Available at: https://digitalcommons.calpoly.edu/theses/3241
Date of Award
3-2026
Degree Name
MS in Civil and Environmental Engineering
Department/Program
Civil and Environmental Engineering
College
College of Engineering
Advisor
Derek Manheim
Advisor Department
Civil and Environmental Engineering
Advisor College
College of Engineering
Abstract
Plugged and abandoned (P&A) oil and gas wells have the potential to serve as conduits for subsurface gas migration to the atmosphere, representing a source of greenhouse gas emissions and potentially hazardous compounds. While methane emissions from P&A wells have received growing attention, the broader suite of volatile organic compounds (VOCs) transported through these wells remains under-studied, as do the soil and physicochemical factors governing their surface emissions. In this investigation, static flux chamber measurements were performed on sixty P&A wells in the San Joaquin Basin, to determine flux of 97 unique chemical compounds. Soil samples were also collected at each site to characterize the influence of soil properties on gas emissions.
Gas fluxes were highly skewed, with two wells accounting for 99.5% of all gas flux. These emissions consisted almost entirely of methane (99.9%), likely from a thermogenic source. Both high-emitting wells were located in the Kern Bluff field, constructed in the late 1940s to early 1950s, had a low ratio of cement volume to total annular volume, and never produced oil or gas, all of which may represent risk factors for elevated emissions. However, the wide variety of mechanisms through which leakage pathways can form, combined with spatiotemporal variability in reservoir characteristics and well iv construction/abandonment quality, makes predicting which wells will become high emitters from records alone extremely difficult, emphasizing the value of direct large scale flux measurements in legacy well monitoring programs.
Net methane flux across the remaining fifty-eight wells was negative; therefore, plugging and abandonment practices in California appear largely effective for the wells sampled. Elevated fluxes of other thermogenic compounds including alkanes and reduced sulfur compounds were observed at the two high-emitting wells, these co occurred with elevated methane flux, suggesting that methane may be a viable indicator for emissions of other reservoir associated gas. Non-thermogenic VOCs did not show significant well-related emissions relative to controls, with observed fluxes largely attributable to atmospheric exchange, compound-specific biogenic production, and other localized sources.
Soil properties including increasing clay content, gravimetric water content, and tortuosity demonstrated moderately strong statistical correlations with reduced flux magnitude, consistent with expected gas transport behavior. Physicochemical properties, particularly molecular weight, also showed meaningful correlations with flux across chemical families. These findings suggest that soil compaction and amendments with fine-grained materials such as bentonite during the plugging and abandonment process may help to mitigate surface emissions over the long-term.