Experimental study on in-situ hydrogen generation from heavy oil: Multi-path conversion and synergistic effect

Amid the global transition toward carbon neutrality, heavy oil-which accounts for more than half of the world’s total crude oil reserves-has emerged as a promising feedstock for in-situ hydrogen generation (ISHG), overcoming the longstanding technical and economic bottlenecks of conventional heavy oil extraction and upgrading. In this study, we systematically investigate the three core ISHG mechanisms (pyrolysis, aquathermolysis, and pneumatolysis) through linear-heating pyrolysis (LHP), high-temperature high-pressure reactor (HTPR), and combustion tube (CT) experiments, with a focus on the regulatory effects of temperature, pressure, water content, reaction atmosphere, and catalysts on hydrogen production performance. Key findings show that pyrolysis enables efficient hydrogen production at temperatures above 500°C, with both water injection volume and pressure exhibiting optimal operating ranges. Catalysts effectively alter the product distribution of pyrolysis and regulate the proportion of hydrogen in the gaseous products. Aquathermolysis presents relatively low hydrogen generation efficiency under conventional low-temperature conditions, which can be significantly enhanced by elevated temperatures, and also benefits from the oxidation and mineralization of formation water. Pneumatolysis is subject to a wider range of influencing factors, where the precise regulation of air, nitrogen, and water injection is the key to achieving efficient hydrogen production. The three mechanisms exhibit complementary advantages: pyrolysis delivers high gasification efficiency in the high-temperature stage, aquathermolysis is adaptable to low-temperature reservoir environments, and pneumatolysis realizes sustained hydrogen production via combustion-gasification synergy. This work clarifies the multi-path conversion mechanisms of heavy oil ISHG, identifies the optimal operating parameters for each mechanism, and provides critical experimental support for the development of efficient, low-carbon technologies for heavy oil recovery coupled with in-situ hydrogen production.