LAUSR

It has been acknowledged that the cores extracted from underground are not always fully representative of the in situ material (e.g., Santarelli and Dusseault 1991; Warpinski and Teufel 1992; Ewy 2015). Even with careful handling and storage, the mechanical properties of the cores are inevitably altered due to stress relief, drainage and cooling, exposure to air or other fluids, etc. These effects can introduce irreversible changes to most rocks, which have been observed in terms of rocks’ permeability, deformability, and velocities (Bernabé 1986; Wang and Simmons 1978; Warpinski and Teufel 1992). Depending on the degree of alteration, the laboratory measurements of the cores may be highly variable and sporadic. Since it affects the attempts to correlate the laboratory results to in situ conditions, it is imperative to evaluate and to minimize the laboratory measurement reproducibility. In the laboratory, the evolution of the measured core properties is often observed when the core is subject to repeated, hydrostatic loading–unloading cycles (Bernabé 1986; Warpinski and Teufel 1992). This evolution is likely to diminish within a number of cycles, and then the measurements are deemed reproducible and reliable. This cycling procedure is often called ‘seasoning.’ Different from other cyclic tests (Haimson 1974; Zoback and Byerlee 1975), seasoning typically subjects the specimen to hydrostatic confining stress and controls the stress between zero and a predetermined maximum level. The notion of seasoning was coined by Bernabé (1986), who suggested that the seasoning process could close some of the microcracks and may presumably yield a non-variable material. Warpinski and Teufel (1992) adopted the seasoning procedure in testing a few tight reservoir rocks and showed the seasoning significantly reduces uncertainties. Boutéca et al. (1998) claimed seasoning led to intrinsic properties of a porous, quartz-rich sandstone. However, Ostermeier (1995) argued seasoning could not improve reproducibility based on their tests on the Gulf of Mexico turbidites. They emphasized that for a viscoelastic material, seasoning may never be achieved. Apparently, the influence of seasoning on the evolution of mechanical properties varies significantly between different rocks. In this study, we seasoned five cores associated with the Bakken tight oil play. Specifically, the objective of seasoning process was twofold: (1) to characterize the evolution of mechanical response of these rocks over pressure cycles and (2) to understand the effectiveness of seasoning on leading to reproducible measurements. During seasoning, we monitored both the static and dynamic responses (deformation and ultrasonic velocities) over multiple cycles until the discrepancy between cycles substantially diminished. Then, the dependency on confinement, loading–unloading hysteresis, and evolution between cycles were examined. * Xiaodong Ma xiaodongma.rocks@gmail.com