TYPES OF STORAGE
There are over 400 natural gas underground storage facilities in operation in the United States today with the capacity to store 3,900 Bcf. Approximately 84% of these facilities utilize depleted reservoirs. The remaining 20% utilize either salt caverns or aquifers. Each type of facility has its own advantages and disadvantages.
SALT CAVERN STORAGE
Salt caverns are leached from salt domes or bedded salt. They are open caverns located at depths several hundred to several thousand feet below the earth’s surface and are accessed by one or more wells per cavern. Most of the domal salt cavern facilities are located in Gulf Coast regions of the United States with some bedded salt caverns located in western Pennsylvania and New York.
- High Injection and Withdrawal Rates
- Multi-cycling capability
- High percentage of total gas volume needed for base gas
- Environmental issues related to brine disposal during construction and operation
- Volume of each cavern has practical and geological limitations Higher operating cost due to corrosive environment
The concept of using aquifers for natural gas storage was first demonstrated in the United States by Louisville Gas and Electric Company with the development of its Doe Run facility in Meade County, Kentucky in 1946. Most of these facilities are located in the upper Mid-Western part of the United States.
- Enables gas storage development in locations where hydrocarbon reservoirs are not readily available or suitable of natural gas storage
- Maintaining integrity of the gas/water interface imposes limits on operating flexibility
- Requires a relatively large base gas to working gas ratio
- Higher potential for water supply contamination
DEPLETED RESERVOIR STORAGE
Depleted reservoir storage utilizes a depleted underground reservoir that originally contained oil and/or gas. The withdrawal process for the gas in storage typically replicates the process originally used to produce gas from the reservoir. In the past, the conversion of a depleted gas reservoir for storage use traditionally involved drilling many new wells that would enable a more rapid withdrawal of gas. Until the early 1990s the development of storage facilities in depleted reservoirs involved the use of vertical wells. This meant that for a reservoir to be a candidate for development into a storage reservoir, the formation had to be fairly thick and porous.
- Minimal disruption to environment beyond that caused by original drilling operations
- Multiple access points to reservoir eliminating service disruptions due to well problems
- Reservoir history is known
- No routine service disruptions for periodic inspections
- Reservoir formation must be fairly thick
- Reservoir characteristics (e.g., low porosity and permeability) can limit injection and withdrawal rates
- Most existing reservoir storage is limited to seasonal service
- On average traditional reservoir storage requires approximately half of total storage capacity to be used for base gas
DEPLETED RESERVOIR STORAGE HORIZONTAL DRAINAGE SYSTEM
As horizontal drilling technology became commercially feasible for exploration and production of oil and gas in the late 1980s and the early 1990s, it became apparent that this same technology could be used to enhance the performance of natural gas storage facilities. By being able to drill laterally through the storage formation, a single horizontal well can achieve performance levels that make many depleted reservoirs feasible for gas storage that were previously deemed not suitable for development.
- All of the advantages of traditional depleted reservoir storage
- Enables economic storage development utilizing reservoirs that would not be feasible otherwise
- Greater geographic application of storage development
- High performance injection and withdrawal can be achieved
- Lower base gas requirements than aquifers, traditional depleted reservoir, and shallow salt cavern storage
- Not all geologic formations are suitable for development with horizontal wells.
- Requires the careful integration of expertise in geology, reservoir science, and drilling engineering