Tracers are commonly used for monitoring fluid transport in oil and water reservoirs. This paper provides results on tracer dispersion experiments in the so-called composite cores. They were performed under steady-state flow of formation water at Sw=100% and Sor, and also under conditions of multiphase flow in a water-alternating-gas process (WAG). The composite core was assembled with 6 plugs from the same sandstone formation, having similar geological characteristics. KCl was used as a passive tracer, for which the chlorine concentration was determined by means of electrical conductivity measurements and tritation of the produced water. From the concentration values at different times, breakthrough curves were obtained and analyzed. Porosity and absolute permeability were measured before packing, and a relative permeability curve for the composite core was obtained. This information was used to history match the production curves in multiphase flow experiments. For the tracer result, a commercial simulator and a 2-D stochastic simulator, developed for this purpose, were used to predict the breakthrough curves. For the latter, an estimate of the dispersivity was obtained by fitting the curve with an analytical solution of the 1-D convection-dispersion equation. Multiphase flow results are in good agreement with the mass balance. For WAG experiments, it can be shown that there is intermediate value of the water-to-oil ratio in the feed that results in an optimum in terms of recovery and economics. The analysis shows that the breakthrough curve can be predicted reasonably well for the condition of Sw=100%. Numerical dispersion turns out to be a difficulty when running the commercial simulator that is naturally overcome in the case of the stochastic simulator. There seem to be effects of saturation on dispersivity as the predictions show.
Introduction
Characterization and monitoring of improved oil recovery processes (IOR), either in the field or in laboratory tests, are necessary tasks for the exploitation of hydrocarbon reservoirs. There are several strategies to fulfill these needs in the field, with 4-D seismic as an outstading example [1–2]. However, some monitoring methods are bound by economical or technical constraints, such that alternative tecniques have to be made available. Tracer technology is an option, that in combination with other strategies, can provide valuable information for various recovery processes. In the laboratory, tracer tests offer a convenient and inexpensive choice.
In recent years, the Venezuelan oil industry has initiated the so-called Integrated Field Laboratories (IFL), with the idea in mind of exploring the application of advance technologies in the field. Several IOR processes are currently under scrutiny, among those, alkali-surfactant-polymer (ASP) [3–4], Nitrogen flooding and water alternating gas (WAG) injection. Regarding WAG processes, these injection schemes have become an important strategy of IOR technique around the world, and have been focus of interest in recent years in Venezuela. This drainage strategy is mainly planned to deal with the major concerns in Venezuelan oil fields: optimizing natural gas resources and increasing oil recovery factors [5].
Among several candidates, VLE area was selected to evaluate an immiscible WAG process as representative of a large number of deep (> 10000 ft) light oil reservoirs in the Maracaibo Lake basin with similar reservoir characteristics, currently under water injection at an advanced stage of depletion. Venezuela has over 14000 MMSTB of oil currently in place in reservoirs with similar conditions. On the other hand, the selection of the VLE field was based on screening criteria for WAG flooding, obtained from successfully and unsuccessfully worldwide projects [5], analytical simulation, experimental and numerical simulation studies [6], as well as availability of water and gas facilities.
