The goal of this project is to field test a technology that would improve the amount of gas that can be extracted efficiently and economically from salt cavern storage.
Rapid City, SD 57703
Product movement to and from compressed natural gas (CNG) storage caverns in salt is accomplished by compression and expansion of the stored gas. The constraint that typically limits the minimum gas pressure for a CNG storage cavern is the potential for salt dilation that can lead to spalling in the cavern roof and/or walls. The gas storage capacity of many caverns can be increased if the minimum level of internal pressure in storage caverns can be reduced without jeopardizing cavern stability. This project investigated the minimum allowable operating pressure of compressed natural gas storage, in salt formations caverns, using a continuum damage mechanics approach.
Geomechanical studies were used to refine and assess the viability of using a state-of-the-art salt mechanics model to predict CNG cavern instability and collapse. The Multimechanism Deformation Coupled Fracture (MDCF) model, originally developed for the disposal of nuclear waste in salt formations, has the capability to predict the onset of dilation. MDCF can also quantify the severity of microfracturing or damage within the salt as a function of time.
This project was the first to determine the minimum allowable gas pressure of actual CNG storage caverns. The differences for minimum gas pressure determined using both stress-based and damage-based criteria were assessed, with lower minimum gas pressures expected for the damage-based criterion. The ability to reduce minimum gas pressure in CNG salt caverns, using a salt damage criterion, is based on the concept that an accurate prediction of salt behavior can be obtained by an advanced model. This model can track the history of damage and healing of the salt.
A total of 50 mechanical and mineralogical laboratory tests were performed on salt core specimens from two natural gas storage caverns (one existing and one planned), owned by Bay Gas Storage Company, Ltd in the McIntosh salt dome located near Mobile, Alabama. The laboratory tests were used to evaluate the parameters of the MDCF model and to aid in model refinement efforts.
Model refinement was performed to improve the predictive capabilities under operating conditions typical of those experienced by natural gas storage caverns in salt formations. Model enhancements included:
Due to time and economic constraints, laboratory testing at stress states other than triaxial compression was limited. Only one, successful complete creep test, in triaxial extension was completed. As a result, complete development of the model was not fully realized during this project. However, development of the model was sufficient for use in numerical calculations of the storage caverns. Using the proposed method compared to a conventional stress-based method to determine minimum working gas pressure, results showed that the working gas capacity of the existing cavern could be increased by 18 percent, and by 8 percent in the planned cavern.
This project has been completed and the final report is available.
Final Report: Salt Damage Criterion Proof-of-Concept Research [PDF-2323KB]