Elastic and creep behavior of fractured shale caprocks in the presence of argon and CO2
A B S T R A C T
A series of triaxial stress, undrained creep tests is reported for fractured/micro-cracked shale specimens from the Eagle Ford formation using argon and CO2 as the pore fluids. Each creep test was composed of a 2-day “loading” creep and a 1-day “recovery” creep stage. Hysteresis cycles and ultrasonic velocities were collected at the beginning and end of each “loading” creep stage. In addition, we conducted a 7-day CO2-injection test on one of the samples and investigated the hysteresis behavior every 24 h. No significant changes in hysteresis were observed after day 2 of the longer-term CO2 creep test, however, the Young’s modulus during unloading (upon concluding the creep test) was notably greater compared to the unloading Young’s modulus of the 2-day CO2 test. We further implemented a conceptual creep model to decompose the elastic, plastic, viscoelastic, and viscoplastic deformations during/after creep tests. Power-law and Burger’s creep models were used to predict the longer-term behavior of these specimens. While fractures/micro-cracks were abundant, the samples exhibited moderate-to-large initial Young’s modulus and low-to-moderate initial creep deformation compared to typical shale rocks. We observed smaller Young’s modulus for the CO2 tests, as confirmed by a greater compliance parameter B in the power-law model and lower E1 parameter in Burger’s model. Creep strain exhibited positive and negative correlations with the power-law exponent, n, and parameter η1 from Burger’s model, respectively. Larger viscoelastic deformations were observed during CO2 tests, as confirmed by the larger n and smaller η1 values, respectively. Lastly, the dynamic Poisson’s ratios (obtained from ultrasonic velocities) were significantly larger than their static counterparts, while the difference in dynamic Young’s moduli during loading and unloading was negligible.