A Universal Formulation for the Prediction of Capillary Pressure

Abstract

Abstract Capillary pressure and associated relative permeability are special core analysis parameters which are vital in accurately describing fluid distributions and movement in porous media when two (or more) immiscible fluids are present. Conventionally, the determination of capillary pressure requires laboratory experiments, which are expensive and time consuming. Assuming that core material is available, typically a limited number of core plugs are considered for testing, often resulting in incomplete reservoir description. This situation then leads to motivation for developing mathematical capillary pressure models, as an alternative to adequately describing fluid behaviour in reservoirs. The well known Carman-Kozeny (C-K) equation (Carman 1937; Kozeny 1927) is commonly used to model permeability as a function of other pore structure parameters. More recently parametric groups were established for the C-K equation, which are in regular use in reservoir characterisation. Specifically, the determination of flow zone units for the description of distinct (litho)facies is now widely used in linking fluid flow to geological descriptions. In this paper, a new and innovative deployment of the C-K equation is investigated to derive capillary pressure relationships. The new mechanistic, drainage capillary pressure model uses the principle of "effective saturation", and is developed and validated with data sets from onshore and offshore Australian hydrocarbon basins, giving a good comparison with laboratory measurements and other established models. Most significantly, the new mechanistic formulation does not just fit capillary relationships but is able to predict drainage curves by knowing or assuming up to six properties: absolute permeability; effective porosity; irreducible water saturation; maximum capillary pressure, which may be related to a particular reservoir situation or laboratory standard; capillary entry pressure; and associated entry water saturation. The last two parameters are typically required for a higher permeability (or better quality) rock. Absolute permeability is incorporated indirectly. Furthermore, there is no requirement for correlation parameters or constants but the analysis determines two parameters which give a measure of heterogeneity of the sample being investigated.