Estimating the hydraulic properties of the fracture network in a sandstone aquifer

Abstract

The potential hydraulic behaviour of the fracture network in a major Triassic sandstone aquifer in the UK has been evaluated. The properties of the fracture network were determined using results from detailed scan line surveys at 10 sites, television and geophysical borehole logging, and packer testing. Six sets of discontinuities common to all sites were identified and statistically characterised (dip, strike, orientation, density, size, and estimated transmissivity). A discrete fracture network model was then used stochastically to investigate the properties of the network. In general, the network is poorly connected: it is estimated that 9% of the discontinuities intersecting boreholes are transmissive. The hydraulic behaviour of the network is generally dominated by one sub-horizontal bedding plane fracture set, although when present, a relatively infrequent north-south striking, sub-vertical set modifies the bulk flow properties significantly. Ignoring this latter set, the network's minimum representative volume is about 35 x 35 x 35 m. The upscaled permeability is anisotropic, being typically 23 times greater in the horizontal than in the vertical. Tortuosity in the north-south direction is around 1.6.

Evaluation of the representative elementary volume (REV) of a fractured geothermal sandstone reservoir

“…Since both aperture distributions from field measurements and hydraulic aperture data from hydraulic test were not gathered, a constant hydraulic aperture of 200 lm was used in the current study, which is in a similar range of previously observed apertures in other sandstone formations (e.g. Steele et al 2006;Hitchmough et al 2007). However, an arbitrary constant hydraulic aperture can be applied on the acceptance of the cubic law by a scale function described in Blum et al (2007).…”

“…Since fracture systems are geometrically complex, DFN models are usually generated using statistical input data of fracture systems (e.g. Long 1983;Priest and Samaniego 1983;Min et al 2004;Blum et al 2005Blum et al , 2007Hitchmough et al 2007;Lee et al 2007). Therefore, the primary requirement is a detailed description of the characteristics and geometries of the fracture systems (e.g.…”

Characterization of a fluvial aquifer at a range of depths and scales: the Triassic St Bees Sandstone Formation, Cumbria, UK

“…Acoustic televiewer logs show that stratabound joints pervade the St Bees Sandstone aquifer; these probably arise due to the lithospheric uplift of NW Europe. Stratabound fractures typically characterize the outcropping Sherwood Sandstone Group across all the UK Triassic basins (Allen et al 1998;Hitchmough et al 2007;Wealthall et al 2001). Additionally, core logs from the deep Sellafield boreholes (Nirex 1996a, b) indicate how such stratabound fractures exist up to~1 km depth in the St Bees Sandstone aquifer.…”

“…Previous hydro-geophysical characterization studies of sandstone aquifers have mostly focused on the first 150 m below the ground surface, providing information on the effects of bedding parallel fractures and faults on water flow (Gellasch et al 2013;Lo et al 2014;Runkel et al 2006;Hitchmough et al 2007;Medici et al 2016). Several petrohydraulic studies conducted on sandstone lithotypes (including fluvial deposits) have been undertaken on successions at depths ≥ 1,000 m using data from hydrocarbon extraction fields (e.g., Baas et al 2007;Corbett et al 2012;Zheng et al 2000).…”

Random neighborhood graphs as models of fracture networks on rocks: Structural and dynamical analysis

“…It has been well documented that such inhomogeneities in the properties of the systems to be modeled play a major role in the diffusion processes taking place, which are quite similar to anomalous diffusion in disordered media. Then, it is normal to model the geostatistics of these reservoirs by a fractional Brownian motion and fractional Lévy motion Hewett (1986);Silva et al (2007).…”