Tectonophysics, 186 (1991) 163-173 Elsevier Science Publishers B.V., Amsterdam 163 The effect of stress-relief on ambient microcrack porosity in core samples from the Kent Cliffs (New York) and Moodus (Connecticut) scientific research boreholes Irene L. Meglis, T. Engelder and E.K. Graham The Pennsylvania Slate University, Department of Geosciences, University Park, PA 16802, USA (Received August 1, 1989; accepted December 21,1989) ABSTRACT Meglis, I.L., Engelder, T. and Graham, E.K., 1991. The effect of stress-relief on ambient microcrack porosity in core samples from the Kent Cliffs (New York) and Moodus (Connecticut) scientific research boreholes. In: J.-C. Mareschal (Editor), Intraplate Deformation, Neotectonics, Seismicity, and the State of Stress in Eastern North America. Tectonophysics, 186: 163- 173. As part of crustal stress studies in the northeastern U.S., two suites of core samples were examined in order to understand the effect of stress-relief on the development of microcrack porosity. Porosity and ultrasonic velocity were measured as functions of confining pressure in cores from the Kent Cliffs, N.Y., borehole, and volumetric strain and ultrasonic velocity were measured as functions of confining pressure in cores from the Moodus, Conn., borehole. Under ambient conditions, properties of the cores are dominated by a microcrack porosity which tends to increase with sample depth from values near zero to approximately 0.6%. This ambient porosity closes at laboratory confining pressures roughly equal to or less than the maximum in situ stress, and is interpreted as forming on relief of stress by drilling. The results of this study suggest that the in situ core microcrack porosity is no greater than 0.05%. The core samples are dominantly granitic gneisses, schists and amphibolites which exhibit moderate to well-developed foliations, oriented subvertically in Kent Cliffs cores and subhorizontally in Moodus cores. Foliation controls the orientation of the stress-relief microcracks, as indicated by the largest linear crack strain occurring normal to the foliation plane. This fabric-control of crack orientations precludes the use of microcrack analysis for estimating directions of maximum, intermediate and minimum in situ principal stress components. However, the general linear increase in ambient porosity with depth correlates with the increase in mean stress with depth as measured by hydraulic fracturing at the two sites. Two cores recovered from highly fractured zones at depth exhibit low stress-relief microcrack porosities which are believed to reflect locally low mean in situ stresses. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Introduction Recovery of core from deep wells initiates ex- pansion of the rock to a new volume which is stable at atmospheric pressure. This expansion, called strain relaxation, is a complex reaction of an elastic aggregate to stress-relief. Following re- laxation, the aggregate is often filled with micro- cracks which may have existed in situ prior to coring or may have nucleated and propagated upon coring. Both the new microcracks, termed stress-relief cracks, and any pre-existing cracks open upon strain relaxation to form a crack volume at atmospheric pressure called the ambient micro- crack porosity. Strain relaxation as a function of time may be divided into two parts: an instanta- neous relaxation and a time-dependent relaxation. Elastic expansion of the component minerals and the bulk of the microcrack expansion occurs rapidly, long before cores reach the surface (Brown, 1989). Time-dependent expansion, which is relatively small compared with the instanta- neous expansion, may be associated with the opening of pre-existing microcracks (Engelder and Plumb, 1984). However, if the rock is relatively free of a pre-existing microcrack and mineral fabric, then the orientations of the principal time- dependent expansions may be controlled by 0040-1951/91/$03.50 0 1991 - Elsevier Science Publishers B.V.