The patent literature splits into three broad routes: thermal methods such as steam and cyclic injection, chemical methods such as polymer and surfactant flooding, and gas-based methods such as CO2 flooding. In this corpus, well-construction mechanics (IPC class E21B) and recovery-fluid material formulations (C09K) account for the largest share of filings, meaning most patented value sits in how fluids and equipment are engineered rather than in monitoring software. Thermal equipment and polymer chemistry form a smaller but still significant secondary layer.

Filings peaked at 99 in 2019 and fell toward a 2022 midpoint of 35, a pattern consistent with a field where the dominant players staked their core claims early and then slowed rather than continuing to expand. It does not necessarily mean interest in EOR is fading commercially — patent activity and commercial deployment do not move in lockstep. Any apparent drop in the very latest year should also be discounted, since publication typically lags filing by around 18 months.

The ranking is led by a recognisable set of major integrated oil companies, oilfield-services firms and specialty-chemical groups, several of which co-file with their own affiliated entities rather than with outside partners. Behind this core sits a long tail of assignees with only one or a few filings each, which is typical of a mature extraction technology rather than an emerging one. Recent-year filing counts for several of these historically active assignees have dropped to zero, though publication lag makes the most recent year an unreliable basis for judging current strategy.

The clearest under-claimed territory sits adjacent to the dense core: digital reservoir-optimisation methods, solar-thermal steam generation for injection processes, enzymatic or biocatalytic flooding agents, and hybrid chemical-thermal cyclic injection schemes all show comparatively thin filing density against the core E21B and C09K claim territory. That thinness reflects lower filing density, not lower technical merit or lower commercial relevance — it means less prior art stands in the way of a well-drafted first claim in these areas.

US20090288826A1 combines stabilized enzymatic fluid treatment with cyclic steam or heated-fluid injection into subterranean formations, which means new filings claiming enzyme-assisted thermal recovery need to design specifically around that combination rather than around either enzymatic treatment or cyclic steam injection alone. Because it merges a chemical mechanism with a thermal one, it sits at the intersection of two of the corpus's core technology classes. Anyone drafting claims in enzymatic or biocatalytic EOR should review this filing's claim scope before finalising language on the injection-cycle mechanism.