Shale rocks with high clay and carbonate content show time-dependent deformation at different conditions of stress. In this paper, we present a series of time-cycling triaxial deformation creep tests conducted on two samples from the Haynesville formation in east Texas. A power-law model was fit to all creep measurements to find the constitutive parameters of the time-dependent deformation of these samples. We obtained these constitutive parameters from triaxial creep experiments over time periods of 3-4 hours, one day, one week and 3-4 weeks. Although further experiments are needed, the experimental results obtained to date show that short-term creep tests are able to provide constitutive parameters that predict long-term deformation of these clay-rich shales. Both Young’s modulus and the viscoelastic properties are anisotropic, with the samples being more compliant and more viscoplastic when deformed normal to bedding.
Understanding time-dependent deformation in unconventional hydrocarbon reservoirs is one of the most difficult technical problems in the oil and gas industry. Viscous deformation can change both mechanical and fluid flow properties of shales, which are of primary importance in determining production capacity of shale gas reservoirs. Clay minerals may be significantly responsible for the inelastic deformation of shale rocks because of their high viscoelastic behavior [1, 2, 3, 4].
Several techniques have been used to determine the inelastic time-dependent deformation of rocks, among which uniaxial and triaxial creep experiments have been used widely [2, 4, 5, 6, 7]. The main problem with conducting these experiments is that time dependent essences of creep experiments make these tests costly and time consuming. Consequently, predicting the longterm inelastic deformation of rocks from short-time experiments has been a matter of interest to the scholars.
In this study, we performed a series of conventional triaxial creep experiments on two clay-rich samples from the Haynesville shale formation to investigate the inelastic deformation process in shales. The triaxial experiments were performed in a time-cycling pattern on a horizontal and a vertical shale sample to understand the bedding orientation effects. This time-cycling pattern includes a series of multi-stage loading/unloading experiments at different time spans. The main goal of performing this type of experiment was to study the authenticity of the few-hours long creep experiments.