ABSTRACT

Abstract:

A recent set of hydraulic fracturing stress measurements in a steeply inclined borehole in Northern South Carolina, has revealed some important facts about the reliability of the method in non-vertical holes yielding non-vertical fractures, and about in situ stress magnitudes at shallow depths (100-250 m). Fourteen hydraulic fracturing tests conducted in the 250 m borehole revealed a state of high horizontal stresses with the maximum stress direction at N66øE. This state of stress was verified by two independent methods of data analysis, and by comparing it with previous results in the vicinity of the test hole as well as in the entire tectonic region. Several unusual test results which may have considerable significance in design are discussed.

INTRODUCTION

A series of hydraulic fracturing (HF) in situ stress measurements were recently undertaken as part of a site feasibility study just south of the North Carolina border and about 50 km north of Seneca, in Oconee County, northwestern South Carolina, in conjunction with a planned pumped-storage underground powerhouse (Figure 1). The test hole is about 8 km ENE of the site of previous I-IF measurements in conjunction with the Bad Creek pumped storage project (Haimson, 1977). The location of the present hole in the Tallulah Falls Formation is only 1 km west of the northeast-striking Brevard Fault Zone, a dominant structure that has been interpreted as either a strike-slip or a thrust fault localized in the weak rocks of the Chauga Belt (Figure

1).

A total of fourteen complete tests (fracturing and packer impression) were carried out in a 250 m N-size (76 mm) hole drilled just above the planned cavern. The stratigraphic column of the drillhole consists of schists and gneiss belonging to the Tallulah Falls Formation. The core shows substantial schistosity, but very few joints.

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