Understanding effective permeability is crucial for predicting fluid migration and trapping in subsurface reservoirs. The Bunter Sandstone of northwestern Europe hosts major groundwater and geothermal resources and is targeted for CO2 storage projects. Here the effective permeability of fluvial facies within the Bunter Sandstone Formation was assessed using facies-scale models. Twelve lithofacies were modeled based on core and outcrop observations of their geometries and dimensions. Permeability values from minipermeameter measurements were assigned to low- and high-permeability lithologies in each facies. The dimensions of a Representative Elementary Volume (REV) in depositional dip, depositional strike and vertical directions were determined by extracting sub-volumes from the models at different scales, calculating values of effective permeability for each sub-volume, and identifying the sub-volume at which the values of effective permeability stabilise as the REV. The REV dimensions vary with facies type and flow direction, but are typically of order tens of centimetres to metres in size, significantly larger than a typical core plug. Having identified the REV, we analyze the effective permeabilities of the different facies types. Normalized values of effective permeabilities in depositional dip, strike and vertical directions (kd, ks, kv), relative to the permeability of low- and high-permeability lithologies in each facies, display a positive linear correlation with the proportion of high-permeability lithology (clay-poor sandstone) for all facies. Therefore, the proportion of clay-poor sandstone, as measured in core data, can be used to predict facies-scale effective permeability in the Bunter Sandstone Formation, as well as in analogous fluvial deposits globally.

Net-transgressive, shallow-marine sandstone reservoirs overlain by thick mudstone seals are prime candidates for storage of CO2, H2 and thermal energy. Although these reservoirs have high net-to-gross ratios, analogous outcrops demonstrate a wide range of sedimentological heterogeneities that are sampled only sparsely or at low resolution in subsurface data. We use a combination of outcrop data, sketch-based reservoir modelling and flow diagnostics to assess the impact of sedimentological heterogeneities on subsurface storage.

The Cliff House Sandstone outcrop example comprises wave-dominated shoreface sandstones arranged in aggradationally-to-retrogradationally stacked parasequences, which overlie and pass up depositional dip into mudstone-dominated coastal plain, lagoonal and tidal flat deposits that encase channelized tidal and tidally influenced fluvial sandbodies. Reservoir models of this outcrop example demonstrate that effective horizontal permeability, flow patterns and displacement, and stratigraphic trapping potential are controlled by: (1) the packaging of shoreface sandstones into laterally extensive parasequences bounded by offshore mudstones; (2) the spatial distribution, connectivity and permeability of channelized sandbodies; and (3) the localized connections between channelized sandbodies and shoreface sandstones. The last two parameters are likely to be poorly constrained in subsurface seismic and well data, and their potential effects require evaluation in reservoir modelling studies.

We describe an efficient and novel method to characterise multiscale geological heterogeneity and its effects on fluid flow using an open-source, sketch-based modelling and flow diagnostics tool (Rapid Reservoir Modelling. RRM). The method has three aims: (1) to generate a nested hierarchy of geologically accurate models of sedimentological heterogeneity at different scales; (2) to determine the representative elementary volume (REV) for each heterogeneity style, in order to calculate effective properties for larger scale models; and (3) to evaluate the impact of sedimentological heterogeneity on fluid flow and trapping in the hierarchy of geological models. The sketch-based modelling approach enables the construction of multiple geometrically accurate geological models and allows us to analyse them quickly using flow diagnostics and simulation tools. We illustrate this approach using examples from Triassic fluvial sandstones of the UK (Bunter Sandstone and Sherwood Sandstone), which host groundwater and geothermal resources and are targets for carbon capture and storage (CCS).