Numerical Simulation of Gas Tracers for Reservoir Characterization, a Case Study

In this paper we describe how incorporating inter-well flow of gas tracers into a numerical simulation model of an oil producing field allowed us to improve the reservoir characterization. Gas tracers evidenced the lateral and vertical reservoir connectivity, identified preferential flow paths and eventually provided an additional tool for the dynamic history match. The field produces light oil with the support of miscible gas injection, from a reservoir composed of two stacked fluvial sandstones units. To improve the reservoir characterization four inter-well gas tracer campaigns involving a total of 13 injector wells have been successfully completed. All these tracer injections have then been modeled at our full field numerical simulation model with the purpose of challenging the reservoir description in it. Input from this exercise have been later-on used during the construction of geomodels, benefiting from our improved reservoir knowledge. The reservoir is composed of two units, Middle (M) and Upper (U), both deposited as laterally amalgamated fluvial channels. Both units appear as vertically separated by an impervious shale interval, only absent in 2 wells out of 30. The shale interval was originally considered as a possible flow barrier by the earliest geological models. However, numerical simulation models were only able to replicate observed tracers arrivals when specific vertical connections existed between both units, indicating the shale interval was no as laterally continuous as formerly suspected. History matching the tracers arrivals in the western field area was also helpful to reveal a fast gas breakthrough between wells which were aligned perpendicularly to the main channel orientation (NW-SE). This finding confirmed that this prevailing channel orientation was not the only responsible for a good reservoir communication, but also the lateral amalgamation (SW-NE) of channels was exerting a significant control in low sinuosity fluvial systems, as well as secondary flow directions in high sinuosity systems. The improved reservoir characterization have been reflected in subsequent reservoir geological models and numerical simulators, avoiding misleading history matching solutions. Also, it is worth to note that this have had a direct impact on the gas injection strategy followed in the field. Quite commonly interwell tracer flow is not fully incorporated into numerical simulators. This is a time consuming process, which, in addition to the conventional model uncertainties, requires sensitivities to the associated tracer parameters. This paper demonstrate through a real case how valuable this additional effort may result, and how it may improve the geological and dynamic understanding of our field.