Low-Salinity Waterflooding Improves Oil Recovery - Historical Field Evidence
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 109965, "Low- Salinity Waterflooding To Improve Oil Recovery - Historical Field Evidence," by Eric P. Robertson, SPE, Idaho National Laboratory, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11-14 November. The paper has not been peer reviewed.

Crude-oil/water/rock interactions can lead to large variations in the displacement efficiency of waterfloods. Laboratory waterflood tests and single-well tracer tests in the field have shown that injection of low-salinity water can increase oil recovery, but testing on a multiwell-field scale has not yet been undertaken. Three Minnelusa-formation waterfloods in the Powder River basin of Wyoming were compared on the basis of salinity of the formation and injection water and of reservoir characteristics.

Introduction

Different wetting states of crude-oil, water, and rock ensembles can yield different oil recoveries from laboratory-waterflood tests. The wetting state, or wettability, of a rock/fluid system can be altered. In the laboratory, wettability can be altered by changing the crude-oil composition, changing the temperature while aging the rock and crude oil, or by changing the temperature during water displacement. Also, water composition can have a significant effect on wettability and on oil recovery. Therefore, cases may exist in which attention to injection-water composition could increase oil recovery, and, likely, increase the economic profitability of a waterflood.

An optimal composition of dissolved solids in the injection water may exist that would yield the highest oil recovery. The composition could involve many variables with respect to ionic composition and concentration, but current knowledge of how and when water composition can be manipulated to increase oil recovery is limited. Several examples of improved recovery by injecting low-ionic-strength brine have been reported for both outcrop- and field-core samples. Fig. 1 shows the potential for increased oil recovery from low-salinity waterflooding. These corefloods were performed on two cores from the CS reservoir under identical conditions except for the composition of the injected water.

The conditions necessary for improved recovery, such as the type of crude oil and rock, composition of the formation and injected waters, and initial water saturation, still are not understood fully. The crude-oil/water/rock interactions that determine displacement efficiency are highly complex. Nevertheless, laboratory observations were sufficiently encouraging to justify more studies aimed at field application.

The Idaho National Laboratory (INL) through funding from the US Department of Energy searched historical waterflood records for anecdotal evidence of increased oil recovery resulting from the injection of lower-salinity water to displace oil in reservoirs with higher-salinity initial formation water. The objective was to research and compare historical field data and to compare waterflood responses from low-and high-salinity injection waters.