Filtering the unfilterable: tuning the mesostructure of precipitating silica gels to improve the filterability of acidic lixiviation slurries

Introduction Previous studies, e.g., by NMR,1,2 fluorescence pho­ tobleaching,3 and dynamic light scattering (DLS),4,5 have shown that the diffusion of noninteracting probe particles in polymer solutions and gels primarily de­ pends on the polymer concentration and the size of the probe. In interpreting these results, it is widely assumed that polymer gels behave like semidilute polymer solu­ tions, while the structural differences due to the pres­ ence of permanent cross-links are ignored. These dif­ ferences have been revealed and characterized by scat­ tering experiments, such as small-angle neutron scat­ tering (SANS) and light scattering, which show signifi­ cant structural rearrangement of the polymer chains upon gelation.6 This observation is corroborated by results from elasticity measurements, indicating that gelation is accompanied by an increase in the elastic modulus of the samples.7 However, the effect of crosslinking on the diffusion of small particles in a gel has yet to be fully elucidated and understood.4,7 In this paper, we demonstrate how fluorescence correlation spectroscopy (FCS) can provide quantitative measurements on the diffusion of particles in polymer systems. We apply FCS to measure the diffusion time of fluorescent TAMRA molecules in poly(vinyl alcohol) (PVA) solutions and gels prepared at various polymer concentrations and cross-link densities. The measure­ ments indicate that the diffusion of probe particles is affected not only by the polymer concentration but also by the cross-link density of the gel. Remarkably, we find a simple linear relation between the diffusion times and the elastic moduli of the same gels.

When probe particles are embedded in disordered media such as semidilute polymer solutions or gels, the inner structure of the disordered media has a significant effect on the motion of the probe particles. From the results of the measurement of the diffusion of probe particles, we can indirectly obtain information on the structure of the media. In this review, studies of polymer solutions and gels relating to this technique are discussed and the experimental results are evaluated. In particular, trapping of a particle at the sol-gel transition, the diffusion of particles in semidilute polymer solution and gels, and the topological structure of these systems in the vicinity of the gelation threshold are described.

Dynamics of probe particles in polymer solutions and gels

Polymer gels are widely used in oil–gas drilling and production engineering for the purposes of conformance control, water shutoff, fracturing, lost circulation control, etc. Here, the progress in research on three kinds of polymer gels, including the in situ crosslinked polymer gel, the pre-crosslinked polymer gel and the physically crosslinked polymer gel, are systematically reviewed in terms of the gel compositions, crosslinking principles and properties. Moreover, the advantages and disadvantages of the three kinds of polymer gels are also comparatively discussed. The types, characteristics and action mechanisms of the polymer gels used in oil-gas drilling and production engineering are systematically analyzed. Depending on the crosslinking mechanism, in situ crosslinked polymer gels can be divided into free-radical-based monomer crosslinked gels, ionic-bond-based metal cross-linked gels and covalent-bond-based organic crosslinked gels. Surface crosslinked polymer gels are divided into two types based on their size and gel particle preparation method, including pre-crosslinked gel particles and polymer gel microspheres. Physically crosslinked polymer gels are mainly divided into hydrogen-bonded gels, hydrophobic association gels and electrostatic interaction gels depending on the application conditions of the oil–gas drilling and production engineering processes. In the field of oil–gas drilling engineering, the polymer gels are mainly used as drilling fluids, plugging agents and lost circulation materials, and polymer gels are an important material that are utilized for profile control, water shutoff, chemical flooding and fracturing. Finally, the research potential of polymer gels in oil–gas drilling and production engineering is proposed. The temperature resistance, salinity resistance, gelation strength and environmental friendliness of polymer gels should be further improved in order to meet the future technical requirements of oil–gas drilling and production.

Impact of cyclic freezing on precipitation of silica in Me–SiO 2–H 2O systems and geochemical implications for cryosoils and -sediments

We have been investigating the preparation of macroporous gel particles of poly(vinyl alcohol) [1-3]. These particles are obtained by the saponification of particles of poly(vinyl acetate) formed in the polymerization of vinyl acetate in a suspension process. The poly(vinyl alcohol) gel particles have been used as a column packing for aqueous separations. However, the gels have a disadvantage in pressure-resistant property because of poor mechanical strength. We expected that a gel having excellent mechanical stability might be obtained by reinforcing poly(vinyl alcohol) with an inorganic component. We report here that reinforcement using silica as an inorganic component is useful for the improvement of performance of poly(vinyl alcohol) gel particles used as a column packing for gel permeation chromatography in aqueous media. Tetraethoxy silane (TEOS) was used as the inorganic component of the hybrid gels throughout this study. The sol-gel method was applied to the preparation of hybrid gel [4-6]. The process is divided into two steps: hydrolysis of metal alkoxides to produce metal hydroxides, followed by polycondensation of hydroxyl groups. By carrying out this reaction in a poly(vinyl alcohol) gel matrix containing many hydroxyl groups, it is possible to copolymerize metal alcoxide with poly(vinyl alcohol). Thus a silica network may be incorporated into the poly (vinyl alcohol) gel matrix. The gel particles of poly(vinyl acetate) (DP = 1000) obtained by suspension polymerization were classified to give a particle size distribution with particle diameters in the range 350-400 #m. The saponification of poly(vinyl acetate) to poly(vinyl alcohol) was carried out at 30 ~ for 1 month by immersing the gel particles in a solution containing sodium hydroxide and methanol in an aqueous saturated sodium sulfate solution [7]. The gel particles of poty(vinyl alcohol) obtained above have sufficient mechanical stability in water below 55 ~ without crosslinking treatment. The gel particles (1 g) obtained above were immersed in 10 ml of ethanol for 24 h and then tetraethoxy silane and hydrochloric acid as catalyst were added to the particles. The reaction was carried out at 30 ~ For the purpose of removal of TEOS homopolymer the reaction product was then subjected to a Soxhlet extraction using tetrahydrofuran. Swelling behaviour and gel permeation chromatographic properties of the hybrid gel particles obtained above were investigated. The ratio of the volume of the particles in ethanol to that on swelling with water (swelling ratio) was evaluated from observations by optical microscopy. As shown in Fig. 1, swelling ratio decreases with increase of reaction time and levels off at about 2 h. This is due to the formation of the silica network in the poly(vinyl alcohol) gel matrix. The swelling behaviour is mainly controlled by the amount of TEOS added. The slurry of hybrid gel particles was packed into a 150 x 4 mm stainless column at a pressure of about 5 MPa. High-performance gel permeation chromatography (HP-GPC) separations were performed with a Shimazu LC-6AD, employing distilled water as eluent. Solutions (40 #1) of individual solutes were injected with an off-column syringe-septum arrangement. Detection of solutes was performed with a Shimazu Refracto Monitor Model RID-6A (cell volume = 10 ~tl, aqueous reference). The samples of poly(ethylene glycol) are designated PEG with a number which is the molecular weight provided by the suppliers (Kanto Chemicals, Tokyo). The calibration curve was established at a flow rate of 0.4cm3/min with a PEG concentration of 2.0% (w/v). The calibration curve of the hybrid gel particles established with PEG samples is shown in Fig. 2, in comparison with the poly(vinyl alcohol) gel particles. The value of the excluded molecular weight is almost the same for the hybrid gel particles and the poly(vinyl alcohol) gel particles. However, the calibration curve for the column packing containing the hybrid gel particles shifts to higher elution volumes. Since separation power is inversely proportional to the slope of the plot of molecular weight versus elution volume, it is clear that the column containing the hybrid gel particles shows better resolution separations. The shape of the slope

ABSTRACT: The Chemical dissolution and precipitation processes in geothermal reservoirs can play a crucial role in optimizing the efficiency, cost-effectiveness, and sustainability of energy production. Mineral dissolution and precipitation occur when the injected fluid is undersaturated or supersaturated, respectively, impacting fracture permeability and the overall system performance. This study investigates the effects of silica and calcite dissolution and precipitation on fracture aperture under geothermal conditions using a single component approach. When undersaturated cold water is injected, silica dissolution enhances permeability by increasing fracture aperture, improving fluid flow and heat extraction. Conversely, silica precipitation in high-temperature regions reduces fracture aperture, restricting permeability. Calcite exhibits similar behavior but with distinct solubility characteristics. Its dissolution is initially prominent near the injection well but declines over time due to reduced reaction rates. In supersaturated conditions, calcite precipitation occurs rapidly where the solute front arrives before the temperature front, leading to permeability reduction. The study highlights the complex dependence of solubility on temperature, with silica exhibiting increased dissolution and precipitation rates at higher temperatures, whereas calcite dissolution is favored due to increased equilibrium concentrations with temperature.

Silica gels doped with bis(2,2′-bipyridine)[2-(2-pyridyl)benzimidazole]ruthenium(II), [Ru(bpy)2(pbimH)]2+, which dissociates a proton from the imino group upon light irradiation, were prepared by the sol–gel method under both acidic and basic conditions. Changes in the absorption spectra of the [Ru(bpy)2(pbimH)]2+-doped silica sols and gels prepared under basic conditions with the transition from sols to wet gels were similar to those of [Ru(bpy)2(pbimH)]2+ in methanol–water solution with a change from basic to acidic conditions. Therefore, the acid–base equilibrium is shifted from the deprotonated form, [Ru(bpy)2(pbim)]+, to the protonated form, [Ru(bpy)2(pbimH)]2+, during the sol–gel transition. At the gelation point, the wavelength of the emission maximum under basic conditions was shorter than that under acidic conditions, indicating that the mobility of the Ru complex in silica gels was more strongly restricted under basic conditions than under acidic conditions.

Oxidation of sulfide minerals releases sulfuric acid and dissolved metals, with iron sulfides pyrite (FeS2) and pyrrhotite (Fe(1−x)S) recognized as the most common acid-forming minerals. Several factors control the oxidation rate including: the oxidant type, sulfide morphology, microbial action, and trace element contents. Whilst metal sulfides such as galena and sphalerite are less acid-forming, they are typically sources of environmentally significant elements such as Cd , Pb and Zn. Common sulfide oxidation reaction products are metal-sulfate efflorescent salts . Dissolution of these minerals is critical to the storage and transport of acids and metals released upon weathering of mineralized rock or mine wastes . Acid formed by sulfide oxidation can be consumed through reaction with gangue minerals. Neutralization is primarily offered by dissolution of carbonate minerals with calcite and dolomite the most effective. Factors affecting carbonate reactivity include: grain size , texture and the presence of trace elements which can influence a mineral’s resistance to weathering. Silicate minerals such as olivine, wollastonite and serpentinite are recognized as effective longer term neutralizers. Lesser neutralizing potential contributions from phyllosilicates , pyroxenes, amphiboles and feldspars have been reported. Micas, clays and organic matter can temporarily adsorb H+ ions through cation exchange reactions, with gibbsite and ferric hydroxide recognized as offering neutralizing capacity under acidic conditions. Ultimately, the balance of acid producing and acid consuming chemical reactions will determine the production of acid rock drainage (ARD).

HDX oilfield is located in Tarim Basin (China) and operated by PetroChina. Horizontal wells are used to develop some reservoirs. Due to high permeability heterogeneity, injection water early breaks through, resulting in a poor sweep efficiency. Conformance control is considered as the most suitable technique for sweep improvement, but such treatment in horizontal wells is more difficult than in vertical wells. Moreover, high temperature (112 ℃) and salinity (290,000 mg/L) increases extra difficulty in the treatment. Because of high temperature and high salinity, dispersed gel (DG) particles were suggested given that they are more insensitive to harsh environments than in-situ crosslinked gels. Considering wide well spacing and discontinuous interlayer, in-depth conformance control using a large volume of gel particles was proposed. To achieve in-depth migration, gel particles should have a slow expansibility and a high deformability. For horizontal well, particles sedimentation in horizontal section is a challenging issue. Therefore, the density of gel particles should be enough low to be well suspended. To meet all these requirements, eventually delayed expansion in water, low-density, highly deformable DG particles were synthesized. The first application was started at October 2016 and finished at June 2017. Multiple slug injection with different particle size and concentration was designed, and gel particles-alternating-water injection was implemented. Incremental oil production response was observed at December 2016 since the second slug injection started. Until June 2019, incremental oil was 45,255 tons, and the input-output ratio is about 48.59. Considering this huge success, the second treatment in another well was implemented from July to September 2019. The incremental oil after 8 months was 4,870 tons, and the treatment continues to be effective until 2022. Moreover, the first treatment is still effective after 3 years, which proves the treatment a great technical and economic success. This paper presents the idea of the operation design of low-density DG particles for in-depth conformance control in horizontal wells. Furthermore, how to precisely evaluate treatment effect are discussed in detail based on production performance together with water injectivity, pressure index, and wellhead pressure drop curve in the injection well. This paper not only presents the idea using low-density gel particles with multistage injection process of gel particles-alternating-water for in-depth conformance control in horizontal wells, but also summarizes important experiences for successful field operation design and control, which will give an updated framework and an important guidance for in-depth conformance control treatment in horizontal wells under high temperature and high salinity conditions.

The use of Enhanced Geothermal Systems (EGS) has been recognized as a viable source of renewable energy in regions with high geothermal temperatures. Nevertheless, geothermal reservoirs may experience reduced permeability during exploration or operation. Research on chelating agents in geothermal environments has been widely disseminated as a complementary method to conventional methods such as hydraulic and chemical stimulation. Previous studies reported fast and significant improvements in permeability in granitic and volcanic rocks using aqueous solutions of glutamic L-diacetate acid (GLDA) under acidic conditions. However, no studies have been conducted with chelating agents applied to volcanic rocks at different pH conditions, since pH determines the ionic species in the solution, and thus, the chemical interactions taking place in a system. Furthermore, the dissolution of minerals in these conditions was not quantified for modeling purposes. In the present study, an aqueous solution of the chelating agent GLDA at various pH conditions (2-10) was applied to improve the permeability of single-fractured intermediate to basic volcanic rocks. According to the results, permeability increases about up 4.3-fold under weak acid (pH 4) conditions, while it increases about 36-fold under alkaline (pH 10) conditions, due primarily to the formation of voids caused by mineral dissolution or groundmass dissolution, respectively. Moreover, channeled samples with mirror-conditions revealed that the formation of voids at acidic conditions was as deep as 135 µm by the selective dissolution of hematite, whereas an average of 4-µm dissolution of quartz was promoted at alkaline conditions. Although the depth of voids formed in alkaline conditions is less than the case of acidic, quartz composes the matrix that surrounds the phenocrysts of volcanic rocks, promoting a preferential fluid path that improved the permeability further at alkaline conditions. This study is the first step in spreading the use of this chemical stimulation technique to different volcanic-rock geothermal systems.Keywords: EGS, chelating agents, permeability enhancement, andesitic rock, selective dissolution of minerals.

Diurnal and seasonal water variations of temperature, pH, redox potential and conductivity in gnammas (weathering pits): Implications for chemical weathering

Experimental and numerical simulation of dissolution and precipitation: implications for fracture sealing at Yucca Mountain, Nevada

Transportation of fugitive dust over long distances because of wind erosion is a severe environmental threat. Different approaches are experienced to control wind erosion, but durability and costs are the main drawbacks of existing techniques. This study hereby investigates sodium silicate usage as an alkaline additive to bind soil particles and control wind erosion. Sodium silicate is an environmentally safe material and the precipitated inorganic silica gel from which has the affinity with soil texture that makes the overall geopolymerization materials and method clean and environmentally friendly. The neutral condition of natural soils can reduce the alkalinity of sodium silicate’s alkalinity upon contact to the silica gel formation and soil geopolymerization. Different water-diluted solutions containing 50, 35, 20 wt% sodium silicate were prepared and sprayed over the soil surface while various specifications of the stabilized part were evaluated. It was found that interparticle cohesion, shear strength, and wind erosion were affected by sodium silicate content, as explained through a series of immersion, direct shear, and wind tunnel tests. A combination of mechanical and chemical forces can explain the interparticle cohesion since no chemical bonding was established between silica gel and soil particles, as described in Fourier-transform infrared spectroscopy analysis. Scanning electron microscopy coupled with energy-dispersive spectroscopy and thermogravimetric experiment displayed soil particles’ aggregation without mineralogical alteration. The achieved results implicate sodium silicate’s promising role as a stabilizer to bind the soil particles and control wind erosion.

Ru nanoparticles in ionic liquids confined by silica gel: A high active iongel catalyst for low temperature water-gas shift reaction