Oil recovery in single capillaries and porous media using wetting nanofluids

Recent experiments and modeling conducted in our laboratory have demonstrated that the spreading of nanofluids, liquid suspensions of nanosized particles, on solids are enhanced due to self-structuring of nanoparticles in the confined three-phase oil-nanofluid-solid contact region. Nanofluids have recently been proposed as agents for enhanced oil recovery (EOR). Despite recently widely conducted research using nanofluids for EOR, the underlying operating mechanism is not well understood. In this thesis, we attempt to understand the mechanism of nanofluid based EOR and evaluate its performance from reservoir core samples and model systems (glass capillary and sintered bead-pack). To visualize how oil displacement in the rock pores by nanofluid, we conducted model study using hexadecane and single glass capillary and showed the oil film dynamics in air and nanofluid after oil was displaced in the capillary. Based on the understanding of the role of nanofluid on oil displacement in capillaries, we conducted imbibition tests using Berea sandstones and flooding experiments in sintered glass-beads. X-ray microtomography was used to visualize and analyze fluid distribution and to see the effect of nanofluid in EOR. We finally considered fractured media by fabricating such structures. The dynamics of a cylindrical hexadecane layer deposited inside glass capillaries after the oil/air displacement was studied experimentally and by modeling. The oil layer subject to surface perturbation becomes unstable forming uniform, regularly-spaced double concave menisci across the capillary that are bridged with dimples (collars). In order to reveal the phenomena of the film thinning and stability between the double concave meniscus and the dimple, we monitored an air bubble approaching a flat glass surface in hexadecane. We found that the oil film thinning in a cylindrical glass capillary and on a flat glass substrate were similar; We adapted the model proposed by Gauglitz and Radke for our system (oil-air displacement) and solved it numerically. The numerical result shows a stable film between the liquid bridge and the dimple, which is consistent with our experimental observations. We also estimated the meniscus-film-dimple thickness profile and found it was in fair agreement with the model prediction.;The dynamics of cylindrical hexadecane film after displacement by a nanofluid in a glass capillary was studied. We found the thick hexadecane film is unstable, and over time it breaks and forms a thin film. Once the thick film ruptures, it retracts and forms an annular rim (liquid ridge) that collects liquid. As the volume of the annular rim increases over time, it forms a double concave meniscus across the capillary and dewetting stops. The thin film on the right side of the double concave meniscus then breaks and the contact angle increases. The process repeats until droplets build along the capillary wall. Finally, the droplets are displaced from the capillary wall by the nanofluid and spherical droplets appear inside the capillary. This is a novel phenomenon not observed during dewetting by a solution without nanoparticles. The theoretical model based on the lubrication approximation using the capillary pressure gradient was developed to estimate the annular rim dewetting velocity. The predicted dewetting velocity is found to be in fair agreement with the experimental value.;We conducted imbibition tests using a reservoir crude oil and a reservoir brine solution with a high salinity and a suitable nanofluid that displaces crude oil from Berea sandstone and single glass capillaries. We present visual evidence of the underlying mechanism based on the structural disjoining pressure for the crude oil displacement using a polymeric nanofluid (our definition of such a fluid means a suspension of polymeric particles in an aqueous substrate) in high salinity brine. The polymeric nanofluid is specially formulated to survive in a high salinity environment and is found to result in an increased efficiency of 50% for Berea sandstone compared to 17% using the brine alone at a reservoir temperature of 55 °C. These results aid our understanding of the role of the nanofluid in displacing crude oil from the rock especially in a high salinity environment containing Ca++ and Mg ++ ions. Results are also reported using Berea sandstone and a nanofluid containing silica nanoparticles.;We conducted a series of flooding experiments at different capillary numbers to quantify the performance of a polymeric nanofluid compared to brine using the sintered glass-beads. A high resolution X-ray microtomography (microCT) was used to visualize oil and brine distribution in a sintered bead-pack before and after nanofluid flooding. The results of flooding experiments showed that an additional oil recovery of approximately 15% is possible with nanofluids compared to brine at low capillary numbers, and is as effective as high capillary number brine flooding. Nanofluid induced additional oil recovery decreases as we increase the capillary number and the total oil recovered shows a marginal decrease. At first glance, these results are opposite of what one expects in the conventional EOR, where oil recovery is known to increase progressively with increasing capillary number. These results cannot be explained based on mobilization theories due to the reduced capillarity. Our results however are consistent with the mechanism of wettability alteration caused by structural disjoining pressure leading to the formation of the wetting nanofluid film between oil and substrate.;We presented experimental studies of nanofluid flooding in fractured porous media formed with sintered glass-beads. The nanofluid injection is conducted at a rate where structural disjoining pressure driven recovery is operational. We found an additional 23.8% oil can be displaced using nanofluid after brine injection with an overall displacement efficiency of 90.4% provided the matrix was in its native wettability state.;In summary, nanofluids are excellent EOR agents and their economic viability needs to be examined.