The goal of this project is to increase the working capacity of conventional natural gas storage facilities in bedded salt by converting them to a refrigerated natural gas storage facility.
Rapid City, SD 57703
In a previous study (Advanced Underground Gas Storage Concepts: Refrigerated-Mined Cavern Storage; Contract No. DE-AC26-97FT34349), DOE and its industry partner, PB-KBB, Inc., determined the feasibility of storing chilled natural gas in a mined hard-rock facility. In salt storage caverns, however, the thermal conductivity of salt is typically significantly higher than that typical of hard rocks such as granite, so the cooling loads required to maintain low temperature gas storage are anticipated to be substantially different. Additionally, the thermal expansion coefficient of salt is much higher than most hard rocks, which can result in larger thermally induced stresses and strains.
This project evaluated the economic and technical feasibility of converting the planned Avoca facility in Steuben County, New York, to a refrigerated natural gas storage facility. The Avoca facility was selected for evaluation because of it's typical storage caverns. These storage caverns are being developed in the salt beds of the Appalachian and Michigan Basins, in the northeastern United States. Phase I of the project was to assess the economic feasibility of increasing the gas storage capacity by chilling the gas to -29oC (-20oF). In Phase II, the technical feasibility of converting conventional storage facility to refrigerated storage was investigated.
The concept of chilling gas before storage, if applicable to planned or existing bedded salt storage caverns, could dramatically increase the storage capacity within a given volume. Currently, facilities in bedded salt are sometimes used to store salt stock and brine for disposal. The salt stock and brine disposal capacity at these facilities are often limited, with expansion by mining of additional caverns uneconomical.
Conversion to a chilled gas storage facility would substantially increase the natural gas storage capacity of such facilities. For example, a site with competent non-salt rock overlying salt, sufficiently tight enough to contain gas, might be used as the cavern roof. The working gas at the facility can then be increased by about 70 percent from 4.4 to 7.5 Bcf, by converting the cavern temperature to –29OC (–20OF).
Although the conversion of the facility examined in this study does not appear to be technically feasible, the study found that there may be cases where chilled gas storage in bedded salt is feasible.
Economic analysis estimated the total equipment and construction costs for converting the Avoca facility from conventional storage to refrigerated storage. They found that an additional $4.29 per million cubic feet (Mcf) would be required to convert this particular site to refrigerated storage. The estimated additional operating costs included a maintenance refrigeration cost and additional processing (refrigeration, dehydration, and heating) cost associated with the throughput of gas. The maintenance refrigeration cost, based on an energy cost of $13.65 per MW-hr ($4 per MMBtu), is estimated to be $316,000 per year after the first year, decreasing over time, as the rock surrounding the caverns are cooled. After 10 years, the maintenance cost is estimated to decrease to $132,000 per year based on the same energy cost. The additional processing costs for refrigerated gas storage and withdraw is estimated to be $0.055 per Mcf), based on an energy cost of $13.65 per MW-hr ($4 per MMBtu).
Because of the large difference between the stored gas temperature (-29°C or -20°F) and that of the surrounding rock (43°C or 110°F), a large amount of heat will be transferred from the rock to the stored gas. This heat gain must be counteracted with refrigeration to maintain a cavern temperature. During Phase I of the project, the cooling load after 1 year of storage operations was estimated to be about 4.1 MW (14 MMBtu per hour). After 10 years of storage operations, the estimated cooling load is reduced to about 1.8 MW (6 MMBtu/hour).
The structural stability study found that conversion of the Avoca facility would not be technically feasible. Upon conversion to refrigerated storage, tensile fractures are predicted to develop in the cavern roof, walls, and floor. Tensile fracturing of the cavern roof may result in gas loss containment and/or integrity loss of the casing shoe, deeming the conversion of this facility technically infeasible. Also, because of the large difference in thermal expansion coefficients between the salt and the shale, it is likely that the contact between the roof salt and shale above it will be broken, resulting in a collapse of the salt cavern roof.
This project has been completed and the final report is available.