Modification of Non-Hydrolytic Sol-Gel Derived Alumina by Solvent Treatments

The effect of wetting non-hydrolytic derived alumina xerogels with water and organic solvents in the 20–70°C range on the alumina's properties was investigated. Wetting with organic solvents does not affect the alumina. However, contact with water was found to change the sharp crystallization at 800°C to a continuous crystallization starting at 450°C. Water treatment for a day at room temperature (RT) followed by second calcination decreased the surface area by 10%. This decrease in surface area is less pronounced with increasing wetting periods. On the other hand water treatment at 50–70°C followed by a second calcination resulted in a surface area increase of up to 15%. Upon water treatment the total pore volume has decreased from 0.65 (cm³/gr) to 0.48 (cm³/gr) and the average pore size decreased from 6.8 nm to 4.1 nm. The Cl content was found to be uneffected by the water treatment, remaining at 2.5% wt. Wetting with water at elevated temperature (70°C) accelerated the morphological changes, eliminating the crystallization peak at 800°C in one hour. A dissolution-reprecipitation mechanism is suggested to explain the results. In addition, Mass-Spectroscopy of the effluent gas during heat treatment revealed the emission of CO₂ and water upon phase transition into -Al₂O₃, at 1150–1300°C.     

Numerical Simulation of a CT-Scanned Counter-Current Flow Experiment

Counter-current flow occurs in many reservoir processes and it is important to understand and model these processes in order to operate them effectively. Both drainage and imbibition processes exist simultaneously during counter-current flow. It has thus proven difficult to model this type of flow using conventional techniques because of the impossibility of assigning a single capillary pressure curve applicable over the entire sample. In the current paper, a new saturation-history-dependent approach has been developed to simulate a counter-current flow experiment done with an X-ray CT scanner. Hysteresis in both capillary pressure and relative permeabilities is considered during simulation. Capillary hysteresis loop and relative permeabilities are extracted through history matching and a family of scanning curves is constructed connecting the two branches of the capillary hysteresis loop. Each gridblock of the sample is assigned a different scanning curve according to the local saturation history. History-dependent modeling of the experiment reproduced two-dimensional saturation distributions over time with good accuracy, which cannot be obtained with traditional simulation using only one capillary pressure curve.     

Modeling the Formation of Fluid Banks During Counter-Current Flow in Porous Media

Fluid banks sometimes form during gravity-driven counter-current flow in certain natural reservoir processes. Prediction of flow performance in such systems depends on our understanding of the bank-formation process. Traditional modeling methods using a single capillary pressure curve based on a final saturation distribution have successfully simulated counter-current flow without fluid banks. However, it has been difficult to simulate counter-current flow with fluid banks. In this paper, we describe the successful saturation-history-dependent modeling of counter-current flow experiments that result in fluid banks. The method used to simulate the experiments takes into account hysteresis in capillary pressure and relative permeabilities. Each spatial element in the model follows a distinct trajectory on the capillary pressure versus saturation map, which consists of the capillary hysteresis loop and the associated capillary pressure scanning curves. The new modeling method successfully captured the formation of the fluid banks observed in the experiments, including their development with time. Results show that bank formation is favored where the p_c-versus-saturation slope is low. Experiments documented in the literature that exhibited formation of fluid banks were also successfully simulated.     

Simulation of three-phase displacement experiments

Three-phase displacement experiments for a water-benzyl alcohol-decane system are simulated. Literature experimental three-phase relative permeabilities for the system are used to describe the relative permeabilities in the three-phase region for different three-phase relative permeability models. Saturation trajectories and elliptical regions are mapped in the three-phase region. Simulations are performed to model displacement experiments including breakthrough and the formation of multiple shocks. The model can be used to predict the results for other displacements. In an experiment where significant gravity segregation is present, the displacement is more accurately modeled by assuming a uniform initial condition than by using the actual vertical saturation and assuming no cross flow. It is shown how different residual saturation values can be measured in the laboratory depending on the initial saturation conditions in the core. The experimental residual saturations can be significantly different than the ‘theoretical’ or model values.     

Microstructure Evolution of Nonhydrolytic Alumina Gels

Nonhydrolitic sol-gel processes of aluminum chloride and aluminum bromide with isopropyl ether and aluminum sec-butoxide were performed at various temperatures. Based on the Arrhenius type variation of the gelation time with temperature, activation energies for the gelation were found to be in the range 19–25 Kcal/mol range. The energies were found to be sensitive to the nature of the aluminum ligands and the chemical scheme. Due to the large activation energy, it is possible to stop the reaction at any time before gelation by cooling the sol to room temperature. Small angle X-ray scattering (SAXS) of sols from the AlClAlCl₃/Pr O system shows unique development of a fractal like structure with nanometer scale order, demonstrated by discrete peaks in the SAXS data. A fractal dimension D = 1.64 was found. An aggregation scheme is proposed to explain this phenomenon. A fractal dimension of 2.4 without small scale ordering found for xerogels prepared from the AlCl₃/ASB system reflects the effect of the different precursors on the microstructure of nonhydrolytic gels.     

Effect of Aging on Nonhydrolytic Alumina Xerogels

Nonhydrolytic sol-gel route is a relatively recent process which enables production of complex, multicomponent oxide materials. This process has some advantages over the conventional hydrolytic sol-gel route due to the ability to produce low-shrinkage, homogeneous, multicomponent gels. The objective of this work was to determine the effects of aging of nonhydrolytic gels on the composition, yield, phase transformations and morphology. Xerogels were prepared from aluminum chloride and isopropyl ether. Properties were studied using AgNO₃ titrations, TGA/DTA, XRD, and BET analysis. We have found that the gels contain significant amount of chlorine where the Cl/Al atomic ratio ranges from 1.1–0.6 depending on the aging time. The crystallization temperature and enthalpy of crystallization decreased with aging time. The decrease of the surface area near the crystallization temperature correlates well with the decrease of the enthalpy of crystallization as a function of aging time. A closed pore phenomenon has been observed in the nonhydrolytic alumina system. Finally, analysis of the condensation degree (CD) yielding Al–O–Al bonds suggests that the rate determining step before the gel point is the alkoxy groups formation. However, during aging of the gels, the CD remains constant since the condensation of chloride with isopropoxy groups is stericly inhibited. Surface areas in the 300–650 m²/g range were obtained depending on the aging time.     

Tracer transport in a fractured chalk: X-ray CT characterization and digital-image-based (DIB) simulation

A digital-image-based simulation methodology is applied to evaluate the influence of heterogeneous porosity on the evolution of tracer concentrations in imaged tracer tests. Maps of computed tomography (CT)-number are calibrated relative to average porosity, and then thresholded to define porosity maps. These data are then used to automate the distribution of parameters within a finite element representation of the geometry. The technique is applied to characterize the variability of the porosity, the hydraulic conductivity, and the diffusivity for an artificially fractured chalk core (30 × 5 cm). X-ray CT was used both to characterize the initial condition of the core, and then to concurrently monitor the transport of an NaI tracer within the fracture and into the surrounding matrix. The X-ray CT imaging is used to characterize the heterogeneous rock porosity, based on which the hydraulic conductivity, and diffusivity of the chalk were defined and were directly imported into our newly developed three-dimensional FEMLAB-based multiple physics simulator. Numerical simulations have confirmed the observed tracer transport behaviors: (1) The different tracer-penetration distances imaged in the matrix above and below the horizontal fracture are indicative of a greater tracer mass penetrating into the lower matrix; and (2) Transport in the matrix below the fracture was enhanced. The computer simulated tracer concentration distributions compare favorably with those monitored by X-ray CT.     

Preparation of Alumina Aerogel Films by Low Temperature CO2 Supercritical Drying Process

Alumina aerogel thin films were formed by a new synthesis route. Sols were prepared by the Yoldas process. Gels were formed by sol evaporation in a few hours. Films were prepared by dip coating glass or alumina substrates into both the sols and the gels. Aerogel films with special morphology were produced for the first time by exchanging the film solvent with acetone after the dip coating, followed by supercritical drying. The morphology of the films, studied by SEM and TEM, consists of fiber-like network of round chains (≈0.1 μm thick), and pores (0.1–0.5 μm in diameter). It is shown that the fibers contain a homogeneous arrangement of sol particles, 2–4 nm in size. Formation of this microstructure can be attributed to phase separation in the alumina-water-acetone system in a 2D film geometry. A conceptual model for the film development is proposed.