Effect of Brine Composition On Recovery of an Alaskan Crude Oil By Waterflooding

Abstract

Abstract Waterflood recoveries of a Prudhoe Bay crude oilfrom 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. For standard waterfloods (no difference between the initial and injected brine composition) Brine 1 (4 wl% NaCI + 0.5 wt% CaCl2) gave 16% higher recovery (% original oil in place) than Brine 2 (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 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 have a considerable impact on both wettability and oil recovery by waterflood.1–3 Sttrprisingly little information is available on the effect ofbrine composition on oil recovery. 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. Favorable 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% NaCI + 0.5% CaCl2, and 2% CaCl2referred to as Brine 1 and Brine 2, respectively. The effect of these brines on oil recovery 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. 1n addition to Brines 1 and 2, waterfloods with synthetic reservoir brine and 4%NaCI brine were also tested. 2. EXPERIMENTAL 2. I. Materials 2.1.1. 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 This procedure gave 7.5% weight of asphaltenes with pentane and 3.8% with hexane. Light ends were removed from the crude oil by evacuation for 1.5 to 2 hours in order to avoid possible evolution of gas bubbles during an experiment. Interfacial tensions of the evaporated A'92 crude oil against Brine 1 and Brine 2 were 21.1 and 21.5 dyn/cm respectively.