Prediction of gas injection performance for heterogenous reservoirs, semi-annual technical report, October 1, 1996--March 31, 1997

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The current project is a systematic research effort that will lead to a new generation of predictive tools for gas injection processes in heterogeneous reservoirs. The project is aimed at quantifying the impact of heterogeneity on oil recovery from pore level to reservoir scales. This research effort is, therefore, divided into four areas: (1) Laboratory Gas Injection Experiments (2) Network Modeling of Three-Phase Flow (3) Benchmark Simulation of Gas Injection Processes (4) Streamline Simulator Development. The status of the research effort in each area is reviewed briefly in the following section. Project Status Laboratory Gas Injection Experiments Gravity drainage of oil in the presence of gas and water has found to result in high recovery efficiency. Numerical representation of the high recovery efficiency requires a good understanding of three-phase relative permeabilities, especially at low oil saturations. Ph.D student Akshay Sahni has analyzed experimental results of selected three-phase displacements in the literature and compared them with the newly developed mathematical theory of three-phase flow in porous media. He approximated the relative permeability of each phase as a polynomial function of the saturation of that phase. An excellent agreement has been obtained between the measured and the calculated saturation paths. The analytical solution has also been checked by performing numerical simulations. Fig. 1 is an example of the comparisons of experiments, mathematical theory and numerical simulations. Fig. 1 shows a situation in which gas is injected into a system with high oil saturation and the formation of an oil bank is observed. The experiments in the literature were generally conducted at relatively high oil saturations. We have designed a series of gravity drainage experiments to measure three-phase relative permeability at low oil saturations. The CT scanner in the Petroleum Engineering Department at Stanford has been modified to measure in-situ saturations of vertically-placed samples, which is necessary in gravity drainage experiments. Akshay Sahni has finished a series of gravity drainage experiments in sand packs using different model oils to calibrate the scanner and to investigate the effect of spreading coefficient on three-phase relative permeability. A procedure has been developed for calculating relative permeabilities from measured in-situ saturations.