3D Modeling, Biodegradation and Local Grid Refinement, a New Approach to Predict Biodegradation at Prospect Scale

Bernard Carpentier, Sylvie Wolf, Sylvie Pegaz-Fiornet, and Isabelle Kowalewski
IFP 1&4 Av. De Bois-Préau 92852 Rueil-Malmaison Cedex France

Predicting the composition of oil is always a critical issue for the evaluation of a petroleum prospect and finding low API° biodegraded oil is probably one of the major risks that the oil companies have to face in deep water exploration. In order to minimise this risk, IFP has been working for a long time on biodegradation processes and has developed conceptual and numerical models designed to evaluate and predict biodegradation at field scale. In the proposed models, biodegradation is supposed to be active at the Oil Water Contact (OWC) and controlled by the filling history of the trap when the reservoir temperature is below 70/80°C.

This biodegradation temperature "window" is detected from basin modelling outputs. The level of biodegradation of the hydrocarbon fluid is controlled by the number of bacteria, related to the depth of the reservoir, and the individual HC consumption rate of the bacteria. Due to biodegradation only being active at the OWC in the transition zone, the level of biodegradation is very sensitive to the rate at which the OWC is moving down during the filling history of the trap and so is largely determined by its geometry. Classical basin models, with cells usually of the size of an entire oil field, are not suitable to such a biodegradation approach.

Nowadays, when reservoir modelling and basin modelling tend to merge in an integrated or combined approach, it is possible to define, in a given basin model, refined areas where the cell's size is small enough to capture the fine description of the field. Such a high definition zone located inside a surrounding basin described with lower resolution is called Local Grid Refinement (LGR). This type of approach is currently available in the IFP basin model "Temis3D".

Taking advantage of this new capability, it is now possible to simulate the filling history of an oil field with a high resolution description of the associated trap. The LGR procedure guarantees a full coupling between the oil field and the surrounding basin behaviour. Using this new functionality in association with a compositional fluid description, it is now possible to simulate biodegradation with the evolution of the composition of the entrapped fluid as an output.

This model was applied on a basin where biodegradation took place. The modelling of generation, expulsion, migration, biodegradation and secondary cracking, fully coupled with the LGR areas located on prospects and fields, demonstrates the potential of such an approach to evaluate, at reservoir and prospect scale, the composition of the biodegraded and thermally cracked entrapped fluid.

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.