Wettability Alteration in Limestone and Dolomite With Brines and CO2

Abstract

Wettability test is one of the most used tools for evaluating rock/fluid interaction in oils reservoir. In the present paper two carbonate outcrop rocks are evaluated for wettability alterations when subjected to brine injections of varying salinities and content of dissolved CO2. The evaluation included a qualitative appraisal via spontaneous imbibition tests and a quantitative assessment by the Amott-Harvey procedure. Rocks refer to limestone and dolomite samples with petro-physical properties similar to the Brazilian pre-salt reservoirs. Testing fluids are a medium gravity crude oil, seawater concentration brine, formation equivalent brine and the carbonated version of these brines. Results show additional oil recovery directly associated with wettability alteration provoked by brine concentration changes. Increments in recovery were observed independently if the brine concentration decreased or increased in the replacement process. For dolomites and limestone wettability changed in the direction of turning the rock from oil-wet to neutral wet. Tests carried out with equivalent carbonated brines show that similar alteration in the wetting properties also occur. Alterations were as well independent of the increase or decrease of the salt concentration in the brine changed. However, CO2 or its derived ions dissolved in the brines seem to inhibit the mechanism of wettability change when rocks are subject to changes in brine salt concentrations.