Advances of Interwell Tracer Analysis in the Petroleum Industry
Authors: Guan L. , Du Y. , Johnson S.G. , Choudhary M.K. 
Published: March 2010

Abstract
The use of interwell tracer tests is becoming increasingly important to the petroleum industry. In addition, the interwell tracer test has proven to be an efficient tool to investigate reservoir flow performance and reservoir properties that control gas and water displacement processes. Tracer data has been used to reduce uncertainty attributed to well-to-well communications, vertical and horizontal flow, and residual oil saturation.
Unfortunately, at present, analysis of the tracer response is still largely qualitative in nature, and most information provided through the monitoring of the tracers is not quantitative. Among the numerous papers found on tracer technology, only a few include history matching of water-tracer test results and only one paper includes history matching of gas tracer test results.
This paper describes the development of interwell tracer analysis methods in the petroleum industry, from the first qualitative tracer study in the 1950s to the latest quantitative tracer study in 2000. The results of our study indicate that only a small number of interwell tracer studies employed advanced numerical modelling methods. In addition, tracer analysis methods in the petroleum industry are not well studied. However, they are far better studied in the hydrology industry. Tracer modelling methods deserve to be given more attention, so that petroleum engineers can take better advantage of results from costly interwell tracer tests.

Introduction
Although tracer tests(1) were developed for tracking the movement of groundwater in the early 1900s, they were neglected by the petroleum industry until the mid-1950s. At this time, petroleum engineers(2, 3) started to conduct tracer tests for determination of fluid flow in waterflooded reservoirs.
In the petroleum industry, solvent is sometimes injected into oil or gas bearing formations for the purposes of producing more hydrocarbons. Tracers can be added to the injected solvent to determine where the injected solvents go. 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 solvents at each injection well with a different tracer and monitoring the tracers that appear at each producing well. Therefore, multiple tracers are often used for interwell tracer tests in the petroleum industry.
Interwell tracers can provide information on flood patterns within the reservoir. This information is reliable, definitive, and unambiguous. Thus, it helps reduce uncertainties about flow paths, reservoir continuity, and directional features in the reservoir. Therefore, petroleum engineers can obtain information on reservoir continuity from the amount of each tracer produced from each well. Reservoir barriers can be identified by non-recovery or delayed recovery of specific tracers. At the same time, tracer test data can help determine residual oil saturation. Tracer test results also provide information on fracture characteristics in a naturally fractured reservoir.
Interwell tracer tests have been applied in many petroleum producing fields across the world. The majority of these fields are located in North America and Europe.