Performance of hydraulic fractures highly depends on formation elastic properties. Empirical rock-physics formulations, developed for the assessment of elastic properties, might not be reliable in organic-shale formations as the result of heterogeneity and anisotropy in these formations. Several models based on effective medium theories have been used to assess elastic properties of organic-shale formations using well logs. Their reliability, however, is still questionable in anisotropic shale formations. Furthermore, many of the existing methods require calibration against core measurements, which can introduce additional uncertainty in the assessment of elastic rock properties.
In this paper, we applied different empirical rock-physics equations and effective medium theories to estimate elastic properties in the Haynesville and the Cana-Woodford shale formations. The inputs to the models include:
acoustic measurements,
well-log-based estimates or laboratory measurements of petrophysical and compositional properties of the formation,
parameters quantifying grain/pore shapes, and
elastic properties of rock constituents.
Next, we performed tri-axial laboratory experiments at different stress levels to measure elastic properties and acoustic-wave velocities in different rock types for both field examples. We then compared the estimates of elastic properties obtained from different methods against core measurements and investigated the reliability of different models.
We showed that the elastic properties obtained from acoustic-wave velocities using conventional empirical techniques are not always in agreement with core measurement. We found the estimates from inclusion-based effective medium theories the most reliable representatives for elastic properties of organic-shale formations. However, the uncertainty in the shape of inclusions still remains a challenge in these techniques. The outcomes of this paper can potentially be a guide for selection of a reliable model for well-log-based assessment of effective elastic moduli in organic-shale formations, which contributes significantly to detecting zones for successful fracture treatment and production.
