The Effective Use of Effective Permeability

Abstract

What is permeability and what is effective permeability?Permeability has a well defined definition in the context of a laboratory coreflood, where it is the transport coefficient in Darcy's Law. What happens in the reservoir, where flow patterns are three dimensional, "cross-sectional areas" are arbitrary, and "pressure drops per unit length" can be defined in many different ways? In this case the definition of effective permeability is, to a large extent, arbitrary, as it depends significantly upon the conceptual experiment which underlies its measurement. This is as true for a direct field determination of permeability as it is when attempting to derive an effective value by an upscaling calculation. This arbitrariness has long been recognised as an issue in the upscaling literature, where the effective permeability may be defined and calculated in a multiplicity of inequivalent ways.In this paper, effective permeability will be treated as an engineering concept. Instead of attempting to derive a "first principle's" definition of permeability, we must first decide upon the use of the effective parameter, and then choose a method of calculation which is designed for that requirement. In particular, three uses are commonly made of permeability: (i) to model the continuity of sands, (ii) to model the continuity of barriers, and (iii) to model fluid flow around barriers. Each of these flow patterns provide different elements of a reservoir characterisation, and require quite distinct calculations.The superiority of this approach to a conventional single definition of permeability is demonstrated with detailed geologic models of the two turbidite reservoirs of the Magnus field. The challenge of one reservoir is that of locating the oil within a mature field, with high risk of rapid water-cut rise for infill targets. The other is low net-to-gross, with the issues of connectivity and large scale pressure support.