The purpose of this project is to develop an improved method to assess the long-term geotechnical integrity of natural gas storage caverns in bedded salt.
Rapid City, South Dakota 57703
Natural gas is often stored in solution-mined caverns in domal salt formations. Salt caverns are an effective means of providing flexible gas storage services that ease dependence on pipeline supply. An advantage of storing natural gas in salt caverns is the immediate availability during peak demands. Most salt deposits are located in the central portion of the United States, with most salt domes along the Gulf Coast. However, most projected growth in natural gas demand is in the northeastern United States. Although a few natural gas storage caverns have been developed within the bedded salt formations in the Michigan and Appalachian Basins, only about four billion standard cubic feet (Bcf) of salt cavern storage currently exists in these regions. The Appalachian and Michigan Basins are near major gas markets in the Northeast, making gas storage in these bedded salts very desirable. However, technical issues have hindered development of solution-mined salt caverns for natural gas storage in this region, as well as in the Midwest. Industry has been reluctant to develop new storage facilities in bedded salt because confidence is lacking in the ability to predict the long-term geomechanical integrity of cavern designs. Industry needs to be assured that the technology for designing caverns is adequate before making a potentially risky financial investment. This research addressed the geotechnical design issues of gas storage caverns in thinly bedded salt.
The prevention of roof collapse is a critical issue for natural gas storage caverns in bedded salt formations. This project evaluated the potential for salt failure over the range of possible stress states experienced by the salt surrounding and overlying a cavern. The effort included investigating the representative stratigraphy for the Appalachian Basin along with characteristic rock properties and in-situ conditions. Laboratory tests were conducted on core samples at stress conditions prevalent in bedded salt forming the cavern roof. The data from these tests were used to develop a failure criterion for the roof salt in bedded salt caverns. Numerical simulation models were developed, together with the new failure criterion, to evaluate the effect of basic design parameters on cavern roof salt stability. Design parameters evaluated include: (1) cavern roof salt thickness, (2) cavern depth, (3) cavern roof span, and (4) cavern operating pressures. Results of the numerical analyses were used to recommend guidelines to enable potential bedded salt cavern gas storage developers to recognize favorable and unfavorable geologic settings for cavern development. These guidelines will provide an added degree of confidence for developers of bedded salt cavern storage projects in the future.
Because damage to the salt enhances the permeability, increases the porosity, and reduces the load-bearing capability, an accurate criterion for predicting salt damage is critical during geomechanical assessments of cavern stability. The new criterion developed under this project is more robust and provides a better representation of observed laboratory measurements than other criteria that are currently being used in the United States. Development of the new criterion was based on the results of laboratory tests used to investigate the effects of shear stress, mean stress, pore pressure, temperature, and load angle on the strength and creep characteristics of salt. The laboratory results indicated that the strength of salt strongly depends on the mean stress and load angle. The strength of the salt does not appear to be sensitive to temperature. Pore pressure effects were not readily apparent until a significant level of damage was induced and the permeability was increased to allow penetration of the liquid permeant.
The numerical simulations performed illustrate the influence that cavern roof span, depth, and roof salt thickness have on the allowable operating pressure range. Comparison of predictions using the new criterion with that of a commonly used criterion indicate that lower minimum gas pressures may be allowed with the new criterion for caverns at shallow depths. However, as cavern depth is increased, less conservative estimates for minimum gas pressure were determined by the new criterion.
A final project report detailing all of the project’s activities is provided below under "Additional Information": “Cavern Roof Stability for Natural Gas Storage in Bedded Salt,” September 26, 2002–June 6, 2005, Principal Authors: Kerry L. DeVries, Kirby D. Mellegard, Gary D. Callahan, William M. Goodman.
Project Final Report - June 2005: Cavern Roof Stability for Natural Gas Storage in Bedded Salt [PDF-5763KB]