Modelling stress-dependent effective porosity-permeability relationships of metre-scale heterogeneous mudstones

Abstract

The importance of shales and mudstones to applied geosciences and in particular to fluid migration in sedimentary basins has never been more recognized than today. Prominent examples are conventional or unconventional petroleum systems, where shales and mudstones act as source, reservoir or cap rock, but also CO2 and nuclear waste storage or hydrogeology. Despite their importance, shales and mudstones are yet not as far well understood as sandstones or carbonate rocks. In particular, the influence of heterogeneity on fluid migration has been poorly addressed in the past, although many authors have identified and studied heterogeneities in shales and mudstones. Nevertheless, their flow properties are fairly well understood when treated as homogeneous on sample scale (centimetre-scale). Typical flow relevant heterogeneities are grain size and thus petrophysical property (e.g. porosity, permeability, capillary entry pressures) variations due to spatial lithological variation induced by primary and secondary sedimentary structures. In this study we investigate flow relevant heterogeneities of shales and mudstones on submetre scale derived from core and borehole images from an off-shore gas field in the Western Nile Delta, Egypt. Thereby, we combine latest models and published measurements of sample-scale petrophysical properties with interpretation, quantitative analyses, advanced modelling and numerical fluid flow simulation to assess the influence of shale and mudstone heterogeneity on fluid flow and hence, fluid migration, retention and mudstone seal capacity. Additionally, the set of mudstone heterogeneities used in this study has been derived from a combined visual and geostatistical interpretation of more than 500 m of mud-rich core and borehole images. As final results, we deliver stress-dependent effective porosity-permeability relationships for a broad range of shale and mudstone heterogeneities, representative model sizes and resolution as well as measures of uncertainty for each heterogeneity type. Moreover, probability density functions describing where and how these heterogeneities appear in larger scale geological units, such as seismic facies or local depositional environments, are provided. As a key result, heterogeneity and lithological variation have great influences on effective permeability and effective permeability anisotropy (Kh/Kv). Furthermore, our results indicate that mudstone heterogeneity is very common in all investigated larger scale geological units (hemipelagites, levees, channels). Modelling of fluid flow through mud-rich sedimentary basins without inclusion of these sub-metre scale heterogeneities of mudstones can therefore lead to misleading results. Thus, effective porosity-permeability (anisotropy) relationships are provided for different lithological variations and mudstone heterogeneities as a final result.