Production in conventional oil and gas reservoirs are obtained by pressure gradients, allowing the oil and gas to flow through the pores of the reservoir and to the producing well. Natural depletion may not be enough for this as pressure may drop below the bubble point leading to dissolved gas coming out of the solution, increasing the viscosity of the remaining oil and reducing its mobility further. Technologies for improved oil recovery (IOR) to increase recovery of mobile oil and enhanced oil recovery (EOR) to recover otherwise immobile oil are therefore key in increasing the recovery beyond current levels which remain at below 40%¹.

Water injection is the standard IOR technology to increase recovery by both improving the sweep across the reservoir and to maintain reservoir pressure above the bubble point. But water still has a limited sweep efficiency resulting in large oil volumes remaining outside the main routes taken by the water. Enhanced oil recovery generally refers to measures to achieve a higher oil recovery than that obtained by water injection alone. This includes solvent (miscible) injections, chemical injections and thermal injections. EOR technologies will address measures to increase both the macroscopic (volumetric) displacement as well as microscopic displacement of oil trapped at pore level. Loosely speaking, flooding and diversion of injected water will increase swept area and thus macroscopic displacement, while additions of surfactants and other products will increase microscopic displacements at pore level.

Injection of CO₂ is currently considered the most effective EOR technology². CO₂ is miscible with oil and it improves the microscopic displacement efficiency by reducing the interfacial tension between oil and water allowing the oil to move more freely to the production well. Core samples tested in the lab at NTNU in Norway³ shows that close to 100% recovery is possible under favourable conditions where pressure and temperature is optimal such that CO₂ is in a liquid state so that there is no capillary forces and no fingering. The limiting factor today for CO₂ to scale is primarily access to large enough volumes for injections and the corrosive nature of CO₂ resulting in high cost to upgrade wells and equipment. The first hurdle may be overcome as more CO₂ is expected to become accessible from increased steam-methane reforming (SMR) of natural gas to produce hydrogen – which is expected to occur in several regions of the world where hydrogen production will grow.

For chemical injections, most focus has been put on polymers as an additive to the injected water. A polymer is a long chain of molecules, which increase the viscosity of the injection water and thus provides better macroscopic sweep of the reservoir. Often such polymer flooding is combined with additions of surfactants and alkalis to also reduce the interfacial tension between the oil and water contacts, and to limit adsorption of the chemicals in the reservoir, respectively. The polymers may also be used to form gel-like plugs in the reservoir to provide flow diversion of water for even better sweep deep in the reservoir. The most used polymer is currently polyacrylamide (PAD), which is any polymer with acrylamide as one of its monomers. The main challenge using such polymers is the fact that it mechanically, thermally and chemically degrades into acrylamide, which is a highly toxic substance and not allowed to be used e.g. in the North Sea⁴. The search is therefore on to find more environmentally friendly and bio-degradable chemical injection products for EOR.

Nanoparticles offer a green attractive alternative to conventional polymers for EOR. Both inorganic and organic alternatives exist today.  Inorganic nano silica particles are abundant and cheap to use. Their properties can also be adjusted to be hydrophilic or hydrophobic depending on needs. Injections of silica nanoparticles has shown to improve microscopic displacements through reductions in interfacial tensions between oil and water, desired wettability alterations and these nanoparticles may plug and block pore channels at sufficient high concentrations and create new desired pathways for water⁵.

Organic nanoparticles based on celluloses offers a green alternative to toxic polymers. Celluloses is the most abundant organic polymer on Earth and the two most common cellulosic sources are wood and cotton. Nanocelluloses can refer to either cellulose nanocrystal (CNC) or cellulose nanofibers (CNF) and are typically in the size range from a few to several thousands nanometres. Nanocellulose can be made from renewables sources or even from unused waste products from paper production. Nanocelluloses is also bio-degradable in high-salinity water and can be discharged directly into the ocean if needed. Nanocelluloses will increase viscosity of injection water and improve macroscopic sweep efficiency. Furthermore, novel multifunctional hybrid polymers offer in-depth placement of gel plugs in the reservoir for advanced water diversions, which will further improve the sweep area⁶. Experimental tests have shown several attractive features of using nanocellulose for EOR. Ideally you want the injection fluid to have low viscosity during injection to easy pumping into the reservoir, while it should have high viscosity when in place to increase macroscopic sweep efficiency. Nanocellulose has shown to have such attractive features as the viscosity increase substantially when he temperature is increased from 60° C to 120° C. Experiments have shown that even small fractions (1%) with aqueous celluloses nanocrystals in a low-salinity brine sandstone core have recovery around 62.2 % whereof 1.2% incremental recovery came from oil produced during the nanocelluloses flooding phase⁷. Log-jamming formations inside the core is believed to be the likely cause of the additional oil recovery during the nano flooding phase. Since the initial recovery in this experiment was as high as 61%, the additional recovery from the nanocelluloses is still considered substantial. Furthermore, this experiment showed that the core sample were homogeneous, and that the viscosity increase observed under porous flow conditions was different from the static laboratory tests. So, for heterogenic reservoirs and with increased temperature-induced viscosity increases of the nanocelluloses, even larger EOR recoveries should be expected⁷.

Opportunities and market impacts

The average recovery factors for oil reservoirs globally remains at less than 40%¹. With existing fields declining at around 4% per annum and few new major discoveries are made, the need to recover more oil and gas from existing reservoirs becomes paramount. Potential for value creation from IOR and EOR is therefore large. In the BP Technology Outlook⁸, it is estimated that an additional 500 billion barrels of oil, or a 10% aggregate increase in total recovery is possible through new technologies by 2050. Estimates for the Norwegian Continental Shelf made by Rystad Energy for OG21⁹ estimate that additional volumes of several billion barrels of oil is possible through EOR technologies such as CO₂ for EOR and advanced water diversion technologies.

Risks and uncertainties

While the recovery potential of EOR measures are high, the long time it takes for physical effects to materialise after injections creates long payback periods for such investments. For nanocelluloses this is exacerbated by the remaining  technical uncertainty related to performance of the technology. Decision uncertainty therefore remains high for these green nano technologies at the present, requiring more pilots and full field test before industrial scaling can be expected. Simultaneous advances in novel tracer technologies is expected to provide additional insights into the detailed performance of the nano technologies, contributing to reduce the technical uncertainty. By 2025 we believe that the technical uncertainty will have been resolved so that the technology should be ready for implementation towards 2030. Main drivers will be larger ultimate recovery and regulations.

Sustainability & Governance

The highly toxic nature of traditional polymers makes green nano particles attractive from an environmental and sustainability aspect. The handling of produced water with traditional polymers requires necessary topside modifications. Nanocelluloses are green and bio-degradable and can be discharged to water directly. Production of the nanocelluloses can be made from waste or bi-products from other celluloses value chains (forestry, paper production, plant production, etc) and production can therefore be fully sustainable.

Contributors

Main author: Frank Børre Pedersen

Editor: Peter Lovegrove