Cement-based materials are of importance in the construction of geothermal wells and high-temperature oil and gas wells. These materials fill the annulus between the well casing and the rock forming a protective layer, known as sealant, which is used primarily to secure and support the casing inside the well. In addition it prevents entry of unwanted fluids into the well and communication between formation fluids at different levels. These cement based sealants need to perform for many years at high temperatures and in severe chemical environments; conditions which can cause the material of the well-casing to degrade resulting in reduced strength and increased permeability. The aim of this study is to develop new materials which will have the potential properties (high strength and low permeability) for use as sealants in geothermal and deep, hot oil wells. In order to do this special cement slurries, based on the CaO−Al2O3−SiO2−H2O (CASH) hydroceramic system, have been synthesised over the temperature range 200 to 350 °C (i.e. the typical working temperature of these wells). The additives used in these cement slurries are silica flour and alumina. A detailed description of a suite of novel hydroceramic compositions over the temperature range 200 to 350 °C is given. X-ray diffraction has been used to determine the mineralogical composition and Rietveld refinement to quantify the known phases present at different temperatures. In addition the chemistry of some of the major phases present has been examined using electron probe microanalysis. Scanning electron microprobe and simulation software have been employed to study the crystal shape of these major minerals. The engineering properties of the hydroceramic materials are very important. A study of the compressive strength and permeability has been carried out over a range of temperature (200 to 350 °C). In addition permeability has been calculated using simulation software and the results compared with experimental values. Hydroceramic formulations with excellent strength and permeability measurements have been found. Some of these formulations have been tested for durability under simulated well conditions. These materials have been immersed into different brines for a certain period of time at temperatures between 200 to 300 °C. Some preliminary results regarding the changes in mineralogy in these samples are presented in this thesis. These experiments have been carried out at the Synchrotron Radiation Source (SRS) using tomographic energy-dispersive diffraction imaging (TEDII).

Considerable research has been done to improve hydrate formation rate. One of the ideas is to introduce mechanical mixing which later tend to complicate the design and operation of the hydrate formation processes. Another approach is to add surfactant (promoter) that will improve the hydrate formation rate and also its storage capacity to be closer to the maximum hydrate storage capacity. Surfactant is widely known as a substance that can lower the surface or interfacial tension of the water when it is dissolved in it. Surfactants are known to increase gas hydrate formation rate, increase storage capacity of hydrates and also decrease induction time. However, the role that surfactant plays in hydrate crystal formation is not well understood. Therefore, understanding of the mechanism through morphology studies is one of the important aspects to be studied so that optimal industrial processes can be designed. In the present study the effect of three commercially available anionic surfactants which differ in its alkyl chain length on the formation/dissociation of hydrate from a gas mixture of 90.5 % methane – 9.5% propane mixture was investigated. The surfactants used were sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS). Memory water was used and the experiments for SDS were carried out at three different degrees of under-cooling and three different surfactant concentrations. In addition, the effect of the surfactant on storage capacity of gas into hydrate was assessed. The morphology of the growing crystals and the gas consumption were observed during the experiments. The results show that branches of porous fibre-like crystals are formed instead of dendritic crystals in the absence of any additive. In addition, extensive hydrate crystal growth on the crystallizer walls is observed. Also a “mushy” hydrate instead of a thin crystal film appears at the gas/water interface. Finally, the addition of SDS with concentration range between 242ppm – 2200ppm (ΔT =13.10C) was found to increase the mole consumption for hydrate formation by 14.3 – 18.7 times. This increase is related to the change in hydrate morphology whereby a more porous hydrate forms with enhanced water/gas contacts. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

The crystal growth of nanoporous materials is different from most other classes of material in that their framework architectures contain periodic arrangement of pores or voids in which there is no direct bonding between adjacent units of the framework. This poses a variety of questions as to how such parts of framework develop during the crystallization process, atomistically and energetically. Here we use the nanoporous metal-organic framework, ZIF-8 as a prototypical material to obtain a basic understanding of the growth of a nanoporous material. The crystals of ZIF-8 produced in the N,N-dimethylformamide solvent [ZIF-8(DMF)] and methanol-co-N,N-dimethylformamide solvent [ZIF-8(MeOH)] are both rhombic dodecahedron in shape with a much smaller crystal size in the latter. In the study of the kinetics of ZIF-8(DMF) crystallization, we get a good agreement in the values of activation energies using both Avrami-Erofe’ev-Hancock-Sharp and Gualtieri’s models, i.e. about 120 kJ mol-1 for nucleation, and 95 kJ mol-1 for crystal growth process. The study of kinetics of ZIF-8 surface growth, by in situ AFM, with ZIF-8(DMF) as seed crystal that are grown in the methanolic growth solution we see faster rate in the <100> directions than the <110> directions, with the most probable activation energy of about 80 kJ mol-1 in both directions. This is the first example of in situ AFM being used to obtain activation energy for a surface growth in MOF. We also reveal here that growth process of ZIF-8 occurs through the nucleation and spreading of successive metastable unenclosed sub-steps to eventually form stable terrace steps of the enclosed framework structure in which this process is reliant on the presence of nonframework species to connect the framework species that have voids between them. The experiments also enable identification of some of the fundamental units in the growth process and the stable crystal surface plane. Further, the spreading of terraces at high supersaturation condition (early state) is fairly isotropic as is seen through the formation of almost-rounded terraces on the surface of ZIF-8. The growth direction becomes clear as the supersaturation condition nears to equilibrium (later stage) by the formation of rhombohedral terraces with pointy ends growing along the <100>, and <110> directions and straight edges growing perpendicular to the <111> direction. Formation of this rhombohedral morphology is explained by a coarse grain approach similar to that used in the Kossel model by making assumptions that the sodalite cage is the growth unit and attachment of one sodalite cage in each growth direction is the rate determining step for the formation of a new row of sodalite cages in each direction. Finally, based on the profiles of growth spirals formed from screw dislocations on the ZIF-8 surface obtained from the ex situ AFM images and ICE theory, plausible screw dislocations with Burgers’ vector of 1/2 <111> and <100>, but not <110>, are deduced. Some of the findings in this work will be applicable to numerous nanoporous materials, and the work in general will support efforts to synthesize and design new framework materials and to control the crystal properties of these materials.

Zeolitic imidazolate frameworks (ZIFs) is a new class of metal-organic frameworks (MOFs) with zeolite-like properties such as permanent porosity, uniform pore size, and exceptional thermal and chemical stability. Until recently, ZIF materials have been mostly synthesized by solvothermal method. In this thesis, further analysis to tune the size and morphology of ZIF-8 is done upon our group’s recent success in preparing ZIF-8 crystals in pure aqueous solutions. Compositional parameters (molar ratio of 2-methylimidazole/Zn2+, type of zinc salt reagents, reagent concentrations, addition of surfactants) as well as process parameters (temperature and time) were systematically investigated. Upon characterizations of as-synthesized samples by X-ray powder diffraction, thermal gravimetric analysis, N2 adsorption, and field-emission scanning electron microscope, the results show that the particle size and morphology of ZIF-8 crystals are extremely sensitive to the compotional parameters of reagent concentration and addition of surfactants. The particle size and morphology of hydrothermally synthesized ZIF-8 crystals can be finely tuned; with the size ranging from 90 nm to 4 μm and the shape from truncated cubic to rhombic dodecahedron.

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