Improved characterisation and modelling of heterolithic tidal sandstone reservoirs

摘要

Tidal heterolithic sandstone reservoirs comprise millimetre- to centimetre-scale intercalations of mudstone and sandstone. Their effective flow properties are poorly predicted by data that do not sample a representative elementary volume (REV), or models that fail to capture the complex 3D architecture of sandstone and mudstone layers. The aim of this dissertation is to develop improved predictions of effective single and multiphase permeabilities in heterolithic tidal sandstones. A surface-based modelling approach has been developed, which honours the geometry of geologic heterogeneity surfaces, integrating easily measurable outcrop derived dataset as input parameters. The workflow uses template surfaces to represent heterogeneities classifiedudby geometry rather than length-scale. The surface-based methodology has been applied to generate generic, 3D mini-models of trough and planar cross-bedded tidal sandstones with differing proportions of sandstone and mudstone. The models closely capture the bedform architectures observed at outcrop and are suitable for flow simulation. Quantitative geometrical input data to construct the models was extracted from an outcrop analogue which records deposition in a tide-dominated deltaic and estuarine setting. The model results demonstrate that effective single-phase permeability of tidal crossbedded sandstones varies with sample volume. A REV of 1 m3 was identified, confirming that the model volume of 9 m3 (> REV) yields representative values. The impact on effective permeability of seven geometric parameters controlling the density of mudstone drapes was determined: mudstone drape coverage, foreset thickness, toeset dip angle, dune climb angle, foreset to toeset ratio, style of cross-bedding (trough or tabular), size of the mudstone drape patches. The impact of seven geometric parameters on effective permeability was determined. Effective permeability decreases as the mudstone fraction increases, and is highly anisotropic. Each parameter investigated can significantly impact on effective permeability, depending upon the flow direction and sandstone fraction, causing considerable spread of the values. During oil/water flow, relative permeabilities are not affected by mudstone drapes, but the irreducible oil saturation varies depending on flow direction and mudstone content. Theudworkflow presented here can be used with subsurface data, supplemented by outcrop analog observations, to generate effective single and multi-phase permeability values to be derived for use in larger-scale reservoir models.