| dc.description.abstract | The main goal of primary cementing is to provide zonal isolation in oil and gas wells. In general, the failure of the cement sheath to maintain long-term zonal isolation is due to:
Low tensile properties
Shrinkage
Shrinkage leads to formation of microannuli and/or tensile cracks in the cement sheath resulting in increased permeability. Expansive additives are used to mitigate bulk shrinkage. In order to compensate effectively for shrinkage, the reactivity of the additive must be such that its expansion occurs mainly in the late plastic phase and in the beginning of the hardening phase. The reactivity of the expansive additive must be regulated to produce the required expansion at the conditions in the actual well.
In addition to shrinkage stresses, most operations in the well impose tensile stresses to the cement sheath, which might result in failure due to poor tensile strength and low flexibility of cement sheath. Incorporation of flexible particles to the cement system is the common technique to improve its flexibility, and thus to relaxing the tensile strength requirement. Addition of flexible particles to the cement system reduces Young’s modulus, but also the tensile strength which is not favorable for long-term zonal isolation.
Lately, several research groups in the oil and gas industry have started to investigate the application of nanomaterials to alleviate problems in oil well cementing. Incorporation of nanosized particles into the cement system can modify a range of properties and improve its functionality. In this thesis we explore two promising applications of nanomaterials for modification of cement systems to provide long-term zonal isolation of oil wells:
Application of nano-MgO (NM) with engineered expansive properties to limit cement system bulk shrinkage.
Application of nanorubber (NR) for enhancing tensile properties of the cement system.
An introductory investigation on the potential use of engineered NM for compensating bulk shrinkage of a cement system was conducted. The reactivity and thus the expansive properties of NM were regulated by heat-treatment. The impact of NM’s heat-treatment condition on morphology and reaction of NMs and cement hydration was investigated. Additionally, the effect of NMs differing in reactivity on tensile properties of cement systems was examined. Addition of only 2 % NM by weight of cement, with proper reactivity, was adequate to compensate for bulk shrinkage of the examined cement system. Increase in pre-treatment temperature resulted in NM reaction retardation, which also affected the hydration process of cement systems containing NMs. In this study, we demonstrate that adjusting the reactivity of NM is a promising technique to mitigate bulk shrinkage of cement systems for the life of the well.
Moreover, a preliminary study was undertaken to investigate the applicability of NR in oil well cement systems, employing the advances in NR-epoxy systems. The NR selected for this study was stable at high temperature and high pH. The effect of NR addition on tensile properties, bulk shrinkage, sonic compressive strength development, setting time and hydration process of cement systems was studied. Cement hydration and sonic compressive strength development were retarded in the presence of NR. NR-cement systems presented reduced bulk shrinkage. Most importantly, addition of NR to the cement system enhanced its capacity to withstand tensile stresses at laboratory condition.
The main contributions of present work are:
Investigating the main causes of cement sheath failure and the requirement to provide long-term zonal isolation.
Review of the research literature on shrinkage mitigation and tensile properties improvement through application of nanomaterials.
Demonstrating improved tensile properties of a cement system in the presence of NR
Controlling the expansive properties of NM by adjusting its reactivity through heat-treatment.
Demonstrating controlled bulk shrinkage of a cement system in the presence of NMs with regulated | nb_NO |
