The physical scale modelling of braided alluvial architecture and estimation of subsurface permeability

Abstract

The quantitative modelling of fluvial reservoirs, especially in the stages of enhanced oil recovery, requires detailed three-dimensional data at both the scale of the channel belt and within-channel. Although studies from core, analogue outcrop and modern environments may partially meet these needs, they often cannot provide detail on the smaller-scale (i.e. channel-scale) heterogeneity, frequently suffer from limited three-dimensional exposure and cannot be used to examine the influence of different variables on the process–deposit relationship. Physical modelling offers a complementary technique that can address many of these quantitative requirements and holds great future potential for integration with reservoir modelling. Physical modelling provides the potential to upscale results and derive reservoir information on three-dimensional facies geometry, connectivity and permeability.\ud \ud This paper describes the development and use of physical modelling, which employs generic Froude-scaling principles, in an experimental basin that permits aggradation in order to model the morphology and subsurface depositional stratigraphy of coarse-grained braided rivers. An example is presented of a 1:50 scale model based on the braided Ashburton River, Canterbury Plains, New Zealand and the adjacent late Quaternary braided alluvium exposed in the coastal cliffs. Critically, a full, bimodal grain size distribution (20% sand and 80% gravel) was used to replicate the prototype, which allows the realistic reproduction of the surface morphology and importantly permits grain size sorting during deposition. Uncertainties associated with the compression of time, sediment mass balance and the hydrodynamics of the finest particle sizes do not appear to affect the reproducibility of stratigraphy between experimental and natural environments.\ud \ud Sectioning of the preserved sedimentary sequence in the physical model allows quantification of the geometry, shape, spatial distribution and internal sedimentary structure of the coarse- and fine-grained facies. A six-fold facies scheme is proposed for the model braided alluvium and a direct link is established between the grain size distribution and facies type: this allows permeability to be estimated for each facies, which can be mapped onto two-dimensional vertical cross-sections of the preserved stratigraphy. Results demonstrate the dominance of four facies based on permeability that range over three orders of magnitude in hydraulic conductivity. Quantification of such variability, and linkage to both vertical proportion curves for facies distribution and connectivity presents significant advantages over other methodologies and offers great potential for the modelling of heterogeneous braided river sediments at the within channel-belt scale. This paper outlines how physical models may be used to develop high-resolution, geologically-accurate, object-based reservoir simulation model

Riparian Tree Establishment on Gravel Bars: Interactions between Plant Growth Strategy and the Physical Environment

“…(1) where D 10 is the grain size, with D in millimetres (Moreton et al, 2002). This is utilized in this case as a surrogate for relative permeability of the substrate, to give an indication of the potential differences in available water between plots.…”

“…*Surface percentage of gravel and sand is based on an 8 mm diameter threshold to distinguish between the two sediment types (gravel > 8 mm, sand < 8 mm), as smaller grain sizes cannot be determined accurately from photographs. †Index of permeability based on the D 10 of the grain-size distribution (see Moreton et al, 2002). ‡Elevation is relative to the High Mixed plot, which was at the level of the vegetated surface of the nearest river island.…”

Grain sorting in the morphological active layer of a braided river physical model

“…The longitudinal shape of bars on the edge define a developed sorting pattern, as also described in Nelson et al (2010). The braided channel in the flume also showed complex and diverse grain sorting patterns of the kind seen in full-scale braided rivers, including lateral sorting at confluences, coarse deposits on bar heads at bifurcations, fine-grained lateral bars on the downstream margins of braid bars, and lateral fining in bends and on bars (see also Moreton et al, 2002;Gardner and Ashmore, 2011).…”

“…Analyses have typically focused on facies patterns and sedimentary structure, but several have mentioned that there is little vertical trend in grain sizes in braided river gravels (e.g., Bluck, 1979;Sambrook Smith, 2000;Heinz et al, 2003;Lunt and Bridge, 2004;Guerit et al, 2014;Marren, 2005;StorzPeretz and Laronne, 2013b). Similarly, physical models of aggraded braided gravel alluvium show patches and threads of distinct facies but no clear trend in grain size (e.g., Moreton et al, 2002). However, these generalities, while useful indications, are based on limited sampling and quantification of trends in particle size sorting.…”

A structure generator for modelling the initial sediment distribution of an artificial hydrologic catchment

“…More sophisticated methods like multi-point geostatistical techniques excel indeed at respecting local data and allow more realistic representations of spatial patterns, but are still dependent on the input of training images that already purport the spatial heterogeneity (Hu and Chugunova, 2008). Given the shortcomings of spatial modelling based on using "pure" geostatistics (Koltermann and Gorelick, 1996), alternative approaches have been developed in recent years: Grunwald et al (2000) implemented the Virtual Reality Modelling Language in combination with soil data, topographical attributes, and kriging to create 3-D representations of soil landscapes; Zappa et al (2006) conditioned 3-D geostatistical simulations using "model blocks" derived from soil profiles that represent facies associations to model a glaciofluvial aquifer; Moreton et al (2002) used a physical model of the subsurface depositional stratigraphy of a braided river system to feed object-based digital spatial reservoir models, while Teles et al (2001) used a multi-agent concept in combination with simple construction rules based on literature and observations to reproduce the structure of an alluvial plain. Sech et al (2009) developed a surface-based spatial modelling approach that enables explicit representation of heterogeneity across a hierarchy of length scales.…”