Using Tracers to Characterize Petroleum Reservoirs: Application to Carmopolis Field, Brazil

Abstract

Abstract A specific area under water injection in the Carmópolis field, Brazil, is been considered a candidate area for a polymer pilot project for mobility control. A reservoir characterization and an evaluation of the polymer performance in this high heterogeneous reservoir were required. For this purpose, radioactive, fluorescent and chemical tracers were applied associated with polymer in a reduced area. The tracer technology has an enormous potential use in Petrobras scenario and this Carmópolis field application was an opportunity to obtain know-how. This paper describes the basic steps from the laboratory tests to the final application including design and programming of field operation. The interpretation of the results using a new approach is also addressed. Introduction Reservoir tracers are substances added to the injection fluid (normally water) and collected by sampling at production wells. Essential clues regarding the fluid path throughout the porous medium may be obtained from the analysis of the tracer output. Directional flow trends, flow barriers and communication between reservoirs are examples of relevant qualitative information, which may be extracted from the so-called well-to-well tracer tests. Moreover, quantitative information as the swept efficiency may be estimated by matching tracer well data against a mathematical model. The choice of a substance to perform the role of a reservoir tracer involves some prerequisites to ensure the technical and economical success of such reservoir characterization operation:non-existent or found in very low concentration in the reservoir,stable and non-reacting with the porous rock at least for the reservoir conditions,allow quantitative analysis at very low concentrations,good solubility in the aqueous phase,safe and easy to manipulate,low cost The tracers applied in the petroleum production industry may be classified into three main groups, namely, radioactive, fluorescent and chemical. Nowadays, the radioactive tracers are the most frequently used1,2. They have an extremely low detection level in the order of ppt (parts per trillion) and, as a consequence, require a tiny injection volume (usually a blister). This is crucial for offshore applications where available space is critical. The manipulation is considered safe under the established limits of the international radiological protection rules. The second most used are the chemical tracers, especially the halogens1. Although effective, as confirmed by stability and compatibility laboratory tests, they present the inconvenience of a relatively high detection level, then requiring larger injection volumes in comparison to radioactive tracers. This may have a negative impact on the final cost in field applications. The fluorescent tracers are inexpensive, safe and easy to manipulate and may be detected visually without laboratory test. The main inconvenience is a certain degree of interaction with the porous rock. As a consequence, their use has been limited to cases in which the residence time is short such as reservoirs with noticeable faults and/or preferential channels. A combination of different types of tracers may also be applied. This paper describes a pioneer and successful application of three tracers to characterize an area of the Carmópolis field. This area is candidate for polymer injection to correct mobility. A radioactive (Tritium), a chemical (Iodide) and a fluorescent (Fluorescein) tracer were approved in laboratory and injected.