ABSTRACT: 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.
1 INTRODUCTION
Strength and deformational characteristics of crystalline solids are often controlled by small variations in impurities. For example, small amounts of carbon are used with iron to strengthen steel and cast iron. In metallurgy, the percentage and type of alloying elements are controlled to produce a material with specific properties. If the mixtures of constituents are allowed to vary, the properties of the products would no doubt be variable. Natural variability is often encountered in rock mechanics, and laboratory experiments usually measure properties of highly heterogeneous crystalline solids. Bedded rock salt is a typical example of a naturally variable crystalline material. Composition of rock salt from different areas or even from different depths in one borehole is often highly variable. General differences are easily visible as differences in color and darkness from impurities, such as clays and anhydrite. Differences in measured mechanical properties are often attributed to specimen-to-specimen variability including impurity content. This investigation evaluates the effects of visible and measurable impurities on the creep of Palo Duro Basin salt.
2 APPROACH AND SCOPE
Specimens from the Woods-Holtzclaw well representing four nominal impurity levels: pure salt, salt containing 10 percent anhydrite, salt containing 10 percent clay, and salt containing 20 percent clay were selected on the basis of visual estimates. A triaxial compression creep test was performed on each of the twelve specimens. The conditions were identical for each test: temperature was 75 °C, confining pressure was 15 MPa, and stress difference was 15 MPa. Each test was run for 3 x 106s (~35 days). Previous tests performed on Palo Duro salt indicate that moderate strains (about 5 percent axial strain) would be obtained at these test conditions over this time period (Senseny et al., I985; 1986). Therefore, if the impurities caused large differences in creep deformation, adequate accuracy and resolution would be available for smaller strains and adequate test machine range would be available for larger strains.
