A laboratory testing program was developed to examine the short-term mechanical and time-dependent (creep) behavior of salt from the Bayou Choctaw Salt Dome. Core was tested under creep and quasi-static constant mean stress axisymmetric compression, and constant mean stress axisymmetric extension conditions. Creep tests were performed at 38 degrees Celsius, and the axisymmetric tests were performed at ambient temperatures (22-26 degrees Celsius). The testing performed indicates that the dilation criterion is pressure and stress state dependent. It was found that as the mean stress increases, the shear stress required to cause dilation increases. The results for this salt are reasonably consistent with those observed for other domal salts. Also it was observed that tests performed under extensile conditions required consistently lower shear stress to cause dilation for the same mean stress, which is consistent with other domal salts. Young’s modulus ranged from 27.2 to 58.7 GPa with an average of 44.4 GPa, with Poisson’s ratio ranging from 0.10 to 0.43 with an average of 0.30. Creep testing indicates that the BC salt is intermediate in creep resistance when compared with other bedded and domal salt steady-state behavior.
Sandia National Laboratories, on behalf of The U.S. Department of Energy (DOE), is evaluating the mechanical integrity of the salt pillars surrounding existing petroleum storage caverns in the Bayou Choctaw Dome (Louisiana) that are part of the U.S. Strategic Petroleum Reserve (SPR). The purpose of this experimental effort is to better characterize the salt strength, dilational strength and creep in the salt section above Cavern 102 and below the overlying abandoned caverns, where casing issues have been observed .
The core used for this experimental study was obtained from a drill hole from depths of 325 to 335 meters below ground surface (bgs) and is above one of the Bayou Choctaw SPR caverns. For reference the top of the salt dome lies between 183 and 213 meters bgs, and the total depth is over 3050 meters bgs. The natural rock salt deposits used in the SPR are ideal for storage of crude oil because of their low (nearly zero) permeability, ease of mining, and proximity to shipping and refining operations.
The mechanical behavior of rock salt is relatively unique when compared with other geologic media due to the well-known ability of salt to show significant time dependent deformation without fracturing when subjected to differential stresses . This can result in the closure of caverns and mines over time, which results in reduced storage in the case of caverns. Under some differential stress states salt is known to dilate due to stress-induced microfracturing .
In this work elastic properties and dilation behavior were determined by performing constant mean stress axisymmetric compression (ASC) and axisymmetric extension (ASE) tests. Creep properties were determined using standard constant differential stress creep tests. While the composition of the salt used in this testing was not quantitatively determined, it was determined, from visual observation, that the salt is relatively pure halite with impurities comprising less than 2-4% by volume. Impurities appeared to be anhydrite.