When a reservoir is flooded with polymer, the mobility ratio between the displaced fluid and the displacing fluid become favourable compared to the conventional water flooding. In the oil and gas industry, the synthetic polymer polyacrylamide in hydrolysed form and the biopolymer xanthan are being used for this purpose. However, the polyacrylamide is susceptible to high temperature and salinity. Also, its synthetic nature makes it harmful to the environment. The biopolymer xanthan has the problem of degradation and both are very expensive. With the shortfall in crude oil price and the high cost of exploitation and drilling new wells, there is need to look inward and think out of the box in formulating new improved polymers that can combat these problems. Natural polymers from agricultural and forest produce are abundant in nature, cheap and environmentally friendly. These agricultural and forest produce contain starch and cellulose which are known to have rigid and long polysaccharide chains that can withstand the harsh reservoir conditions. But the design of a polymer flood or a permeability-modified process involving polymer requires knowledge about the polymer flow mechanism and the rheological behaviour of the porous media. This paper, therefore, reviews the available natural polymers that can be used for enhanced oil recovery applications and the mechanism affecting their flow behaviour in porous media. The emphasis is on the physical aspect of the flow, the microscopic rheological behaviour of the natural polymers. The dominant mechanism of the flow process was adsorption, mechanical entrapment and hydrodynamic retention. It was observed that the polymer exhibited non-Newtonian, pseudoplastic and shear-thinning behaviours. The literature review on oil displacement test indicates that natural polymers can recover additional oil from an oil field. Environmental application issues associated with the application of natural polymers have opened new frontier for research and are also highlighted herein.

Nanotechnology has found its way to petroleum engineering, it is well-accepted path in the oil and gas industry to recover more oil trapped in the reservoir. But the addition of nanoparticles to a liquid can result in the simplest flow becoming complex. To understand the working mechanism, there is a need to study the flow behaviour of these particles. This review highlights the mechanism affecting the flow of nanoparticles in porous media as it relates to enhanced oil recovery. The discussion focuses on chemical-enhanced oil recovery, a review on laboratory experiment on wettability alteration, effect of interfacial tension and the stability of emulsion and foam is discussed. The flow behaviour of nanoparticles in porous media was discussed laying emphasis on the physical aspect of the flow, the microscopic rheological behaviour and the adsorption of the nanoparticles. It was observed that nanofluids exhibit Newtonian behaviour at low shear rate and non-Newtonian behaviour at high shear rate. Gravitational and capillary forces are responsible for the shift in wettability from oil-wet to water-wet. The dominant mechanisms of foam flow process were lamellae division and bubble to multiple bubble lamellae division. In a water-wet system, the dominant mechanism of flow process and residual oil mobilization are lamellae division and emulsification, respectively. Whereas in an oil-wet system, the generation of pre-spinning continuous gas foam was the dominant mechanism. The literature review on oil displacement test and field trials indicates that nanoparticles can recover additional oil. The challenges encountered have opened new frontier for research and are highlighted herein.

The rate of replacement of produced oil and gas reserves by new discoveries is in a state of steady decline. Instead of searching for rare new oil fields, it is more economically justified to improve production from the existing and known fields. This is often achieved using enhanced oil recovery (EOR) technologies. The application of EOR in the North Sea dates to the mid-1970's with most of the fields being flooded with gas due to their light oils. Following a critical review of relevant published literature, the EOR methods in the past five decades are: water alternating gas (WAG), miscible gas injection (MGI), foam assisted water alternating gas (FAWAG), simultaneous water and gas (SWAG), and microbial enhanced oil recovery. The first part of this paper explores the advantages and limitations of the field implementation of gas EOR methods in North Sea oil fields. In the second part, new screening criteria of WAG, SWAG, MGI and FAWAG were developed by performing statistical analysis of the data from the past field experiences, especially in the North Sea. The screening criteria of the future methods are clearly documented in the literature and therefore not covered in this study. From the screening criteria, it has been identified that most North Sea fields qualify for WAG. This explains why WAG has been the most common scheme in the North Sea. FAWAG should also be implemented either after WAG or SWAG when the residual oil saturation is < 20%.

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Polymers play a significant role in enhanced oil recovery (EOR) due to their viscoelastic properties and macromolecular structure. Herein, the mechanisms of the application of polymeric materials for enhanced oil recovery are elucidated. Subsequently, the polymer types used for EOR, namely synthetic polymers and natural polymers (biopolymers), and their properties are discussed. Moreover, the numerous applications for EOR such as polymer flooding, polymer foam flooding, alkali–polymer flooding, surfactant–polymer flooding, alkali–surfactant–polymer flooding, and polymeric nanofluid flooding are appraised and evaluated. Most of the polymers exhibit pseudoplastic behavior in the presence of shear forces. The biopolymers exhibit better salt tolerance and thermal stability but are susceptible to plugging and biodegradation. As for associative synthetic polyacrylamide, several complexities are involved in unlocking its full potential. Hence, hydrolyzed polyacrylamide remains the most coveted polymer for field application of polymer floods. Finally, alkali–surfactant–polymer flooding shows good efficiency at pilot and field scales, while a recently devised polymeric nanofluid shows good potential for field application of polymer flooding for EOR.

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