Modeling and Application of Tracers for Reservoir Characterization

There were two main objectives of this research. The first objective was to develop an accurate and efficient tracer model to study gas tracers used in both single-well and interwell tracer tests. The second objective was to develop an advanced technique for the description of reservoir characteristics using an interwell tracer test. The first objective was achieved by implementing a tracer option in UTCOMP (an equation of state compositional simulator). The code was made efficient by separating the tracer components from the phase equilibria calculations. Data from two interwell perfluorocarbon gas tracer tests at Elk Hills Naval Petroleum Reserve (NPR) were analyzed and 3D simulation studies were conducted using the newly developed tracer features in UTCOMP. A new tracer test for estimating reservoir heterogeneity called vertical tracer profiling (VTP) was developed and tested. This interwell tracer test is done by injecting tracers or collecting tracer samples at different locations along the depth of a reservoir. One of the most important and difficult problems in analyzing interwell tracer data is the solution of the inverse problem, that is, the inference of the reservoir heterogeneity from the tracer production data. A combinatorial optimization method called simulated annealing was used to solve this problem. One of the advantages of simulated annealing is that more than one type of data can be used simultaneously to generate the reservoir description. In this case, both vertical variograms, which can be estimated from core data, and tracer data were used. This approach was applied to both conventional tracer production data and tracer production data using VTP. Results show that the new tracer test can give the same or better reservoir description compared to a conventional tracer test. Since a lot of computational time is required, an alternative procedure that takes much less computational time and yet produced superior results was developed. The permeability field generated from this improved procedure was shown to be effective in matching simulated waterflood data at high endpoint mobility ratios. We next investigated the accuracy of the swept volume calculated from the first-moment method using tracer production data from an interwell tracer test in a heterogeneous reservoir. Results of swept volume calculated from the first-moment method of tracer production data are compared with ones from reservoir tracer concentration contours from simulations. It is. shown that the accuracy of the first moment method depends on the detectable limit or the cutoff concentration of tracer production data. The first-moment method was extended to partitioning tracers to determine the residual oil saturation between wells. This method gives an accurate estimate of the residual oil saturation even for a very heterogeneous reservoir and a nonuniform (local) distribution of residual oil. A field application is illustrated using data from a gas tracer test in the Shallow Oil Zone (SOZ) reservoir of the Elk Hills NPR. The use of the Peng-Robinson equation of state to predict the partition coefficients of perfluorocarbon gas tracers (PFT's) at different conditions was also investigated. The first-moment method was applied to VTP tracer production data to obtain an estimate of the distribution of the residual oil saturation in the reservoir. These results show that VTP data can provide insight into the flow pattern of a reservoir and the accuracy of the estimate depends on the degree of vertical crossflow. The use of selective injection combined with VTP is shown to provide the most information about crossflow and reduce the chance of the tracers bypassing the residual oil.Petroleum and Geosystems Engineerin