Elsevier

Journal of Contaminant Hydrology

Volume 221, February 2019, Pages 82-97
Journal of Contaminant Hydrology

Experimental and numerical investigations on the effect of fracture geometry and fracture aperture distribution on flow and solute transport in natural fractures

https://doi.org/10.1016/j.jconhyd.2018.11.008Get rights and content

Highlights

  • Conservative solute transport in single fractures for multiple dipole configurations

  • Good agreement between the experiments and complex spatial models

  • Simpler models fail to describe the experimental breakthrough curves

  • Clear impact of fracture geometry on flow and solute transport

  • Necessity to incorporate complex flow fields in transport models

Abstract

The impact of fracture geometry and aperture distribution on fluid movement and on non-reactive solute transport was investigated experimentally and numerically in single fractures. For this purpose a hydrothermally altered and an unaltered granite drill core with axial fractures were investigated. Using three injection and three extraction locations at top and bottom of the fractured cores, different dipole flow fields were examined. The conservative tracer (Amino-G) breakthrough curves were measured using fluorescence spectroscopy. Based on 3-D digital data obtained by micro-computed tomography 2.5-D numerical models were generated for both fractures by mapping the measured aperture distributions to the 2-D fracture geometries (x-y plane). Fluid flow and tracer transport were simulated using COMSOL Multiphysics®.

By means of numerical simulations and tomographic imaging experimentally observed breakthrough curves can be understood and qualitatively reproduced. The experiments and simulations suggest that fluid flow in the altered fracture is governed by the 2-D fracture geometry in the x-y plane, while fluid flow in the unaltered fracture seems to be controlled by the aperture distribution. Moreover, we demonstrate that in our case simplified parallel-plate models fail to describe the experimental findings and that pronounced tailings can be attributed to complex internal heterogeneities. The results presented, implicate the necessity to incorporate complex domain geometries governing fluid flow and mass transport into transport modeling.

Introduction

Fractures or shear zones are dominant structures for fluid flow and mass transport in crystalline rock of low or negligible matrix permeability. A full comprehension and prediction of fluid flow and coupled mass transport is of paramount importance for many applications of geoengineering ranging from low flow rates, as expected in the nuclear waste disposal far-field (Geckeis et al., 2004; Vilks and Bachinski, 1996), to high flow rates, e.g., in geothermal systems (Schill et al., 2017). In this respect, the quantification of fluid flow in fractures is crucial and was extensively studied for decades (Bodin et al., 2003; Brown, 1987; Sahimi, 2011; Tsang, 1984; Witherspoon et al., 1980; Zimmerman et al., 2004).

In general, mass (solute and particulate) transport through fractured media is accompanied by hydrodynamic dispersion (Roux et al., 1998) including (i) Taylor dispersion, due to the parabolic velocity profile, induced by friction between two no-slip fracture walls; (ii) macrodispersion, due to spatial heterogeneities resulting in flow channeling (Bodin et al., 2003); and (iii) molecular diffusion due to a developing concentration gradient. Natural rock fractures possess complex geometries with rough and irregular fracture surfaces (asperities), highly variable aperture distributions and contact areas (asperity contacts) of different sizes between the opposite surfaces (Brown et al., 1998; Durham et al., 2001; Hakami and Larsson, 1996). Fracture and fracture network solute transport is directly affected by spatial heterogeneities inducing complex flow fields. Flow channeling is a typical fracture flow feature, which is provoked by strongly pronounced aperture distribution variabilities resulting in preferential flow paths of lower hydraulic resistance (Bodin et al., 2003; Huber et al., 2012; Keller et al., 1999; Moreno et al., 1988; Neretnieks et al., 1982; Tsang and Neretnieks, 1998). With respect to the entire fracture, elevated flow velocities prevail in those regions (Brown et al., 1998). In close proximity to these channels, depending on the geometry and given hydraulics, there are often hardly accessible areas for the main flow (low flow zones), where features like recirculation zones may form (Boutt et al., 2006). Solutes or particles may reach those areas by hydrodynamic dispersion and/or molecular diffusion. The increased residence time of solutes and colloids, and therefore, increased time scales for chemical and physical interaction with the fracture surface, may then lead to higher retardation and retention (e.g. sorption kinetics, reduction kinetics or matrix diffusion). In consequence, the residence time distributions in systems described above show much more pronounced tailings.

Numerous analytical and numerical approaches are postulated to describe mass transport in single fractures and fracture networks. A very simplified and elementary model of a single fracture is a smooth parallel-plate simplification with a constant aperture (Louis, 1969; Oda, 1986; Parsons, 1966; Snow, 1965). In this case, the hydraulic fracture conductivity (volumetric flow) is a cubic function of the aperture (Snow, 1965; Witherspoon et al., 1980). Neglecting rough fracture surfaces and tortuous flow paths, these geometrically simplified models cannot describe the flow dynamics (Brown, 1987; Sisavath et al., 2003) and overestimate fracture flow rates (Konzuk and Kueper, 2004; Nicholl et al., 1999). Consequently, this parallel-plate approach is rather applicable for smooth and wide fractures.

In order to simulate precisely the fluid flow in rough and heterogeneous fracture geometries, complex equations, such as the non-linear Navier-Stokes equations are used, which fully describes the motion of Newtonian fluids in a continuum media (Batchelor, 1967; Wilkes and Bike, 1999). Since high computational and numerical efforts are necessary to solve the Navier-Stokes equations, numerous simplified assumption, e.g. the local cubic law (Zimmerman and Yeo, 2000) or the Stokes equation (Zimmerman et al., 1991), are often applied to describe the fluid flow in fractured media. But such simplifications are only valid under certain criteria and requirements, which require an explicit knowledge of acceptable limitations, which are not always easy to determine (Brush and Thomson, 2003).

The isolated impact of fracture geometry and aperture distribution is less well investigated and a distinction between the contribution of hydrodynamic dispersion and diffusion to low-velocity zones is challenging (Cardenas et al., 2007). Due to this lack of information, tailing effects in the residence time distribution are often attributed to reactive interaction with the fracture surface or to matrix diffusion. However, effects of fracture geometry and hydrodynamic condition were found to be of influence and even more important with increasing Reynolds number, e.g., in 2-D numerical modeling of fluid flow and mass transport in a CT-scanned fracture (Cardenas et al., 2007). Here, velocity contrasts are generated by geometry-triggered features like eddies. Boutt et al. (2006) inferred the trapping of colloids in intra-fracture recirculation zones from 2-D numerical simulations for an aperture of 3.5 mm, with the upper fracture surface being rough while the lower side was considered flat. 3-D models indicated that flow heterogeneity and additional dispersion are generated by asperity contacts and surface roughness, even for small Reynolds numbers (Re = 0.001) (Zou et al., 2017).

However, direct comparison of experimental data and numerical simulations of the same setting (e.g. Huber et al., 2012; Tenchine and Gouze, 2005) are rare, which creates a lack in benchmarking of these numerical simulations. A crucial step to tackle this is high-resolution scanning of the fracture geometry. In particular, for laboratory experiments, tomographic methods such as computed tomography (μ-CT) represents a non-destructive way to image the internal, often complex structure of fractured media (Keller, 1998) and can be implemented in 2-D or 3-D numerical models (Huber et al., 2012; Petchsingto and Karpyn, 2009).

Due to the lack of direct comparison, in the present study, we investigate the impact of fracture geometry in the x-y plane and of aperture distribution on solute transport experimentally and numerically. For this purpose, a hydrothermally altered and an unaltered natural granitic fracture were utilized. Due to the alteration process, the surface and fracture features (e.g. roughness and aperture distribution) were expected to differ significantly between both drill cores (Sausse, 2002). In order to induce different dipole flow fields and to investigate the impact of spatially variable apertures on flow and solute transport, the injection and extraction locations of the conservative fluorescent solute tracer Amino-G were systematically varied for two flow rates, 24 or 12 mL/h. μ-CT scans of both fractures provided the basis for 2.5-D numerical models on fluid flow and solute transport to which the experimental results were compared. Here the term 2.5-D describes a 2-D model geometry in the x-y plane, which was extended by the aperture distribution in z-direction via the mapping of an interpolation function to the 2-D geometry. Therefore, the aperture distribution was not part of the geometry itself, rather it was considered hydrodynamically by resistance forces in the fluid flow simulation. Moreover, an additional model including matrix diffusion was generated to investigate any matrix diffusion effects on the residence time behavior.

Section snippets

Fractured drill core samples and flow cell preparation

Both fractures were sampled from drill cores of the well EPS-1 at the Enhanced Geothermal System (EGS) reference site at Soultz-sous-Forêts, France, which is located about 70 km north of Strasbourg at the NNE-SSW trending western boundary of Upper Rhine Graben. At Soultz-sous-Forêts, three naturally fractured, granitic reservoirs with permeabilities of about 3E–17 to 3E–16 m2 (Schill et al., 2017) and locally up to 3E–14 m2 (Kohl et al., 1997) occur underneath the Cenozoic and Mesozoic

Fluid flow modeling

The modelled flow regimes in both fractures with the flow rate of 24 mL/h are depicted in Fig. 5, Fig. 6. The displayed color range indicates high flow velocities in red and low flow velocities in blue. The highest flow velocities predominate in the inlet and outlet tubes, which is due to the small cross-section area of the tubing compared to the bigger cross-section area of the fracture. For better visibility of the flow field, the color range is limited to flow velocities between 0 and

Summary and conclusions

In this work, the impact of fracture geometry in the x-y plane and of aperture distribution on solute transport was investigated on two differently altered, fractured, granitic drill cores. By scanning the drill cores using μ-CT the internal structures of the fractures are accessible, showing the altered fracture to be more complex in geometry with a smaller mean aperture compared to the unaltered one. In contrast, the unaltered fracture showed to have a geometry with a minor number of asperity

Acknowledgment

MS and ES thank the HGF portfolio project “Geoenergy” under the Helmholtz topic “Geothermal Energy Systems” and FMH and TS the Helmholtz Program NUSAFE for support. The work has received partial funding by the Federal Ministry of Economics and Technology (BMWi) under the joint KIT-INE, GRS research projects “KOLLORADO-e” (02E11203B) and “KOLLORADO-e2” (02E11456A), and the European 7th Framework Program (FP7/2007-2011) under grant agreement no. 295487 (BELBaR Project). We want to thank GEIE

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