Massive salt formations and their proximity to pipeline systems and power plants make Arizona attractive for natural gas storage.
Salt is abundant and valuable in Arizona. Two of the nine known deposits are thicker than the Grand Canyon is deep. Total thickness of salt (common table saltsodium chloride) in the other seven deposits is unknown. Interpretations of eighteen unexplored areas may have potential salt deposits in the subsurface (see map).
Salt is the basis for two industries in Arizona, solution mining and subsurface storage.
At the Morton Salt Co. facility near Glendale, subsurface salt is dissolved with fresh water and the brine pumped into evaporating ponds.
Salt harvested from the ponds is used for industrial and commercial purposes such as water treatment systems, deicing highways, and making ice cream.
Caverns dissolved in subsurface salt are used to store LPG at the Ferrellgas Partners LP facility near Holbrook and the AmeriGas Partners LP facility near Glendale. Ferrellgas has 11 caverns with a total capacity of 85 million gal. AmeriGas has three caverns with a total capacity of 150 million gal.
Both existing LPG-storage facilities are served by the Burlington Northern Santa Fe Railroad. Recent interest in building new gas-fired electric power plants in Arizona has focused attention on storing natural gas in the large subsurface salt deposits near Phoenix and Kingman.
Opportunities
The potential is good to expand current facilities or develop new facilities in the nine known salt deposits. In addition, other salt deposits, currently untested, may be present beneath numerous areas of the state.
Recent interest in building new gas-fired power plants in Arizona has led three companies to investigate the feasibility of storing natural gas in Arizona salt.
Copper Eagle Gas Storage LLC is evaluating the suitability of storing up to 3.2 bcf of working gas in the Luke salt deposit west of Phoenix.
Copper Eagle was formed by Phoenix-based Pinnacle West Energy, division of Pennacle West Capital Corp., and Shreveport, La.-based Crystal Gas Storage Inc., an affiliate of El Paso Corp., Houston. The Copper Eagle storage facility would tie-in to the El Paso Natural Gas pipeline system. An open season for the project ended in November 2001.
Copper Eagle reported receiving 15 bids for a total capacity that exceeded the 3.2 bcf offered. The facility would be capable of withdrawing the entire working gas volume in 10 days and injecting the same volume in 20 days, resulting in about 12 complete cycles/year.
To date, Copper Eagle has drilled two stratigraphic holes to study the characteristics of the salt for storage and the adjacent sedimentary units for disposal of brine. Core sampling of the salt has been completed.
Test results of the core samples indicate that the physical qualities of the salt are similar to other salt formations in the country where natural gas is safely stored. Depending on additional studies and a final decision to move forward, the Copper Eagle storage facility could be placed into service by mid-2005.
Desert Crossing Gas Storage & Transportation System LLC is studying the feasibility of storing up to 10 bcf of working gas (inventory capacity) in the Red Lake salt deposit in the Hualapai Valley north of Kingman, Ariz. Desert Crossing is sponsored by Allegheny Energy Supply Co., Arizona’s Salt River Project, and Sempra Energy Resources, a Sempra Energy company.
The Desert Crossing storage facility would include multiple deep salt caverns and connect to an associated pipeline system upon completion.
The associated pipeline system, with a projected capacity of 800 MMcfd, would extend 300 miles from near Las Vegas to southwestern Arizona and connect the storage facility with all the major interstate natural gas pipelines in Arizona and southern Nevada.
An open season for the project ended in February 2002. The pipeline is projected to be in service no earlier than June 2004 with initial storage withdrawal capacity of 200,000 decatherms/day (200 MMcfd) no earlier than May 2005.
Desert Crossing reported that upon completion, scheduled for February 2008, the facility would have firm withdrawal capability of 800 MMcfd and firm injection capability of up to 667 MMcfd resulting in up to 13 complete cycles/year.
Actual dates would be set and site-specific geologic and engineering studies would start upon positive results of the open season.
Aquila Inc., a wholly owned subsidiary of Kansas City-based UtiliCorp United Inc., is studying the feasibility of storing up to 12 bcf of working gas in the Red Lake salt deposit about 30 miles north of Kingman. Aquila recently announced that it had purchased the Red Lake gas storage project from Southwest Gas Corp., including 36,000 acres of fee property with accompanying water and mineral rights in Arizona’s Hualapai Valley.
The proposed Aquila storage facility would include two storage caverns and connect to interstate pipelines operated by El Paso Natural Gas, Transwestern Pipeline Co., and Questar Pipeline Co. Open season for the project was Feb. 5-Mar. 15, 2002.
The first 6 bcf of withdrawal capacity (450 MMcfd) is to be in service by January 2004 with the full 12 bcf of capacity in service by January 2005.
Aquila has reported that, upon completion, the facility would have a withdrawal capacity of 900 MMcfd and an injection capacity of 450 MMcfd, resulting in about nine complete cycles/year.
Most of the geologic and engineering studies were completed before Aquila purchased the property from Southwest Gas Corp.
Storing natural gas and LPG in Arizona’s subsurface salt deposits has several benefits.
Arizona is the only state in the Southwest US with salt bodies large enough for storing large volumes of natural gas and LPG between the main sources of supply and regions of demand.
Salt deposits in Arizona offer exceptional off-peak energy storage possibilities because of their proximities to interstate pipelines, railroads, and the several new natural gas-fired power plants being built in Arizona.
Salt-solution caverns provide an economic alternative to surface storage in steel tanks.
The high deliverability of natural gas stored in salt caverns is a distinct advantage over storage in depleted oil and gas fields and aquifer reservoirs. Subsurface salt is ideal for gas storage because it is impermeable, making it impossible for gas to escape.
Finally, special technology and expertise make solution mining and cavern development in subsurface salt safe, efficient, and environmentally friendly.
The Arizona Geological Survey recently published Circular 30, ‘Arizona Has Salt!,‘ to summarize information about Arizona’s salt deposits, including the literature, drilling, and gravity data that define the major deposits.
Circular 30 documents the relationship between gravity data and salt deposits, points to areas where additional deposits may be present, and shows the proximities of highways, railroads, and pipelines to the known and potential salt deposits.
Distribution, thickness
Salt is present in all three physiographic regions of Arizona (see map). Information about subsurface strata in the three regions comes primarily from gravity modeling, seismic surveys, and sparsely located drill holes.
The most extensive deposits are in the Colorado Plateau Province where salt underlies more than 3,500 sq miles in the Holbrook Basin.
Salt there attains a maximum aggregate thickness of 655 ft and includes the potassium minerals sylvite, carnallite, and polyhalite near the top of the salt interval.
The potassium-rich unit underlies about 600 sq miles, ranges up to 40 ft thick, and has an estimated potential of about 285 million tons of minable potash.
The thickest known salt deposits are in the deep intermountain basins in the Basin and Range Province.
Gravity data, seismic profiles, and a few deep drill holes indicate that the salt deposits in Mohave County north of Kingman and in Maricopa County west of Phoenix are at least 6,000 ft thick and may be thicker than 10,000 ft.
As much as 100 cu miles of salt may be present in the Hualapai Valley north of Kingman; more than 15 cu miles may be in the Luke Basin west of Phoenix.
These two salt deposits, and the anhydrite deposits in the Picacho Basin between Phoenix and Tucson, are among the thickest bedded evaporite deposits in the world.
Very little deep drilling has been done in Arizona. Statewide, the average drilling density is only about one well/250 sq miles.
In many of the basins in the Basin and Range Province, the average drilling density is closer to one well/400 sq miles. For this reason, the total extent and thickness of the massive salt deposits in the Basin and Range are not well defined. No drill holes have penetrated the entire thickness of the main mass of any of these salt deposits.
Only a few holes have penetrated more than a few thousand feet of the salt. More drilling is needed to define these salt bodies further and shed light on the sedimentary units adjacent and beneath the salt.
These sedimentary units, like the productive units that are intimately associated with salt deposits around the world, may also have potential for oil and gas.
Age, origin; potential depositis
Marine and nonmarine salt deposits are present in Arizona. Salt in the Holbrook Basin on the Colorado Plateau was deposited during the Permian period (285 to 245 million years ago). At that time, east-central Arizona was an area of very low relief adjacent to open seas to the south.
Salt was deposited in coastal sabkhas that were inundated only occasionally and in which wave energy was low.
The thick deposits in the Basin and Range Province were deposited in a nonmarine environment during Miocene time (15 to 12 million years ago).
Prolonged periods of internal drainage, arid climate, and high evaporation rates resulted in the deposition of salts in closed basins.
The abundance of salt in Arizona and the gravity anomalies associated with many of the salt deposits suggest that salt may be present in less-explored basins.
Eighteen basins have gravity anomalies similar to those associated with the known salt deposits.
Circular 30 includes descriptions of the available gravity data and seismic profiles, as well as information from sparsely located deep drill holes in the 18 basins.
The abundance of salt in Arizona suggests that salt may also be present in basins other than the 18 described. As discussed in Circular 30, deeply buried salt deposits may be present in basins that do not have pronounced gravity anomalies.
The author
Steven L. Rauzi is oil and gas administrator for the Arizona Geological Survey, Tucson. He joined the Arizona Oil & Gas Conservation Commission in 1988 before it merged with the Geological Survey in 1991. Rauzi worked for Texaco Inc. in Los Angeles as an exploration and development geologist from 1980 to 1987, after receiving BS and MS degrees in geology from Utah State University. He is a member of AAPG.