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Geomechanic Characteristics of the Eagle Ford Shale – A Comparative Study for Outcrop and Subsurface Formation Properties


This paper is focused on integrating the mechanical, acoustic and geological properties of the Eagle Ford shale formation in Texas, and is intended to provide clues that assist towards the optimization of stimulation jobs in the subsurface. The unconventional reservoir facies of the Eagle Ford are associated with oil production from mudstones related to transgression-regression cycles. They have produced about 2.69 BCF of gas and 0.6 MMBBLS of liquids in the first 6 months of 2013, becoming one of the most prolific shale plays in the USA in terms of production growth. Extensive studies have been conducted on subsurface characteristics along with the Boquillas outcrop belt in terms of their depositional model, mineralogy content, microfacies and mechanical properties. However, from the geomechanics perspective, there is scarce published data that relates the mechanical and acoustic properties of the outcrops with those of the subsurface Geology such as fabric, organic content and mineralogy. In this study, the lower section of the Eagle Ford, which contains the highest TOC in subsurface, is first correlated with outcrop samples from Del Rio County (Texas) through analysis of log and geological data. Triaxial experiments with simultaneous measurements of dynamic and static moduli have been performed on outcrop sample plugs and preserved cores from the subsurface, in order to identify their mechanical and acoustic distinctiveness. Thin section interpretation, CT scans, XRD, SEM and TOC measurements from the same core samples have been integrated with the geomechanical dataset in order to find the most relevant parameters affecting the mechanical properties of the lower Eagle Ford formation. Due to the similarity in their facies’ attributes and mineralogy and divergence in organic content, outcrop to subsurface comparison was used to understand the role of these properties on the brittleness of the formation.