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In-Situ Stress and Physical Property Measurements in Appalachian Site Survey Boreholes

Mark D. Zoback, Daniel Moos, Brian Coyle, Roger N. Anderson

Four shallow core holes have been drilled as part of the site survey investigations. These holes provided the opportunity to continuously sample several key rock units to be encountered in a proposed deep hole, and to make a suite of important geophysical measurements. These measurements include heat flow, in-situ stress, P-wave and S-save sonic velocity, electrical resistivity, fracture density and orientation, and natural gamma radiation. The in-situ stress measurements confirmed that this area is subjected to the same northeast-southwest-directed compressive stress field that characterizes the central and eastern United States. Thus, stress measurements in the proposed deep hole should test a stress field that is characteristic of a broad region in the southeast. The i -situ velocity, resistivity, and natural gamma measurements can be combined with similar measurements on core samples at elevated pressures to estimate rock properties at depth, as well as to calibrate these measurements in the proposed hole. Sonic velocity measurements in the shallow holes and on the core provide additional velocity control for interpreting seismic reflection profiles. Data on the distribution and orientation of natural fractures have important consequences for anticipating deep drilling conditions. For example, one hole is sited in the Brevard shear zone; however, relatively few macroscopic fractures were encountered in the hole, and the quality of the mylonitized rock exhumed from the well is excellent. Therefore, no drilling problems are expected in the proposed deep hole.

A hole drilled in the Henderson Gneiss was highly fractured and eventually abandoned. However, on the basis of fracture data in the other three wells, we believe the dense fracturing found in the Henderson Gneiss hole represents a local condition that should not affect the proposed deep hole if it is properly sited.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.