Abstract

The determination of in-situ rock-mechanical properties is important for a variety of reservoir engineering purposes like hydraulic fracturing, estimation of removable reserves, prediction of wellbore stability and subsidence. The absence of direct measurement techniques forces the industry to rely on indirect methods for predicting mechanical properties. Measurements of ultrasonic acoustic velocities and quasi-static measurements of change in strain under increasing load were conducted on samples of highly lithified, tight, quartzose channel sandstone along a well-exposed outcrop of an asymmetric anticline in south-central Arkansas. Results from thin- section analyses were used to explain the variation of mechanical properties for samples from either side of the anticline. Presence of more microcracks in samples from the steeply dipping limb of the anticline resulted in larger discrepancy between static and dynamic elastic moduli compared to samples from the gently dipping limb of the anticline where the static and dynamic moduli more closely matched each other. The discrepancy between static and dynamic moduli was also related to confining pressure. Comparatively higher failure strength of samples from the gentle limb of the anticline has been facilitated by the distribution of grain sizes. The combination of microcracks and presence of micaceous minerals made fracture propagation easier in samples from the steep limb of the anticline and lowered the failure strength of these rocks.