Enzymes for Enhanced Oil Recovery (EOR)
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Primary oil recovery by reservoir pressure depletion and secondary oil recovery by waterflooding usually result in poor displacement efficiency. As a consequence there is always some trapped oil remaining in oil reservoirs. Oil entrapment is a result of complex interactions between viscous, gravity and capillary forces. Improving recovery from hydrocarbon fields typically involves altering the relative importance of the viscous and capillary forces. The potential of many EOR methods depends on their influence on fluid/rock interactions related to wettability and fluid/fluid interactions reflected in IFT (Green and Willhite, 1998). If the method has the potential to change the interactions favorably, it may be considered for further investigation, i.e. core flooding experiment, pilot and reservoir implementation. Enzyme-proteins can be introduced as an enhanced oil recovery method to improve waterflood performance by affecting interactions at the oil-water-rock interfaces. An important part of this thesis was to investigate how selected enzymes may influence wettability and capillary forces in a crude oil-brine-rock system, and thus possibly contribute to enhanced oil recovery. To investigate further by which mechanisms selected enzyme-proteins may contribute to enhance oil recovery, groups of enzymes with different properties and catalytic functions, known to be interfacially active, were chosen to cover a wide range of possible effects. These groups include (1) Greenzyme (GZ) which is a commercial EOR enzyme and consists of enzymes and stabilizers (surfactants), (2) The Zonase group consists of two types of pure enzyme, Zonase1 and Zonase2 which are protease enzymes and whose catalytic functions are to hydrolyze (breakdown) peptide bonds, (3) The Novozyme (NZ) group consists of three types of pure enzyme, NZ2, NZ3 and NZ6 which are esterase enzymes and whose catalytic functions are to hydrolyze ester bonds, and (4) Alpha-Lactalbumin ( -La) which is an important whey protein. The effect of enzymes on interactions in the oil/brine/solid system was studied. It was found that enzymes can change the adhesion behavior of the crude oil on glass surfaces from adhesion to non-adhesion when they are added to the brine solution. This was confirmed by contact angle measurements, which showed that contact angles became more water-wet (i.e. decreased) after exposure to enzyme solutions. Possible mechanisms giving rise to these observations, including catalysis of ester hydrolysis and enzyme adsorption, were discussed and tested. An experimental study of changes in oil-water interfacial properties by addition of enzymes and proteins, including measurements of interfacial tension and electrophoretic mobility, has been performed. It was found that the effect of enzymes on oil-water properties is minor compared to their effect on oil-water-solid properties. Their contribution to change interfacial tension between oil and water is not significant while they affect the electrophoretic mobility of emulsified oil in enzyme-brine solution to some extent. Attempts were also made to study changes in both oil and water phase composition after equilibration with enzymes. However, since the chemical composition of crude oil is highly complex, a model oil was used in some of the experiments. The model oil was chosen to be a water insoluble ester (ethyl decanoate) solved in mineral oil in an effort to verify the possible role of catalysis of ester hydrolysis. Dynamic core displacements using sandstone and carbonate rocks were conducted to show the potential of improved oil recovery by enzyme- and combined enzyme-surfactant flooding. Most of the core flooding experiments commenced with water flooding from initial water saturation, Swi, (established with synthetic sea water) which will be referred to as secondary mode displacements. Accordingly, tertiary oil recovery processes were used to describe injection of enzyme and/or enzyme-surfactant solutions from residual oil saturation, Sor, established by the secondary displacements. The core floodings were conducted on various cores of the same type to check the reproducibility of the experiments. Flooding carbonates and aged Berea sandstone cores, waterflooded to residual oil saturation, with Greenzyme added to the water phase gave an additional recovery of between 3 and 11 % OOIP. One experiment on aged sandstone core and two on carbonate cores performed with one of the esterase enzymes also showed a reduction in residual oil in the same ranges as that observed for Greenzyme. From a capillary desaturation point of view, the reduction in interfacial tension obtained by adding Greenzyme is not sufficient to induce mobilization of residual oil. Further, a reduction in residual oil saturation was found after flooding with one of the esterase enzymes, which did not affect the oil-water interfacial tension. Based on these observations, we expect wettability changes to be the main factor contributing to mobilization of oil remaining after regular waterflood. To explore this hypothesis further, micromodel experiments were undertaken. Micromodel experiments showed change in amount of residual oil saturation by enzyme-brine flooding when 1wt% Greenzyme and NZ2 were added to the brine solution. The amount of change in residual oil saturation was consistent with the incremental oil recovery produced in the core flooding experiments. Micromodel experiments also showed that the pattern of residual oil saturation change significantly to more distributed oil patches by injecting enzyme-brine solutions although the additional oil production was relatively low. This change in pattern of residual oil saturation is likely related to wettability alteration toward more water-wet state induced by enzyme-brine solution. The evidences of wettability alteration made by micromodel experiments could validate our proposal, wettability alteration, as the main mechanism contributing to increasing oil recovery.