A Study of Mineral/Alkali Reactions—Part 2

Abstract

Summary In a study of alkalinity loss by interactions of alkali with minerals and reservoir sands, static tests were conducted with high concentrations of NaOH and a variety of minerals, mineral mixes, and reservoir sands at high temperature. Reactions lasted up to 2 months in some cases. Solutions were analyzed for pH and titratable alkali, silicon, and other elements. The reacted alkaline solution from a reservoir-sand/alkali reaction was used for interfacial tension (IFT) and contact-angle measurements and for sandpack oil-recovery tests. Alkalinity loss was greatest for kaolinite and gypsum; moderately high for montmorillonite, illite, dolomite, and zeolite; moderately low for feldspar, chlorite, and fine quartz; lowest for quartz sand; and insignificant for calcite. Alkalinity loss was dominated by chemical reactions for kaolinite, by dissolution/precipitation equilibria for gypsum and dolomite, by ion-exchange and silica dissolution for montmorillonite, and by ion exchange and chemical reaction for illite. The alkalinity loss by multimineral mixtures and reservoir sands agreed reasonably well with the sum of contributions by individual minerals. The contribution by highly reactive minerals was disproportionate to their weight fraction and was found empirically. The contributions by other minerals (not very reactive or ion-exchange-dominated) were approximated proportionately to their weight fractions. For reservoir sands, identifying exposed minerals and estimating their exposed surface areas by scanning-electron (SEM) and thin-section microscopy with several samples were important. In an attempt to determine the influence of dissolved species from reaction, reacted alkali after sand/alkali contact was tested for contact angle, IFT, and oil recovery for a U.S. gulf coast reservoir system. More oil-wet (quartz) conditions were found with reacted alkali than with fresh alkali with one oil sample, and highly oil-wet conditions were found with both fresh and reacted alkali for another oil sample. Minimum IFT values were slightly lower, and no detrimental effects of dissolved species on oil recovery were observed.