Numero Capilar

SPE 108724 Mobilization of Remaining Oil-Emphasis on Capillary Number and Wettability
Else B. Johannesen and Arne Graue, University of Bergen
Copyright 2fJ07, Sodety 01Petroleum Engiooo", This paper was prepared for presentation at the 2007 Inlemational Oil Conference and Emibition in Mexico held in Veracruz, Mexico, 27-30 June 2007. This paper was selected fo< presentation by an SPE Progl3fllCommillee following review 01 information contained in an abstract subm itted by the au1hOi(s). Conlents of the paper, as presenled, have nol been reviewed by the Society 01 Petroleum Enginee,.. and are subject 10 correction by the author( s). The malerial, as presented. doe s not necessanly raned any position of the Society of Petroleum Engineers . ns offlC8fS , or members . Papers presented al SPEmeetings are subject to publication review by Editorial Committees of the Society . 01 Petroleum Enginoors. EJeclronic raproduction. distribution. 0< storage of any part of this paper for comm ercial purposeswithout !he written consent of the SocieIy of Petroleum Engineers Is prohib iled. Permiss ion 10 reproduce 10 print is restricted .to an abstract of not more than 300 """Ids; illusttalionsmay not be copied . The abstract must contain conspicuous acknowledgment of where and by whom !he paper was presented . Wrae Librarian. SPE, P.O. Box 833836. Richardson , Texas 75083-3836 U.SA. fax 01-972-952-9435.

the reservoir is determined. Oil recovery depends strongly on the wettability condition and the interaction between the capillary and viscous forces will accordingly change withwettability [1, 2]. This study emphasizes the impact from wettability on the residual oil saturation during increasing differential viscous pressure drops at various wettabilities. Capillary forces are responsible for fluid entrapment during an immiscible displacement in porous media. Laboratory studies have shown that more of the remaining oil may be recovered in immiscible displacements if increasedviscous displacement is applied. By exceeding the capillary forces trapped residual oil may be mobilized [3]. The capillary forces are determined by the wettability conditions and the oil/water interfacial tension (IFT). The potential to mobilize capillary trapped oil depends on the pore geometry. The required viscous force needed to mobilize trapped oil is determined by the fluid dynamics of thedisplacing phase, Thus an important parameter determining mobilization of capillary trapped oil during immiscible fluid displacements is the capillary number; exhibiting the ratio of viscous forces to the capillary forces. There are several expressions for the capillary number, but the most commonly used is [4]:

Abstract The ratio between the viscous and capillary forces, commonly denoted theCapillary Number No> is crucial in determining the remaining oil saturation. The impact on residual oil saturation by a systematic increase in N; is determined in homogeneous chalk at wettabilities varying from nearly neutral-wet to strongly-water-wet conditions. In fractured chalk reservoirs waterflood residual oil saturation is strongly dependent on the wettability. The current results provideassistance in determining the potential target for tertiary oil recovery by measuring the amount of mobile oil at various Nc ' A series of displacements of oil by water injection at constant pressure were canied out to determine the relation between oil recovery and applied capillary number in waterfloods at different wettability conditions.

N c,l - -vf-i CT


(1)


Maximum oil recovery at constant N;occurred at wettability conditions reflecting an Amott Index to water at 0.3. The remaining oil decreased with increasing capillary number and significant trapped oil after completed spontaneous water imbibition was mobilized at moderately water-wet to nearly neutral-wet conditions. Similar results as reported in the literature for waterflooding residual oil saturations as function of wettability...