Light oil fields in Lake Maracaibo have been produced for several decades under depletion and waterflooding. Due to reservoir heterogeneity and complexity the traditional waterflooding oil recovery is not expected to exceed 30% of initial oil in place. Potential Improved Oil Recovery (IOR) methods capable to boost production from the mature fields are now under investigation and testing by the State Venezuelan Oil Company PDVSA. PDVSA's main IOR technology strategy is based on Integrated Field Laboratory (IFL) philosophy. IFL allows concentrating resources on modern technology evaluation, design IOR processes to solve regional problems, reduce the gap between laboratory screening and the field application phase for most promising recovery methods. The Water Alternating Gas (WAG) pilot at VLE–305 area of Lagocinco field is one of such IOR laboratory fields with the first immiscible gas injection pilot in Lake Maracaibo started in May 2000. The WAG pilot encompasses five production wells, one dual completion injection well and one observation well. The producing reservoir C-23 is a stratified sandstone with areal and lateral heterogeneities. Separator hydrocarbon gas is injected in the dual completion injection well in the WAG mode. Different sensitivity simulations were performed to design and optimize WAG injection strategy. The WAG pilot is now in its second stage of implementation and evaluation. Simulations are focused on tracer modeling and interpretation of production wells response. Since available injection volumes of hydrocarbon gas for large scale WAG injection projects are not available in the area, other IOR alternatives are being considered. The VLE pilot is again used as a laboratory for screening other potential IOR technologies. Nitrogen injection in WAG and continuum mode, and cyclic water injection are strategies under consideration. Nitrogen injection facilities will not be operational in the near future, therefore cyclic water injection is considered as an alternative for short-term production improvement under waterflooding. Cyclic injection can accelerate oil production and reduce water cut making production more economic and not requiring significant additional investments in the field. The preliminary evaluation of cyclic injection in the pilot area gives oil production improvement by 2–3%, and expansion of the process over a larger field area could give additional sweep improvement. WAG injection is estimated to improve oil recovery by 10–12% of STOOIP. WAG with nitrogen as injection gas may give 4–6% of STOOIP in increased oil recovery in comparison with waterflooding. Introduction PDVSA s light oilfields in Lake Maracaibo have been under exploitation for more than four decades. As these resources approach maturity, it has been realized that improving recovery will demand efficient IOR processes considering specific regional conditions. The Integrated Field Laboratory (IFL) philosophy is one of PDVSA s main technology strategies since 1998 designed to accomplish these goals [1]. Several IOR processes are currently under field evaluation, among those, alkali-surfactant-polymer (ASP) and hydrocarbon gas WAG injection. Nitrogen injection, steamflooding, pulse water injection and air injection in mature light and medium oilfields are other IOR methods under scrutiny. During the last decade WAG injection has become an important IOR technique around the world [2], and has been focus of interest in recent years in Venezuela [3].

It is widely accepted that comprehensive data acquisition programs are necessary for WAG management and pilot project interpretations. Common data acquisition methods used to monitore WAG processes are frequently analysis of separator testing, fluid composition, production/injection rates, pressures injection surveys, gas-oil ratios (GOR) and saturation logging. Chemical tracers have been used as a tool for monitoring water and gas injection, whereas for WAG process no experience in a field has been reported in Latinamerica. At the Lagocinco field, C2/VLE-305 reservoir (located in Maracaibo Lake basin) a WAG pilot project is currently being developed, using a chemical tracers program with Perfluorocarbon and fluorinated Benzoic acids. Five chemical tracers have been injected in both phases (water and gas) during WAG test as a surveillance tool for the process. The aim to inject gas and water tracers was related to use breakthrough time and tracer production/injection history to get a better understanding of dynamic reservoir behavior and to support and upgrade the reservoir model. This paper briefly describes the monitoring process achieved in Lagocinco pilot project, and some results of this monitoring in relation to the WAG process. The result of the first water tracer test has indicated early breakthrough in four wells of the pilot project. One of the wells has increased oil production rate and three of them have maintained oil production rate and have decreased water cut and GOR as consequence of WAG process. The rest of the wells do not show a clear trend in tracer production, confirming heterogeneities in the pilot area. Zones not drained were identified and connection between the injector well and producer were characterized. The results of the first gas tracer have shown different distributions and velocities between gas and water. Finally, the second water tracer breakthrough was observed 3.3 months after its injection, just in one well, declaring that water distribution or path has changed as a consequence of WAG process. Introduction WAG injection processes have become an important IOR technique around the world [1], and have been focus of interest in recent years in Venezuela. This drained strategy is mainly planned to deal with the mayor concerns in Venezuelan oil fields: optimizing natural gas resources and increasing oil recovery factors [2]. Among several candidates, the VLE area was selected to evaluate an immiscible WAG process as representative of 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. In Venezuelan western reservoirs there are over 1.1 MMMSTB of oil currently in place in reservoirs with similar conditions. The selection of the VLE area as an Integrated Field Laboratory (IFL) [3] was based on screening criteria for WAG flooding, obtained from successful and unsuccessful worldwide projects [2], analytical simulation, experimental and numerical simulation studies, as well as availability of water and gas facilities. In this paper is described the first immiscible WAG injection pilot at VLE-305 in Maracaibo Lake focused on the monitoring process achieved with chemical tracers and productions curves. A better design, models and operational description of WAG process can be found in previous paper [4].