An often-overlooked statement in the Energy Resource Conservation Board (ERCB)/Alberta Energy Regulator (AER) suggests procedures for gas migration testing, as outlined in Directive 20, reads: “Testing is to be done only in frost-free months. Periods immediately after a rainfall have to be avoided.”
Temporal and seasonal changes can play a major role in altering the soil gas compositions used to characterize gases migrating to surfaces from deeper source zones. Frozen and/or wet ground can limit gas flow rates to the surface, and change the dynamics of the secondary microbial processes that affect the key geochemical indicators used to assess gas migration. A decrease in the rate of gas migration to the surface will increase the residence time of these gases in the soil, leading to larger shifts in the concentration of methane. This is of particular concern, considering that one of the most common screening processes for gas migration relies solely on methane concentrations measured at the surface, and/or in shallow soil gas monitoring wells. Therefore, the ideal time to evaluate gas migration issues based on soil gas measurements is in the dry summer months when the water table is low. Heavy rainfall, cold temperatures, or any other factor(s) that affect the moisture content of the soil, or the level of the water table in a particular area, can limit the effectiveness of gas migration testing by decreasing the amount of methane migrating to the surface. Ultimately, artificially lowered methane concentrations measured at the surface will lead to well abandonments at locations that have gas migration issues.
It is nearly impossible to guarantee that every gas migration investigation will be completed under ideal conditions; however, if the conditions are not ideal, these locations should be flagged for additional review. Overlooked gas migration issues early in the screening process are a major concern, because the costs associated with post-abandonment gas migration investigations, and well re-entry, are significant.
