Separating Environmental Modulation From Source Intermittency in Methane Leakage From a Legacy Well in British Columbia

Abstract

Abstract Legacy oil and gas wells are an incompletely quantified source of greenhouse gas emissions and offer leakage pathways for subsurface gas migration if inadequately isolated. We present a high‐resolution, process‐based characterization of leakage dynamics at an integrity‐compromised legacy gas well in northeastern British Columbia using five spatial flux surveys and approximately 70 hr of continuous hotspot measurements of methane (CH 4 ) and carbon dioxide (CO 2 ), integrated with co‐located soil and meteorological observations. Multivariate statistics and machine learning were used to separate environmental modulation from intrinsic source variability. Methane emissions formed a compact, persistent core (∼1 m 2 ), whereas CO 2 exhibited a broader (∼30 m 2 ), evolving footprint consistent with near‐surface oxidation and redistribution during upward gas migration. Baseline CH 4 flux was strongly modulated by soil thermal–hydrological and atmospheric conditions, whereas episodic high‐flux events were not explained by surface drivers, indicating transient subsurface source dynamics. Consequently, similar surface flux magnitudes can arise from different combinations of environmental state and source strength, demonstrating that flux magnitude alone is not a unique indicator of leakage severity. These findings show how short‐duration or spatially limited surveys can mischaracterize well‐scale emissions and that CH 4 ‐only measurements may under‐detect leakage where oxidation broadens the CO 2 signal. By resolving environmental modulation and source intermittency, this study provides process‐level insight into legacy‐well leakage and a framework for interpreting surface flux measurements and designing multi‐species monitoring strategies across emissions assessment, abandoned‐well management, and subsurface gas storage.