Thermal-Chemical EOR for Heavy Oil: Onshore–Offshore Adaptability, Engineering Challenges and Low-Energy Optimization

Heavy oil constitutes a vital unconventional energy resource for securing global oil supplies, yet prominent discrepancies exist between onshore and offshore heavy-oil exploitation conditions. As widely documented in global field practices, steam flooding, the dominant thermal recovery technology for heavy oil, suffers from high energy consumption and severe steam channeling, while standalone chemical enhanced oil recovery (EOR) encounters rapid agent degradation under high-temperature marine reservoir conditions. Existing review papers mostly investigate onshore steam flooding, offshore thermal recovery, or chemical flooding in isolation. They lack comprehensive cross-scenario comparisons of onshore-offshore reservoir adaptability, as well as integrated analyses of thermal-chemical coupled low-carbon development strategies. This paper critically reviews reservoir-matching criteria and practical limitations of onshore steam flooding, and summarizes unique marine-related engineering constraints and optimization countermeasures for offshore steam thermal recovery. Furthermore, platform-specific bottlenecks and optimization paths for offshore chemical EOR are fully clarified. On this basis, the microscopic synergistic oil-displacement mechanisms of thermal-chemical composite systems are elaborated, and differentiated coupling development modes suitable for onshore and offshore heavy-oilfields are proposed. Numerical simulation results based on representative onshore and offshore pilot block parameters indicate that such differentiated schemes can achieve 8–14% incremental oil recovery compared with pure steam flooding scenarios. A set of integrated energy-saving and low-carbon optimization technologies, including waste-heat recovery, geothermal-assisted steam generation, and intelligent injection-production control, is analyzed to mitigate development-related energy losses. Finally, universal reservoir, engineering, economic, and environmental bottlenecks restricting heavy-oil thermal-chemical EOR are summarized, and future research directions covering green displacement polymers, lightweight offshore integrated injection equipment, and digital energy-efficient exploitation are prospected. This review offers comprehensive references for the low-carbon and efficient development of both onshore and offshore heavy-oil resources.