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.