The application of interwell tracer tests is becoming increasingly important to the petroleum industry. Interwell tracer tests, as a proven and efficient tool, have been used to investigate reservoir flow performance and reservoir properties that control gas and water displacement processes. Tracer data have been used to reduce uncertainties attributed to well-to-well communications, vertical and horizontal flow, and residual oil saturation.
This paper describes the development of interwell tracer tests in the petroleum industry, from the first qualitative tracer test in the 1950s to the latest quantitative tracer test in the 2000s. The results of our study indicate that poor sampling is the most frequently encountered problem that leads to a failure tracer test and only a small number of interwell tracer tests have employed the advanced numerical modeling methods to analyze the test data. In addition, the interwell tracer tests in the petroleum industry are not well studied as hydrology industry. Therefore, the interwell tracer tests interpretation methods deserve to be paid more attention, so that petroleum engineers can take better advantage of the 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 mid 1950s.At this time, petroleum engineers[2,3] started to conduct tracer tests for determination of flow of fluids in waterflooded reservoirs.
In the petroleum industry, solvent is sometimes injected into oil or gas bearing formations for the purpose 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 solvent at each injection well with a different tracer and monitoring the tracers appearing 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. However, interwell tracer tests are considerably time-consuming and they must last long enough (from several days to several weeks or even months) for injected tracers to flow from injectors to producers.
The interwell tracer tests have been applied in many fields across the world. The majority of the fields are located in the North America and Europe. This study gives a review of the development of inter well tracer tests as it is found in the open literature search in the petroleum industry. Unfortunately, not all field tests are adequately described and this review is limited to the publicly accessed papers.
The scope of this review is interwell field tracer tests and studies in the petroleum industry. Consequently, experimental works and theoretical studies on interwell tracer flow are not included in this study. Although single well tracer tests are useful for the determination of residual oil saturation, they are also excluded from this review.
