Spontaneous precipitation of calcium silicate hydrate in aqueous solutions

Calcium silicate hydrates (C‐S‐H) are very important not only for their contribution to the development of cement and concrete properties but also for use as fillers and in silicate glasses. In the present work, the thermodynamics and the kinetics of the spontaneous precipitation of C‐S‐H from aqueous solutions were investigated over the pH range 10‐12 at 25 °C. The thermodynamic driving force was calculated taking into consideration all equilibria involved in the supersaturated solutions. In the range of the solution supersaturation values examined the precipitation occurred spontaneously, with the exception of the series of experiments done at pH 12.0, where induction times preceded the appearance of the precipitate. The rates were measured at constant pH as a function of the solution supersaturation and were found to depend strongly on the solution supersaturation, pH and on the total calcium to total silicate molar ratio in solution. Fit of the kinetics results in a power law relating rates of precipitation with respect to C‐S‐H precipitated, suggested a surface diffusion controlled mechanism for the formation of C‐S‐H. The precipitated solids did not show significant morphological differences at different pH values. From the induction times preceding the spontaneous precipitation at pH 12.0, a value of 30 mJm^‐2 was calculated for the surface energy of C‐S‐H. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sandbed Consolidation with Mineral Precipitation

A new method has been developed to prevent sand reentrainment during oil production from unconsolidated or poorly consolidated reservoir formations. Consolidation of the zformation around the well is achieved through in situ precipitation of a sparingly soluble salt, namely, calcium phosphate. Control of the depth of salt formation is achieved by alternating injection, mixing, and reaction of two aqueous solutions of calcium chloride and potassium phosphate. Calcium phosphate crystals precipitate and grow on the grain surfaces, forming sufficiently uniform coatings. The formation of relatively uniform coatings on the grains causes an acceptably small decrease of the permeability, which is a feature of primary importance for oil production. The grains are gradually "cemented" with bridges of calcium phosphate crystallites and form a consolidated and still porous structure. As a result, the rate of hydrocarbon production for the problematic reservoir can be increased considerably without undesirable reentrainment of sand. The proposed method for consolidation has been successfully tested in sandbeds. Several series of experiments have been carried out under diverse conditions to establish the optimum parameter values for the implementation of this method. A set of optimum conditions at 25 degrees C were determined and these conditions gave satisfactory consolidation with permeability loss of ca. 60% of the initial value. The conditions of precipitation were chosen so that the precipitated phase was octacalcium phosphate [Ca(4)H(PO(4))(3).2.5H(2)O], along with its byproduct hydroxyapatite [Ca(5)(PO(4))(3)OH]. Experiments were also carried out at 70 degrees C and have shown that it is feasible to consolidate loose sandpacks at oil reservoir conditions. Copyright 2000 Academic Press.     

Precipitation of sparingly soluble salts in packed sandbeds in the presence of miscible and immiscible organic substances

Sparingly soluble salts precipitation, e.g. calcium carbonate or calcium sulfate, results in pore clogging in rock formations and in the concomitant reduction of the local permeability of oil wells during the oil extraction processes. On the other hand, in situ controlled salt precipitation is desirable in various applications e.g. waterproofing of concrete constructions suffering from leakages, etc. In the present study, calcium carbonate (CaCO₃) precipitation in sandbeds was investigated, in the presence of organic solvents simulating the conditions prevalent in oil‐well zones. CaCO₃ precipitation was investigated from supersaturated solutions prepared by in‐situ mixing of NaHCO₃ and CaCl₂.2H₂O solutions before the inlet of sandbeds. The solution resulting from the mixing of the two solutions was supersaturated with respect to all calcium carbonate polymorphs. Three types of experiments were performed depending on the supersaturated solutions: a) aqueous solutions b) aqueous supersaturated solutions in contact with sandbeds pre‐saturated with n‐dodecane c) aqueous solutions containing monoethylene glycol (MEG). Results showed that oil–water interfaces enhanced the heterogeneity of the supersaturated solutions and accelerated crystal growth of calcium carbonate at the inlet of the sandbed, resulting in early pore clogging and limitation of local permeability. Maximum sandbed consolidation was obtained with the solutions containing MEG.