Tracers are increasingly being used as effective reservoir monitoring and surveillance (M&S) tool in the oil and gas industry. They can qualitatively or quantitatively gauge how fluid flows through the reservoir. Tracer surveys, conducted either as interwell tests or single-well tests, are one of the enabling technologies that can be deployed to investigate reservoir flow performance, reservoir connectivity, residual oil saturation and reservoir properties that control displacement processes, particularly in improved oil recovery (IOR) and enhanced oil recovery (EOR) operations.

An inter-well chemical tracer test (IWCTT) was designed and implemented in a carbonate field with a diffuse fracture network on peripheral water-flood to determine reservoir connectivity, water breakthrough times, and fluid saturations. The reservoir is heterogeneous with layers of high to low permeability limestone with porosity a range of 20-30%, and interspersed with patchy layers of dolomites and a very competent anhydrite seal. The IWCTT was conducted on a very mature part of the field near the peripheral water injectors to determine the effectiveness of infill injector wells. Four distinct tracers were injected into four individual injectors and the residuals were monitored in four “paired” producers.

This paper reviews the complete design and implementation of the test, operational issues, and the analyses and interpretation of the results. The breakthrough times of the tracers are reported and interwell connectivity between the paired and cross-paired injectors and producers are analyzed. Tracers were detected in three of the paired producers, albeit at different breakthrough times. One tracer was also detected in a cross-paired producer. The tracer results were modeled using the current reservoir simulation model with fairly good matches. The tracer data were then used to fine tune the current reservoir simulation model. The IWCTT has been very effective in analyzing well interconnectivity in the reservoir and has led to better reservoir description and an improved dynamic simulation model. Plans for further implementation of this important M&S tool will also be shared.

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