Proposal Before the start of an enhanced oil recovery project it is important to adequately describe the architecture of sand bodies, preferential flow directions and permeability barriers. As a consequence of project implementation operating costs tend to increase so in order to guarantee the economic success of the venture, reservoir surveillance plays a major role. This is mandatory in highly heterogeneous reservoirs where it becomes strictly necessary to identify and adjust any deviation from the anticipated response to injection. This paper presents the results of the use of chemical and radioactive tracers in Grupo Neuquen reservoir, Loma Alta Sur field, Argentina. The objectives have been to improve the dynamic description of the reservoir previous to the injection of costly polymers and surfactants and to optimise the secondary flood by increasing volumetric sweep efficiency. This field has been in production since 1990. Water injection started in 1993 and due to field observations of poor reservoir connectivity an intensive programme of tracers was recommended. Three injectors were inoculated, one of them with ammonium thiocyanate and the rest with tritiated water. Fifteen producers were monitored throughout twelve months. Field data is presented in this report. It shows that hydraulic connectivity is higher than expected although fluid migration among wells is highly variable depending on reservoir transmissibility and channel orientation. For wells separated only 300 feet arrival times of few hours were reported. From these observations it was concluded that many sand bodies still produce under primary recovery. Sands with better transmissibility and continuity are constantly recycling injected water contributing little to reduce oil saturation. As a consequence a full field reservoir description in a sand by sand basis was initiated before the implementation of an EOR process. Introduction Loma Alta Sur field is located in the south of Mendoza province, Argentina (Fig.1) in what is known as the folded belt of Malargüe. Enhanced oil recovery is being considered as an alternative to increase the final recovery factor of the field. Rock permeability curves combined with high oil viscosity, typically higher than 150 cp, produce a very unfavorable mobility ratio of 17. The use of polymers and surfactants in conjunction with hot water injection has been the choice in order to improve volumetric and microscopic sweep efficiencies. Loma Alta Sur field Discovered in 1990 it is now on a mature stage of development. After three years of primary production a peripheral waterflood pilot was devised with two injectors and six producers which was extended to the whole field between 1994 and 1995. In 1997 two infill wells were drilled with the objective of injecting hot water to reduce oil viscosity. However upon completion, productivities were so high that they have never been converted to injectors and today are still producing. Formation pressures taken during logging indicated that while some sands had responded to water injection, others were still at initial reservoir conditions. A complete lack of vertical communication was then inferred. Since then in all the infill wells that have been drilled, pressures were measured in a sand by sand basis to monitor waterflooding. Loma Alta Sur has been divided in four stratigraphic sequences labeled from bottom to top, V, VI, VII and GNS. The latter is not productive. Besides each of these sequences has a lot of sand bodies evidenced by the gamma ray and SP logs (Fig.2) In general the vertical pressures profiles indicated that only sequence V had responded to water injection, while sequences VI and VII were producing under primary recovery (Fig.3) This would be because sand bodies from the upper producing sequences would have less lateral continuity. The thickness and width of fluvial channels are related to each other in such a way that in accordance with the thickness of the sands present in this field they would not extend aerially more than 200 m. This was the main reason why the average well spacing was reduced from its original 260m to 205m and then to 170m, evidencing an increase in recovery factor (Fig.4). Actual distances for the pilot tracer program are in the order of only 120m.

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