ABSTRACT

Creep deformation in salt rocks plays a significant role in the structural integrity and long-term stability of underground storage. Experimental measurements of salt creep behavior help predict underground gas repositories’ long-term geomechanical behavior. Previous time-scaling creep experiments have focused on the axial strain of unconsolidated sands under zero lateral strain conditions. This study is the continuation of previous time-scaling creep tests conducted earlier on the Spindletop salt. A comparative testing procedure and analysis method is conducted on Himalayan Pink Salt. Multistage Triaxial tests and creep tests for different holding time durations and stress regimes were performed. The multistage triaxial test showed evolving deformational mechanisms under the mapped yield surface based on the irrecoverable to recoverable strain ratio. At low stresses, crack closure or conformance dominates. Then plasticity becomes important, ending at early crystal failure. Unlike unconsolidated sands, salts showed both time and strain amplitude scaling. The axial and radial strain data both show scaling behavior under both low and high levels of deviatoric stress. The salt showed only axial creep response at low deviatoric stress distally from the yield surface (no radial response). In contrast, the salts showed strain amplitude scaling both axially and radially at high deviatoric stress proximal to the yield surface. Microstructural images showed creep damage under high deviatoric stress associated with parallel planes of dislocation-intergranular slip, microcracking, and compaction-induced radial strains. The period of scaling is interpreted as regions where a single mechanism is dominating. Strain amplitude scaling for both low and high deviatoric creep stress tests provides inputs for a model of creep response. The scaled data provides an understanding of the magnitude of mechanical damage associated with the stress-strain curves in salts. This data allows extrapolated estimates to be made for the structural integrity of salt caverns during fluid injection and depletion.

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