Hydraulic conductivity | EPFL Graph Search

In science and engineering, hydraulic conductivity (K, in SI units of meters per second), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on the intrinsic permeability (k, unit: m2) of the material, the degree of saturation, and on the density and viscosity of the fluid. Saturated hydraulic conductivity, Ksat, describes water movement through saturated media. By definition, hydraulic conductivity is the ratio of volume flux to hydraulic gradient yielding a quantitative measure of a saturated soil's ability to transmit water when subjected to a hydraulic gradient. Methods of determination There are two broad categories of determining hydraulic conductivity: *Empirical approach by which the hydraulic conductivity is correlated to soil properties like pore size and particle size (grain size) distributions, and

Fonctionnement hydrique de différents types de placages sableux dans le sahel burkinabè

Dial Niang

The study was performed in the Sahelian part of Burkina Faso. The country is subject to difficult climatic conditions, a strong demographic growth and a continuous decrease in soil fertility. The resulting degradation affects the processes which govern ecological systems and leads, on the long run, to a modification of the ecosystems. This is particularly the case of the eolian formations, the only ecological units susceptible of being useful in the area, since they present good infiltration capacity and support the main part of the vegetation. Field measurements (rainfall, runoff, saturated hydraulic conductivity, bulk density, hydraulic conductivity function, soil water content and pressure head) have been done to asses the impacts of the environmental degradation on the hydrodynamic behavior of these formations. Within this context, an experimental device made of seven measurement sites has been installed in three contrasting zones. The results show significant differences in the hydrodynamic behavior of the soils, according to their surface properties. On one hand, the sites located on an erosion crust as well as on a drying crust in transition towards an erosion crust, are characterized by a low infiltration capacity which favors runoff. The quantity of infiltrated water is very low when compared to total precipitation. Water is stored near the soil surface (in the first 30 cm) favoring evaporation in the days following rainfall events. Thus, little water is available for plants. The resulting water stress causes a weak density, or even complete absence of plants which have difficulties to colonize such surfaces. Surface runoff is favored with runoff coefficients ranging from 50 to 80 %. On the other hand, the sites situated on drying crusts are characterized by good hydraulic conductivity and infiltration capacity. The storage of water in the root zone is more important. Water flows deeper into the soil and, occasionally, drains at the reference depth of 50 cm. The runoff coefficients were found to be lower than in the other sites (30 to 40 %). This differentiated behavior is one of the main factors affecting the formation of different landscapes in the area through the existence of a relationship between soil surface properties, the type of landscape and the ratio runoff/infiltration. The sites located on an erosion crust and those in transition towards an erosion crust behave like impluviums, while those situated on drying crusts are more favorable to infiltration. Observation of the evolution of the hydraulic properties of the soil surface shows a progressive alteration that frequently leads to strongly degraded mediums unfavorable to plants. Control and rehabilitation methods considered in the study, namely "restoration" and "mise en défens" have only partial and time-limited effects; they do not make it possible to stop durably degradation or to favor rehabilitation. Among the other measures assessed on the INERA plots, only the restitution to the soil of the beforehand mown plants causes an improvement of the soil surface properties; the restitution causes the formation of an important macroporal system favoring water infiltration and circulation in the soil. The effects of those different measures occur mainly in the first years after their introduction and concern especially vegetation regeneration, soil protection against degradation factors, improvement of hydrodynamic soil properties, reduction of runoff and erosion. They make it possible to slow down soil degradation, but not to stop it definitively. Within this context, proposals of further studies focussed on other measures to control and rehabilitate degraded areas have been suggested, taking into account the specificity of the sahelian zone, the endogenous knowledge of the agro-pastors, as well as socio-economic and ecological conditions.

EPFL2006

Transmissivity of rock fractures: Normal loading and shear reactivation

Monia Woehner

Enhanced Geothermal Systems represent a major field of study in the context of renewable energy resources. To create extractable energy from those reservoirs, a high enough fluid flow rate for production needs to be achieved. This fluid flow rate is directly related to the permeability of the fracture system in the reservoir. Hence, by increasing the reservoir’s permeability the fluid flow increases as well. For this purpose, shear stimulation has proven itself effective but remains a challenging field of study. Many aspects of this mechanism are still unknown and have to be explored further. This study focuses on the fracture transmissivity evolution of low porosity rocks (i.e. granite and marble) with smooth fracture surfaces subjected to progressing shear displacement until several millimetres of offset. The aim of this study was to record fracture transmissivities continuously throughout the experiment without stopping the mechanical loading processes and thereby using the oscillatory pore pressure method. In this context, a new experimental procedure was created making a direct shear configuration using a standard triaxial apparatus possible. Both samples were exposed to a constant confining pressure of 25 MPa which can be directly reflected as normal stress on the fracture. The granite fracture transmissivities were in the range of 6.8317·10−20 to 3.4429·10−19 m3. They first followed a decreasing trend until reaching the minimum value. Then a strong peak could be observed after which transmissivities were increasing again until the end of the experiment. Numerous peaks and deviations from the linear trend could also be observed. The marble fracture transmissivities were in the range of 1.4604 · 10−20 to 2.1110 · 10−20 m3 and were generally lower than the granite’s. Also, the transmissivity evolution followed a different path, continiously and slowly decreasing until the final position. The curve was generally more flattened and did not show any strong peaks. These results showed that rock fractures behave in different manners according to their lithology and more precisely, to the material’s hardness. Rock fractures often form wear products which can either obstruct the flow paths or enhance them as a function of the wear mechanism and wear volume. The granite sample encountered the debris formation mechanism that would transition from mild to severe wear. This transition could be observed in its fracture transmissivity passing from a decreasing to an increasing trend. On the other hand, the marble sample was subjected to a continual asperity smoothing mechanism, thereby slowly closing up the fracture and decreasing its fracture transmissivity. Within the frame of EGS, the results of this work can have interesting implications for production rates. Results showed that not only loading conditions can affect the fluid flow through rock fractures. Rock lithologies and ranges of shear displacement also play an important role and should be explored further.

2020

Contribution à l'étude du comportement thermo-hydraulique du stockage de chaleur en aquifère

Stéphane Dupasquier

In this work, a parametric study is done with computational methods for groundwater flow and heat transport in order to describe the dominant physical phenomena in an Aquifer Thermal Energy Storage (ATES). Optimum operating conditions and geological parameters are deduced from it. The intended use of the ATES is the domestic water or space heating. The stock is seasonal and functions without heat pump. Parametric study is organized as a series of reference simulations relating to the thickness of the aquifer and the injection flow rate. The other parameters and operating conditions of are kept constant. Other simulations are done and compared to the reference simulations in order to describe the influence of injection temperature, hydraulic conductivity, anisotropicity, injection frequencies, phases of rest, outflow rate and doublet wells. The cells of natural convection are strongly affecting the performances of an ATES. The thermal losses by conduction trough bedrock and caprock is an unfavourable factor as well, but to a lesser extent however. The increase of inflow rate always improves the efficiency, whereas an optimum aquifer thickness exists for each inflow rate. This optimum is characterised by weak natural convection phenomena. A greater thickness means strong density-viscosity effects, while a thinner aquifer forms a horizontally stretched stock, with increased conductive losses through the bedrock and cap rock. A maximum thermal recovery rate exists for a given flow rate. This maximum increases with inflow rate. The injection temperature behaves on the stock in the same way as the aquifer thickness for a given inflow: there is an optimal temperature. However it is in general preferable to increase the injection temperature instead of the inflow rate. In terms of optimization of the thermal power at disposal, the most favorable scenario is an increasing flow during the discharge. The precise scenario is different for each problem. The performances of a ATES are the complex result of the many parameters which enter in plays. One will recommend an inflow rate of at least 100m33/d during 6 months, and even rather 500 - 1000m3/d with an injection temperature of 80°C at least. An injection temperature of 60°C will have to be associated with an inflow of 1000m3/d at least. The geological targets proposed in Switzerland are the quaternary alluvial deposits filling the former glaciated valleys, fluvioglacial deposits and alluvial fans.

EPFL2001