Investigation of the effects of fractured porous media on hydraulic tests—an experimental study at laboratory scale using single well methods

Abstract.During a risk assessment procedure as well as when dealing with cleanup and monitoring strategies, accurate predictions of solute propagation in fractured rocks are of particular importance when assessing exposure pathways through which contaminants reach receptors.Experimental data obtained under controlled conditions such as in a laboratory allow to increase the understanding of the fundamental physics of fluid flow and solute transport in fractures.In this study, laboratory hydraulic and tracer tests have been carried out on an artificially created fractured rock sample. The tests regard the analysis of the hydraulic loss and the measurement of breakthrough curves for saline tracer pulse inside a rock sample of parallelepiped shape (0.60 × 0.40 × 0.08 m). The convolution theory has been applied in order to remove the effect of the acquisition apparatus on tracer experiments.The experimental results have shown evidence of a nonDarcy relationship between flow rate and hydraulic loss that is best described by Forchheimer's law. Furthermore, in the flow experiments both inertial and viscous flow terms are not negligible.The observed experimental breakthrough curves of solute transport have been modeled by the classical onedimensional analytical solution for the advection-dispersion equation (ADE) and the single rate mobile-immobile model (MIM). The former model does not properly fit the first arrival and the tail while the latter, which recognizes the existence of mobile and immobile domains for transport, provides a very decent fit.The carried out experiments show that there exists a pronounced mobile-immobile zone interaction that cannot be neglected and that leads to a non-equilibrium behavior of solute transport. The existence of a non-Darcian flow regime has showed to influence the velocity field in that it gives rise to a delay in solute migration with respect to the predicted value assuming linear flow. Furthermore, the presence of inertial effects enhance non-equilibrium behavior. Instead, the presence of a transitional flow regime seems not to exert influence on the behavior of dispersion. The linear-type relationship found between velocity and dispersion demonstrates that for the range of imposed flow rates and for the selected path the geometrical dispersion dominates the mixing processes along the fracture network.

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“…The surface of the block sample has been sealed with transparent epoxy resin (Leven et al, 2004) (Fig. 1b).…”

Section: Methodsmentioning

confidence: 99%

Evidence of non-Darcy flow and non-Fickian transport in fractured media at laboratory scale

Cherubini

Giasi

Pastore

2013

Hydrol. Earth Syst. Sci.

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“…The surface of the block sample has been sealed with transparent epoxy resin (Leven et al, 2004). A hole of 1 cm diameter has been opened for each discontinuity in correspondence to the boundary of the block.…”

Section: Characterization and Preparation Of Block Samplementioning

confidence: 99%

Bench scale laboratory tests to analyze non-linear flow in fractured media

Cherubini

Giasi

Pastore

2012

Hydrol. Earth Syst. Sci.

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Abstract. The knowledge of flow phenomena in fractured rocks is very important for groundwater resources management in hydrogeological engineering.A critical emerging issue for fractured aquifers is the validity of the Darcian-type "local cubic law", which assumes a linear relationship between flow rate and pressure gradient to accurately describe flow patterns.Experimental data obtained under controlled conditions such as in a laboratory increase our understanding of the fundamental physics of fracture flow and allow us to investigate the presence of non-linear flow inside fractures that generates a substantial deviation from Darcy's law.In this study the presence of non-linear flow in a fractured rock formation has been analyzed at bench scale in laboratory tests. The effects of non-linearity in flow have been investigated by analyzing hydraulic tests on an artificially created fractured rock sample of parallelepiped (0.60 × 0.40 × 0.8 m) shape.The volumes of water passing through different paths across the fractured sample for various hydraulic head differences have been measured, and the results of the experiments have been reported as specific flow rate vs. head gradient. The experimental results closely match the Forchheimer equation and describe a strong inertial regime. The results of the test have been interpreted by means of numerical simulations. For each pair of ports, several steady-state simulations have been carried out varying the hydraulic head difference between the inlet and outlet ports. The estimated linear and non-linear Forchheimer coefficients have been correlated to each other and respectively to the tortuosity of the flow paths.A correlation among the linear and non-linear Forchheimer coefficients is evident. Moreover, a tortuosity factor that influences flow dynamics has been determined.

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“…Since cracks have relatively high permeabilities (Leven et al 2004;Miao et al 2004), the steady-flow method is selected here, and specimen dimensions, flux, and seepage pressure difference between input and output must be measured for calculating the permeability. The Non-Darcian permeability equation is deduced as follows.…”

Section: Experiments System and Non-darcian Approachmentioning

confidence: 99%

“…In addition, Gascoyne and Wuschke (1997) conducted a point-station survey, by collecting gas flow velocity and arrival time at fixed points, to investigate helium gas transport through a water-saturated granitic fracture zone at about 40-m deep underground. Leven et al (2004) built a single well/hole method, by measuring pneumatic discharge of pressure and flux through a pre-bored well/hole, to provide an insight into processes of air flow occurring in unsaturated fractured sandstone blocks. Of late, Davy et al (2007) have tried combining external pressure change with fracture permeability measurement.…”

Section: Introductionmentioning

confidence: 99%

Gas Permeability of Fractured Sandstone/Coal Samples under Variable Confining Pressure

2009

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Rock fractures transmit underground gas effectively once they have sufficient widths and interconnection. However, the fracture geometries needed for gas transport are strongly influenced by surrounding pressure conditions. In order to inspect and quantify the influence of surrounding pressure, we design and manufacture a set of gas flow apparatus that can be connected to the MTS815 material testing system, which provides loads and exhibits external pressures in the experiment. With the apparatus and MTS815, we test the fractured samples of sandstone and coal and obtain their relationship between permeability and external pressure. In particular, our permeability calculation based on collection of gas flux and pressure difference has involved the influence of non-Darcian flow. In addition, our study also includes a numerical simulation in the RFPA software platform to display the internal field changes of cracks. The results show that fracture permeability strongly depends on confining pressure, and a critical pressure probably occurs, about 1.5-2 MPa in our experiments, to split each of the permeability curves into two stages, a slow climb and an exponential rush. As a complement, the numerical simulation also demonstrates one more stage for the permeability curve, the post-rush steady fluctuation.

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Investigation of the effects of fractured porous media on hydraulic tests—an experimental study at laboratory scale using single well methods

Cited by 18 publications

References 23 publications

Evidence of non-Darcy flow and non-Fickian transport in fractured media at laboratory scale

Evidence of non-Darcy flow and non-Fickian transport in fractured media at laboratory scale

Bench scale laboratory tests to analyze non-linear flow in fractured media

Gas Permeability of Fractured Sandstone/Coal Samples under Variable Confining Pressure

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