Mechanistic Analysis of Stage-Dependent In Situ Hydrogen Production from Heavy Oil Under Nitrogen Atmosphere

In situ gasification-assisted hydrogen production from heavy oil has the potential to simultaneously achieve in-reservoir upgrading and hydrogen generation, offering a promising pathway for the clean and efficient development of heavy oil resources. To investigate stage-dependent hydrogen-formation pathways and the thermal conversion behavior of the heavy oil-water system, batch reactor experiments were conducted to simulate heavy oil gasification under reservoir-like conditions. Experiments were carried out under a nitrogen atmosphere at different temperatures and reaction times. Combined with gas chromatography and elemental analysis, the product distribution and reaction characteristics were investigated. The results show that, under an inert N2 atmosphere with an oil-to-water mass ratio of 1:1, in situ hydrogen production from heavy oil can be qualitatively divided into three approximate stages: a deoxygenation activation stage (300–400 °C), a water–gas shift hydrogen-production stage (400–500 °C), and a coke conversion hydrogen-production stage (500–550 °C). Temperature was identified as the key factor governing the reaction pathway, and the highest H2 mole fraction of 10.84% was obtained at 450 °C for 6 h. Reaction time mainly affected thermal conversion and gas distribution, while excessively long residence time at high temperatures promoted hydrogen-consuming reactions. Elemental analysis of the high-temperature residues further revealed enhanced dehydrogenation, aromatization, and coking, indicating that coke served as an important intermediate for sustained hydrogen generation. A stage-dependent mechanism of in situ hydrogen production from heavy oil at 300–550 °C was proposed, and the dominant reactions controlling H2 generation in different temperature intervals were clarified. These findings provide a basis for process optimization and numerical simulation of in situ hydrogen production from heavy oil.