Elastic and creep behavior of fractured shale caprocks in the presence of argon and CO2

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.