Abstract

This paper presents new models and their solutions for the flow of tracers in reservoir, both radioactive and chemical, under constant mass flux conditions, which can be used for the design, evaluation and interpretation of injection test. The models developed consider homogeneous and naturally fractured reservoirs (NFR). For the case of NFR, a transient fracture-matrix tracer transfer is assumed. The models and the solutions acquired include the tracer lost to the matrix inaccessible (dead end) pore volume. The dimensionless groups used in this work resemble as much as possible those used in well test analysis.

Short and long time approximate analytical solutions for these tracer flow conditions are derived, which are useful to validate the general Laplace space solution, and for the interpretation of the tracer response in observation wells.

The solutions presented in this work were applied to real field interwell (observation) tracer responses, allowing the estimation of important fluid and reservoir parameters, such as the radial dispersion coefficient, Dr, and for NFR the average fracture aperture (width) and the block size.

The evaluation and interpretation of a pulse tracer test in a complex naturally fractured and stratified carbonate reservoir is evaluated and analyzed. In this test a chemical tracer Perfluoromethylcyclopentane (PMCP) was used.

Introduction

Tracers are substances that are added to the injected fluid with the purpose of studying the trajectory of the fluids inside a reservoir, as they advance toward the producing wells. A tracer should meet certain characteristics, among others: not interfere with the fluid flow, require a small concentrations and easy detection. Another purpose of introducing a tracer in the injected fluid is to detect the fractures preferential orientation or discontinuities in the reservoir. In any project of fluid injection, the channels of high permeability in the reservoir can quickly establish "short-circuits" for the injected fluid, reducing the efficiency of the process drastically, and in some cases can cause the failure of the project. The field tests with well to well tracer injection and the data analysis with the correct model, provide the possibility for the improvement of the reservoir characterization.

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