Erik Svenson

Defense Talk

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A Method for Measuring Transient Head and Aperture Changes During Air-Slug Tests: Theory and Application

MS Thesis

Introduction

Understanding the hydraulic properties and boundary conditions that control the movement and storage of fluids within an aquifer is of great importance to the management of groundwater resources (Witherspoon and Gale, 1983; Wang et al., 1977). Among the various well testing techniques, slug tests are perhaps the most commonly used method for characterizing hydraulic properties in fractured rock aquifers (Karasaki et al., 1988; Bredehoeft and Papadopulos, 1980; Black, 1985; Barker and Black, 1983). Investigators often prefer these tests over other hydraulic tests (e.g., pumping and injection tests), because they are relatively inexpensive, easy to perform, and require modest equipment. The basic slug test procedure involves changing the total head in the wellbore suddenly and measuring the falling or rising head as a function of time until an equilibrium state is reached in the aquifer ( Butler, 1998). Interpretation typically involves matching the transient head data to type curves generated from analytical solutions (e.g., Cooper et al., 1967; Karasaki, 1986; Barker and Black, 1983; Black, 1985) in order to estimate values of transmissivity and storativity. These data are often incorporated into numerical models of flow and transport to evaluate and predict groundwater movement and contaminant migration within the subsurface.

However, the transient pressure signal produced by a slug test is weakly sensitive to storativity, so the standard slug test is best suited to obtain flow properties (Butler, 1998; Murdoch and Germanovich, 2006). Interpreting features of a fracture network (e.g., leakage and large-scale heterogeneities) from a slug test can also be challenging because a variety of formation parameters can produce similar pressure signals (Karasaki, 1988; Shapiro and Hsieh, 1998). As a result of this ambiguity, obtaining reliable estimates of such parameters from a slug test can be difficult (Shapiro and Hsieh, 1998).

Previous investigations (Rutqvist, 1995; Rutqvist et al., 1998) have revealed that fractures deform when the fluid pressure within them changes and measuring this deformation along with head changes during a well test holds the potential to markedly increase the accuracy of estimating such properties as storativity. Well testing methods using precision borehole extensometers have made it possible to measure fracture deformation and head changes during well tests to determine formation properties, although applications of these tests are rare (Gale, 1975; Thompson and Kozak, 1991; Martin et al., 1990; Hesler et al., 1990). Those studies that have measured fracture deformation during a well test have shown that estimates of storativity, transmissivity, and normal stiffness can be inferred from these measurements.

This thesis describes a new in situ slug test method that makes use of a borehole extensometer to measure head and aperture changes of gently dipping fractures during an air-slug test conducted between a straddle-packer. These measurements offer a means of directly measuring the vertical stiffness of individual fractures, as well as obtaining estimates of local transmissivity and storativity within an aquifer. The aperture displacement and wellbore head measurements, moreover, can be used to identify fracture network features away from a single borehole that would otherwise be difficult to distinguish from head data alone using conventional slug test methods. Repeating these tests at different locations in a borehole using inflatable packers provides a means of producing vertical profiles of these parameters within an aquifer.

A theoretical analysis designed to simulate flow and deformation of an idealized, flat-lying fracture intersected by a borehole was used to interpret formation properties from the transient head and aperture data. Graphical techniques based on the theoretical analysis were used to obtain preliminary estimates of local transmissivity, storativity, and normal stiffness from the field data. Thereafter, a numerical model was inverted to obtain estimates of formation heterogeneities away from the borehole.

Objective

The objective of this thesis was to develop the field methods for measuring transient head and aperture changes during an air-slug test and then analyze those measurements with the theoretical analysis to obtain estimates of formation properties. These methods were applied in a borehole that penetrates fractured crystalline rock at a site in Clemson, South Carolina. The purpose of the investigation was to characterize the spatial distributions of storativity, transmissivity, and normal stiffness of the aquifer underlying the site, as well as to determine locations of network features away from the borehole.

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AIR-SLUG LOW-PRESSURE STRADDLE-PACKER SYSTEM TO FACILITATE CHARACTERIZATION OF FRACTURED BEDROCK

Abstract. Straddle-packer tests are used for determining aquifer properties in fractured rock, but packer tests are often avoided because they can be expensive. We have developed a simple, inexpensive, lightweight air-slug straddle packer system that can measure transmissivity as well as head variations along boreholes.

The air-slug packer system was used at a site in Clemson, South Carolina to evaluate the transmissivity and head distributions in a borehole. The results show that transmissivity in the aquifer range from 10 -7 to 10 -4m 2/s. Transmissivity distributions show three permeable intervals (T≈10 -4m 2/s) at depths of 24, 34, and 50m separated by relatively low permeability material (T ≈ 10 -7 m/s). Upward vertical head gradients of 0.013 and 0.0016 were measured in the well. Similar results were obtained with standard, high-pressure packers.

This paper appeared in the 2005 Georgia Water Resources Conference. Click here for the full paper.

Last Updated: August 28, 2007 -- Questions or comments, contact Larry Murdoch.
School of Environment, Department of Geological Sciences
340 Brackett Hall, Clemson, SC 29634  USA -- Telephone: (864) 656-3438, Fax: (864) 656-1041
© 2004-2005 Clemson Univeristy, Clemson, SC, USA. All rights reserved.