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  • Publication
    Accès libre
    Hydraulic testing of low-permeability formations: A case study in the granite of Cadalso de los Vidrios, Spain
    (2009)
    Mejías, Miguel
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    In recent years, the study of low-permeability geological formations has undergone considerable development. This is mainly due to the use of natural geological barriers to confine waste disposals, preventing leaking water from bringing contaminants into contact with the biosphere and the groundwater resources.
    In that context, the Spanish Geological Survey (IGME, Spanish acronym), supported by the National Radioactive Waste Management Agency (ENRESA, Spanish acronym) and with the technical advice of the Swedish Nuclear Fuel and Waste Management Company (SKB) designed and built a Mobile Hydrogeological Unit (UMH, Spanish acronym) for low-permeability formations characterization, which has been operational since 1997 and has been used for different purposes. Among other possibilities, the UMH allows conducting: constant transient-state flow-rate injection tests, constant-head transient-state injection tests, pseudo-stationary state injection tests, pressure fall-off tests, slug tests and pulse tests.
    The main objectives of this article are to describe the hydraulic characterization methodology used by IGME to carry out hydraulic tests in low permeability environment, to compare different testing methods and to summarize the results that have been obtained when characterizing the leucogranites of Cadalso de los Vidrios in Spain. The study area presents an increasing interest for granite production inside the Community of Madrid. The petrological and structural characteristics of the granite rocks and the core-samples extracted from a 200 m deep borehole investigation are described. The packer tests are conducted with the Mobile Hydrogeological Unit.
    The tests are interpreted with the help of analytical solutions. The main software used is Hytool, an open source matlab toolbox that provides a library of analytical solutions and a set of routines to facilitate hydraulic tests interpretation.
    The results allow the elaboration of a comparative analysis of the applied hydraulic tests and to define the hydraulic conductivity optimum application interval most suited for each of the used methods.
    Hydraulic conductivity values obtained varies from 3.20•10−7 m/s, for the upper weathered layer, to 2.80•10−12 m/s, for the test section from 97.36 to 116.15 m depth.
    Finally, the hydraulic conductivity values obtained in this area are compared with other case studies of granite formations around the world.
  • Publication
    Accès libre
    Understanding diagnostic plots for well-test interpretation
    (2009) ; ;
    Mejias, Miguel
    Dans le domaine de l’interprétation des essais de pompage, un graphe de diagnostic est un graphe simultané du rabattement et de sa dérivée logarithmique en fonction du temps. Ce graphe est habituellement représenté en échelle bi-logarithmique. Les avantages principaux et les limites de cet outil sont discutés à l’aide de trois exemples hydrogéologiques réels. Des recommandations pratiques sont proposées afin d’utiliser ces graphes pour sélectionner un modèle conceptuel d’écoulement. Lorsque les mesures de rabattement sont bruitées, le calcul de la dérivée logarithmique présente des artefacts que l’on peut minimiser en échantillonnant le signal. Lorsque le débit de pompage varie, ou lors d’un test de récupération, les graphes de diagnostic peuvent aussi être utilisé à condition d’effectuer une déconvolution des données au préalable. Enfin, les effets d’une erreur d’estimation du temps de début d’un test sont discutés. Tous ces exemples montrent que les graphes de diagnostic constituent un outil extrêmement utile pour l’interprétation des essais hydrauliques., In well-test analysis, a diagnostic plot is a scatter plot of both drawdown and its logarithmic derivative versus time. It is usually plotted in log–log scale. The main advantages and limitations of the method are reviewed with the help of three hydrogeological field examples. Guidelines are provided for the selection of an appropriate conceptual model from a qualitative analysis of the log-derivative. It is shown how the noise on the drawdown measurements is amplified by the calculation of the derivative and it is proposed to sample the signal in order to minimize this effect. When the discharge rates are varying, or when recovery data have to be interpreted, the diagnostic plot can be used, provided that the data are pre-processed by a deconvolution technique. The effect of time shift errors is also discussed. All these examples show that diagnostic plots have some limitations but they are extremely helpful because they provide a unified approach for well-test interpretation and are applicable in a wide range of situations., En los análisis de ensayos de pozos, un gráfico de diagnóstico es un gráfico de dispersión de las depresiones y de sus derivadas logarítmica en función del tiempo. Usualmente se grafica en una escala log – log. Las principales ventajas y limitaciones del método se revisaron con la ayuda de tres ejemplos hidrogeológicos de campo. Se proveen pautas para la selección de un modelo conceptual apropiado a partir de un análisis cualitativo de la derivada logarítmica. Se muestra cómo el ruido en las mediciones de las depresiones resulta amplificado por el cálculo de las derivadas y se propone muestrear la señal con el objeto de minimizar este efecto. Cuando los ritmos de descarga varían, o cuando los datos de recuperación deben ser interpretados, el gráfico diagnóstico puede ser utilizado siempre que los datos sean pre-procesados por una técnica de deconvolución. Asimismo se discute el efecto de los errores de desplazamiento del tiempo. Todos estos ejemplos muestran que el gráfico diagnóstico tiene algunas limitaciones pero son extremadamente útiles porque proveen un enfoque unificado para la interpretación de ensayos de pozos y son aplicables en una amplia gama de situaciones., Nel dominio dell’interpretazione delle prove di pompaggio, un grafico diagnostico è un grafico simultaneo dell’abbassamento e della sua derivata logaritmica in funzione del tempo. Questo grafico è solitamente rappresentato in scala bi-logaritmica. I principali vantaggi ed i limiti di questo strumento sono discussi attraverso tre esempi idrogeologici reali. Delle raccomandazioni pratiche sono proposte al fine di utilizzare questi grafici per scegliere un modello concettuale di flusso. Quando le misure di abbassamento contengono del rumore, il calcolo della derivata logaritmica presenta degli artefatti che sono riducibili campionando il segnale. Quando la portata del pompaggio varia, o durante un test di recupero, i grafici diagnostici possono anche essere utilizzati a condizione di effettuare una preliminare deconvoluzione dei dati. Infine, gli effetti di un errore di stima del tempo di inizio di un test sono discussi. Tutti gli esempi mostrano che i grafici di diagnosi costituiscono uno strumento estremamente utile per l’interpretazione delle prove idrauliche.