Monitoring Anisotropic Hydrational Strain of Shale with High Resolution via Distributed Fibers

Summary Heterogeneous and anisotropic hydration behaviors of shale play a key role in petroleum drilling, which can induce strength degradation and wellbore instability. Herein, we innovatively use the technology of distributed fiber-optics sensing (DFOS) based on Rayleigh frequency shift to strictly monitor the anisotropic hydrational strain of shale with high resolution for the first time. Specifically, we developed a universal experimental framework for measuring hydrational strain under an unconfined free soaking condition combined with distributed fibers and strain gauges, where the shale core sample is immersed in deionized water in a hydrostatic balance and a reserved slack fiber is placed on the surface to measure hydration-induced temperature change. We also conducted a serial fiber winding strategy that included two independent geometric configurations to determine anisotropy. Under the assumption of axisymmetric deformation during hydration, we separated and determined circumferential and axial hydrational strain components after adopting the strain decomposition equation. In addition, we performed two extra tests to validate such axisymmetric assumption. The experimental results indicate that the hydrational strain of shale measured by fibers and gauges showed a good agreement with an average relative error not exceeding 5% and the impact of temperature change caused by hydration on hydrational strain (0.75 με) was so slight as to be ignored. Further, the validated tests confirmed the axisymmetric assumption that the circumferential and axial hydrational strain components were not dependent on the azimuth angle. The results also demonstrated that the hydrational strain increased rapidly for the initial 6 hours and then tended to be convergent, with the maximal circumferential hydrational strain (950 με) being nearly three times larger than that of the axial one (300 με), especially for the central region of shale. These phenomena implied the significant intrinsic hydration anisotropy and the potential boundary effect of shale in such a configuration. A natural parameter η (the hydration self-similarity ratio) was defined to evaluate and scale the hydration anisotropy. It showed that η varied at different heights (3.50 ±0.01) but did not vary much at long-term hydration, which implied heterogeneous hydration. It also showed that η increased over time in the initial period, while it later became constant, which implied progressive anisotropic hydration dynamics. The constitutive curve of hydrational strain vs. water adsorption revealed that there existed three evolutional regimes during hydrational deformation, including dynamic η (≤1 and §amp;gt;1) for the initial time and stable η for the long-term behavior (due to hydration balance). Another interesting result indicated that two hydrational strain components could be fitted by an exponential law vs. water adsorption. These results highlight the particular advantages and reliable applications of DFOS in sufficient hydrational strain measurement of shale, which can also enhance our understanding of anisotropic hydrational deformation and induced cracking.