Published: March 10, 2011 r2011 American Chemical Society 1605 dx.doi.org/10.1021/ef101572n | Energy Fuels 2011, 25, 1605–1616 ARTICLE pubs.acs.org/EF Mobilization of Fine Particles during Flooding of Sandstones and Possible Relations to Enhanced Oil Recovery Andrew Fogden,* ,† Munish Kumar, ‡ Norman R. Morrow, § and Jill S. Buckley § † Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra ACT 0200, Australia ‡ DigitalCore Pty Limited, 73 Northbourne Avenue, Canberra ACT 2600, Australia § Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071, United States b S Supporting Information ABSTRACT: The mounting evidence that waterflooding of clay-containing sandstone reservoirs using floodwater with reduced salinity can enhance oil recovery, but with unpredictably large variation in responses, demands improved understanding of the underlying mechanisms. Mobilization of clays and other fines is one candidate mechanism. Flow experiments in Berea sandstone plugs were designed such that the change in their fines distribution from before to after the oil and water injections could be imaged in exactly the same pores using scanning electron microscopy. This technique also allowed imaging of the wettability distribution on pore surfaces and was coupled to spectroscopic analysis of the adsorbed asphaltene amounts. One-phase flows switching from high- to low-salinity water led to only a low level of fines mobilization, compared to two-phase experiments in which high- or low-salinity water displaced crude oil from mixed-wet prepared plugs. The images reveal that loosely bound, partially oil-wet fines lining sandstone grains are stripped by the adhering oil during its recovery and redeposited on grains further downstream. Reduced salinity increases the fraction of fines thus mobilized by weakening their bonds to grains and strengthening their bonds to oil. Evidence suggests that these more oil-wet fines stabilize the water-in-oil curved menisci, which can aid in maintaining the connectivity of the oil phase and thus enhance oil recovery. ’ INTRODUCTION There is widespread interest in the mechanisms by which oil recovery is increased by either displacement of oil with low- salinity water at high initial oil saturation or by mobilization of residual oil established by high-salinity flooding. Laboratory results indicate that the onset of increased recovery by low-salinity flooding, if observed, occurs at salinities below 5000 ppm. This cutoff is not well-defined and is dependent on the specific components of the water phase and the properties of the rock and crude oil. Tang and Morrow 1,2 concluded that increased recovery from sandstone by low-salinity flooding requires the presence of clay, an initial water saturation, and crude oil. Later, the study of the displacement of refined oil from mixed-wet rocks showed that the presence of a crude oil/water interface is also necessary. 3 The sufficient conditions are still uncertain, as evi- denced by examples of a lack of increase in oil recovery when all necessary conditions are fulfilled. 4,5 Nevertheless, successful field tests, 6,7 other field-wide analyses, 8 and recent laboratory studies that have identified positive responses in an increasing range of outcrop and reservoir rocks 9-12 have spurred interest in identify- ing targets for low-salinity projects. In the clay-bearing Berea sandstone first investigated by Tang and Morrow, 2 an increase in the effluent pH was observed and ascribed to ion exchange with high-surface-area clays. However, effluent interfacial tensions were diminished by less than 30% from ∼25 mN/m, insufficient for improved recovery by lowering the ratio of capillary to viscous forces. 13 Further, for many subse- quently investigated crude oil/rock combinations that responded to low-salinity flooding, the effluent pH ranged from a little below to just above neutral. 3,14 Tang and Morrow 2 noted that increased recovery was accom- panied by the production of water-wet fine particles that sank to the bottom of the oil/water separator and the possible presence of partially oil-wet fines in a small rag layer between produced oil and water. It was hypothesized that improved oil recovery involved the release of mixed-wet fines from pore walls, possibly together with the dislodgement of water-wet particles. Sarkar and Sharma 15 earlier reported that the rate of fines production in response to the decreased salinity of the injected water was dimi- nished in the presence of crude oil. For many crude oil/water/ rock combinations, the production of fines in the effluent was not observed, and, accordingly, Lager et al. 14 argued that the in- creased oil recovery did not involve their movement. However, a lack of observation of fines production from cores does not preclude substantial local mobilization and migration at pore scales, especially in the case of predominantly oil-wet fines, for which relatively small populations in the produced oil would be difficult to detect visually. The interpretation of pressure mea- surements with respect to fines migration in low-salinity water- floods is inconclusive. Dissolution of rock minerals was also discounted by Lager et al. 14 as a feasible explanation for improved oil recovery by Received: November 21, 2010 Revised: February 9, 2011