The Role of Diesel Fuel in Salt Dome/Cavern Natural Gas Storage
According to the U.S. Energy Information Administration, approximately 32.31 trillion cubic feet of natural gas was used in the U.S. in 2022, equal to about 33% of the total U.S. primary energy consumption. Five states accounted for 39% of total U.S. natural gas consumption in 2021, with Texas consuming nearly double the amount used by the each of the next four highest states (California, Louisiana, Pennsylvania and Florida).
Natural gas is commonly stored under pressure in three types of facilities: depleted reservoirs in oil and/or natural gas fields, aquifers and salt caverns. Of those choices, 78% of natural gas storage in the U.S. is housed in depleted natural gas or oil fields as the equipment from former production facilities can easily be converted for use in storage and transfer/pipeline connections making this the option widely accessible.
The least common form of underground natural gas storage is in salt domes or salt caverns, which make up only 7% of total U.S. underground natural gas storage capacity. Currently, there are approximately 36 underground natural gas salt cavern facilities in the U.S., most of which are located in the Gulf Coast states. Salt domes or salt caverns are utilized for natural gas because they are dry and geologically stable, allowing for storage of large quantities of natural gas in a safe manner.
In this post, we examine the role of diesel fuel in making the underground storage of natural gas in salt caverns or domes viable.
Naturally Occurring Salt Formations: A Storage Solution Millions of Years in the Making
The formation of Gulf Coast salt deposits began millions of years ago, likely during the Jurassic period, when the area was covered by a shallow sea which continually evaporated and refilled, leaving behind a thick deposit of salt, called Louann salt. Fast forwarding through time, the Louann salt was buried by sediment, most of which came from predecessors of the Mississippi and Rio Grand River systems. As the buried salt was less dense than the deposited sediment, pressure and temperatures at significant depth made it possible for the salt to flow pliantly upwards through the overlain deposits over millions of years, resulting in what we presently call salt domes. The term salt dome refers to the tops of these columns of salt.
Developing Caverns for Natural Gas Storage
The size of salt caverns varies widely based upon storage needs but can reach a diameter of one mile and height of 30,000 feet. Caverns are typically located between 1,500 and 6,000 feet below ground surface.
Developing a natural gas storage cavern out of a salt dome is a simple process. A well is drilled from the ground surface through the cap rock of the salt dome and water is injected to dissolve the salt and create the desired storage space for natural gas. The process is referred to as “solution mining” or “leaching.” Solution mining involves the installation of several individual casings at the well head for water injection, brine extraction and injection/extraction of a “blanket fluid.” Water is injected into the inner casing to begin the dissolution process with the resulting brine displaced to the surface through the inner annulus of the casing. Then a blanket fluid is injected into the outer annulus to control upward dissolution and protect the borehole from collapse. This blanket fluid is used to shape the cavern and stabilize the cavern roof. The fluid, most commonly diesel fuel, is less dense than the injected water and brine, and creates a separate layer on top of the brine. By protecting the salt at the top of the cavern from further dissolution, the blanket fluid prevents cave in or collapse.
Blanket Fluid Alternatives
While the most common practice, using diesel fuel as a blanket fluid in the development of salt caverns to serve as underground storage for natural gas is not without risk. But finding an ideal alternative is not an easy task.
To be considered for use as a blanket fluid, a chemical must meet the following criteria:
- Health, Safety & Environment (HSE): toxicity to humans, environment and aquatic environments must be low.
- Density: the density of the blanket fluid must be lower than that of the injected water and resulting brine to ensure that the blanket fluid is buoyant enough to protect the cavern roof.
- Solubility: blanket fluid solubility in brine needs to be low so as not to allow for mixing and prevent unwanted leaching.
- Measurability: to measure and maintain cavern growth, there needs to be a distinction between cavern brine and blanket fluid to determine blanket fluid depth.
- Freezing, boiling and flash temperature: the physical state of the blanket fluid must be such that it will remain liquid within the working temperature range within the cavern.
- Stability: chemical stability under high temperatures, pressure or microbial decomposition may result in loss of effect of the blanket fluid.
- Handling: ease of handling is required for effective utilization of the blanket fluid, i.e. low enough viscosity.
- Price: the volume of blanket fluid needed to support cavern development is very large and varies by cavern size but can reach hundreds of thousands of gallons of diesel per cavern, for example.
Well Engineering Partners, a Dutch consultancy firm, completed an evaluation of seven different groups of chemicals to identify an alternative to use of diesel fuel as a blanket fluid. Their findings are as follows:
- Gases: Inert to salt and therefore would provide dissolution protection to the cavern roof. However, with densities three orders of magnitude lower than most liquids under standard conditions, the head exerted on the liquid is reduced, creating high wellhead pressures. Petroleum gas is highly flammable so would not pass the HSE requirement. Nitrogen is widely available and has low reactivity and low toxicity; however, due to small molecule size nitrogen can permeate through salt cavern walls, requiring continual monitoring and re-injection of the chemical to maintain blanket pressure. In addition, while nitrogen is inexpensive, handling of the gas requires specialized high-pressure equipment.
- Esters: Generally highly flammable and miscible in water therefore would not pass HSE and measurability requirements. Biodiesels, while having the same physical properties as diesel, have been reported to allow for microbial activity when in contact with water. The potential exists for the partial conversion of biodiesel to methane due to this microbial activity, creating a risk for fire and explosion therefore the HSE requirement cannot be met. Renewable diesel does not attract water like biodiesel, but costs 2 to3 times as much as diesel, and so does not meet the price criteria.
- Insoluble Fatty Acids: The main constituent of vegetable oils; they generally have low toxic effects to humans and the environment, high flash point (low flammability), lower density than that of water, can be handled in a large range of temperatures, their smoke point is higher than average working temperatures, and they are insoluble in liquids. However, microbial growth under salt cavern conditions is unknown and vegetable oils are reported to be biodegradable under aerobic and anaerobic conditions, with 70 to 100% biodegradation occurring over 28 days according to studies conducted by Emmanuel O. Aluyor et. al. in 2009. The potential degradation of vegetable oil requires continual monitoring and re-injection, and the price varies from 2 to 10 times the price of diesel.
- Refined Petroleum Products: Surdyne B140, ExxsolD100, and gas to liquid (GTL) were evaluated as these alternatives are similar to diesel. All three have lower toxicity to humans and aquatic organisms compared to diesel; however, their potential for degradation is unknown at this time and therefore the HSE risks cannot be evaluated. Long-term downhole testing would be required to identify potential microbial deterioration. GTL has a lower flashpoint than diesel, increasing its flammability risk, and therefore increasing HSE risks. The price of all three alternatives is 1.15 to 2 times that of diesel.
- Glycols: Generally less toxic to humans and the environment, less flammable than diesel, and have a higher density than fresh water. Glycols are also fully miscible in fresh water, which renders them unsuitable as a blanket fluid.
- Ethers: The toxicity and reactivity of ethers are generally low; however, the flash points are also very low making them highly flammable, rendering them unsuitable as a blanket fluid. Additionally, the costs are 4 to 7 times that of diesel.
- Alcohols: Toxicity of alcohols to humans and the environment varies; methanol and ethanol are toxic to humans but have a low toxicity to aquatic organisms, as does the solubility and flash point, with low molecular weight alcohols being very flammable. The price of alcohols is generally 2 to 6 times the of diesel fuel. For these reasons, alcohols are not a suitable alternative to diesel.
A Cautionary Tale: When Diesel Fuel is Released to the Surface
The importance of proper handling and storage of fuel during the solution mining portion of cavern development to protect areas surrounding natural gas storage facilities is illustrated here:
VERTEX was retained by an environmental/pollution insurance carrier to assist in its investigation of a claim involving the surface release of 16,000 gallons of diesel fuel at a natural gas storage, transmission and distribution facility in Texas. The facility was in the process of developing an underground natural gas storage salt cavern when it was discovered that one of the on-site frac tanks containing the diesel blanket fluid had failed and released the entirety of its contents overnight. Approximately 4,000-gallons of diesel fuel was captured in the frac tank secondary containment, however, approximately 16,000-gallons of diesel fuel flowed to the surrounding unpaved ground surface and migrated to a drainage channel with an outfall to an adjacent marsh area and tributary. Approximately 1.6 acres area of land and 3+ miles of tributary were affected by the leak. Remediation was completed under both State and Federal regulatory oversight and included:
- the installation and maintenance of absorbent booms within the tributary
- fuel recovery
- impacted groundwater, surface and stormwater recovery and disposal
- soil excavation and disposal
- vegetative rinsing along the banks of the tributary
- tributary and marsh restoration activities
Emergency response and remedial efforts required off-site disposal of the following:
- approximately 1M gallons of impacted groundwater, surface water and stormwater
- 3,510-gallons of impacted sludge/sediment
- 8,880-cubic yards of impacted soil
- 220 cubic yards of spent absorbent pads/boom and vegetative debris
- 270 cubic yards of impacted shrubs and trees
- 357 tons of contaminated timber mats used to create access roadways along the tributary
Remediation costs to address the release approached $10 million. The exact cause of the frac tank failure has yet to be determined.
While use of diesel fuel as a blanket fluid may pose an environmental risk at the surface during solution mining activities, based on the limited research available to date, it is effective as a blanket fluid for salt cavern development. Additional studies are needed to assess biodegradation of refined petroleum products and insoluble fatty acids as suitable blanket alternatives in the future. In the meantime, it is important to be aware of the surface release risks associated with the use of diesel fuel as a blanket fluid.
For more information on best practices regarding natural gas storage and environmental concerns please contact Kristen Wieland or call 888.298.5162 or submit an inquiry.
