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Updated December 2022

This fact sheet explains what a critical sour well is, the special safeguards such a well requires, and how these safeguards protect Albertans from sour gas blowouts.


What is a critical sour well?
A critical sour well is a well that could potentially release large quantities of hydrogen sulphide (H2S), causing significant harm to people nearby. When deciding if a sour gas well should be considered critical, the Alberta Energy Regulator (AER) examines factors such as the complexity of drilling the well and how many people live in the nearby community.

What is sour gas?
Sour gas is natural gas that contains some amount of H2S. H2S is the chemical formula for hydrogen sulphide, a toxic gas formed by the breakdown of organic materials. It can be found in natural gas, oil, sewage, swamps, stockyards, and pulp and paper processing. The gas is colourless, but you can recognize its “rotten egg” smell even at low concentrations.
At higher concentrations, it stops people and animals from breathing, so if not handled properly, it can be deadly. Because H2S is heavier than air, it tends to accumulate in low-lying areas.

How do you decide to classify a well as critical?
All applications to the AER to drill a well must consider the possibility of encountering sour gas. If the AER’s initial evaluation shows that H2S may be a factor, we will examine the application further.

We consider two major criteria to determine whether a sour well should be classified as critical:

  • the distance of the well from an urban centre or public facility (e.g., a recreational facility), and
  • the potential H2S release rate during drilling.

The potential H2S release rate is determined by the percentage of H2S in the gas and the rate at which H2S can be delivered to the surface. The release rate is measured in cubic metres per second (m3/s) at standard pressure and temperature (101.3 kilopascals, 15.5°C).

For example, a well with a weak flow of gas and a 1% H2S content would be classified as critical if it were very close to a town. But a gas well with 10% H2S content in a remote location with no people nearby might not be classified as critical.

What special planning is required for a critical well?
A critical well requires a detailed drilling plan that addresses all aspects of a proposed operation, including 

  • well design,
  • equipment,
  • drilling procedures,
  • training and supervision,
  • inspections, and
  • emergency response planning.

How does an operator prevent equipment failure?
When drilling a critical well, equipment must be able to resist the harmful effects of sour gas (i.e., corrosion) and contribute to blowout prevention. For example, the drill pipe used for critical wells must be high quality and inspected to ensure it meets the latest standards.

What is a blowout?
A blowout is an uncontrolled flow of gas, oil, or other well fluids from a wellbore into the atmosphere. A blowout usually results from a combination of factors, such as human error and equipment failure.

In case equipment does fail, do critical wells have backup equipment?
Yes. Backup equipment provides a second line of defence to ensure problems with a well are controlled early before resulting in a blowout. For example, a site must have a backup degasser (a device that removes unwanted gas from drilling fluids) and keep twice the necessary amount of drilling mud in reserve. Mud is the liquid mixture circulated through the wellbore during drilling and is important in holding back subsurface pressure.

Can human error be prevented?
Regulation cannot guarantee that workers at a critical well will not make a mistake. However, errors can be reduced with proper training and supervision and by following policies, procedures, standards, and best practices. The basic rule is that qualified and experienced drilling crews will work on critical wells. Rig managers and supervisors must have a current Energy Safety Canada Second Line Supervisor BOP Well Control certificate. All supervisors, rig managers, and drillers must have H2S Alive certification and experience with drilling sour wells.

Who inspects critical wells?
The responsibility for inspection rests primarily with the company drilling the well and the well-site supervisor. Daily and weekly inspections are conducted by both company and the drilling contractor personnel.

Our field staff will check the company’s inspection records and conduct independent inspections. For most critical wells, there is at least one AER inspection before or during drilling a critical zone. Critical well inspections are detailed. If serious problems are found, drilling operations are suspended if safe to do so until the deficiencies have been corrected. For more information on AER inspections, see EnerFAQs Inspections and Enforcement of Energy Developments in Alberta.

How do I find out if a well near me is critical?
The best place to start is with the company drilling the well. If you know the company name, call its nearest office. However, if you would like more information, contact the nearest AER field centre.

As part of the detailed emergency plan for a critical well, each company must contact everyone within a certain distance of the proposed well site. The company should provide you with information about its plans and seek your input. We will not grant the company a drilling licence until it completes this work.

If the company cannot satisfy your concerns or answer your questions, contact the nearest AER field centre, and we will assist you.

What if the worst happens and a critical well does blow out?
If blowout prevention procedures fail, complementary emergency response plans would be triggered to protect people’s health and safety. Response plans might include igniting the well (setting it on fire). Ignition will convert H2S to sulphur dioxide, which is less toxic and disperses more effectively because the heat from the fire will carry it upwards and lower the ground-level concentration.

Are emergency response plans required for drilling or operating a critical sour well?
Every company drilling or operating critical sour wells must have an AER-approved emergency response plan (ERP). An ERP establishes the processes and procedures that a company will follow during an emergency. If you live in an area where sour gas drilling is likely, be assured that we will not license a company to drill a critical well until it has prepared an ERP specifically for that well. The ERP will include details on weather patterns, terrain, nearness of people, the expected release of H2S, and other site-specific considerations.

How is the emergency planning zone (EPZ) around a critical well determined?
An emergency response zone (EPZ) is the area around a well where residents and visitors (e.g., campers or hunters) within the zone would potentially be at risk if a critical well blows out. The size of the EPZ depends on the potential release rate of H2S and other specific circumstances. If you live inside an EPZ, the company will meet with you to discuss what measures should be taken in an emergency and any special needs you may have, such as transportation or special health considerations.

What if I live outside the EPZ? How am I protected in case of a blowout?
If a blowout occurs, the AER will establish an emergency operations centre to coordinate the work of the provincial emergency response team. The operator is expected to keep people within the EPZ and the surrounding area informed of any action required to protect their health and safety. If necessary, the AER has the authority, tools, and capacity to direct any aspect of the response.

Monitoring air quality downwind from the well is one of the first activities initiated in a sour gas blowout. Mobile equipment will be set up to track the plume and identify gas concentrations in and around the EPZ. If the emergency response team determines a danger to human health, residents will be evacuated from the EPZ or the well ignited to protect the public or both.

What happens once a critical well has been drilled? How am I protected then?
The emergency provisions for drilling critical wells extend to their ongoing operations and maintenance. Once a critical well is ready to be placed on production, the ERP will include protections specific to this phase of development.

Why drill critical sour gas wells at all?
The AER must decide if a company should be allowed to drill a critical well having regard for the broad public interest—that is, what is best for all Albertans—while still being concerned about possible negative effects on individuals.

The sour gas industry is a vital part of Alberta’s economy. Natural gas heats our homes, generates electricity, and supplies us with many valuable consumer products.

Sulphur, a by-product of sour gas, is used in making fertilizers and many other chemical products. Canada is one of the world’s largest exporters of sulphur. Drilling is the only sure way to find and produce natural gas and to determine Alberta’s natural gas reserves to meet future needs.

Additional Information
For more information on the AER and its processes or if you wish to speak to your local field centre or have questions about energy resource development in Alberta, contact the AER’s Customer Contact Centre: Monday to Friday (8:00 a.m. to 4:30 p.m.) at 1-855-297-8311 (toll free).

This document is part of the EnerFAQs series, which explains the AER’s regulations and processes relating to specific energy issues. Please visit to read more of the EnerFAQs series.

Each year the AER collects, compiles, and publishes a large amount of technical data and information about Alberta’s energy development and resources for use by industry and the public. This includes raw data, statistics, hearing materials, and information on regulations, policies, and decisions.

Publications may be downloaded free of charge from the AER website ( or made available through the Products and Services Catalogue by contacting Data & Information Services (email: @email).

AER Head Office
Suite 1000, 250 – 5 Street SW
Calgary, Alberta T2P 0R4
1-855-297-8311 (toll free)

Energy and Environmental 24-hour Response Line (emergencies and complaints): 1-800-222-6514 (toll free)