A chemical model 1s presented for extrapolating laboratory data on mineral-alkali reactions to reservoir time scales. Minerals 1n sandstone and ions in caustic solution were identified which control consumption of sodium hydroxide during alkaline flooding. This work may help operators to predict when significant dissolution of the formation will occur and where precipitates will deposit.
The dissolution of eight silicate minerals in caustic solution at 24° and 70° C was determined. The minerals studied were quartz, two feldspars (microcline and albite), two micas (muscovite and blotite), and three clays (kaolinite, montmorillonite, and chlorite). The concentrations of the ions: sodium, silicate, aluminate, and hydroxide were measured periodically during bottle tests.
Balanced chemical reactions are written for quartz, kaolinite, and phillipsite, and the corresponding equilibrium quotients are defined. A kinetic model is presented which includes rate constants, solid-liquid ratios, and equilibrium quotients to account for the effect of solution composition. The model was tested 1n detail for kaolinite at 24° and 70° C. The equilibrium quotient for kaolinite was found to be three times higher at 70° C than at 24° C. Dissolution rate data were fitted successfully with the kinetic model.
Kaolinite was observed to cause a supersaturated condition with respect to sodium aluminosilicate minerals when the hydroxide-ion concentration was above 0.07 mol/dm3 at 70° C.
The resulting onset of precipitation was characterized by a short induction time predicted by the model. Sodium and hydroxide 1ons were consumed at equal rates from solutions containing silicate and aluminate 1ons. In the presence of excess sodium hydroxide, within 2 months, each kilogram of kaolinite removed 5 equivalents of sodium hydroxide.
The reaction of Kern River reservoir sand with sodium hydroxide was studied to test the model. Consumption of sodium hydroxide occurred as predicted. However, caustic consumption was delayed in a slim tube packed with 99% quartz and 1% kaolinite, probably because of reduced nucleation of new minerals.