Anomalous Elastic Behavior in Berea Sandstone
Davis, Eric Sean 1991-
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Berea sandstone is a well-studied oilfield reservoir material that is commonly used as a reference material when the exact constituents of a reservoir are unknown. Although it has been extensively studied, there are still several unanswered questions that remain due to the complexity of the mechanical system, being a network of hard grains cemented together with a soft bonding system. In particular, the origin of the nonlinear and nonequilibrium dynamics in Berea is still poorly understood. In this dissertation, the anomalous elastic behavior of Berea sandstone in which the sandstone softens with cooling for a set temperature range is explored using multiple experiment sets to shed light on the possible mechanisms involved. By using a combination of Resonant Ultrasound Spectroscopy (RUS), Through-Transmission technique (TT), and resonance tracking, several qualitative and quantitative aspects of this anomalous elastic behavior are detailed. Specifically, it was found that the bulk modulus of Berea sandstone decreases with cooling from 205 °C to 110 °C and then increases an almost identical amount with additional cooling from 110 °C to room temperature. Resonance tracking determined that equilibration time did not seem to affect the anomalous elastic behavior with cooling. Higher starting temperatures did not affect the critical temperature in which the anomalous behavior returned to normal behavior with cooling. The anomalous behavior was only found in Berea sandstone and Buff Berea, whereas several other reservoir materials exhibited normal behavior. Normal elastic behavior was also found in fused silica, making it less likely that the anomalous elastic behavior originates from the constituents of Berea sandstone. The anomalous behavior did not extend to heating in Berea sandstone, causing a hysteresis loop in the elastic constants when heated and then cooled. Finally, the anomalous behavior was seen in both heating and cooling for Buff Berea, however, and the qualitative behavior for both heating and cooling was near identical.