Cement / rock interaction in geothermal wells

<p>One of the main issues associated with the exploitation of geothermal energy is the durability of the cement that is used downhole to cement the steel casing to the formation. Cement durability can have a major impact on the lifetime of geothermal wells, which do not usually last as long as desirable. The cement formulations used in the construction of geothermal wells are designed to provide mechanical support to the metallic well casings and protect them against the downhole harsh environment, which often leads to corrosion. This research is focused on the way that these formulations interact with the surrounding rock formation in geothermal environments, and aims to understand whether these are likely to affect the cement durability and, consequently, the geothermal well lifetime. The experimental work in this thesis consists of examining the changes in the interfacial transition zone (ITZ) that forms between geothermal cements and the volcanic rocks, after hydrothermal treatment. Holes were drilled in blocks of volcanic rocks and cement slurries with distinct formulations were poured into the cavities. The assemblages were autoclaved under typical geothermal conditions. The main variables under study were the cement formulation, the temperature of curing (150°C and 290°C), the presence of drilling mud, CO₂ exposure and the type of rock. The results show that with all the Portland cement based systems a series of chemical reactions occur at the interface between the cement and the rock, the ITZ, where migration of Ca²⁺ and OH⁻ ions occurs from the cement into the rock pores. These reactions are ongoing, which occur faster during the first days/few weeks of curing, mostly driven by physical process of cement movement into the rock, followed by a slower second stage, controlled mostly by chemical driving forces. This work highlights the interdependence between the chemical and physical interactions between geothermal cements and volcanic rocks which are complex. Variables such as temperature and time of curing and silica addition affect the cement phases that form, while the amount of amorphous silica and rock permeability dictate the extent of rock interaction. The presence of carbon dioxide influences the extent of rock/cement interaction and this can be controlled by the rock permeability and cement formulation. Consequently, most of the above mentioned variables were found to have an impact on the geothermal cement durability, which depends on the way these factors are combined.</p>

Cement / rock interaction in geothermal wells

<p>One of the main issues associated with the exploitation of geothermal energy is the durability of the cement that is used downhole to cement the steel casing to the formation. Cement durability can have a major impact on the lifetime of geothermal wells, which do not usually last as long as desirable. The cement formulations used in the construction of geothermal wells are designed to provide mechanical support to the metallic well casings and protect them against the downhole harsh environment, which often leads to corrosion. This research is focused on the way that these formulations interact with the surrounding rock formation in geothermal environments, and aims to understand whether these are likely to affect the cement durability and, consequently, the geothermal well lifetime. The experimental work in this thesis consists of examining the changes in the interfacial transition zone (ITZ) that forms between geothermal cements and the volcanic rocks, after hydrothermal treatment. Holes were drilled in blocks of volcanic rocks and cement slurries with distinct formulations were poured into the cavities. The assemblages were autoclaved under typical geothermal conditions. The main variables under study were the cement formulation, the temperature of curing (150°C and 290°C), the presence of drilling mud, CO₂ exposure and the type of rock. The results show that with all the Portland cement based systems a series of chemical reactions occur at the interface between the cement and the rock, the ITZ, where migration of Ca²⁺ and OH⁻ ions occurs from the cement into the rock pores. These reactions are ongoing, which occur faster during the first days/few weeks of curing, mostly driven by physical process of cement movement into the rock, followed by a slower second stage, controlled mostly by chemical driving forces. This work highlights the interdependence between the chemical and physical interactions between geothermal cements and volcanic rocks which are complex. Variables such as temperature and time of curing and silica addition affect the cement phases that form, while the amount of amorphous silica and rock permeability dictate the extent of rock interaction. The presence of carbon dioxide influences the extent of rock/cement interaction and this can be controlled by the rock permeability and cement formulation. Consequently, most of the above mentioned variables were found to have an impact on the geothermal cement durability, which depends on the way these factors are combined.</p>

Damage Evolution and Fracture Events Sequence Analysis of Core-Shell Nanoparticle Modified Bone Cements by Acoustic Emission Technique

In this research, damage in bone cements that were prepared with core-shell nanoparticles was monitored during four-point bending tests through an analysis of acoustic emission (AE) signals. The core-shell structure consisted of poly(butyl acrylate) (PBA) as rubbery core and methyl methacrylate/styrene copolymer (P(MMA-co-St)) as a glassy shell. Furthermore, different core-shell ratios 20/80, 30/70, 40/60, and 50/50 were prepared and incorporated into the solid phase of the bone cement formulation at 5, 10, and 15 wt %, respectively. The incorporation of a rubbery phase into the bone cement formulation decreased the bending strength and bending modulus. The AE technique revealed that the nanoparticles play an important role on the fracture mechanism of the bone cement, since a higher amount of AE signals (higher amplitude and energy) were obtained from bone cements that were prepared with the nanoparticles in comparison with those without nanoparticles (the reference bone cement). The SEM examination of the fracture surfaces revealed that all of the bone cement formulations exhibited stress whitening, which arises from the development of crazes before the crack propagation. Finally, the use of the AE technique and the fracture surface analysis by SEM enabled insight into the fracture mechanisms that are presented during four-point bending test of the bone cement containing nanoparticles.

Effect of Incorporating Nanoporous Metal Phosphate Materials on the Compressive Strength of Portland Cement

Nanoporous metal phosphate (NP-MPO) materials are being developed for removal of contaminant oxyanions (As(OH), , and ), and cations (mercury, cadmium, and lead) from water and waste streams. Following sequestration, incorporation of metal laden NP-MPOs as a portion of cement formulation would provide an efficient and low-cost way to immobilize metal laden NP-MPOs in an easily handled waste form suitable for permanent disposal. There are no known investigations regarding the incorporation of NP-MPOs in concrete and the effects imparted on the physical and mechanical properties of concrete. Results of this investigation demonstrated that incorporating of NP-MPO materials requires additional water in the concrete formulation which decreases the compressive strength. Thus, incorporation of NP-MPOs in concrete may not serve as an efficient means for long-term disposal.