Validation of results obtained from static and dynamic experiments

measurements, respectively, in first part of experimental work to test the enzymes’ effect on those properties. Adding enzymes to the brine phase gave a significant decrease in contact angle for Greenzyme and the three different esterase enzymes tested (see Chapter 7).

However, only Greenzyme was found to have an effect on oil-water interfacial tension, which was reduced by a factor of approximately 3 when Greenzyme was added (see Chapter 8). In second part of experimental work, flooding aged Berea sandstone and carbonate cores waterflooded to residual oil saturation, with Greenzyme added to the water phase gave an additional recovery of between 3 and 11 % OOIP. Three experiments performed with one of the esterase enzymes also showed a reduction in residual oil in the same ranges as that observed for Greenzyme.

Using micromodels can contribute to improved understanding of basic mechanisms involved in oil recovery techniques by providing direct observations of pore level events. In this work, micromodels were used to gain insight into the mechanisms by which enzymes may contribute to recovering incremental oil (see chapter 10). Glass micromodels are two-dimensional models of pore system which can be a representative of porous media. Such models can connect our understanding from static measurements on flat surfaces and dynamic measurements in cores by providing visualization of flow within the pore space. The latter includes observations of the effect of pore and fluid variables on the trapping and subsequent mobilization of residual phases during simulated secondary and tertiary recovery processes.

While glass micromodels have been used mostly as tools for qualitative study, some researchers have used image-analyzing techniques to measure the oil saturation for each step of flooding (e.g. Jeong et al., 2000; Sohrabi et al., 2000 and 2004). In this study, a computerized image processing system was employed for oil saturation measurement before and after enzyme-brine treatment.

Micromodel experiments showed changes in the residual oil saturation by enzyme-brine flooding when 1wt% Greenzyme and NZ2 were added to the brine solution. The incremental oil recovery by enzyme-brine flooding seems to be independent of the amount of the residual oil after waterflooding. Experiments with Sor of 0.16 and 0.22 (saturation unit) after waterflooding, produced almost the same amount of incremental oil after injection of

enzyme-Chapter 11. Summary of the main results

brine solutions. Reduction in oil saturation by enzyme-brine injection, as well as the apparent independency between incremental oil and initial residual oil saturation after water flooding was consistent with the results of the core flooding experiments which were presented in chapter 9.

As discussed in the core flooding chapter, wettability alteration may be the main factor contributing to mobilization of oil remaining after waterflood. Wettability alteration of flat surfaces by adding enzymes to the brine solution was shown and discussed in detail in chapter 7. Micromodel experiments showed that the pattern of residual oil saturation change significantly after enzyme-brine flooding, and that the observed changes can be the result of wettability alteration towards a more water-wet state. Figure 11.3shows an example of the change in oil saturation distribution through the micromodel. As Figure 11.3shows, the residual oil pattern is modified to more distributed oil patches by injecting of enzyme-brine although the additional oil production was relatively low.

Figure 11.3: An example of micromodel @ Sor after different stages of flooding (a) after waterflooding and (b) after enzyme-brine flooding.

The enzyme-brine solution could mobilize oil from areas which were not swept by water in the waterflooding step. By injecting enzyme-brine solution in the second stage of the flooding, the flow mechanism seemed to behave more like the water-wet system and reach to

(a) (b)

Chapter 11. Summary of the main results

the by-passed oil zones in the model through water films. By continuing the injection the water filaments surrounding the oil present in the larger pore-bodies, thicken progressively and leave oil filaments in the middle of pores. This filling mechanism finally causes oil snap-off at the pore throats. At a later stage some of the by-passed oil starts to move along the model, which leads to the distribution and re-trapping of small oil patches along the model (see Figure 11.3). Figure 11.4 shows some examples of snap-off which occurred after about 3 PV enzyme-brine injections into the model.

Figure 11.4: Examples of snap-off event by injection of enzyme-brine in experiment No.3. Time interval between images is about one minute.

Wettability alteration to a more water-wet state can also be investigated by considering the curvature of the oil-water interface (see e.g. Buckley, 1996; Sohrabi et al., 2000 and 2004). In mixed-wet systems where contact angle between oil and water is relatively low (around 90 degree), the oil-water interfaces are fairly flat (Buckley, 1996). Investigations of high resolution photos from the model showed significant changes in the curvature of oil-water interfaces from intermediate-wet (flat interfaces) to water-wet status after exposure of the model to enzyme solution. Comparison of Figures 11.5-a and b shows the change in oil-water interfacial curvature. In Figure 11.5-a the majority of the interfaces show non-water-wet behavior. The curvature of the oil-water interfaces display intermediate-wet behavior, where oil-water interfaces are fairly flat, or even more oil-wet behavior where the curvature of the oil-water interface is towards the oil phase. However, after exposing the micromodels to

(a)

(f) (b)

(e)

(c)

(d)

Chapter 11. Summary of the main results

enzyme-brine solution (see Figure 11.5-b), the oil-water interfaces appear to be more water-wet with the interfacial curvature towards the water phase. This change in behavior to a more water-wet state was observed in all experiments after flooding with several pore volumes of enzyme-brine solution.

Figure 11.5: Example of effect of wettability change on residual oil saturation in micromodel experiments after enzyme-brine flooding (experiment NO.3). (a) after waterflooding (b) after about one day enzyme-brine

flooding.

The evidences of wettability alteration made by micromodel experiments can validate our proposal, wettability alteration, as the main mechanism contributing to increasing oil recovery. Observations of wettability change were in agreement with contact angle measurements on glass surfaces and incremental oil recovery after enzyme-brine injection in micromodel experiments were also in agreement with core flooding experiments.

(a)

(b)