SCA2014-037 1/7 A SYSTEMATIC INVESTIGATION OF WETTING STABILITY IN AGED CHALK M. Steinsbø, A. Graue and M.A. Fernø Department of Physics and Technology, University of Bergen, NORWAY This paper was prepared for presentation at the International Symposium of the Society of Core Analysts held in Avignon, France, 8– 12 September, 2014. ABSTRACT Waterflood performance of fractured chalk reservoirs are strongly influenced by wettability, and spontaneous imbibition of water from fractures to matrix is a key mechanism for oil production. To determine the enhanced oil recovery (EOR) potential for induced wettability change, for instance during a surfactant injection, stable and uniform wetting preferences representing reservoir conditions must be established during core analysis to avoid laboratory artifacts. A systematic investigation of wettability stability in laboratory aged core plugs was performed by repeated cycles of spontaneous and forced imbibitions. Altered wetting states, ranging from strongly water-wet to moderately water-wet conditions were established by aging outcrop Rørdal chalk samples in crude oil at 80°C for 2 to 6 days at a range of initial water saturations during aging. The Amott-Harvey water index (I w ) was found stable over two cycles of spontaneous imbibition tests, with an average change of I w ±0.04 between the first and second cycle. The oil recovery rate in partly waterflooded regions and transition zones was evaluated by varying the initial water saturation. INTRODUCTION Residual oil saturation in fractured chalk reservoirs, after primary and secondary water or gas injections, is often high because large areas may be unswept due to fingering, or bypassed due to fluid flow preference along fractures, and gravity segregation. It is well known that the wettability preference of a reservoir is a major factor for oil recovery because it controls the location of fluids in the pore space and the interaction of the driving forces and thus determining the ultimate recovery [1]. The wettability of a rock surface is determined by the presence of water and/or oil layer [2]. To alter the wettability preference during laboratory core analysis, in particular when working with outcrop analogue rocks, an aging process is often used to allow polar components from the crude oil to be absorbed on the rock surface. The established wetting preference depends on aging time, initial water saturation during aging, temperature and the crude oil/rock/brine system, and may reflect different reservoir wetting conditions ranging from strongly water-wet to near neutral-wet. A dynamic aging method, capable of establishing uniform wetting conditions by continuously flowing crude oil through core samples [3, 4], was used to establish different wetting preferences, representing typical wetting conditions for partly waterflooded areas, transition zones or residual oil zones (ROZ). 12 originally strongly water-wet outcrop chalk core plugs (6 pairs of 2 similar core plugs) resembling a North Sea reservoir were aged with North Sea crude oil at 80°C at six different initial conditions varying both the aging time and the initial water saturation; short (2 days) and long (6 days) aging, and, at three different initial water saturations; low, medium and high. The initial water saturation ranged from S w =0.23 to S w =0.35 and the measured Amott-