Hydrogeology Journal

, Volume 15, Issue 8, pp 1629–1642 | Cite as

Flow and transport predictions during multi-borehole tests in fractured chalk using discrete fracture network models

Report

Abstract

Multi-borehole pumping and tracer tests on the 10 to 100-m scale were conducted in a fractured chalk aquitard in the Negev Desert, Israel. Outcrop and core fracture surveys, as well as slug tests in packed-off intervals, were carried out at this site to obtain the parameters needed for construction of a stochastic discrete fracture network (DFN). Calibration of stochastic DFNs directly to the multiple borehole test data was inadequate. Instead, two equivalent deterministic DFN flow models were used: the vertical-fractures (VF) model, consisting of only vertical fractures, and the fractures’ intersections (INT) model, consisting of vertical and horizontal fractures with enhanced transmissivity at their intersections. Both models were calibrated against the multi-borehole response of one pumping test and their predictions were tested against three other independent pumping tests. The average accuracies of all transient drawdown predictions of the VF and INT models were 65 and 66%, respectively. In contrast to this equality in average drawdown predictions of both models, the INT model predicted better important breakthrough curve features (e.g., first and peak arrival times), than the VF model. This result is in line with previously assumed channeled flow, derived from analytical analysis of these pumping and tracer tests.

Keywords

Fractured rocks Discrete fracture network (DFN) Numerical modeling Israel Solute transport 

Résumé

Des tests de pompage et de traçage sur plusieurs puits à l’échelle de 10 à 100 m ont été réalisés dans un aquitard crayeux fracturé du Désert du Négev, Israël. Les levés des affleurements et des fractures traversés par les carottages, ainsi que des slug tests réalisés sur des intervalles isolés par des packers, ont été réalisés sur le site pour obtenir les paramètres nécessaires à la construction d’un réseau stochastique discret de fractures (DFN, en anglais). Une calibration des DFNs stochastiques, directement sur les données des forages, était inadéquate. A la place de cette approche, deux modèles d’écoulement de DFN, équivalents et déterministes, ont été utilisés : le modèle de fractures verticales (VF) qui comme son nom l’indique ne comprend que des fractures verticales, et le modèle à intersections de fracture (INT), comprenant des fractures verticales et horizontales et des transmissivités augmentées aux intersections. Les deux modèles ont été calibrés suivant la réponse d’un test de pompage suivi sur plusieurs puits, et leurs prédictions ont été testées suivant trois autres essais de pompage. L’exactitude moyenne de toutes les prédictions transitoires de rabattement des modèles VF et INT était de 65 et 66%, respectivement. Contrairement à la coïncidence dans les prédictions de rabattement moyen des deux modèles, le modèle INT prédit mieux les caractéristiques importantes des courbes de restitution (e.g., temps de la première arrivée et du pic de concentration) comparé au modèle VF. Le résultat est consistant avec des écoulements de chenaux supposés précédemment, dérivés d’analyses analytiques de ces tests de pompage et de traçage.

Resumen

Se realizaron pruebas de bombeo en varios sondeos y pruebas con trazadores en una escala que varió de 10 a 100m en un acuitardo fracturado calcáreo en el Desierto Negev, Israel. Se llevaron a cabo en el sitio levantamientos en afloramientos y fracturas de núcleo, así como pruebas con remoción/adición rápida de agua en intervalos sin empaque, para obtener los parámetros necesarios en la construcción de una red de fracturas discreta estocástica (RFD). La calibración de RFDs estocásticas directamente con los datos de pruebas de sondeo múltiples fue inadecuada. En vez de esto, se utilizaron dos modelos de flujo RFD determinísticos equivalentes: el modelo de fracturas verticales (FV), que consiste únicamente de fracturas verticales, y el modelo de intersección de fracturas (INT), que consiste de fracturas horizontales y verticales con transmisividad realzada en sus intersecciones. Ambos modelos fueron calibrados en relación con la respuesta de una prueba de bombeo en sondeos múltiples y sus pronósticos fueron evaluados en relación a otras tres pruebas de bombeo independientes. Las imprecisiones promedio de todas las predicciones transitorias de descenso de los modelos INT y FV fueron 65 y 66%, respectivamente. En contraste con esta igualdad en las predicciones promedio de descenso de ambos modelos, el modelo INT pronosticó mejor las características de curvas de avance importantes (por ejemplo, tiempos de llegada inicial y tiempos pico) que el modelo FV. Este modelo es congruente con el flujo de canales que se asumió previamente derivado del análisis analítico de las pruebas con trazadores y pruebas de bombeo.

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  1. 1.Bureau of Economic Geology, Jackson School of GeosciencesThe University of Texas at AustinAustinUSA
  2. 2.The Seagram Center for Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality SciencesThe Hebrew University of JerusalemRehovotIsrael
  3. 3.The J. Blaustein Institutes for Desert Research, Zukerberg Institute for Water Research and Department of Environmental Geological SciencesBen-Gurion University of the NegevNegevIsrael

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