to claim 1, wherein the first reagent is one of alcohol dehydrogenase, methanol dehydrogenase, alcohol oxidase, persulfate, permanganate, chromate, lead tetraacetate, periodate, periodic acid, boronic acid, glycerol dehydrogenase, monoethylene glycol dehydrogenase or ruthenium complexes such as [Ru(III)(bpy).sub.3].sup.3+.

13. The method according to claim 1, wherein the treatment chemical is methanol and the first reagent is alcohol oxidase.

14. The method according to claim 1, wherein the treatment chemical is monoethylene glycol and the first reagent is periodate.

15. The method according to claim 1, wherein the fluid immiscible with water and reagents are mixed, rotated, vortexed, shaken, sonicated, inverted, filtered, heated or centrifuged or a combination of these actions carried out simultaneously.

16. The method according to claim 1, wherein the fluid immiscible with water is heated above ambient temperature for a period of time to accelerate the reaction.

17. The method according to claim 1, wherein a treatment of the fluid sample is used to remove interfering chemicals from the fluid immiscible with water.

18. The method according to claim 17, wherein the treatment step comprises desalting the fluid immiscible with water.

19. The method according to claim 17, wherein the fluid immiscible with water is passed through a desalting column to remove salts.

20. The method according to claim 17, wherein ethylenediaminetetraacetic acid (EDTA) or other ion chelator is added to the fluid immiscible in water either before or at the same time as the first reagent and/or second reagent.

21. The method according to claim 17, wherein catalase is added to the fluid immiscible in water either before or at the same time as the first reagent and or second reagent.

22. The method according to claim 1, wherein the signal is detected using fluorescence, chemiluminescence, colourimetry, IR, Raman or UV-visible spectroscopy.

23. The method according to claim 1, wherein the signal is detected using fluorescence detector, luminescence detector, Raman detector, optical microscope, CCD camera, photographic film, fibre-optic device, photometric detector, MEMS device, single photon detector, spectrophotometer, chromatography system or by eye.

24. The method according to claim 1, wherein at least one positive and/or negative control samples are used.

25. The method according to claim 24, wherein the negative control contains inactivated first and/or second reagents.

26. The method according to claim 24, wherein the negative control contains only the second reagent.

27. The method according to claim 24, wherein the value of any optically detected background signal from the negative control is subtracted from the optically detected value obtained from a sample of fluid immiscible with water being analysed.

28. The method according to claim 24, wherein the positive control is aqueous or fluid immiscible with water known to contain treatment chemical.

29. The method according to claim 24, wherein calibration samples are used in order to create a calibration curve for the purposes of quantification.

30. The method according to claim 24, wherein the positive control or calibration samples are used to identify and compensate for variations due to different reagent activities.

31. The method according to claim 1, wherein a calibration factor is used to account for variations due to the differing extraction efficiencies from different fluids immiscible with water.

32. The method according to claim 1, wherein the range of detection is extended by altering the ratio of the fluid immiscible with water sample and aqueous phase.

33. The method according to claim 32, wherein the ratio of the fluid immiscible with water sample and aqueous reagents is altered by changing volume of aqueous phase and/or fluid immiscible with water.

34. The method according to claim 33, wherein the ratio of fluid immiscible with water and detection reagents is altered by diluting the fluid immiscible with water or a second fluid immiscible with water which does not contain the treatment chemical prior to mixing with aqueous phase.

35. The method according to claim 34, wherein the second fluid immiscible with water is heptane, hexane, petroleum ether, octane or kerosene.

36. The method according to claim 1, wherein the range of detection is altered by changing the conditions to alter speed of reaction, yield or detectability.

37. The method according to claim 36, wherein the temperature or timings, the pH of reagents, or the detection method is altered.

38. The method according to claim 1, wherein the fluid immiscible in water is added to the first and/or second reagent.

39. The method of claim 1, wherein the fluid sample is a single or multiphase fluid.

40. The method of claim 1, wherein the addition is carried out under pressure.

41. The method of claim 1, wherein the addition is carried out by bubbling the fluid immiscible with water through water or an aqueous solution of one or more reagents.

42. The method according to claim 1, wherein the method or steps within the method are automated.

43. A kit for monitoring of water-soluble treatment chemicals in a fluid that is immiscible with water according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described with reference to the following figures, in which:

(2) FIG. 1 is a diagram illustrating the separation of the aqueous and water-immiscible phases in a sample holder, with a beam of light that can be used for detection passing through the aqueous phase;

(3) FIG. 2 is a graph showing the signal generated from a range of concentrations of MEG in oil, data is the average of triplicate samples and error bars (where visible) represent standard deviation;

(4) FIG. 3 shows the reproducibility between samples of MEG in oil run at different times, results are the average of three assay runs and error bars represent standard deviation;

(5) FIG. 4 is a graph showing the reproducibility between four replicate samples each spiked separately, this was carried out at two different MEG concentrations (20 and 10 ppm). A comparison of the results with the expected absorbance based on a linear fit of aqueous controls is also shown;

(6) FIG. 5 is a graph showing the signal generated from a range of concentration of MEG in condensate, absorbance measured at maximum;

(7) FIG. 6 is a graph showing the signal generated from a range of concentration of MEG in condensate, absorbance measured at suboptimal maximum;

(8) FIG. 7 is a graph showing signal generated from corrosion inhibitor in oil

(9) FIG. 8 is a graph showing signal generated from ethanol

(10) FIG. 9 is a graph showing signal generated from testing a number of reagents when used in the presence of methanol-containing oil

(11) FIG. 10 is a graph showing that the signal generated can be proportional to the methanol concentration in produced fluids

(12) FIG. 11 is a graph showing benefits, in this case reduced variability, of using an automated mixing step

(13) FIG. 12 is a graph showing how the range of the method may be extended by using reduced volumes of sample

(14) FIG. 13 is a graph showing how the range of the method may be extended at the higher end by using suboptimal conditions

DESCRIPTION OF PREFERRED EMBODIMENTS

Experiment 1: MEG in Crude Oil

(15) An experiment was conducted in order to investigate the suitability of the MEG assay to the oil and gas industries. Firstly, to determine whether the reagents would be able to function in the presence of crude oil, which contains numerous compounds that may interfere. Secondly, to assess whether it was possible to detect a signal from the sample without first separating the oil and aqueous phase.

(16) Various concentrations of MEG were spiked into black crude oil. A stock solution of 0.1% (1,000 ppm) MEG was prepared and then serial diluted to create a range of MEG concentrations.

(17) The assay was conducted by adding sodium periodate in buffer (50 mM) to oil samples. The samples were mixed by rotating for 15 min, then an equivalent of Fluoral-P was added. After heating for 30 min the samples were centrifuged and then the absorbance recorded. Four concentrations of MEG (20, 10, 5 and 0 ppm) were run in quadruplicate.

(18) The results clearly show an increasing signal with increasing MEG concentration (FIG. 2). The data was fitted linearly with R.sup.2=0.9989. The errors represent standard deviation between quadruplicate samples and are very small indicating the assay repeatability is extremely good.

(19) In the field, a sample may be taken from an oil or gas producing, refining, distilling or processing plant; from oil fields; from fuels; from produced or overboard fluids or from hydrocarbon streams. The samples may be taken in-line, at-line, on-line or offline. The reagents may be added to the sample by hand, or by automatic injector. The latter would offer the possibility for automating the system, which would make it especially reliable and high-throughput.

Experiment 2: Reproducibility and Accuracy of MEG Assay

(20) To check the reproducibility of the assay, oil samples containing 20, 10, 5 and 0 ppm MEG were analysed on different days. The data from the three runs was averaged and is displayed with standard deviation error bars in FIG. 3. The data was fitted linearly with R.sup.2=0.9944.

(21) The agreement between samples spiked separately was also investigated to ensure the spiking method was reproducible. The signal generated by four samples, at both MEG concentrations (20 and 10 ppm), were very similar indicating the spiking method is reproducible and giving further evidence that the assay is repeatable (FIG. 4).

(22) The accuracy of the method was determined by comparing the signal generated from oil samples containing MEG to samples which were run in the absence of oil i.e. aqueous only samples. The data from the aqueous samples was fitted linearly and the results compared with those for the oil samples (FIG. 4). The comparison between the aqueous controls and the oil samples was very good with very similar absorbances produced for both sample types.

(23) This experiment demonstrates that in an organic fluid environment that might be expected to contain impurities which would interfere with the chemical reactions involved in detecting MEG at such low concentrations the assay still functions well.

(24) In the field, a sample may be taken from an oil or gas producing, refining, distilling or processing plant; from oil fields; from fuels; from produced or overboard fluids or from hydrocarbon streams. The samples may be taken in-line, at-line, on-line or offline. The reagents may be added to the sample by hand, or by automatic injector. The latter would offer the possibility for automating the system, which would make it especially reliable and high-throughput.

Experiment 3: MEG in Condensate

(25) A further experiment was conducted in order to investigate the suitability of the assay to the oil and gas industries and in particular whether the assay was compatible in a range of oil and gas fluids. Unlike experiments such as Experiment 1 which used crude oil, condensate was tested here to determine if the method worked with this different fluid.

(26) Standard solutions were prepared to give a final concentration of MEG of 0, 0.78, 1.56, 3.1, 6.25, 12.5, 25, 50 and 100 ppm when 1 l of the solution was added to 2 ml water. A solution of sodium metaperiodate in sodium acetate was also freshly prepared. 1 l of the MEG solutions were added to 2 ml condensate, before adding 500 l water, shaking, 500 l of the periodate solution, and 1 mL of fluoral-P and heating. Following the incubation step the solution was placed in a cuvette and absorbance read. The results are shown in FIG. 5 and a clear increase in signal with increasing MEG concentration can be observed. As a number of the measurements are above 1, the samples were also analysed at suboptimal wavelength to determine if alternative wavelengths could be used for more concentrated samples. FIG. 6 shows this is possible. The absorbances measured during this experiment are all higher than those in experiments 1 and 2 since a modified method was used.

(27) In the field, a sample may be taken from an oil or gas producing, refining, distilling or processing plant; from oil fields; from produced or overboard fluids; from fuels; from MEG reclamation or regeneration plants or from hydrocarbon streams. The samples may be taken in-line, at-line, on-line or offline. The reagents may be added to the sample by hand, or by automatic injector. The latter would offer the possibility for automating the system, which would make it especially reliable, simple and high-throughput.

Experiment 4: Testing Corrosion Inhibitor in Oil

(28) To determine the presence of corrosion inhibitor in oil, a sample of oil to which a water dispersable corrosion inhibitor had, or had not, been added was tested. 5 uL of corrosion inhibitor was added to 5 mL of light condensate before being mixed vigorously by shaking. No corrosion inhibitor was added to a second 5 mL of condensate. 1 mL was transferred to a cuvette (in triplicate) before 2 mL of freshly made 60 nM Nile Red in water was added. The cuvette was inverted 10, then left to stand for 30 seconds, to allow the phases to settle before being read on a handheld fluorometer with rhodamine filter sets. FIG. 7 shows an increased fluorescence signature from corrosion inhibitor-containing sample compared to the control.

Experiment 5: Measuring Ethanol Concentration

(29) The following method was used to measure ethanol concentration using alcohol dehydrogenase to generate a detectable product. 1.3 mL phosphate buffer (50 mM, pH 8,8), 0.1 mL aqueous ethanol solution, 1.5 mL NAD.sup.+ (15 mM) and 0.1 mL alcohol dehydrogenase (ADH) containing 0.1% bovine serum albumin were added to a cuvette. A control was measured with the ADH omitted. The ethanol concentrations analysed were 95% and 3% and the ADH concentrations used were 0.75 U/mL and 3.4 U/mL. The absorbance at 340 nm was recorded. FIG. 8 shows the signal produced from different ethanol and enzyme concentrations.

Experimental 6: Testing Oil Samples with Detection Reagents

(30) Treatment chemicals in oil samples are traditionally tested first by extracting them from oil, using water and then removing the aqueous layer with a separation step then adding detection reagents. This is because the oil can inactivate the detection reagents, opaque oil may scatter optical signals so introducing inaccuracies and reducing sensitivity, products of reactions can be solubilised back into the oil making them undetectable in the water layer and because where multiple reagents are used they may work under incompatible conditions such as temperature and pH so reducing sensitivity. Reactions with oil which contained methanol were set-up using four different sets of reagents, these were alcohol oxidase and amplex red, alcohol oxidase and MBTH (in presence of FeCl.sub.3), alcohol dehydrogenase and NAD.sup.+, alcohol dehydrogenase and Fluoral-P. All reactions were carried out in a single vial with the oil present. Two of the reactions gave signals, one gave a very low signal and one did not work. FIG. 9 shows the results for the successful reactions which used amplex red and ADH and Fluoral-P. The reaction with MBTH was unsuccessful and only a very low fluorescent signal was seen for ADH and NAD.sup.+. It should be noted that the same reaction without oil present did work with MBTH indicating it is not clear or obvious that reactions which combine reagents and produced fluids in the same vial will work.

Experiment 7: Methanol in Crude Oil

(31) For some applications it is important that methanol concentrations in produced fluids can be quantified. The following experiment demonstrates that the size of the signal generated can be proportional to the concentration of methanol present. Crude oil was spiked with methanol to final methanol concentrations of 500, 375, 250, 125, 62.5, 31.25, 15.6, 7.8 and 0 ppm. A portion was added to the aqueous reagents (alcohol oxidase, fluoral P with buffer) and heated and rotated end over end for 35 min, then the absorbance recorded. FIG. 10 shows the linear relationship between absorbance and methanol concentration.

Experiment 8: Methanol in Crude Oil and the Influence of Automation

(32) To determine whether automation may reduce inter-user variability methanol-containing oil was mixed by hand (shaking) and with an automated end over end mixer. Methanol in dichloromethane was added to crude oil (final concentration 20 ppm). Water was added to extract the methanol from oil and the extraction was either achieved by shaking the sample manually (vigorously, or gently), or by automated mixing with heating in an adapted end over end mixer. The water phase was separated and detection reagents alcohol oxidase and fluoral P were added to the water extract and this mixture heated before absorbance was read.

(33) Absorbance readings were converted to methanol concentrations. FIG. 11 shows that differences were observed in manual methods (vigorous vs gentle) but that automation generated more consistent results.

Experiment 9: Extending the Range of the Methanol Assay

(34) The wider the dynamic range of any test the more applicable it is. Absorbance readings are only accurate up to 3 a.u. and given that the method disclosed here does not use a separation step dilution after extraction is not possible and any dilution is limited by the volume of the vial used, which is limited in turn by the dimensions of the detection equipment. Altering the volume of oil sample:detection reagent ratio allowed a range of concentrations to be quantified. FIG. 12 shown that the nominal methanol concentration does not vary greatly when using between 0.25 and 3 mL of oil sample, lines shown are linear trendlines. Altering the experimental conditions or detection wavelength to something suboptimal can extend the range to higher treatment chemical concentrations. FIG. 13 shows how signal from 100 ppm methanol in oil can be near the limit of detection (near 3 a.u.) under one set of conditions and markedly reduced under another.