The Climate Impact of US Offshore Oil and Gas Production

The combustion of coal, oil, and natural gas is the primary driver of contemporary climate change. Mitigation efforts must contend with the reality that fossil fuels will remain a large fraction of global energy use well into the 21st century. The climate impact of fossil fuels varies across fuel types and within a fuel type. Variability in the latter is strongly driven by different practices and infrastructure across different production fields. The metric, carbon intensity (CI), reflects the ratio of greenhouse gas emissions to energy and can be used to compare climate impacts of fuels. Policy and investment decisions can leverage knowledge of the carbon intensity of fuels produced in different locations to choose fuels that reduce climate impacts of current fossil fuel use. Oil and gas (O&G) production is a large source of greenhouse gases, emitted as carbon dioxide (CO2), from combustion operations, and methane (CH4), from fugitive leaks and intentional venting. Numerous studies have shown that inventory CH4 emissions from onshore fields tend to be underestimated in comparison to flux estimates calculated from atmospheric observations. However, observations of CH4 from offshore fields are sparse and few studies have incorporated existing observation-based updates to CH4 into CI calculations. This dissertation quantifies the greenhouse gas emissions from offshore O&G production using atmospheric observations and presents results as CI. First, we analyze a limited airborne sample of CH4 emissions from offshore platforms in the Gulf of Mexico and evaluate the inventory. Next, we return to the Gulf to gather a larger and more representative sample of CH4, CO2, and nitrogen oxides (NOx=NO+NO2) emissions. We combine this data-set with all facility-level fluxes of CH4 gathered by previous studies to evaluate the inventory and calculate a CI. Finally, we extend our airborne campaign to the other offshore production basins in the US: offshore the North Slope in Alaska, Cook Inlet in Alaska, and offshore California. We estimate regional CIs and combine with results from the Gulf of Mexico to calculate a CI of total US offshore production. We find that the CI of total US offshore O&G production in 2021 is 5.7 [4.5-6.8, 95% confidence interval] grams of CO2 equivalent per Megajoule of O&G produced (gCO2e/MJ). This is over twice as high as the corresponding CI from the US Environmental protection Agency (EPA). This gap is explained by elevated CH4 emissions from primarily gathering and processing platforms in shallow waters of the Gulf of Mexico. These facilities have a disproportionate contribution to total emissions, similar to onshore super-emitters, and can be targeted for emission reductions. We also find that inventory platform counts are incorrect: overestimated in federal waters and missing in state waters. In response, the EPA has updated their inventory, which we evaluate with observations. Regional CIs vary from a low of ~1 gCO2e/MJ in the newly developed deep waters of the Gulf of Mexico to higher values in declining fields that range from ~7-43 gCO2e/MJ, either due to CO2 or CH4. This pattern of higher intensities with age is expected but the range may reflect different practices, infrastructure, and regulations. These findings arrive ahead of a series of expansions in US offshore production and may be used as a baseline CI for current operations in environmental impact statements and reflects the importance of strategies that maintain low CIs as fields age.PhDAtmospheric, Oceanic & Space ScienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/176676/1/agorchov_1.pd