Print this pageThe Integration of Structural Uncertainty into Fault Seal Predictions
Fault geometry and associated property prediction uncertainties can have a large impact on the simulated fluid communication within a reservoir. A minor change in modelled throws can have a large influence in heterogeneous permeability systems (the majority of reservoir systems), and significant differences can also be observed for high net-to-gross systems.
Uncertainties in the modelled fault throws can be due to the limitations of seismic imaging combined with the geocellular modelling process. In general, a proportion of the deformation around a seismically-observed fault (folding and/or damage zones) is accommodated within the wall rocks of the main fault plane, but cannot be imaged by seismic data. The geocellular modelling process also introduces uncertainties: the form of seismic markers immediately adjacent to a fault is often ignored, and data outboard of the fault is projected into the modelled fault surface. Vertical uncertainties of tens of metres are thus common. This amount of uncertainty may be considered irrelevant on the bulk reservoir scale because it does not greatly affect the bulk form of reservoir juxtapositions; however, it does affect the detailed distribution of permeability connections that are developed across the fault. In the majority of cases where the reservoir permeability is heterogeneous and vertical permeability is low compared to horizontal permeability, this change in permeability connections across the fault can have a significant impact on simulated cross-fault fluid flow. By varying the fault throw within realistic uncertainty bounds and recalculating the fault transmissibility properties, both enhanced and reduced connectivities, varying water breakthroughs and different sweep efficiencies are all evident.
In this contribution we describe a technique that allows the routine incorporation of structural geometric and fault property uncertainty analyses in the reservoir evaluation process, and demonstrate the impact of such uncertainties for a high net-to-gross reservoir. This system of throw modification developed and implemented within Petrel allows fault offsets within fully populated geocellular grids to be automatically modified ‘on the fly’ and the results rapidly fed through to simulation. Structural geometric uncertainty, together with a variety of fault property uncertainties, can then be incorporated into uncertainty workflows and this forms part of a more robust geological approach to history matching.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil