Influence of impurities on the creep of salt from the Palo Duro Basin

Twelve triaxial compression creep tests were performed on Lower San Andres salt specimens from the Palo Duro Basin to assess the influence of impurities on creep deformation. Composition of the specimens was measured by methods of wet chemistry coupled with ethylene diaminetetraacetic acid (EDTA) digestion and point counting on full-size polished sections. Overall, the 12 specimens comprise 71.6 to 96.6 percent halite, 2.4 to 7.5 percent anhydrite, and 0.2 to 24.7 percent clay. All specimens were tested at a temperature of 75 ° C, a confining pressure of 15 MPa, a stress difference of 15 MPa, for approximately 35 days. Strain versus time data for individual tests are fitted to the exponential-time creep law which has three fitting parameters: (1) amount of transient strain, (2) steady-state creep rate, and (3) a rate parameter. The parameter values are correlated with the amounts of halite, anhydrite, and clay in each specimen. Anhydrite content has the strongest influence on the creep response. Increase in anhydrite content reduces the steady-state creep rate and the related axial strain. No statistically significant correlation exists between the amount of clay or halite and the creep response. Variability of test results is not readily explained by amount and type of visible impurities.