Will Tracer Move the Same Velocity as Its Carrier?
Authors: Guan L. , Du Y. 
Published: December 2007

Abstract
The interwell tracer technology has been proved to be an efficient tool to investigate reservoir flow performance and reservoir properties that are controlling gas and water displacement processes. Tracer test data have been used to reduce the uncertainty on well-to-well communications, vertical and horizontal barriers to reservoir fluids and estimation of residual oil saturation. Unfortunately, at present, the analysis of the tracer response curve is still qualitative in nature.
In our tracer modeling study on the X oil field, we observe that the breakthrough of the water tracer and its carrier are not identical and which is not what we expected. The tracers and their carriers are always breakthrough at different times and the difference can be relatively very large. The injected water usually breakthrough faster than water tracers, while the gas tracer usually breakthroughs later than its carrier.
After discuss the flow of tracer and its carrier in the subsurface reservoirs, we derive the analytic solution to the differences in breakthrough time in this paper. The result from the analytical solution matches the breakthrough time differences. The results of our study will improve the understanding of tracer flow in the subsurface hydrocarbon reservoirs and provide valuable insight in the future interwell tracer test designs in the petroleum industry.

Introduction
In the petroleum industry, water is often injected into oil or gas bearing formations for the purpose of producing more hydrocarbons. Tracers can be added to the injected water to determine where the injected water goes. The subsurface flow in the reservoir is anisotropic, and the reservoirs are usually layered with significant heterogeneity. As a result, solvent movement in the reservoir is difficult to predict, especially in reservoirs containing multiple injectors and producers. However, the flow paths can be identified by tagging a solvent at each injection well with a different tracer and monitoring the tracers appearing at each producer. Therefore, multiple tracers are often used for interwell tracer tests in the petroleum industry.
The interwell tracer technology applied during water/gas injection programs provides the reservoir engineer with additional information on the flood pattern in the reservoir. This information is reliable, definite and unambiguous, thus it reduces many uncertainties about the flow paths, reservoir continuity and directional features in the reservoir. Petroleum engineers can establish the reservoir continuity based on the information from different tracers produced from various wells, and reservoir barriers can be identified by non-recovery or delayed recovery of specific tracers between injectors and producers. Tracer test data also can be used to determine the residual oil saturation, and characterization of naturally fracture reservoirs.
The computer development during recent years has added a dimension to tracer technology and has placed new emphasis on mathematical modeling and simulation of tracer response curves. It is the reservoir heterogeneity that controls the flow pattern in the reservoirs; it has a strong influence on the production of oil, gas, and water. In order to improve identification and determination of the scale and location of the heterogeneity, it is essential to be able to simulate the flow of tracers inside the reservoir. The tracer flow between a pair of well gives definite information about reservoir between the two wells.

History of Interwell Tracer Analysis
The interwell tracer tests were first developed for tracking the movement of groundwater in the early 1900s, but they were neglected by the petroleum industry until mid 1950s. Since then, petroleum engineers have started to conduct tracer tests for determination of fluids flow in waterflooded reservoirs. At that time, the simple qualitative analysis method was the only available technique to analyze the tracer test data.
In 1965, Brigham and Smith1 first developed a semi-analytic model for predicting tracer breakthrough times and peak concentration at a five-spot pattern. In this study, they assumed that the tracers moved radially from the injector to the producers through homogenous, non-communicating layers, with longitudinal dispersion in the direction of flow. The number of layers, thickness of layers, and layers' permeability were used to represent the reservoir heterogeneity.