RU2473885C2 - Quantitative analysis of thiourea and sodium fluorescein simultaneously present in formation water - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: quantitative analysis wherein an analysed sample is separated from oil; cleaned from mechanical impurities; clarified in a centrifuge; alkali is added for quantitative determination of sodium fluorescein using a luminescent technique; and concentration of thiourea and sodium fluorescein is determined by an interpolation technique from results of three combined measurements, one of which is associated with the sample and the other two are associated with model solutions prepared from the analysed sample by fixed dilution with formation water and fixed addition of the analysed indicator in such an amount that the signal of one of the model solutions is greater, and for the other - less than the signal of the analysed sample. To analyse thiourea, a fixed amount of sodium pentacyano-aqua-ferrite and sodium fluorescein in an amount equal to the amount in the sample which is measured by a luminescent technique are added to the analysed sample.

EFFECT: high accuracy of measuring concentration of thiourea and sodium fluorescein.

1 ex, 2 tbl

Description

The invention relates to spectrophotometric methods of analysis and can be used in the oil and gas industries for quantitative determination in reservoir waters specially injected into productive formations of various water-soluble, poorly sorbed rocks and environmentally friendly organic and inorganic compounds called indicators or tracers.

It is known (see: Sokolovsky EV, Soloviev GB, Trenchikov Yu.T. Indicator methods for studying oil and gas bearing strata. M: Nedra, 1986. - 158 pp.) That fluorescein is used as indicators in oil fields sodium, ammonium thiocyanate, sodium chloride, nitrates, urea, thiourea and other substances. The most difficult stage of indicator research is the quantitative determination of a labeling substance (indicator) in formation fluids. This is due to the multicomponent composition of the reservoir fluids and their high contamination. Therefore, a sample of produced water is separated from the oil before measurement, filtered and appropriate reagents are added for analysis.

There is a known method for determining the concentrations of a four-component composition of indicators (sodium fluorescein, ammonium nitrate, urea, thiocarbamide) when they are combined in formation waters using spectrophotometry and special calibration dependencies for each individual indicator (see: Chernorubashkin A.I., Makeev G.A. ., Gavrilenko G.A., Shamkin V.N. // Oilfield business. VNIIOENG, 1980. No. 5. P.15-15).

A known method of spectrophotometric determination of the concentrations of three different indicators in formation waters (sodium fluorescein, potassium thiocyanate and urea) by interpolation without constructing calibration dependencies using two or more solutions with a fixed content of the studied components as external standards (see: Onuchak L.A. , Arutyunov Yu.I., Kudryashov S.Yu., Sizonenko G.M., Deineka O.V. RF Patent No. 2003134880 dated 01.12.2003).

Labeled liquid is also known for monitoring the development of an oil and gas field containing thiourea as a labeling substance. Thiourea in formation water is quantitatively determined by the reaction with potassium iron-hydrogen sulfide in the presence of acetic acid (see: Soloviev GB, Sokolovsky EV, Saakov SA USSR author's certificate No. 646036 of 02/05/1979 at the request of Severo - Caucasus State Scientific Research and Design Institute of the Oil Industry No. 2554605 of 12/15/1977).

However, the known methods for determining the quantitative content of both individual indicators and individual indicators when they are present together in formation waters (with the exception of sodium fluorescein by luminescence) have relatively low sensitivity and accuracy of concentration measurements due to unaccounted for calibration errors introduced by the process studies by the composition of formation waters and the inconsistent background level of previously undetected indicators for measurement results.

Closest to the proposed invention by the combination of essential features is a method for determining the quantitative content of indicators in formation waters, in which a sample containing sodium fluorescein in the presence of a multicomponent composition of indicators (sodium nitrate, potassium thiocyanate, urea) is purified from mechanical impurities, clarified by centrifugation, in the resulting solution is added alkali for the quantitative determination of sodium fluorescein by the luminescent method according to previously performed grades full-time dependence, and the concentration of individual indicators in the sample is determined by interpolation according to the results of three aggregate spectrophotometric measurements at wavelengths fixed for each individual indicator, one of the measurements is carried out for a purified test sample with the addition of appropriate reagents, and two other measurements are carried out for model solutions prepared from the source formation water (without indicators) with the addition of sodium fluorescein in an amount equal to that measured in the sample according to the calibration curve the property and weighed portions of the indicator under study in such a quantity that the signal of the spectrophotometer for one of the model solutions is larger and less for the other than the signal of the sample under study (see: Onuchak L.A., Arutyunov Yu.I., Kudryashov S. Yu., Sizonenko G.M., Astrov V.I. RF patent No. 2301409 dated 06/20/2007 at the request of SamSU No. 2005124417 dated 01/08/2005).

In the known method, the quantitative content of sodium fluorescein is determined by a previously performed calibration dependence using model mineralized solutions of distilled water with fixed additives of sodium fluorescein at pH 9.

The disadvantages of this method are the inability to spectrophotometrically determine the content of thiourea with the simultaneous presence of sodium fluorescein in the sample due to the superposition of its absorption spectrum on the spectrum of thiourea, as well as the decrease in the accuracy of measuring the concentration of sodium fluorescein from a previously performed calibration on model solutions of distilled water due to the influence of composition changes and formation water properties in the study of oil and gas fields.

The objective of the invention is to improve the accuracy of spectrophotometric determination of the concentration of thiourea and sodium fluorescein with their joint presence in formation waters.

This problem is solved due to the fact that during the quantitative analysis of thiourea and sodium fluorescein, when they are present together in the formation water, the test sample is separated from oil, purified from mechanical impurities, clarified in a centrifuge, alkali is added to the resulting solution to quantify sodium fluorescein by the luminescent method, moreover, the concentrations of thiourea and sodium fluorescein are determined by the interpolation method according to the results of three cumulative measurements, one of which relates to the test sample, and the other two are for model solutions prepared from the test sample, with a fixed dilution of the source formation water (without indicator) and a fixed addition of the test indicator in such an amount that the signal of one of the model solutions is greater, and for the other, less than the signal of the test sample, for thiourea analysis add a fixed amount of sodium pentacyanoquaferriate and sodium fluorescein to the test sample in an amount equal to that measured in the test sample by the luminescent method.

When solving this problem, a technical result is created, which consists in the following:

- measurement of the concentration of thiourea and sodium fluorescein with their joint presence in the sample is carried out by the interpolation method according to the results of three cumulative measurements, one of which relates to the analysis of the test sample, and the other two to model solutions prepared from the test sample by fixed dilution and a fixed additive of the test indicator in such a quantity that the signal for one of the solutions was greater, and for the other, less than the signal of the test sample;

- adding sodium fluorescein to the test sample in an amount equal to that measured by the luminescent method in the sample when analyzing thiourea reduces the spectrophotometric measurement error associated with the superposition of the absorption spectra of sodium fluorescein on the spectrum of the complex compound of thiourea and sodium pentacyano-aquaferriate sodium at a wavelength of 590 nm;

- the interpolation measurement method allows to reduce the measurement error compared to the absolute calibration method by linearizing only the plot of the calibration dependence, at the measurement point between two fixed external additives, one of which provides a larger and lower concentration of the indicator relative to its concentration in the test sample , and not in the entire working range, as in the case of constructing a calibration dependence on the minimum to maximum possible indicator concentration.

An example of a specific implementation of the method

The proposed method for the quantitative analysis of thiourea and sodium fluorescein with their combined presence in formation waters is performed using serial fluorescence and spectrophotometric analyzers, for example, Fluorate 02-3M (Lumex, St. Petersburg) and KFK-2.

The method is as follows. A sample of produced water from an oil well containing sodium fluorescein in the presence of thiourea is preliminarily separated from oil in a separatory funnel, mechanical impurities are removed by filtration through a FOFS-17 blue ribbon paper filter. Then the sample is clarified by precipitation of colloidal impurities using a coagulant FeCl ₃ in an alkaline medium. The resulting solution is poured together with the precipitate into centrifuge tubes and centrifuged at 8-10 thousand rpm until the sample becomes transparent (without visible opalescence). Prepared by the method described above, formation water samples are subjected to the following additional steps to determine the concentration of sodium fluorescein and thiourea:

1) determination of sodium fluorescein. A few drops of 2N are added to the sample. NaOH to obtain a solution pH of 9. At the same time, the fluorescence intensity increases significantly, the measurement of which using the Fluorat 02-3M instrument at a wavelength of 525 nm ensures the determination of sodium fluorescein concentration by interpolation. For this, additional measurements of sodium fluorescein concentration are carried out in two model solutions prepared from the test sample by fixed dilution of the source formation water (without indicators) and a fixed addition of sodium fluorescein in such an amount that the indicator concentration for one of them is higher and less for the other than measured by the device in the test sample;

2) determination of thiourea. To 25 cm ^{3 of the} test sample, add 0.5 cm ^{3 of a} solution of sodium pentacyanoacquaferriate, which is prepared as follows. In 20 cm ^{3 of} distilled water, 1 g of sodium nitroprusside and 1 g of hydroxylamine hydrochloride are successively dissolved. Then add 2 g of sodium bicarbonate. After the evolution of CO _{2 has} ceased, 0.1 cm ^{3 of} pure liquid bromine is added, stirred, filtered and diluted with distilled water in a volumetric flask to 50 cm ³ . The reagent retains its properties for about two weeks.

The measurement of the concentration of thiourea is carried out after 40-50 minutes of exposure to complete the reaction of the formation of a complex of thiourea and sodium pentacyano-aquaferriate.

The concentration of thiourea is determined on a KFK-2 photocolorimeter with a 3 cm thick cuvette and a wavelength of 590 nm by the interpolation method according to the results of three cumulative measurements, one of which is the test sample and the other two are model solutions prepared from the test sample with a fixed dilution of the source formation water and a fixed addition of thiourea in such an amount that the signal of the device for one of them is greater, and for the other, less than the signal in the sample. To increase the accuracy of the analysis, in connection with the superposition of the absorption spectra of sodium fluorescein on the spectrum of the complex compound of thiourea at a wavelength of 590 nm when determining the concentration of thiourea, sodium fluorescein in an amount equal to that measured according to claim 1 is added to the test sample. interpolation method on the device Fluorat 02-3M.

An experimental evaluation of the implementation of the proposed and known methods for the quantitative analysis of thiourea and sodium fluorescein with their joint presence in formation waters was carried out using the example of analysis of three mixtures of these indicators. The first and second mixtures contained the minimum and maximum concentrations of the analyzed indicators, respectively, measured with the Fluorat 02-3M and KFK-2 devices. The third mixture contained the average values of the concentrations of indicators in the measuring range of the devices. To determine the content of individual indicators in the analyzed sample in a known manner, calibration dependencies of the form were used

where D _i is the signal of the spectrophotometer; C _i - indicator concentration; and _i and b _i are the coefficients of the calibration dependence.

Calibration solutions were prepared using mineralized formation water with the addition of appropriate reagents for each individual indicator. When constructing the calibration dependence for the determination of thiourea, sodium fluorescein with an average concentration within the measurement range was additionally added to the calibration solutions in order to reduce its effect on the result of measuring the thiourea concentration due to the superposition of absorption spectra when measuring the indicators at the selected wavelength.

The calibration results of the devices are presented in table 1.

Table 1 The results of the calibration of devices for determining thiourea and fluorescein sodium No. ⁿ / _n Name of indicators and measuring device Linear range of measurement of concentration, mg / l Calibration Coefficients and Confidence Measurement Intervals b ± Δb a ± Δa one Fluorescein Sodium Fluorate 02-3M 0.02-0.2 0.9 ± 0.045 0.11 ± 0.055 2 Thiourea KFK-2 2.0-20.0 0.06 ± 0.003 0.21 ± 0.02

The concentration of indicators was measured by the proposed method by the interpolation method according to the results of three cumulative measurements according to the equation

where C _i is the concentration of the i-th indicator in the test sample; C ₁ and C ₂ are indicator concentrations in two model solutions; D _i , D ₁ and D ₂ are the spectrophotometer signals for the i-th indicator in the sample and in model solutions 1 and 2, respectively, with D ₁ > D _i > D ₂ .

The experimental results are summarized in table 2.

table 2 Comparative data of experimental verification of the known and proposed methods No. ⁿ / _n Test mixtures The concentration of the indicator, mg / l Known method The proposed method The signal of the device, D _i Concentration measured by (1), mg / l δ ^* ,% Instrument signal Concentration measured according to (2), mg / l δ ^* ,% D ₁ D ₂ D _i one Fluorescein sodium 0.04 0.144 0,038 5,0 0.137 0.155 0.145 0,039 2,5 Thiourea 2.0 0.315 1.75 12.5 0.401 0.224 0.326 1.93 3,5 2 Fluorescein sodium 0.2 0,304 0.216 8.0 0,305 0.256 0.297 0.208 3.8 Thiourea 20,0 1,576 22.76 13.8 1,582 1.23 1.46 20.86 4.3 3 Fluorescein sodium 0.12 0.211 0,112 6.6 0.221 0.198 0.214 0.116 3.3 Thiourea 11.0 0.94 11.7 6.4 0.916 0.814 0.891 11.35 3.2

* Relative error in determining the concentration of the i-th indicator according to equations (1) and (2).

As can be seen from the data in table 2, the proposed method provides a significant increase in the accuracy of determining the quantitative content of thiourea and sodium fluorescein with their joint presence in the studied samples in comparison with the known method. So, when analyzing all three investigated mixtures, the maximum relative error in determining the concentration of thiourea does not exceed 4.3%, while for the known method, the maximum error in determining the concentration of thiourea is 13.8%.

The determination of the concentration of sodium fluorescein and thiourea by the interpolation method in the proposed method instead of constructing the calibration dependence ensured a decrease in the relative measurement error by an average of more than two times, which, apparently, is associated with a partial exclusion of the nonlinearity of the signal from the concentration on the measurement results.

The use of the proposed method for the quantitative analysis of thiourea and sodium fluorescein in formation waters improves the accuracy of determining multicomponent indicator compositions in indicator studies in oil fields due to signal non-linearity at the measurement point, and also reduces labor costs during measurement by eliminating the calibration dependence.

Claims (1)

Quantitative analysis of thiourea and sodium fluorescein with their joint presence in formation waters, in which the test sample is separated from oil, purified from mechanical impurities, clarified in a centrifuge and added to the obtained alkali solution for quantitative determination of sodium fluorescein by the luminescent method, characterized in that the concentration of thiourea and sodium fluorescein are determined by interpolation according to the results of three cumulative measurements, one of which relates to the test sample, and the other two to model solutions prepared from the test sample by fixed dilution of the source formation water (without indicator) and a fixed addition of the test indicator in such an amount that the signal of one of the model solutions is larger and the other is smaller than the signal of the test sample, and for the analysis of thiourea in the test the sample is added a fixed amount of sodium pentacyanoquaferriate and sodium fluorescein in an amount equal to that measured in the test sample by the luminescent method.