Abstract - We outline the main factors affecting the incorporation in reservoir flow models of faults having negative impacts on production time-scale fluid flow. Upscaling and representation of faults in flow models are examined with respect to the significant fault zone property parameters, the accuracy with which they can be determined, and how both the fault properties and geometries can be incorporated realistically in flow models of conventional resolutions.
Representation of a fault as a transmissibility surface conflates the effects of four variables, fault zone thickness and permeability, grid-block size and matrix cell permeability. As outcrop data and, relatively sparse, well data show that fault displacement/thickness ratios and permeabilities each vary widely, a range of values should be tested. The permeabilities of the two principal types of fault rock occurring in fault zones, cataclastic deformation bands and shaley gouge, can be estimated from existing outcrop data and from empirical knowledge of comparable reservoirs.
Flow simulation results are expressed in terms of the fractional effective permeabilities of model volumes with/without faults or fault arrays represented. For simple single fault models, results for a range of representations in numerical models are compared with those of analytical and statistical methods. The spatial variation of transmissibility factor values, representing a variety of combinations of thickness and permeability variations within a fault zone, is captured by a uniform transmissibility factor the value of which is calculated for the arithmetic means of the lognormally distributed permeability and thickness values. Spatial correlations between fault zone thicknesses and permeabilities and differences between along- and across-fault zone permeabilities are accommodated by systematic adjustments to the upscaling procedure. The method provides an objective basis for incorporation of realistic fault zone thicknesses and permeabilities in flow models without the need for grid-block refinement. The hydraulic properties of sub-surface faults remain the principal source of uncertainty in the specification of a flow model.
Abstract of talk given to:
Modelling Permeable Rocks. Institute of Mathematics and its Applications Conference, Cambridge, March 1998