STRONTIUM ISOTOPES AS NATURAL TRACERS IN RESERVOIR OILFIELD AND IN GROUNDWATER SYSTEMS

Transcription

1 009 International Nuclear Atlantic Conference - INAC 009 Rio de Janeiro, RJ, Brazil, September7 to October, 009 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: STRONTIUM ISOTOPES AS NATURAL TRACERS IN RESERVOIR OILFIELD AND IN GROUNDWATER SYSTEMS Marcos E. Santos, Helena E. L. Palmieri and Rubens M. Moreira Centro de Desenvolvimento da Tecnologia Nuclear, CDTN/CNEN Avenida Presidente Antônio Carlos, Belo Horizonte, MG mes@cdtn.br; help@cdtn.br; rubens@cdtn.br ABSTRACT The radioactive beta (ß - ) decay of 87 Rb to 87 is an important isotope system that has been widely applied for geochronological purposes and in identifying ground water sources, aquifer interactions and as a tracer for a secondary recovery process in offshore oilfields via seawater injection. The 87 / 86 ratio of present seawater is constant worldwide, while formation waters in hydrocarbon reservoirs have various values are in most cases higher than modern seawater. This can be the basis for a natural tracer technique aiming at evaluating the performance of seawater injection processes by evaluating the 87 / 86 ratio and the total content of formation waters in the reservoir prior to injection, followed by monitoring these values in the produced water as injection proceeds. Inductively Couple Plasma Mass Spectrometry ICP-MS is a technique that has potential to be used in studies with tracers in the environment in the determination of isotope ratios and element traces in a sample. This work describes the methodology that will be used for the determination of variations in the isotopic composition of and presents the preliminary results obtained determination of the strontium isotope ratios ( 87 / 86 ) using Inductively Coupled Plasma Mass Spectrometry (ICP-MS).. INTRODUCTION The element strontium (atomic number 8) has four stable isotopes with mass numbers 84, 86, 87, and 88. For most elements, the isotopic composition is constant in nature. However, this is not the case with strontium. A steady increase in the 87 / 86 ratio over long time spans is observed, due to the radioactive decay of 87 Rb (half-life of 48,8 x 0 9 years) to 87. The radioactive beta (ß - ) decay of 87 Rb to 87 is an important isotope system that has been extensively applied for geochronological purposes. More important it is extensively used to determine rates and fluxes involved in a wide range of geological processes operating from within the hydrosphere of the soil/rock system to the deep mantle. In addition, recently strontium isotopes have increasingly been applied as tracers for defining provenances in many different scientific disciplines (biology, nutrition, medicine, forensic and art history) []. Strontium isotopic composition is also an important tool in hydrogeology, enabling tracing of ground-water flow and water mixing []. In oilfields the natural production mechanism, or primary production, contributes to the extraction of about 5% of the original oil from the reservoirs. This means that over 75% of the originally existing oil remains in the pores and fissures of the oil bearing rocks. The production flow rate depends on the differential pressure between the permeable layer and the bottom of the well, the average permeability, the layer thickness and the oil viscosity. The

2 main natural production mechanism are the expansion of oil, water and gas and in certain cases the water influx from aquifers connected with the reservoir oilfields []. When primary oil production decreases inside a reservoir due to the reduction of the original pressure, water is usually injected to increase the oil production anew. Water injected into special wells (injection wells) forces the oil remaining in certain layers to emerge at other wells (production wells) close to the injector. This technique, commonly called secondary recovery, contributes to the extraction of up to 50% of the original oil in reservoirs. The injected fluid may be spiked with a tracer for the purpose of assessing the efficiency of the secondary oil recovery process. Samples taken at production wells are then analyzed for the tracer, the results yielding important information; the breakthrough of the injection fluid at the production well sampled can thus be detected. Presently used tracers usually fall into one of two categories, chemical (including fluorescent dyes) or radioactive. However, such tracers may be subject to several limitations [4], especially when working in crowded offshore platforms. Besides this, in many applications multiple tracers may be required. This is where the search for novel tracers comes in. The 87 / 86 ratio of oilfield formation waters is significantly higher than that of seawater, in so far as the former have stay in contact with the reservoir rocks for extended periods of time. Strontium is relatively abundant in seawater (~8 ppm). Numerous measurements of 87 / 86 in ocean water have demonstrated that this ratio is constant throughout oceans worldwide at ~ (relative to a value of for the NBS 987 standard). In contrast to seawater, oilfield waters show rather variable isotope contents (< >0.70) [4]. The aim of this work is to develop a methodology for the determination of the 87 / 86 ratio using inductively coupled plasma mass spectrometry (ICP-MS). This ratio will be used as the natural tracer for seawater injection, as well as a tracer of groundwater flow and mixing.. EXPERIMENTAL.. Instrumentation Traditionally, thermal ionization magnetic sector mass spectrometry (TIMS) is the method of choice for measuring the isotope ratio, because most commonly used applications, such as in geochronology, require a precision of better than 0.0% relative standard deviation (RSD) of the 87 / 86 ratio. In comparison with TIMS, the precision that can be obtained with quadrupole-based ICP-mass spectrometry (ICP-QMS) is relatively poor ( 0.% RSD). However, for applications for which the ultimate level of precision is not required, ICP-QMS is an attractive alternative to TIMS, owing to its ease of operation, widespread availability of instruments, and much higher sample throughput [5]. As in ICP-MS, sample preparation for TIMS is laborious and requires chemical isolation of the strontium since the 87 signal is corrupted by the isobaric superposition due to the presence of the parent 87 Rb in the sample.

3 isotope ratios have always been difficult to analyze at low concentrations (less than 0 ppm), because there are large differences in the abundances of the isotopes 84, 86, 87 and 88. In many materials the low abundances of rubidium and their relatively low ages result in small isotope variations. Whenever analyzing solid materials, multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) coupled with a laser ablation system should be able to perform a isotope analysis within minutes, while maintaining the spatial resolution and avoiding the extensive wet chemistry and warm up times characteristic of the TIMS instrument []. In this work, two alternatives are proposed for the determination of the 87 / 86 ratio in water samples. The first one, in course of implementation, will be based on an ion-molecule reaction in a dynamic reaction cell (DRC) coupled to an ICP-MS instrument, as proposed by Moens et al [6]. In the DRC + ions are reacted with CH F and converted to F + ions that can be measured free from the isobaric interference by the Rb + ions, which do not react with that gas. No chemical separation is needed; consequently the easiness and the throughput are considerably increased. The second alternative is based on the measurement of the 87 / 86 ratio using an ICP-MS DRC-e in the standard mode, after separation of Rb from by cation exchange chromatography... Interferences Results from mass spectrometric measurements of strontium isotopes have to be corrected from effects resulting from isobaric and molecular interferences. The presence of krypton (containing 86 Kr) in an argon ICP source may lead to erroneous measurements. This can be avoided using pure argon gas. Interferences in the strontium mass region in samples of natural water and carbonate matrices are due to the superposition of the 48 Ca 40 Ar + signal on 88. Due to the isobaric overlap of the 87 + and 87 Rb + ion signals, strontium and rubidium must be somehow separated from one another prior to the isotope ratio measurements. Other than the use of the DRC, rubidium interference has been eliminated either through mathematical corrections [] or sample pre-treatment. Although mathematical corrections are an easy way out, they become a significant source of uncertainty and may even fail to work properly when the []/[Rb] ratio is below 000 [7]. Alternatively, the use of a chromatographic separation is particularly suitable for the elimination of not only rubidium but also other elements present at very high levels such as calcium (as high as 500 mg/l). Selective extraction of strontium can be accomplished mainly with commercial resins using crown compounds ((4, 4, (5 ) di-t-butyl-ciclohexane-8-crown-6) or sulfonic acid as functional groups... Mass discrimination correction

4 It is necessary to use this correction when sample pre-treatment via cation-exchange is done, because traces of Rb remain in the solution. The isotopic mass discrimination of the mass spectrometer will be corrected by using an 86 / 88 internal standard assumed to be constant in nature. The use of internal normalization for mass discrimination correction is a common practice in TIMS and MC-ICP-MS. It consists in employing the exponential law: F= log(r true /R obs )/log(m 86 /m 88 ) () where R obs is the measured ratio, m 86 and m 88 are the exact masses of 86 and 88, respectively, F is the correction factor per atomic mass unit and R true is the recommended value. As a convention used by the International Union of Geosciences the value R true = 0.94 is used (USGS) [].. RESULTS AND DISCUSSION A Perkin Elmer strontium carbonate stock standard solution containing 999 µg/l and unspecified 87 / 86 isotope ratio, has been used for the tests. Solutions in the range 5-00 µg/l were prepared by appropriate dilution of the stock standard solution in HNO % and ultra-pure water. Tlabe indicates the level of purity of the stock solution. Table. Trace metal impurities in the stock solution via ICP/ICP-MS analysis The instrument used for the isotope ratios measurements of these solutions is a Perkin Elmer ELAN DRC-e ICP-mass spectrometer, in standard mode. The operational conditions are summarized in Table.

5 Table. Operational conditions of the Perkin Elmer ELAN DRC-e ICP-mass spectrometer ICP Rf Power 40 Nebulizer Gas Flow.0 L/min Plasma Gas Flow 5.00 L/min Sampling cone Nickel Skimmer Nickel Lens Voltage 6.00 Analog Stage Voltage Pulse Stage Voltage 750 RPq 0.5 The diluted standards were analyzed in three replicates. The measured 87 / 86 isotope ratio and their standard deviations for 0 µg/l and 00 µg/l solutions are shown Table. The reference value of the 87 / 86 ratio in the NIST 987 standard is Table. Results of the 87 / 86 ratio measurements 0 µg/l solution Replicate Isotope Ratio (Norm) µg/l solution Replicate Isotope Ratio (Norm) Summary Analyte Mass Ratio Mean % RSD 0.70 Summary Analyte Mass Ratio Mean % RSD Thus the average deviations between the results of the measured isotope ratios and that of

6 strontium carbonate standard are 0.05 and 0.0 (corresponding to. % and.8 % relative deviations) for the 0 µg/l and 00 µg/l solutions, respectively. 4. CONCLUSIONS The results thus far obtained are preliminary ones, but they can be considered rather promising. As was expected the experimental method using the ICP-MS ELAN DRC-e (in the standard mode) resulted quite efficient for isotope ratios determinations with concentrations of the order of some tens of ppb. The 87 / 86 isotope ratio values measured in solutions prepared from the same stock are not divergent between themselves, nor do the appreciably diverge from the NIST 987 standard. The observed relative standard deviations of the isotope rations are higher than those obtained with TIMS or MC-ICPMS, as predictable. It must be stressed that the here considered application of the 87 / 86 isotope ratio as a tracer aims at tracing waters in oil reservoir and aquifers. These applications do not exact the same rigorous accuracy of the measurement of isotope ratios as other applications do, as for instance in determining of the age of rocks. ACKNOWLEDGEMENT Work supported by the Minas Gerais State FAPEMIG (Fundação de Amparo a Pesquisa do Estado de Minas Gerais). REFERENCES. P. Z. Vroon, B.Wagt, J. M. Koornneef, G. R. Davies, Problems in obtaining precise and accurate isotope analysis from geological materials using laser ablation MC-ICPMS, Anal. Bioanal. Chem., 90, pp (008)).. S. Ehrlich, I. Gavrieli, L Dor, L. Halicz, Direct high-precision measurements of the 87 / 86 isotope ratio in natural water, carbonates and related materials by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS), J. Anal. At. Spectrom., 6, pp 89-9 (00).. Technical Report Series nº 4, Radiotracer Applications in Industry: A Guidebook, pp (IAEA 004). 4. P. C. Smalley, A. Räheim, J. A. D. Dickson, D. Emery, 87 / 86 in waters from the Lincolnshire Limestone aquifer, England, and the potential of natural strontium isotopes as a tracer for a secondary seawater injection process in oilfields, Applied Geochemistry,, pp (988). 5. F. Vanhaecke, G. Wannemacker, L. Moens, J. Hergoten, The determination of strontium isotope ratios by means of quadrupole-base ICP-mass spectrometry: a geochronological case study, J. Anal. At. Spectrom., 4, pp (999).

7 6. L. J. Moens, F. F. Vanhaecke, D.R. Bandura, V. J. Baranov, S. D. Tanner, Elimination of isobaric interferences in ICP-Ms, using ion-molecule reaction chemistry: Rb/ age determination of magmatic rocks, a case study, J. Anal. At. Spectrom., 6, pp (00). 7. C. Brach-Papa, M. Bocxstaele, E. Ponzevera, C. R. Quétel, Fit for purpose validated method for the determination of the strontium isotope signature in mineral water samples by multi-collector inductively coupled mass spectrometry, Spectrochimica Acta Part B, 64, pp 9-4 (009).