The Use of Tracers for Reservoir Characterization

Abstract

Abstract: The use of tracers in the reservoir studies shows that this is the best method to evaluate the downhole region of the well and interwell space. Traditionally, the reservoir studies have combined the integration of the geological, geophysical, and well tests as well as geochemical data and production history to construct geologically realistic models. Tracer methods give new opportunities to evaluate the fluid-flow properties and continuation or compartmentalization of reservoirs. Radioactive and chemical tracers have been used in Volgograd Region to construct hydrodynamic models of clastic and carbonate reservoirs. The method of radon indication has been used for downhole region evaluation. After radon injection, the permeability along the vertical section in a well can be calculated according to the level of radioactivity on a log. The convective nature of tracer-tests provide additional information about the reservoir. The following parameters are used for interwell space evaluation: tracer velocity (time of flight from injection to production wells) in a reservoir; tracer concentration as the consequences of tracer dissolution in formation fluid flow; frequency of the tracer presence in samples - a number of tracer pathways from injection to production wells. The use of tracer survey method can evaluate the vertical fluid flow barriers, reservoir continuity in every block, fluid-flow properties, and understanding of the reservoir architecture in the interwell space. The main result of tracer method is the ability to detect the ultra high permeability zones that cannot be found by other methods of reservoir analysis. Introduction Geological analysis of reservoir based on core data is a principal instrument for reservoir modeling. **The comparison between cross-plots of porosity and permeability enabled to derive permeability from core data and to calculate the parameter related to porosity obtained from geophysics and well logging. However, core data are often very limited to characterized reservoir as a whole, especially for fractured carbonate reservoirs (3). Large fractures are typically more widely spaced than the diameter of a borehole, therefore it is very difficult to get a core in such failure area and obtain reliable samples. Direct evidence of fracture spacing is lacking in well logs data due to small volume of fractures. On the other hand, long fractures having apertures 1–2 mm are much more conductive than matrix with high porosity and they make the main contribution to the permeability of the reservoir. To understand the reservoir quality, it is necessary to detect the real high permeability zones and their relationship with storage capacity of different parts of the reservoir. Pumping test technology is widely used by the oil industry for dynamic reservoir characterization. However, evaluation of available test data has low selectivity and tests do not cover the reservoir profile. Simultaneous analysis of pumping test and tracer test data would make it possible to receive the layer-to-layer variations in permeability. The radon tracer method was widely applied to show the permeability profiles of many reservoirs in the Volgograd Region. Possibility of different methods for reservoir characterization related to the scale of observed object is shown on fig. 1. Some radioactive and chemical tracers have been used for interwell space evaluation to demonstrate continuity or isolation of different parts of reservoir (), fault and fracture detection and leakage in production and injection wells. Permeability profile and interwell flow pattern may be a basis for 3D reservoir modeling and could become an alternative to conventional streamline models. Tracer investigations show that tracers can be considered as a useful tool for reservoir characterization.