Role of Silicate and Aluminate Ions in the Reaction of Sodium Hydroxide With Reservoir Minerals
- S.D. Thornton (Natl. Inst. for Petroleum and Energy Research)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1988
- Document Type
- Journal Paper
- 1,153 - 1,160
- 1988. Society of Petroleum Engineers
- 5.4.6 Thermal Methods, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.3.2 Multiphase Flow, 2.5.2 Fracturing Materials (Fluids, Proppant), 2.4.3 Sand/Solids Control, 2.4.5 Gravel pack design & evaluation, 4.3.4 Scale
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Summary. A chemical model is presented for extrapolating laboratory data on mineral/alkali reactions to reservoir time scales. Minerals in sandstone and ions in caustic solution that control sodium hydroxide consumption during alkaline flooding were identified. This work may help operators 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 deg. C [75 and 158 deg. F] was determined, The minerals studied were quartz, two feldspars (microcline and albite), two micas (muscovite and biotite), and three clays (kaolinite, montmorillonite, and chlorite). The concentrations of 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 that includes rate constants, solid/liquid ratios, and equilibrium quotients to account for the effect of solution composition. The model was tested for kaolinite. The equilibrium quotient for kaolinite was found to be three times higher at 70 deg. C [158 deg. F] than at 24 deg. C [75 deg. F]. Dissolution-rate data were fitted successfully with the kinetic model.
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.
Reservoirs containing acidic crude oils have been flooded with alkaline solutions, such as sodium hydroxide or sodium orthosilicate for EOR. Alkaline flooding has been proposed as an inexpensive means of reducing interfacial tension (IFT) with natural surfactants formed in situ or as a cost-effective alternative to micellar/ polymer flooding. A study by the Natl. Petroleum Council has concluded that the alkaline flooding process must be improved if it is to recover a significant amount of tertiary oil in the U.S.
The reaction of sandstone with alkali is recognized as a major problem in alkaline flooding. Mineral reactions consume alkalinity. Also, caustic solutions weaken sandstone by dissolution. Finally, alkaline solutions deposit scale in producing wells. Several authors have presented models of mincral/alkali reactions because prediction of the reactions is important.
In this paper, a reaction model is presented that includes both kinetics and equilibria of individual mineral reactions. Model parameters are determined experimentally. New data on the reaction of individual silicate minerals with caustic solution are presented. This paper is intended to promote further understanding of mineral/ alkali reaction mechanisms.
Theory on Reaction Kinetics
Silicate and aluminate ions play a central role in the reaction of silicate minerals in caustic solution. Silicate minerals consist primar-ily of SiO and Al O , which dissolve in caustic solution as sili cate and aluminate ions, respectively. Studies indicate that the silicate and aluminate ions exist primarily as H SiO and Al(OH) , respectively, in the pH range of 9 to 13. When a caustic solution contacts an individual mineral, the silicate- and aluminate-ion concentrations in the aqueous solution will increase until the solution reaches equilibrium with the mineral: however, when a caustic solution contacts a number of silicate minerals in close proximity. it cannot reach equilibrium with all of them simultaneously. In this case, the most-soluble minerals dissolve, and the least-soluble minerals precipitate. The identities of precipitating minerals depend on the availability of mineral-constituent cations, such as sodium, calcium, and magnesium, in solution.
The difference in composition between dissolving minerals and precipitating minerals accounts for long-term caustic consumption by silicate minerals. Minerals that precipitate from caustic solution have a higher sodium content than minerals that dissolve. This effect is described below by the example of kaolinite and quartz (which contain no sodium) dissolving and phillipsite (which contains sodium) precipitating.
It is recognized that silicate solution chemistry involves various degrees of acid/base dissociation and polymerization. It is believed that an exhaustive solution model is not required for predicting, on a first order, the conditions leading to the dissolution of reservoir minerals and the precipitation of new minerals that are less soluble. For simplicity, neither activity coefficients nor ion speciation is included in this work. The goal is to make this technique computationally simple. Quartz, kaolinite, phillipsite, and gibbsite are chosen for a reaction model because quartz and kaolinite are both reactive and abundant, and phillipsite has been found to be a product of mineral/alkali reactions. The new mineral species actually formed in the present work were not identified, but the general mechanism is demonstrated by a mass balance of the solution species: sodium, silicate, aluminate. and hydroxide (cf. Table 1).
Balanced chemical reactions are written as a basis for defining the equilibrium between the solution and each mineral. The equilibrium condition for each mineral is defined in terms of an equilibrium quotient, Q. A representation of the reaction for quartz is
and the corresponding equilibrium quotient is
where the brackets represent concentration in mol/dm. The equilibrium quotient for quartz accounts for the fact that in alkaline so-lution, the solubility of quartz is proportional to a gooapproximation to hydroxide-ion concentration. Thornton and Radke measured the solubility of quartz in alkaline solutions and presented a thermodynamic model that predicts quartz solubility as a function of hydroxide-ion concentration.
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