Divalent Ion Exchange With Alkali

Abstract

Abstract Exchange of hardness ions is important in EOR with chemical additives. In both micellar/polymer and caustic flooding processes, multivalent ions released from rock surfaces can interact with anionic surfactants, rendering them preferentially oil soluble and/or insoluble in water. Because hardness cations are sparingly soluble and precipitate in alkaline solutions, such solutions may be more efficient as surfactant-flood preflushes than are softened brines. Multivalent ion precipitation may also occur in alkaline water flooding. To permit design of such processes, this paper presents a chromatographic theory for simultaneous ion exchange with precipitation of divalent ions. Theoretical effluent histories and concentration profiles are presented for the cases of finite pulses and continuous injection of hydroxide ions into linear cores. Complete capture of the insoluble salt particles is assumed. Results are given for the case of instantaneous equilibration of the solution with the precipitate, as well as for the case of complete nonequilibration in which the solid precipitate does not redissolve. These two physical extremes predict field performance and laboratory results, respectively. Data for Berea sandstone and an argillaceous sand compare favorably with the proposed theory. The efficiency of alkaline preflushing is shown to depend on the exchange isotherm, initial divalent loading of the rock, injected pH and salinity, the solubility product of the precipitated salt, and pulse size. The effect of pulse size on complete equilibrium removal of hardness ions is reduced efficiency with increasing size until a critical volume approximating continuous injection is reached. Increasing injected pH and salinity provides a more favorable response. The theoretical model, when applied to field conditions, predicts redissolution zones that have not been previously recognized because solution residence times in laboratory columns are too short. Calculations show that precipitate redissolution by the low-pH solutions following alkaline pulses may introduce high concentrations of calcium behind the preflush where interference with micellar or polymer solutions is likely. These results suggest that reservoir preflush design from laboratory tests, while possible, must be made carefully. Introduction The presence of multivalent cations in reservoir brines can profoundly affect the oil-recovery efficiency of micellar or polymer slugs. These chemicals can react with hardness cations present to produce water-insoluble constituents. Removal of divalent cations, such as magnesium or calcium, with softened brines has been studied extensively. Problems with reservoir heterogeneities and incomplete sweep are legion. Equally important, however, is the strong preference of most reservoir rock for calcium, which means that large volumes of preflush are required even in the swept zones to obtain the desired low concentrations of hardness ions. Campbell and Holm and Robertson have proposed using chemicals such as sodium hydroxide or sodium orthosilicate in a preflush to react with multivalent cations by precipitation. Currently, a field test of this idea is under way. SPEJ P. 657^