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New Optical Transparent Sensors for Colorimetric Chemical Analysis of  Tracers for Oil Field Applications

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We present the results of theoretical and experimental work on the development of transparent sensors for oil tracers for application in oil reservoirs. The 15 newly proposed tracer molecules are commercially available

Results are presented of implementing a new kind of optical analytical method, in which, as a sensor, membranes are used that are created of reagents immobilized on optically transparent solid polymer matrices. Interacting with a substance under investigation the immobilized reagents change the matrix color. This color change indicates a content of a  oil tracer determined and persists for a long time. The color difference obtained can be transformed to numerical value that is treated as analytical information. The registration of not reflection and transmission of the light radiation through transparent sensor is promising to reach larger sensitivity up to order of magnitude 102 at low optical absorption.

Rapid analytical testing is required in different production processes (for example, in determination of tracers in oil emulsions; in inspection of water chemistry of power installations, water cleaning and water conditioning); in environmental monitoring.  In the class of optical analytical methods the spectrophotometry is traditionally used. In this case, optical density or transmission of solutions are evaluated with subsequent their recalculation in terms of parameters of substances under investigation sought for. Frequently, the optical spectrum obtained is so complicated that cannot be easily interpreted. Also, traditional spectrophotometers needs a sample preparation and have unsatisfactory mass and dimensional characteristics (5-8 kg) that restricts their applications to large scale quick tests.

Our Polymeric optodes


In order to produce optodes that virtually are a transparent polymeric material containing functional groups being able to sorb and extract oil tracers or other analytical reagents. By means of the installation a radical block polymerization of a methacrylic monomers is fulfilled in form of plates 0.5-0.6 mm thick. Then the plates are parted into necessary size chips. Each of the chips is the polymethacrylate matrix (PMM). Then the immobilization of analytical reagents in a static mode is implemented where the PMM is exposed in water, water-organic (oil-water emulsions) or organic reagent solutions for 5-15 min (figure 1). After the immobilization, the matrix remains transparent and uncolored.

 As a result of the contact of the matrix with the solution containing oil tracers into the PMM, under certain pH, a colored coordinatively saturated complex is formed, a color of which is in one-to-one correspondence with the concentration of the determined tracer in a sample. A particular color depends mainly on the reagent and determined component, and its luminosity – on the component amount.

The optodes developed were tested in practical techniques for detection of a lot of tracers (Table) and shown highly satisfactory outcomes. The transparent polymeric optodes changing dye can be applied both in solid phase spectrophotometry and in visual rapid determination of substances. The latter can be automated using different means of color images capturing by a computer and their subsequent digital processing. This gives a base to develop a new analytical method.

Laboratory-scale experiments were conducted to evaluate the physicochemical properties (e.g., detection, stability, and toxicity) and performance of the molecules as potential oil tracers. Two and more tracers simultaneously detectable using the new transparent optical sensor  and can therefore be used in the same tracing campaign without any aquipment or by photometric indication for the detection of tracer in oil reservoir, which is economically desirable. 

 


The oil industry increasingly needs more precise information to describe and understand oil reservoir geometry. Among the different reservoir characterization tools available [i.e., production flow rates of reservoir fluids, four-dimensional (4D) seismic analysis, pressure measurements, tracing tests], the interwell tracing technique using water tracers has proven to be a useful and efficient experimental tool in complex oil reservoirs, for which data are difficult to obtain with other techniques.
 

For this technique, two types of water tracers are generally used: the passive tracer, also called the "non-partitioning", "ideal", or "aqueous" tracer, which always follows the water phase without interacting with other phases (i.e., oil), and the partitioning tracer that partitions between the aqueous and oil phases. 

The tracing test using passive water tracers has been implemented for many years in a broad range of oil reservoirs throughout the world. However, the limited number of chemical passive water tracers available represents a major problem for the oil industry.

We have developed new chemical sensor for tracers for reservoir applications and have identified and evaluated their performance based on transparent polymer. Detection is a key parameter because very low limits of detection and quantification (LOD and LOQ) are required for the injection of very low quantities of tracer into oil reservoirs to limit environmental impacts.