Geometric and Property Uncertainty Modelling Within Complex Fault Zones

Simon D. Harris, Steve Freeman, Rob J. Knipe, and Zinedine Khatir
RDR Ltd, School of Earth Sciences, University of Leeds, Leeds, United Kingdom

Efficient methods are required to optimise the fault juxtapositions and property distributions within reservoir simulation models and to assess the likely impact of the uncertainty relative to the required tolerances. The fundamental issues are that the seismic-based model could: tend to overestimate the throw on the main slip surfaces; not include drag effects or secondary faults; ignore deformation within the host as well as within the main fault zone.

This may result in an underestimation of the bulk reservoir connectivity and the connections between different layers. The true connectivity would probably allow many more pathways than the geocellular model can physically represent.

Compounded with the geometric effects, simple fault rock property estimates will be in error. Single intrinsic permeability values with specific fault thickness relationships will fail to model the true range. This is important because the fluid flux distributions will be dominated by the high-transmissibility regions that generally form a relatively small proportion of the fault area.

The composite effect of geometric and property spatial variabilities means that models will tend to underestimate communication across faults. This observation will be counteracted by multiphase flow effects in zones of higher water saturations.

Fluid flow simulations have been performed on representative reservoir permeability and fault rock property distributions to quantify the impact of varying geometry and property values. These have been contrasted against those values typically produced by existing software systems. This approach has enabled the development of algorithms for creating simple modified grids that better model the flow across fault zones within reservoir simulators.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas