Abstract

Determination of in situ tectonic stresses at depth can supply information which may be useful in predicting future movement in geologically active areas. These stresses can be determined by hydraulic fracturing and use of a new device, the deep stress probe (DSP). The DSP is hydraulically operated tool which is a combination straddle-packer and-impression packer which can determine the orientation packer which can determine the orientation (inclination) and azimuth of induced fractures.

This paper discusses results of two field tests recently conducted in the Granite Mountain Quarry near Marble Falls, Texas, to determine in situ tectonic stresses. This site had been used previously for near surface stress determination by over-coring. The experiments were performed in a 4 3/4-inch diameter hole, 1150 feet deep in granite. The borehole was fractured at various horizons, using different fracturing fluids to determine the most suitable system for future tests in hard rock. Downhole pressure readings were taken and oriented impression packers were used to determine the azimuth of the induced fractures. A downhole camera will be used to verify the fractures and to disclose any tendency of the fractures to heal.

Results of the DSP tests are compared with earlier results obtained by overcoring. The results suggest that lateral stresses at the depths tested are less than may be inferred from the surface tests.

Introduction

Determination of in-situ tectonic stresses at depth is an important aspect of various scientific and engineering problems related to underground construction and excavation. The behavior of rocks can supply information needed in the planning of underground construction, monitoring of tectonic activity, etc. It is felt that data gathered by monitoring tectonic activity in geologically active areas can be used to forecast probable periods of major disturbance. This could result in increased safety and decreased surface damage.

For the past decade, studies have been conducted at the University of Minnesota to develop a technique of measuring stresses at great depths.

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