Effect of Brine Composition On Wettability And Oil Recovery of a Prudhoe Bay Crude Oil
Authors: Yildiz H.O. , Valat M. , Morrow N.R. 
Published: March 2010

Abstract
Waterflood recoveries of a Prudhoe Bay crude oil from Berea Sandstone were determined for two brine compositions used previously in a study of the effect of brine composition on the recovery of Moutray crude oil. These brines will be termed Brine 1 and Brine 2. For standard waterfloods (no difference between the initial and injected brine composition) Brine 1 (4 wt% NaCl + 0.5 wt% CaCl2) gave 16% higher recovery (% original oil in place) than Brine 2 (0 wt% NaCl + 2 wt% CaCl2). Recovery by injection of Brine 1, with Brine 2 as the initial brine, or vice versa, gave recoveries, even at breakthrough, that were intermediate to the results of standard waterfloods (no change in brine composition). Standard waterflood recoveries for synthetic reservoir brine were comparable to those obtained with Brine 1. The results demonstrate that brine composition can have a large effect on oil recovery and that displacement efficiency is not necessarily dominated by the composition of the initial (connate) brine.

Introduction
Reservoir wettability is a dominant factor in determining waterflood recoveries and their economic limits. In the laboratory, different wettability states can be induced by exposing outcrop core samples to crude oil. This process is referred to as aging or marination. Crude oil type, aging conditions, and initial water saturation (Swi) have a considerable impact on both wettability and oil recovery by waterflood(1–3). The effects on oil recovery of extreme changes in pH and interfacial tension have been investigated, but there is surprisingly little information available on the effect of brine composition. For waterfloods that are about to be implemented, judicious choice of injected brine when more than one source is available, or modification of the brine through addition of salts, could result in higher ultimate oil recoveries. Favourable adjustment of the injected brine composition for a mature waterflood offers a possible approach to reducing the water/oil production ratio and extending the productive life of the reservoir.
A major part of this work concerns two somewhat arbitrary brine compositions: 4% NaCl + 0.5% CaCl2 and 0% NaCl + 2% CaCl2, referred to as Brine 1 and Brine 2, respectively. The effect of these brines on oil recovery by waterflooding and spontaneous imbibition from Berea Sandstone has been investigated in detail for Moutray crude oil(4). Special attention was given to the effect of differences in the initial (connate) brine and the injected brine, and the effect of changing brine composition during the course of a waterflood. This paper reports a companion study using the same core material and brines and a crude oil sample from Prudhoe Bay, designated A'92. In addition to Brines 1 and 2, waterfloods with synthetic reservoir brine and 4% NaCl brine were also tested.

Experimental
Materials

Fluids
Approximately 0.02% NaN3 was added to the brines to prevent bacterial growth. A'92 was a stock tank crude oil. Asphaltene content was measured using a 9:1 ratio of precipitant (pentane or hexane) to crude oil, following the procedure described by Jadhunandan(5).