The effect of stress-relief on ambient microcrack porosity in core samples from the Kent Cliffs (New York) and Moodus (Connecticut) scientific research boreholes

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.