Greenhouse gas intensity of geologic hydrogen produced from subsurface deposits

Geologic hydrogen (H2) deposits could be a source of climate friendly energy. In this work, we perform a prospective life cycle assessment of a generic geologic hydrogen production and processing system. While it is still too early in the development cycle to estimate precise life-cycle carbon intensities (CI) we can use fundamental engineering physics and chemistry to estimate CI. Our baseline case includes gas with 85 mol% H2, and remainder N2 (12%) and CH4 (1.5%) and other inert gases (1.5%). We set baseline productivity, depth, and other producing parameters using data from US natural gas wells drilled to date. We use a modified version of the OPGEE open-source oil & gas life cycle assessment tool to model the energy use and emissions from geologic hydrogen systems. Producing, dewatering, separating, and (if needed) compressing the gas results in a site-boundary GHG intensity of ~0.4 kg CO2eq. GHG per kg of H2 produced for the baseline case, or ~3 gCO2eq./MJ LHV H2. The largest sources of GHG emissions are fugitive losses from the system and embodied emissions in constructed wellbores and equipment. Sensitivity analysis is performed by examining varying gas compositions, as wells other resource parameters and production methods. We find that results are most sensitive to the gas composition – mainly the amount of H2 and CH4 in the raw gas stream. Other key drivers include how the process is powered and fueled, and the methods of handling and disposing of the separated non-H2 species.