On November 4, 2025, the Alberta Energy Regulator released AERH2S, an updated version of its computer model for calculating Emergency Planning Zones (EPZs) for sour gas applications. AERH2S replaces the longstanding ERCBH2S model and is available for immediate use by industry. According to AER Bulletin 2025-34, the use of AERH2S will become mandatory for all applications filed under Directive 056: Energy Development Applications and Schedules, including amendment applications, starting January 1, 2026. The continued use of ERCBH2S is permitted for updates to existing EPZs (and any EPZs established before January 1, 2026) under Directive 071: Emergency Preparedness and Response until further notice.
Background
An EPZ is a calculated geographical area around a sour gas well, pipeline or facility where, because of the presence of a hazardous substance (H2S), the licensee is required to design and implement specific emergency preparedness measures in accordance with Directive 071. The size of the EPZ and its contents (for example, surface developments or egress routes) dictate the measures that a licensee must include in its emergency response plan.
The AER's predecessor, the Energy Resource Conservation Board (ERCB), first released ERCBH2S in 2008, replacing manual calculations and nomographs. Initially, the ERCBH2S model employed a "worst-case" combination of various factors in calculating the extent of the EPZ. In 2009, the model was updated to use historical meteorological averages in the EPZ calculation as opposed to simply assuming all conditions are simultaneously at their most extreme. Aside from minor technical fixes, the ERCBH2S model remained substantially unchanged after 2009.
Overview of AERH2S
AERH2S marks the first substantial update to the AER's sour gas EPZ modeling tool in over 15 years. This new model maintains consideration of key dispersion conditions, such as wind, weather and release type, but replaces the conservative "worst-case scenario" approach used by ERCBH2S with a "reasonable worst-case" methodology. By focusing on expected operational conditions and functioning mitigation systems, as opposed to considering only the most extreme circumstances, AERH2S produces EPZ calculations that more accurately reflect real-world operations and weather conditions.
Among the most notable changes introduced with AERH2S is a change in how EPZ distances are calculated and reported under Directive 071. Unlike ERCBH2S, which permitted the submission of EPZ distances calculated either with or without mitigation measures, the updated AERH2S model requires licensees to submit only "with mitigation" EPZ distances. Using this approach, emergency shutdown device settings are configured to reflect typical operating conditions and ensure that release volumes used in the model are as low as reasonably practicable. This is a clear example of the AER's shift from conservative "worst-case scenario" modelling under ERCBH2S, toward the "reasonable worst-case" framework under AERH2S that incorporates the presence and functionality of mitigation systems. While individual results will vary, EPZs generated using AERH2S will generally be smaller and more representative of real-world operations.
Impacts to Industry
The shift to a "reasonable worst-case" modelling approach may have tangible implications for energy companies with sour gas assets. Because EPZ size often impacts the nature and scope of emergency planning measures required under Directive 071, even modest changes to an EPZ can lead to significant shifts in the associated emergency response plan. For instance, in cases where a "no mitigation" EPZ was previously filed with the AER, the shift to a "with mitigation" calculation may result in certain residents or egress routes being excluded from the EPZ, thereby simplifying a company's emergency response plan. According to AER Bulletin 2025-34 and until further notice, licensees are not required to update their existing EPZ calculations unless a Directive 056 application is also involved.
The Energy Regulatory practice group at Bennett Jones has considerable experience acting for energy companies in the context of applying for, developing, and operating sour gas assets. If you have any questions concerning the impacts of these developments or the regulation of sour gas operations more broadly, please contact the authors of this post.
