The Origin of Reversible Hydroxide Uptake on Reservoir Rock

When reservoir solids reversibly consume hydroxide, the impact on alkaline-waterflood performance can be significant. Only recently has this reaction been recognized as a principal factor influencing oil recovery rates and chemical-pulse depletion. This paper considers the origin of the reversible hydroxide uptake to be ion exchange of sodium for hydrogen ions. Using a simple, mass-action equilibrium model, we describe the alkali exchange isotherm. Because hydronium and hydroxide concentrations in water are never zero, hydroxide uptake must be reported relative to a reference pH and salinity. With the recognition of a reference state pH and salinity. With the recognition of a reference state and with the mass-action model, we predict qualitatively the effects of pH, salt concentration, and temperature on the measured hydroxide uptake isotherms for the Wilmington, Ranger-zone sand. Mineral sites that exchange ions of sodium for hydrogen may also exchange calcium for hydrogen or for sodium. Using simple mass-action equilibria again, we demonstrate that reversible hydroxide uptake depends on hardness concentration and that calcium/sodium exchange is pH dependent. Introduction Alkaline flooding is a technique in which chemical interactions with reservoir minerals are of paramount importance to success or failure. Hydroxide consumption falls into three broad categories:reversible rock adsorption or ion exchange,congruent and incongruent mineral dissolution, andprecipitation of insoluble hydroxides. All three loss mechanisms have been considered in various levels of detail. Reversible hydroxide ion uptake, which was overlooked in earlier work on alkali/rock interactions, is, perhaps, the least transparent consumption reaction. Its perhaps, the least transparent consumption reaction. Its existence and importance have come to light only recently. In modeling the alkaline oil recovery process, de Zabala et al. point out that equilibrium hydroxide ion uptake causes a chromographic lag in caustic and in the accompanying in-situ generated surfactants, which in turn slows oil-production rates. Likewise, Bunge and Radke demonstrate that hydroxide ion uptake alone can diminish an alkaline chemical pulse to ineffective concentration levels. Thus, even if caustic consumption by dissolution and precipitation could be eliminated, ion-exchange delay can precipitation could be eliminated, ion-exchange delay can limit the success of alkaline EOR. Therefore, when a reservoir is considered for possible alkaline flooding, understanding and quantifying any alkali/rock ion exchange is necessary. This paper presents a simple, mass-action treatment of reversible hydroxide ion uptake by sodium/hydrogen ion exchange followed by a reaction to form or dissociate water. We refer to this overall reaction scheme as hydroxide uptake, or reversible hydroxide consumption. By using the mass-action model and by paying careful attention to the measurement of alkali exchange isotherms, we show how the effects of pH, salt content, and temperature on the hydroxide uptake isotherms may be explained. The relationship between sodium/hydrogen exchange and calcium/sodium exchange is also explored. We demonstrate that calcium/sodium exchange is a combination of sodium/hydrogen and calcium/hydrogen exchange. Therefore, calcium/sodium exchange isotherms generally must be a function of pH, and hydroxide exchange isotherms must depend on calcium concentration. Finally, the connection between the hydrogen exchange capacity (HEC) and the calcium exchange capacity (CEC) is elucidated. Sodium/Hydrogen Exchange Fig. 1 gives the reversible hydroxide uptake on Wilmington, Ranger-zone sand as a function of hydroxide concentration at three temperatures with and without NaCl. Although there is considerable scatter in the data and few points at the lower pH values, the uptake isotherms appear Langmuir in shape. Also, alkali exchange increases with increasing temperature but decreases when salt is added. Reversible alkali exchange occurs not only with the unconsolidated reservoir materials shown here but also on consolidated Berea sandstone. To quantify the behavior of reversible surface uptake of hydroxide ions on reservoir minerals, we adopt our previous. ture of weak-acid sodium/hydrogen cation previous. ture of weak-acid sodium/hydrogen cation exchange: MOH+Na+ + MONa+H+................... (1) where M represents a mineral exchange site. For oxides (such as silica) MOH denotes a hydrolyzable acid site, while for clay minerals (such as kaolinite or montmorillonite MOH denotes a negative lattice exchange site occupied by a hydronium ion. SPEJ P. 711