Date of Award

Fall 1993

Document Type

Thesis

Degree Name

Master of Science in Environmental Engineering - (M.S.)

Department

Civil and Environmental Engineering

First Advisor

John R. Schuring

Second Advisor

Paul C. Chan

Third Advisor

Earl David Shaw

Abstract

This thesis investigates the flow of compressible fluids in pneumatically fractured formations. Pneumatic fracturing is a recently developed technique for increasing permeability in geologic formations by the controlled injection of high pressure air. The artificially induced fractures enhance the flow rate of liquids and vapors in the subsurface, and can be applied to in situ remediation of hazardous waste sites, and for other hydrogeological applications.

A flow model for discrete fractures is derived based on the assumptions of viscous, laminar fluid flow through planar fractures (Poiseuille type flow). The model takes into account non-linearity introduced by gas compressibility effects. Provision is also made for turbulent conditions which can result from high flow velocity and/or surface roughness of fractures. The model is presented in both linear and radial flow formats.

Model validation is accomplished by analyzing pressure and flow data from a siltstone formation which had been pneumatically fractured in the vadose zone. Air flows were observed to increase from a baseline of 0.3-0.4 SCFM before fracturing, to 4.0-71 SCFM after fracturing. Using this model, the total equivalent (single) aperture for the 8.4 ft. test zone was found to increase from 86 microns in the prefracture condition, to 516 microns in the postfracture condition. Analysis of fracture flow velocities and associated Reynolds numbers indicated that although laminar flow conditions exist in natural fracture networks, some degree of turbulence may be encountered in pneumatically induced fractures owing to aperture enlargement.

Detailed study of borehole video tapes of the fracture well indicated the principal mechanism of flow enhancement was aperture dilation of existing natural fractures, and improvement of fracture connectivity. A minor amount of new horizontal and vertical fractures were also noted. Statistical analysis of the video also indicated an improvement in flow uniformity along the borehole profile, as a result of fracturing. A physical model of the fracture zone is presented which is useful for analysis of contaminant mass transport in the formation, especially applications involving molecular diffusion, heat transfer, and biodegradation processes.

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