A practical methodology to screen oil recovery processes involving spontaneous imbibition

International audienceWe present a general novel technique to monitor saturation changes on small rock samples of only 15 mm in diameter and 20 mm in length for the purpose of assessing the kinetics of spontaneous imbibition processes. With a fully-3D imbibition configuration involving counter-current flows through all faces of the sample, the method is based on a NMR technique in which the sole oil phase present within the sample is monitored. The experimental method is fast for two reasons that are i) the possibility to perform accurate measurements on tiny samples, ii) the adoption of a 3D flow geometry. The kinetics of oil desaturation during spontaneous imbibition is analyzed with the help of an analytical 3D diffusion model, according to which the kinetics is proportional to the value of a "capillary" diffusion coefficient. For the purpose of demonstrating our methodology, we used this technique to compare the spontaneous imbibition of restored sandstone miniplugs from a sandstone reservoir, with and without alkali in the imbibing brine. The imbibition kinetics was quantified as capillary diffusion coefficient values. The studied case results revealed mixed impacts of alkali on the spontaneous imbibition kinetics, involving both a brine-oil interfacial tension change and a wettability alteration of the rock, the latter requiring further investigation beyond the scope of this article. List of symbols EOR Enhanced oil recovery IFT Interfacial tension [N/m] NMR Nuclear magnetic resonance D, 2r Diameter of the core (r: radius) [m] D c Capillary diffusion coefficient [m 2 /s] D e Effective molecular diffusion coefficient [m 2 /s] D m Molecular diffusion coefficient [m 2 /s] K Single-phase permeability [m²] k rw , k ro Relative permeabilities to water (w) and oil (o) L, 2l Length of the core (l: half length) [m]  w,  o Relative mobility for water (w) and oil (o) [Pa-1 .s-1 ]  w ,  o Viscosities of water (w) and oil (o) [Pa.s] Pc Capillary pressure [Pa]  Porosity of the core S w , S o Water (w) and oil (o) saturation S w *, S o * Scaled water (w) and oil (o) saturation (cf eq (3)) S wi , S oi Initial water (w) and oil (o) saturation S of Final oil saturation T 2 Transverse relaxation time [s] V Bulk volume of the sample [m 3 ] V p Porous volume [m 3 ] V o Oil volume in the core [m 3 ] V oi Initial oil volume in the core [m 3 ] ACKNOWLEDGEMENTS We thank OMV for allowing us to publish the results. Wolfgang Hujer is also acknowledged for providing laboratory data and rock description