Comparison of Short-Term and Long-Term Creep Experiments in Shales and Carbonates from Unconventional Gas Reservoirs

We carried out a series of long-term creep experiments on clay- and carbonate-rich shale samples from unconventional gas reservoirs to investigate creep over both relatively short-term (4-h) and long-term (4-week) periods. Results from each set of experiments were compared to evaluate the ability to predict the long-term behavior of reservoir rocks using relatively short-term creep experiments. The triaxial deformation experiments were performed in a time-cycling pattern, which included a series of four stages of loading, creep, unloading and recovery experiments conducted over different time spans. The loading conditions (tens of MPa) reflect current reservoir conditions and were far below the strength of the samples. Experiments were conducted on both horizontal and vertical shale samples to address anisotropy introduced by the bedding. A power-law model was fitted to the creep data to predict the long-term behavior of shale samples. Regardless of the applied loading history, results of the experiments show that the shale samples follow a single trend representing their creep behavior through time. We show that the simple power-law model is capable of describing creep over multiple time periods. Additionally, the value of the creep compliance factor is consistent over different creep testing periods and it is possible to characterize the behavior of these samples from relatively short-term (1 day) creep experiments.