Tracer Testing for Reservoir Description
Authors: Brigham William E. , Abbaszadeh-Dehghani Maghsood 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Summary
When a reservoir is studied in detail for an EOR project, well-to-welltracers should be used as a tool to help understand the reservoir in aquantitative way. Tracers complement the more traditional reservoir evaluationtools. This paper discusses the concepts underlying tracer testing, theanalysis methods used to produce quantitative results, and the meaning of theseresults in terms of a conceptual picture of the reservoir. Some of thelimitations of these analysis methods are discussed, along with ongoingresearch on tracer flow.

Introduction
Whenever reservoir engineers consider an EOR project, questions should ariseconcerning the detailed knowledge of the reservoir in which EOR is beingconsidered. Such questions are often asked (and partially answered) whenwaterflooding has occurred, but the far greater cost of any EOR processcompared with waterflooding puts a greater emphasis on the need for a detaileddescription of the reservoir to be exploited.
Traditional studies include geology, geophysics, coring, well logging, andwell testing to help define reservoirs and their heterogeneities. It isimportant that these studies be coordinated so that the unified picture of thereservoir is compatible with all the various sources of information. For EOR evaluation, the most important data to be sought concern the nature of the flowfrom well to well, so tests that reflect that kind of information are ofparticular value.
Two types of tests used to a larger extent in recent years are well-to-wellpressure-transient tests and well-to-well tracer tests. Both can be a great aidin quantitatively determining the nature of the preferred flow paths ofreservoirs. Also, both types of tests are considerably more time-consuming thanpressure-transient tests within a well. The well-to-well pressure-transienttest must last long enough for pressure changes to be seen and interpreted atthe offset wells. This may require several hours to several weeks, depending onthe reservoir properties and the interwell distances. The tracer tests requirean even longer time, for they must proceed long enough for fluids to flow fromwell to well.
Some reservoir engineers think that it is not necessary or important to runboth pressure interference tests and well-to-well tracer tests, because bothtests are designed to determine well-to-well flow characteristics. This feelingis not correct. The two types of tests measure differentcharacteristics. The pressure interference tests tend to measure averageproperties between wells and are very good for determining whether directionalpermeability trends exist. The tracer tests can indicate the extent ofheterogeneity that exists to make up those averages measured by theinterference tests. They give quantitative clues as to the early breakthroughcharacteristics of a flood that strongly affect its ultimate recovery. Thusthese two testing techniques are complementary, not competing.
In this paper, we discuss in some detail the nature of the response one canexpect from well-to-well tracer tests and how one analyzes the results fromsuch a test to evaluate the heterogeneities. In addition, we address the workbeing done to analyze fractured systems. This work, although moving rapidly, isstill in its primitive stages. Finally, we briefly discuss the use ofsingle-well tracer tests, their utility, and the type of research needed toenhance the value of tracer tests for reservoir description.