ABSTRACT

INTRODUCTION

The Underground Research Laboratory (URL), near Pinawa, Manitoba has been excavated in granitic rock belonging to the Lac du Bonnet batholith, one of many intrusions of the Canadian Shield. The batholith is 75x25 km in surface area and extends downward about 10 km. Its age has been estimated at 2680 Ma. Two major thrust faults dipping 25-30 ΓΈ to southeast have been encountered at URL, Fracture Zone 3 at about 100 m depth, and Fracture Zone 2 at about 280 m (Figure 1). Fracture Zone 2 in particular appears to be the boundary between two separate stress fields. Based on previous stress measurements using a variety of techniques, such as USBM overcoting, microseismic, convergence, and hydrofracing there are strong indications that the principal horizontal stress magnitudes increase considerably below Fracture Zone 2, and that their directions rotate with respect to those established at the higher elevations (Figure 1). However, no measurements have been conducted below the 420 level. We carried out a total of 14 hydraulic fracturing tests and 9 oriented-packer impressions in borehole HF1, a 100 m vertical N-size hole drilled downward from the floor of 420 level, URL. Details of the procedures, results, interpretations, and stress calculations are presented in the following chapters.

TESTING EQUIPMENT AND PROCEDURE

The equipment used in these hydraulic fracturing measurements was especially prepared for the very high breakdown pressures anticipated. Previous tests were on occasion unsuccessful in fracturing the rock below Fracture Zone 2 due to equipment pressure limitations (typically 0-60 MPa). The hydraulic system used in the present tests was upgraded so that it can be operated safely at pressures reaching 100 MPa. Downhole pressure transmitters were placed in a sealed housing on top of the straddle packer for the purpose of downhole monitoring of test-interval and packer pressures. During testing, the downhole packer and test-interval pressures and the flow rate were continuously monitored and recorded simultaneously on a 3-channel strip chart analog recorder, a data tape recorder, and a microprocessor (PC) via an analog-to-digital converter. Testing procedures were similar to those described in earlier publication by Haimson. We used an impression packer orienting tool to obtain an oriented trace of the induced hydraulic fracture on the test-hole wall. Fracture traces on the impression packer were marked with respecto a scribe line on a transparent sheet for later detailed analysis.

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