The geology of gas storage

Posted on March 28, 2017

We shared the following information in The Chronicle Herald on March 28, 2017 about the geology of natural gas storage and the Alton project:

In response to a March 15 opinion article on the geology of the Alton Natural Gas Storage Project, I would like to set the record straight and share the following information.

Other companies have looked for oil and gas in the reef structures located well below the salt formation where the Alton project is being built. Those exploration programs are not Alton.

Alton is a natural gas storage project being constructed near Stewiacke. Gas storage caverns will be located about one kilometre underground in the Stewiacke salt formation. The salt was deposited by evaporation of an ancient sea millions of years ago and is essentially pure salt with very few impurities. To create the caverns, the salt will be dissolved using tidal water from the nearby Shubenacadie estuary.

As described in the project’s environmental assessment, salt is ideal for natural gas storage. It dissolves easily in water, making cavern formation possible and is often found in large, relatively homogeneous deposits. Unlike rock, which can fracture in a brittle manner, salt shapes in a plastic manner throughout geological periods (millions of years) and is devoid of fractures. Salt forms a tight seal through which stored gas can’t escape.

Salt caverns in geologic basins in Alberta, Saskatchewan, throughout the U.S. and around the world have been operating safely, dating back in many cases over 60 years.

At Alton, our caverns will be completely surrounded by salt. The geological structure at Alton is a salt pillow contained entirely within the Stewiacke formation of the Windsor group. It is a layer of pure salt beneath a layer of very hard rock within the Shubenacadie sub-basin. We have used modern seismic data, geophysical data and a variety of information from numerous well cores to place our salt caverns in a safe location in accordance with regulatory requirements and best engineering practices.

Geologic formations on top of the Stewiacke formation include limestone, dolomite, sandstone, siltstone, anhydrite, gypsum and other salt layers. These rock layers will not affect the salt cavern operation.

The process to create the caverns at Alton dissolves the salt at a very slow rate, taking two to three years to complete. This best practice ensures no impact below ground that could potentially result in earthquake activity.

We will use sonar equipment and other survey data during the life of this project to monitor the caverns.

The rigorous Canadian regulatory framework ensures projects are carefully scrutinized before they can proceed. There is ongoing oversight and monitoring during operations.

It is mandatory in all Canadian jurisdictions that design and operation of cavern storage facilities conform to the requirements of CSA Standard Z341, Storage of Hydrocarbons in Underground Formations. Alton Natural Gas Storage will meet or exceed CSA Z341 and all federal and Nova Scotia regulations. The standard requires that extensive geological analysis be undertaken and reviewed by the regulatory authority.

In Nova Scotia, oversight for safe development of the Alton caverns is provided by the Nova Scotia Utility and Review Board (UARB). The permit to construct the caverns was issued in September, 2013, and required third-party experts retained by the UARB to review the cavern design and operations. The third-party’s approval for Alton was received in the summer of 2016 after extensive review of the cavern design and the salt formation.

Storing hydrocarbons in salt caverns and underground formations has been the safest and most secure storage method. It is a proven technology with a solid safety record. These caverns are deep underground and provide a solid enclosure for natural gas.

Storage plays a critical role in balancing North American natural gas supply and demand and helping to ensure reliable and affordable energy for customers. For additional information on Alton, I encourage readers to visit the project’s website www.altonnaturalgasstorage.ca.

Charles Lyons is Vice President of Environment, Health, Safety, Security and Sustainability for AltaGas.