Many improved recovery processes involve injection of a fluid whose properties are different than water. The fluidhydrocarbon- rock interactions lead to improved recovery. An example of an improved recovery process is polymer flooding. In this method, polymer, a water-soluble chemical, is added to improve the phase mobility ratio, which leads to improved sweep efficiency.

This paper describes the modeling of some improved oil recovery processes using tracers in an unstructured grid simulator. The flow of chemicals, such as polymers, is modeled as the flow of a negligible tracer. Depending on the process, the tracer can flow either in the oil phase, the water phase, or both. Negligible tracers can also be used to model the effect of salinity on improving oil recovery for processes such as enhanced waterflooding. Another application of the tracer approach is to model single-well chemical tracer tests, which are used to determine the residual oil saturation before and after applying an improved recovery method. Other phenomena that can be modeled using this approach include mixing oils of different API gravity, mixing of waters with different salinities and vaporization of oil in a black-oil model.

A major advantage of using tracers to model improved oil recovery processes is the decoupling of the tracer equations from the other component equations and solving the equations in a sequential form. This paper will also describe the system of equations, and the numerical methods used to solve the system. Several numerical simulations of the recovery processes mentioned earlier are described in this paper to demonstrate the application of modeling improved recovery processes through tracers.

1. Introduction

Flow tracers are used to track detailed fluid movements: injection fluid front advance, sweep efficiency and breakthrough calculations, fluid migration across faults, detection of flow barriers, etc. Because of the complexities involved in interpreting the field tracer tests, a reservoir simulator is used to simulate the flow of tracers to assist in interpretation of these tests.

Tracer transport is typically modeled by the advection-diffusion equation. For most practical cases, tracer diffusion may be neglected. The advection-diffusion equation can be added to the regular set of reservoir simulation equations to model several improved recovery methods that use chemicals to alter fluid properties and rock-fluid interactions. This work describes the implementation of the tracer equation to model active and passive tracers as well as partitioning and non-partitioning tracers and their use in modeling several improved recovery mechanisms such as polymer flood, enhanced waterflooding and accurate oil vaporization modeling.

In this work, tracers (e.g. polymer, salt, surfactant, etc.) are modeled as additional components in the simulation. Each tracer is associated with a regular component (e.g. oil, gas, water, CH4, CO2, etc.) and the phases (e.g. liquid, vapor, aqueous) in which the associated component flows. The liquid phase is defined to primarily contain the oil component, the vapor phase the gas component and the aqueous phase the water component. Passive tracers do not alter physical properties of the phase in which they are present. These tracers are mainly used to track injected fluid. Examples include (1) tracking location of injected versus resident gas, (2) tracking movement of components from one location to another, e.g. across a gas-oil-contact, (3) tracking movement of injected fluid from several wells. Figure 1 shows such an example where a gas tracer and two water tracers are used to track injected gas and water. A four-layer reservoir was simulated with both gas and water injection. The figure shows the number of moles (per grid block) of the injected component in the reservoir.

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