The goal of this study was to compare constitutive parameters derived from short-term (four hour) and long-term (several-week) creep experiments. We conducted a series of creep experiments on clay and carbonate-rich shale samples from unconventional gas and oil reservoirs at room temperature, principally on samples from the Haynesville and Eagle Ford formations. Samples with different mineralogies were subjected to a series of multi-stage loading/creep, unloading/recovery cycles conducted over different time spans. All creep stages of the experiments were performed at a constant differential stress level; only the testing time of each creep/recovery stage was varied. Following Sone and Zoback (Jour. Petrol. Sci. and Eng., 2014), a power-law creep model was used to obtain the creep constitutive parameters. Results of these experiments show that the shale samples follow the same creep trend through time, regardless of the loading history. Also, we show that the simple power-law model is capable of describing creep over multiple time periods. Using this model, we are able to characterize viscoplastic behavior of shale rocks from relatively short-term (1 day) creep experiments.
A common challenge in laboratory creep studies is to know how long laboratory experiments need to be carried out to accurately measure the parameters that make it possible to predict rock deformation over relatively long periods of time. This time-dependent deformation affects mechanical and flow properties of sedimentary rocks [1, 2, 3, 4, 5, 6]. Initial laboratory studies of viscoplatic deformation were carried out via uniaxial and triaxial creep experiments [7, 8, 9, 10]. Because of the time-consuming nature of these experiments, relatively few papers have addressed this topic.
In this paper we carried out a series of creep experiments on shale samples with various amounts of carbonate and clay to investigate the constitutive law and creep parameters obtained in loading steps ranging from several hours to several weeks. We extend the comprehensive short-term (several hour) creep experiments reported by Sone and Zoback (2014) who investigated the role of clay and organic content on viscous deformation of shales from unconventional gas reservoirs . They used a relatively simple (two parameter) power-law model to fit the data.
Several mechanisms have been proposed to explain low temperature creep in sedimentary rocks. Although Peterson and Wong (2004) argue that the deformation of rocks at temperatures below 1200 K can generally be considered brittle , other studies have shown that it is possible to observe inelasic deformation of rocks at lower temperatures due to mechanical compaction processes . For example, Sone and Zoback (2014) argued that the time-dependent deformation they observed was accommodated by changes in the pore volume.