Date of Award

8-2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Mechanical Engineering

Advisor

Huey, Jr., Cecil O.

Committee Member

Biggers , Sherrill B.

Committee Member

Joseph , Paul F.

Abstract

Fractures in rock hold important stores of water and petroleum, and slight changes in fracture aperture accompanying drawdown from pumping wells play a key role in recovering these resources. Four accomplishments are described that advance insight into the behavior and characterization of fractured crystalline rock.
First was the development of two removable borehole extensometers that enable small axial displacements to be measured during hydraulic well tests. The extensometers consist of four major components: 1.) a pair of anchors; 2.) a displacement transducer 3.) a registration system, and; 4.) a temperature-compensated reference rod. One extensometer uses an axial reference rod with multiple, low-profile anchors, whereas another uses an offset reference rod with a single pair of anchors. Both designs can be readily mobilized and are capable of resolving sub-micron displacements in boreholes.
Second, hydromechanical well tests were developed using the extensometers to measure the axial displacement of borehole walls during conventional slug and constant-rate pumping tests. These displacements were dominated by changes in fracture aperture. Results from well tests in fractured gneiss near Clemson, SC, were characterized by maximum head changes up to 10m and accompanying maximum aperture changes ranging from 0.4 μm to 14.0 μm. Plotting the aperture change as a function of drawdown yielded plots having distinctly different shapes for different formation properties.
Third, a numerical model that couples elastic deformation and fluid flow in a single fracture was used to predict aperture changes and pressures resulting from well tests. Analytical solutions for pressure were used to validate the model for idealized conditions. Model predictions of displacement for idealized fractures provided fundamental insights that were used to understand the response of more complicated formations found in the field.
Fourth was the development of procedures to estimate subsurface properties. Field measurements of pressure and displacement, obtained using the extensometer, were used to infer hydraulic, mechanical, and geometric properties of the subsurface formations. Analytical methods were used to obtain initial estimates, which were then refined using an optimization software package together with the numerical model. The results indicated the characterization of hydraulic well tests in fractured rock could be improved by measuring and interpreting displacements along with pressure changes.

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