PR-317-10702-R01 Testing for the Dilation Strength of Salt

A laboratory testing program on rock salt specimens was performed using test conditions that are consistent with the stresses that are experienced near the surfaces of salt caverns during storage operation. The proposed work effort focuses on improving the methodology for defining the onset of dilation for rock salt. Geomechanical studies use dilation criteria to assess the potential for salt damage that can lead to spalling in the cavern roof and/or walls and subsequent damage to the cavern or hanging string. This constraint is often the one that limits the minimum gas pressure in a natural gas storage cavern. This report documents the PRCI funded follow-on activities to the recently completed Gas Storage Technology Consortium project [DeVries, 2010]. The work activities completed include the following:

Laboratory dilation strength testing of eight specimens having preconditioning durations longer than 10 days.
Numerical modeling to identify and optimize an appropriate specimen shape for dilation testing in triaxial extension states of stress.
Laboratory constant mean stress extension testing on the optimized specimen shape.

DeVries [2010] documented the effects of the preconditioning durations on the dilation strength of salt specimens. Preconditioning of specimens is the process whereby specimens are subject to a relatively high hydrostatic stress for a specified period of time. It is believed that preconditioning mitigates some of the damage to the specimens induced by coring, transporting, and specimen preparation. The study documented by DeVries [2010] suggests that increasing the preconditioning duration increases the dilation strength of salt, with the maximum precondition duration limited to 10 days. This project expands upon these findings through additional testing to determine if preconditioning durations longer than 10 days has any additional benefit.

In addition to the preconditioning task, this study will also investigate the variability issues observed during dilation strength tests performed under triaxial extension states of stress. It is hypothesized that the high variability seen in extensional test results might be attributed to end effects caused by (1) the friction at the specimen-platen interface and (2) specimens breaking outside the range measured by gages. To help reduce frictional effects and breakage location issues, numerical models of alternate specimen shapes were created to provide a basis for testing a new specimen geometry. Laboratory tests were performed on the new specimen geometry to validate any of its possible benefits.