Anton Kaiser

Authors:

Anton Kaiser, Qingwang Yuan, and Mohamed Mehana

Natural and stimulated geologic hydrogen has the potential to emerge as a new, clean energy source. Understanding the mechanisms of hydrogen generation in natural settings and stimulated conditions is crucial to evaluating and optimizing the potential of a source rocks. Most of the research focuses on the closed system, there is a lack of a model for simulating hydrogen production and evaluating it in an open system. The goal of this study is to provide reliable geochemical models for hydrogen production from serpentinization of mafic and ultramafic rocks in closed and open systems. In this work, two geochemical models were created that allow to estimate the potential volume of hydrogen produced for a given volume of rock and its variations with time for different conditions.

Geochemical program PHREEQC is used in this study to model kinetic reaction rates and thermodynamic equilibrium conditions for serpentinization. Based on experimental conditions, two models were created: 1) a kinetic reaction model for hydrogen generation and rock alteration over time; and 2) a thermodynamic model showing hydrogen generation at equilibrium conditions. After achieving a good match to experimental results, the model was used to test sensitivities of hydrogen production to various factors: temperature, pressure, rock composition, and pH. Running the models as open and closed systems allows to assess the increase of generated hydrogen from a source rock as well as the impacts of changing parameters that control the reactions and equilibrium.

The results from validated models show how all different factors affect hydrogen generating reactions. Temperatures above 400°C and a pH above 8 stopped all hydrogen production. Specifically, rock composition has a major impact on hydrogen production when temperature varies. Lower Fe(II) bearing rocks (Fo70 and above), higher temperatures around 300°C are required to maximize hydrogen production as Mg(II) consuming reactions that do not produce H₂ are favored otherwise. This leads to low Fe(II) bearing rocks only generating significant volumes in a small temperature region. Rocks with Fe(II) bearing components above 30% yield best results at temperatures around 150°C. Extracting hydrogen from the system (i.e., an open system) showed that that hydrogen generation can be increased by a factor of more than 2 compared to that of a closed system, and generation rate is accelerated by up to 6 times. Enabling an open system towards stimulation conditions, higher hydrogen yields are also observed at lower temperatures than its closed system counterpart. Peak hydrogen generation is observed at 500bar with reactions stopping at pressures around 800 to 950bar depending on initial rock composition.

The work provides reliable geochemical models for analyzing, assessing, and optimize hydrogen production that is not available previously. Adjusting both pressure and temperature can significantly increase the total hydrogen yield and generation rate. Even for iron-rich rocks with low Fe(II) content, hydrogen production can be increased by more than 10 times at optimized pressure and temperature conditions. These findings provide important insights for analyzing the potential hydrogen generating source rocks in natural settings and the possibility of increasing hydrogen yield through stimulation.