ABSTRACT:

This paper presents the results of an in-situ stress measurement campaign that employed several complementary hydraulic fracture-based stress measurement techniques at the 4100-ft level (1250 m below ground surface) of the Sanford Underground Research Facility (SURF). Multiple techniques were used to have redundancy in the measurements to minimize ambiguity of the results, and to employ techniques that can infer the complete stress tensor rather than just the minimum value. The testing protocol included fracture initiation and propagation, shut-in pressure decline, controlled flow back, and two types of re-opening tests. Despite careful planning and execution of this testing program, several of the test zones yielded rather ambiguous results that allow placing of relatively wide constraints for the principal stresses. A further complication is that, contrary to what has been observed in other nearby measurement campaigns, the induced fractures were not vertical, which complicates the stress interpretation. This results in a much more complicated near-wellbore stress state that poses an additional challenge to the interpretation of the re-opening tests. The paper will present a preliminary interpretation of these tests using an analytical borehole stress solution for arbitrary in situ stress orientations to provide an estimate of the full stress tensor and offer some observations about the relative merits of each technique employed.

1. INTRODUCTION

This paper will present the results of a stress measurement campaign on the 4100 foot level of the Sanford Underground Research Facility (SURF), which is the former Homestake mine. We will first provide a brief background of previous stress measurement campaigns at SURF, then a brief introduction to each measurement technique employed with the objective of presenting the advantages and disadvantages encountered with each during this measurement campaign.

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