Elevated-Temperature Caustic/Sandstone Interaction: Implications for Improving Oil Recovery

Abstract

Abstract Caustic in the form of sodium hydroxide solutions is shown to interact strongly with sandstone at elevated temperature (185°F). Such interaction has substantial influence on the success of secondary and tertiary oil-recovery sodium hydroxide floods carried out in sandstone formations of more ordinary temperatures. Caustic in the form of sodium hydroxide interacts with sandstone at elevated temperature to promote (1) significant dissolution of the more susceptible silicate minerals, predominantly clay and large-surface-area silica minerals; (2) sandstone weight loss; (3) increased porosity; (4) propagation of significant concentrations of water-soluble silicates, including sodium orthosilicate; (5) in-situ formation of new immobile aluminosilicate material; (6) changes in permeability; and (7) hydroxide ion consumption. Caustic/sandstone interaction resulting from sodium hydroxide dissolution of silicate minerals is limited by kinetics. The interaction increases with increasing temperature, increasing sodium hydroxide concentration, and increasing caustic/sandstone contact time. The rate and the amount of interaction are sensitive to sandstone mineralogy and lithology. Although not studied during this work, the presence of crude oil, along with crude-oil type, may affect the rate and the amount of interaction. Based on the laboratory study of elevated temperature caustic/sandstone interaction involving silicate-mineral dissolution by sodium hydroxide, it is concluded that, at lower temperatures, the much slower dissolution interaction has implications for field application of sodium hydroxide for improving waterflood sweep efficiency and enhancing oil recovery. Specifically, during field applications, the slow interaction could deplete the active hydroxide ions. More common low-temperature field applications are especially susceptible because of long caustic/sandstone contact times and because, in general, relatively small concentrations of sodium hydroxide have been used historically. This study points out that proper extrapolation of laboratory caustic-flooding results to field conditions should account for slow kinetic phenomena. The study also helps explain detrimental caustic/sandstone interactions and lower-than-expected oil recoveries experienced during a number of long-duration low-temperature enhanced oil recovery (EOR) field tests and floods. On the positive side, the dissolution interaction, especially at elevated temperatures, generates in-situ significant concentrations of water-soluble silicates, including sodium orthosilicate. Water-soluble silicates have been reported as candidates for improving oil recovery and for use in preflushes to condition formations for other EOR techniques. Other than noting their in-situ formation and propagation, we did not study the alkaline water-soluble silicates in detail, and they are not discussed.