Despite their inadequacy, assumptions of isotropy and homogeneity are often applied to the analyses of in situ permeability tests. In this paper, we exam the effects of anisotropy and heterogeneity on Pressure Pulse Test (PPT) and Constant Head Injection Test (CHIT) by means of numerical approaches. Some new findings that offer practical considerations in properly interpreting in-situ permeability tests are presented.

Malgre leur manque de precision inherente, les assomptions d'anisotropie et d'heterogeneite sont souvent utilisees lors de test de permeabilite in situ. Dans cette etude, nous examinons les effets de l'anisotropie et de l'heterogeneite sur le test par pression pulsee et le test à injection à charge hydraulique constante par le biais de la simulation numerique. Des nouveaux resultats permettant une interpretation pratique et claire des test de permeabilite in-situ sont presentes.

Trotz ihrer inharenten Ungeeignetheit werden haufig Annahmen ueber Isotropie und Homogenitat auf Analysen von in-situ Permeabilitatstests angewendet. In diesem Papier untersuchen wir die Effekte von Anisotropie und Heterogenitat auf Durck-Impuls-Test (PPT) und Konstant-Kopf- Einspritztest (CHIT) mit numerischen Ansatzen. Einige neue Befunde, die zu praktischen Erwagungen zur richtigen Interpretation von in-situ Permeabilitatstests fuehren, werden vorgestellt.

Introduction

The safe disposal of radioactive wastes is given particular attention in all countries engaged in nuclear power generation. It is also a concern in many countries making use of radioactive materials for medical, industrial, or research purposes. The basic concept of final disposal is to bury or store the radioactive wastes into stable, deep, rock formations having low permeability. Thus hydraulic characterizations of such low permeability geological formations are of fundamental necessity, and errors in the assessment could lead to significant misinterpretation of the suitability of a potential waste facility.

In general, hydraulic characterization of a low permeability geological formation can be performed with laboratory methods by testing representative samples taken from the site under simulated stress conditions and/or with in situ permeability tests. The effects of anisotropy and heterogeneity of a rock mass may be evaluated relatively easer in laboratory studies by systematically conducting permeability tests on rock specimens cored in different directions and locations, such as those partially completed by Zhang et al.1. In field studies, however, assumptions of isotropy and homogeneity are routinely applied to the analyses of in situ permeability tests, in spite of their inherent unfitness.

In this study, we exam the effects of anisotropy and heterogeneity on Pressure Pulse Test (PPT) and Constant Head Injection Test (CHIT) that are two principal techniques currently used for evaluating the hydraulic properties of low permeability geological environments. Numerical approaches based on the Finite Element Method are adopted. The effects of anisotropy and heterogeneity on each permeability test are evaluated by

  1. first producing experimental results for each test method using an anisotropic and heterogeneous model;

  2. then back-calculating both the values of permeability and specific storage using a routinely used isotropic and homogenous model and

  3. comparing the differences between the values of permeability and specific storage back-calculated from the isotropic.

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