EOR (EOR)

Enhanced Oil Recovery (EOR) is the third stage of crude oil extraction, applied to mature reservoirs after primary recovery (natural pressure) and secondary recovery (water flooding) have left 60–70% of the original oil trapped in pore spaces. EOR techniques inject substances — most commonly CO₂, sometimes nitrogen, steam, or chemical surfactants — that physically and chemically alter the reservoir to free additional oil. CO₂-EOR is the largest and most established form, used in the United States Permian basin since the 1970s and increasingly considered for India's mature onshore fields.

The mechanism involves three effects when CO₂ contacts reservoir oil at high pressure (typically above 10 MPa):

• Miscibility — CO₂ dissolves into the oil, reducing its viscosity and interfacial tension with the rock
• Swelling — dissolved CO₂ expands the oil volume, displacing it toward production wells
• Pressure maintenance — injection sustains reservoir pressure that would otherwise decline

For Indian CBG operators, EOR represents an emerging market for biogenic CO₂ off-gas. ONGC and Cairn Oil & Gas have run pilot CO₂-EOR projects at fields in Gujarat, Rajasthan, and Andhra Pradesh, with potential demand of several million tonnes per year if commercialised at scale. A CBG plant producing 5 TPD of biomethane simultaneously generates 8–10 TPD of CO₂ that could, in principle, be liquefied (₹2,500–4,000 per tonne in transport-ready form) and sold to oil companies for EOR.

The trade-offs limiting CBG-to-EOR sales in India are practical. EOR-grade CO₂ requires 95–99% purity and very low water content (under 50 ppm) to avoid corrosion of injection wells. CO₂ delivery requires either pipeline infrastructure (essentially non-existent in India) or cryogenic tanker transport at distances under 500 km to remain economic. And EOR projects need long-term, contracted CO₂ supply at predictable prices, which small CBG plants cannot reliably provide. As a result, EOR remains a theoretical rather than realised offtake for most Indian CBG operators today.