Scaling Spontaneous Imbibition of Aqueous Surfactant Solution into Preferential Oil-Wet Carbonates Eli J. Høgnesen, † Dag C. Standnes, ‡ and Tor Austad* ,† Stavanger University College, Ullandhaug, Postbox 8002, 4068 Stavanger, Norway, and Centre for Integrated Petroleum Research (CIPR), University of Bergen, Norway Received March 12, 2004. Revised Manuscript Received July 8, 2004 Carbonate reservoirs are usually strongly fractured, with very high permeability contrasts between matrix blocks and fractures. Normally, a simulation of fluid flow with a dual porosity model is used, which is based on a fluid-exchange term where the dimensionless time is a key factor. Besides traditional reservoir rock and fluid parameters, the scaled dimensionless time must include the influence of capillary and gravitational forces. A very recent publication [Li and Horne, SPE Paper No. 77544, 2002] examined an “analytical” model that involved all these effects. In the present paper, we have tested the model for spontaneous imbibition of aqueous surfactant solution into preferential oil-wet carbonate cores. A chemical reaction occurs between the surfactant and adsorbed polar organic components/carboxylates at the carbonate surface, ahead of the fluid displacement process, as has been discussed previously [Standnes and Austad, J. Pet. Sci. Eng., 28, 123, 2000]. It was of great interest to determine if this new analytical model could handle such a process. The ranges for the scaling parameters were as follows: interfacial tension (IFT), 0.3-0.8 mN/m; permeability, 3-350 mD; initial water saturation (S wi ), 0-0.5; core height, 5-30 cm (but with the same diameter); diameter, 2.5 and 3.5 cm (but with the same core height); temperature, 40-70 °C; and sulfate concentration, 0-1.7 g/L. Temperature change has a great influence on the imbibition rate, because of changes in IFT, critical micelle concentration, and fluid viscosity. Sulfate, being a potential-determining ion toward CaCO 3 [according to Pierre et al., J. Dispersion Sci. Technol., 11, 611, 1990] was observed to have catalytic effects on the wettability alteration process in chalk at low temperature [according to Strand et al., Energy Fuels, 17, 1133, 2003]. When the characteristic length (L a ) is used as the shape factor of the cores, all the parameters scaled very well, except for the height of the core and the diameter of the core at low IFT (low temperature). However, when just the height of the cores was used as the shape factor, all the parameters scaled quite well when the normalized oil recovery was plotted versus dimensionless time. The fit of the scaling, using the height of the core as the shape factor, suggested that gravitational forces were very active in the oil recovery mechanism. Introduction Imbibition has been described as a process by which a wetting fluid is drawn into a porous medium by capillary action. The process is important in oil produc- tion, and several papers in the literature, during a 50-year period, that address spontaneous imbibition have been tabulated and summarized. 1 Very recently, Morrow and Mason 2 gave a review about oil recovery by spontaneous imbibition that especially involved the influence of rock wettability and scaling of the imbibi- tion process. Oil recovery from highly fractured carbon- ate reservoirs with low-permeability matrix blocks is an example where the spontaneous imbibition of water is an important improved oil recovery (IOR) technique. 3 Unfortunately, ∼90% of the carbonate reservoirs are neutral to oil-wet, which implies that spontaneous imbibition of water will not occur. 4 Viscous flooding will normally give low sweep efficiency, because of the high permeability contrast between the fractures and the matrix blocks. Al-Hadhrami and Blunt 5 discussed sev- eral IOR techniques (viscous forces, gravitational forces, reduced oil-water interfacial tension (IFT), gas injec- tion, and wettability alteration) to overcome the capil- lary barrier to invade an oil-wet rock matrix and displace the oil in a secondary drainage process. Wettability alteration toward more water-wet condi- tions, which seems to be an actual method, can be obtained through the use of chemicals 6,7 or steam. 5 * Author to whom correspondence should be addressed. Telephone: +47 51832296. FAX: +47 51831750. E-mail: tor.austad@tn.his.no. † Stavanger University College. ‡ University of Bergen. (1) Zhou, X.; Morrow, N. R.; Ma, S. Presented at the 1996 SPE/DOE Tenth Symposium on Improved Oil Recovery, Tulsa, OK, April 21- 24, 1996, SPE/DOE Paper No. 35436, 1996. (2) Morrow, N. R.; Mason, G. Curr. Opin. Colloid Interface Sci. 2001, 6, 321-337. (3) Chen, H. L.; Lucas, L. R.; Nogaret, L. A. D.; Yang, H. D.; Kenyon, D. E. SPE Reservoir Eval. Eng. 2001, 3 (February), 16-25. (4) Rao, D. N. Wettability Effets in Thermal Heavy Oil Recovery Operations. Presented at the 1996 SPE/DOE Tenth Symposium on Improved Oil Recovery, Tulsa, OK, April 21-24, 1996, SPE/DOE Paper No. 35462. (5) Al-Hadhrami, H. S.; Blunt, M. J. SPE Reservoir Eval. Eng. 2001, 4, 179-186. (6) Standnes, D. C.; Nogaret, L. A. D.; Chen, H. L.; Austad, T. Energy Fuels 2002, 16, 1557-1564. 1665 Energy & Fuels 2004, 18, 1665-1675 10.1021/ef040035a CCC: $27.50 © 2004 American Chemical Society Published on Web 09/04/2004