REPRESENTATION AND SCALING OF FAULTS IN FLUID-FLOW MODELS

Fault representation and scaling in flow models are examined with resp ect to fault zone properties, the accuracy with which they can be dete rmined, and how these variables and fault geometries can be incorporat ed realistically in to flow models. Outcrop data show that fault displ acement/thickness ratios and permeability vary widely. For simple sing le fault models, results for numerical models are compared with analyt ical and statistical methods. Representation of a fault as a transmiss ibility surface conflates the effects of four variables - fault zone t hickness and permeability, grid-block size and matrix (host-rock) cell permeability. Random spatial variation of transmissibility factor val ues is well represented by a uniform transmissibility factor which is the arithmetic mean of the values representing log-normally distribute d permeability and thicknesses. Realistic ranges of fault zone thickne sses can be represented without grid-block refinement by an upscaling method based on simple transformation of transmissibility factor curve s derived from a range of coarse grid-block models. Sub-seismic faults have significant effects on effective permeability of model volumes a t kilometre scales only when the faults are assigned a permeability le ss than c. 0.001 of the matrix permeability.