Kinetics Of Silica Precipitation Research Articles

Kinetics of silica precipitation in geothermal brine with seeds addition: minimizing silica scaling in a cold re-injection system

The utilization of geothermal energy remains underdeveloped, mainly due to the technical problem of silica scaling. The scaling can eventually disrupt the electricity production process due to frequent pipe maintenance. Although inevitable, scaling can be controlled by accelerating the precipitation process through the addition of silica seeds. Silica gel has an affinity to bind with dissolved silica in geothermal brine that therefore reduces the likelihood of silica to form scale on the pipeline surfaces. In the present work, brine was taken from geothermal well Unit 3A–3B at the Dieng geothermal power plant with an initial silica monomer concentration of approximately 420 ppm. Silica gel seeds were added to the brine at a precise pH and temperature and dissolved silica concentration was analyzed by detecting silica monomers with UV–visible spectrophotometry using the vanadate/molybdate (yellow) method. Experimental results showed that the silica concentration in the liquid phase could be reduced by the addition of these seeds. Silica precipitation was determined by mass transfer of silica monomers from the fluid phase onto solid surfaces, and it was found that precipitation decreased as pH and temperature increased. Calculations also showed that the mass transfer coefficient was enhanced by fluid agitation. The silica precipitation process was optimal at a pH of 7, a temperature of 40 °C and agitation speed of 800 rpm; the result was a mass transfer coefficient of 0.5924 cm/s. In a dimensionless correlation, the mass transfer coefficient can be expressed in the equation (kc·dp/DAB) = 1.4242·Re0.529·Sc0.3333.

The simulation of thermo-hydro-chemical coupled heat extraction process in fractured geothermal reservoir

Chemical reaction effect is a critical issue in the heat extraction process of a fractured geothermal reservoir. In this study, the thermo-hydro-chemical coupled condition is simulated based on a unified pipe-network method (UPM) by considering interconnected fracture networks embedded in the rock mass. The chemical reaction kinetics of silica precipitation and dissolution is incorporated to capture and examine the effects of the silica-water interaction on the evolution of fracture apertures. The chemical influenced heat transfer between solid and fluid phases in the fractured porous media is characterized by a local thermal non-equilibrium (LTNE) model introduced based on two energy balance equations. A pipe equivalent technique is used to discretize the multiphysical coupled equations and unify the fracture-matrix information at the interface through the superposition principle in the UPM framework. Convergence tests are performed to verify the proposed model with one large fracture embedded in a doublet system for geothermal development. Fracture networks are then introduced in a case study where the influences of different injection temperature, saturation condition, injection pressure differential and initial fracture aperture on the silica reaction related alteration of fracture apertures as well as the heat production are analyzed.

Silica Precipitation Kinetics: The Role of Solid Surface Complexation Mechanism Integrating the Magnesium Effects from 25 to 300°C

The results presented in this paper allow identifying and integrating the role of magnesium in the kinetic rate of silica precipitation. The basic thermodynamic and kinetic approaches are not sufficient to comprehensively describe the kinetic rate of silica precipitation as well as the simultaneous changes of the solution properties such as pH variations. Trial-error modelling tests reveal that it is necessary to take into account the silica solid surface complexation reactions, in particular the protonation reactions of the silanol sites outcropping of silica surface, in the kinetic law to reproduce measured properties (pH, dissolved silica concentrations, etc.). This newly developed kinetic law is able to correctly describe silica precipitation in presence of magnesium as well as chemical changes in the aqueous phase up to high temperatures (300°C).

Experimental study on the kinetics of silica polymerization during cooling of the Bouillante geothermal fluid (Guadeloupe, French West Indies)

Despite many studies, our understanding of silica precipitation from natural waters remains limited, in particular for geothermal waters. Here we present a detailed study on the kinetics of silica polymerization as a function of fluid temperature and pH using high-temperature (250-260°C) seawater-derived geothermal fluids as those discharged from the Bouillante geothermal site. We monitored the on-site decrease in monomeric silica concentration (initial SiO2 concentration of about 600mgl−1) with time using the molybdenum blue spectrophotometric method on samples of separated water collected from the high-pressure separator at 167°C and cooled to 25, 50, 75, and 90°C and for pH values ranging from 5 to 8. During all these experiments, only silica polymerization was observed, with the formation of colloidal particles in suspension in the solutions. No scaling of amorphous silica was formed. The collected data were after modeled in order to determine the useful kinetic parameters for predicting and preventing amorphous silica precipitation in the production wells during fluid exploitation in the specific context of Bouillante. Results under the investigated experimental conditions show that the kinetics of silica precipitation is affected more strongly by pH than by temperature change. The reaction in the acidic Bouillante solution begins with a transition period that significantly decreases the kinetics of silica polymerization. Modeling the experimental data indicates that the silica polymerization up to the state of equilibrium is characterized by a 2nd-order kinetic law relative to dissolved silica; this could indicate that the polymerization is controlled mainly by the formation of dimers. However, the first hour of the experiments is better characterized by a 4th-order kinetic law, suggesting more complex polymerization reactions in the initial stage with the formation of nanocolloidal particles containing 3 to 4 monomers. In both cases, the corresponding kinetics rate constant k is linearly dependent on pH for pH between 5 and 7. The activation energy Ea for the overall reaction, calculated using the Arrhenius equation under the considered pH and temperature conditions, ranges between 41±9.8 and 54±9.6kJmol−1. To complete this work, the colloid particles formed at the end of the kinetics experiments were extracted and analyzed by SEM and TEM microscopy, X-ray Diffraction and the BET method. Results show that the size of the colloids increase and their specific surface decrease with increasing pH and are thus dependent on pH. Globally, our work provides a reliable database for understanding silica polymerization kinetics in natural geothermal brines or geologic waters characterized by a near neutral pH and moderate dissolved silica concentrations.

Kinetics of silica precipitation on quartz seeds at 200–300 °C

Kinetics of silica precipitation on quartz seeds at 200–300 °C

Kinetics of silica precipitation on quartz seed crystals at 200°C

Kinetics of silica precipitation on quartz seed crystals at 200°C

The effect of cyanobacteria on silica precipitation at neutral pH: implications for bacterial silicification in geothermal hot springs

In this study, we performed silica precipitation experiments with the cyanobacteria Calothrix sp. to investigate the mechanisms of silica biomineralization. Batch silica precipitation experiments were conducted at neutral pH as a function of time, Si saturation states, temperature and ferrihydrite concentrations. The experimental results show that in solutions undersaturated with respect to amorphous silica, the interaction between Si and cell surface functional groups is weak and minimal Si sorption onto cyanobacterial surfaces occurs. In solutions at high Si supersaturation states, abiotic Si polymerization is spontaneous, and at the time scales of our experiments (1–50 h) the presence of cyanobacteria had a negligible effect on silica precipitation kinetics. At lower supersaturation states, Si polymerization is slow and the presence of cyanobacteria do not promote Si–solid phase nucleation. In contrast, experiments conducted with ferrihydrite-coated cyanobacteria significantly increase the rate of Si removal, and the extent to which Si is removed increases as a function of ferrihydrite concentration. Experiments conducted with inorganic ferrihydrite colloids (without cyanobacteria) removes similar amounts of Si, suggesting that microbial surfaces play a limited role in the silica precipitation process. Therefore, in supersaturated hydrothermal waters, silica precipitation is largely nonbiogenic and cyanobacterial surfaces have a negligible effect on silica nucleation.

Precipitation of quartz during high‐temperature, fracture‐controlled hydrothermal upflow at ocean ridges: Equilibrium versus linear kinetics

We have addressed quartz precipitation resulting from cooling of silica‐saturated, high‐temperature hydrothermal solutions ascending through crack‐controlled permeable regions beneath an ocean ridge crest. The results of this work indicate that for systems with high initial permeabilities, such as black‐smoker‐like systems, fluid channels can exist for several decades without serious sealing as a result of quartz precipitation, provided the kinetics of silica precipitation are considered. In lower‐permeability systems, even though kinetics of precipitation remain important, reduction of permeability as a result of quartz precipitation can occur on a time‐scale of a decade, leading to a decline in heat output and temperature of fluid as it encounters lower‐temperature regions. In addition, we show that kinetics considerations result in the preferential sealing of thin cracks. This result is consistent with seismic velocity data, which suggest that the increase in seismic velocity with depth and age in the oceanic crust may result from the preferential closing of thinner cracks. These results also suggest that precipitation of quartz should occur near the dike/lava transition zone or in mineralized zones where large temperature gradients may exist as a result of mixing between cool seawater and hot hydrothermal fluids. The results of this model are thus consistent with the observed increase in quartz veins in such regions beneath the TAG mound and in ophiolites.

Determination of the rate-limiting mechanism for quartz pressure dissolution

A model of quartz pressure dissolution via grain boundary diffusion through an adsorbed water layer, incorporating the effects of silica precipitation kinetics is used to derive a simple expression indicating whether densification by pressure dissolution is rate-limited by mass transport of precipitation kinetics. Using values of the diffusivity of thin films derived from pressure dissolution experiments, it is shown that at temperatures normally associated with pressure dissolution in sedimentary environments kinetics will be rate-limiting for most fine-grained sandstones and for coarser sediments, where quartz precipitation is inhibited. However, due to the strong temperature dependence of the quartz precipitation rate constant, it is demonstrated that at the elevated temperatures used in pressure dissolution experiments (typically >300°C) surface reactions are unimportant for particles > 2–6 μm in size.

Role of silica precipitation kinetics in determining the rate of quartz pressure solution

Role of silica precipitation kinetics in determining the rate of quartz pressure solution

A numerical model for porosity modification at a sandstone‐mudstone boundary by quartz pressure dissolution and diffusive mass transfer

ABSTRACTIn many sandstones quartz cements significantly reduce porosity. The origin of these cements is often unclear. This paper investigates a possible mechanism for the generation of silica for quartz cement by pressure dissolution in interbedded mudstones.Theoretical models of quartz pressure dissolution, including the effects of silica precipitation kinetics, show that the concentration of dissolved silica in the pore fluids of a compacting sediment increases with decreasing grain size and silica precipitation constant. Quartz precipitation is strongly inhibited by the presence of small amounts of clay within a sediment, suggesting that siltstones and quartz‐rich mudstones which are undergoing pressure dissolution may act as a source of dissolved silica for export to nearby, coarser sediments.A computational model for the diagenetic modification of a sandstone‐mudstone interface due to pressure dissolution is described. Both sandstone and mudstone layers are assumed to be actively compacting by pressure dissolution, and mass transport by molecular diffusion is considered. As quartz precipitation in the mudstone layer is relatively slow compared to that in the sandstone, significant amounts of dissolved silica become available in the mudstone, and may be exported into the adjacent sandstone. In the absence of pore‐fluid advection, this may result in the formation of extensive secondary quartz within the sandstone, close to the interface.The volume of silica exported from the mudstone is limited by the length scale over which diffusion through the mudstone is effective. This is typically 3–5 m. The volume of silica available therefore suggests that extensive porosity modification within the adjacent sandstone can only occur close to the mudstone. Thus it is possible that thin sandstones could become cemented by slow diffusive transfer of silica, but that in thicker sandstones the silica may become dispersed by pore‐fluid advection.

The role of silica precipitation kinetics in determining the rate of quartz pressure solution

Traditionally, models of intergranular pressure solution have assumed that grain boundary diffusion, probably through an adsorbed water layer, is the rate‐limiting process (Weyl, 1959; Rutter, 1976). However, observations by Heald (1956, 1965) and experiments by Cecil and Heald (1971) show that the presence of clays may strongly inhibit the growth of secondary quartz. Under these circumstances, silica precipitation kinetics may also be important in determining the pressure solution rate. A model of quartz pressure solution including the rate of silica precipitation kinetics is developed, and strain rate equations are derived for a compacting sediment. Laboratory data for the precipitation constant k_, for quartz, indicate that precipitation kinetics should not be important in determining the rate of pressure solution in clean quartz sand. However, an estimate of the in situ value of k_ for the Dogger Beta formation, made from the data of Füchtbauer (1983), suggests that in a geological setting, k_ may be much smaller than under laboratory conditions. In consequence, it is asserted that silica precipitation kinetics may influence the rate of quartz pressure solution when secondary quartz growth is inhibited.