Examining Lateral Gas Migration at Energy Wells - Insights for Risk Management, Monitoring, and Research Directions

Gas migration, the flow of gas outside the surface casing of a well (BC OGC, 2018), is a well integrity issue affecting an estimated <1% up to ~10% of energy wells, depending on geographical region, drilling era, geology, gas source, and other factors (e.g., Davies et al., 2014; Bachu, 2016; Lackey et al., 2017). The risk associated with a given case of gas migration may be defined as the likelihood for gas phase and/or dissolved phase methane to migrate away from a well over a distance to a potential receptor (e.g., a residence or water well), combined with associated potential consequences in relation to human health, safety, and the environment (Welch and Cahill, 2017). All reported cases of gas migration in British Columbia (150 cases, BC OGC, 2018) and Alberta (3276 cases, Bachu, 2017) are based on field observations of surface gas efflux proximal to the wellhead, typically extending radially less than 6 m but up to ~20 m (BC OGC, 2018; Forde et al., 2019). In rarer documented case studies, however, surface gas efflux indicators or water well impacts have been observed at significantly greater distances of 1 km or more (e.g., Chilingar and Endres, 2005; Hammond, 2016). Additionally, recent experimental and numerical modelling work has demonstrated the importance of geologic anisotropy and heterogeneity in influencing gas migration extent (e.g., Cahill et al., 2017; Moortgat et al., 2018). It is, therefore, reasonable to infer there would be a large range in extent of free phase and/or dissolved phase natural gas plumes associated with energy wells exhibiting gas migration. This presentation reviews previous research and data that informs potential lateral gas migration distances for cases of gas migration to examine the likelihood aspect of risk potential. Insights are extracted regarding setbacks, investigation distances, key attributes for assessment, and research needs.