ETD Collection

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2017 - 20193

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    Tribology of drill bit material in deep oil drilling process: variation of weight on bit (WOB) and bit rotating speed
    (2019) Seloanyane, Matsolo
    The oil and gas industry is faced with great challenges when it comes to the drilling process due to a shift in drilling shallow onshore layers, to deep and ultra-deep offshore layers. The depletion in the onshore oil and gas reserves has resulted in the need to explore alternative reserves, to meet the constantly increasing energy demand. Due to increased drilling depth, the drilling equipment faces challenges of increased torque and drag, which lead to friction and wear, as a result of complicated drilling environments. These challenges eventually lead to serious equipment failure, resulting in increased production costs. It is therefore of paramount importance to find different combinations of drilling parameters that will give the highest production rates at minimum costs while also paying attention to strict environmental and safety regulations. In this research, different drilling parameters such as weight-on-bit (WOB) and drill bit rotating speed, including different drilling environments such as dry and wet environments were studied, to determine how their different combinations affect the coefficient of friction (COF), when used in three different rock types; Fine-grained Arkose sandstone, Coarse-grained Arkose sandstone and Quartzite. In dry environments, varying the WOB and drill bit rotating speed greatly affects the COF. Higher speeds and low loads resulted in reduced COF while low speeds and high load portrayed increased COF. For wet environments, different nanoparticles were used in water-based and oil-based fluids. The oil used was environmentally friendly and biodegradable vegetable oil named castor oil. The nanoparticles (NPs) of bentonite, attapulgite, sepiolite and cellulose nanocrystals (CNC), were used as additives in drilling muds, to reduce the COF. These NPs were successful in the reduction of COF, but they performed better in the oil-based fluids than in the water-based fluids
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    Tribology study in deep downhole drilling processes: influence of contact pressure and sliding speed on friction under newtonian and non-newtonian lubrication
    (2017)
    Tribology study in deep downhole drilling processes: Influence of contact pressure and sliding speed on friction under Newtonian and NonNewtonian Lubrication. Drilling processes are complex and costly processes. Mechanical friction is predominant as the limiting factor in attaining high recovery rate of target recoverable oil resources. As a result it is of essence to reduce the cost incurred as a result of mechanical friction by putting in place the methods to minimise the friction encountered during drill string/casing and drill string/formation contact. However to best perform this, laboratory simulation of the conditions and / understanding of the mechanisms found in downhole, in particular, deep and ultra-deep downhole is a necessity. The work presented herein simulates the drill string/casing and drill string/formation or rock contact under aqueous and non-Newtonian lubrication. Furthermore contact pressure and drilling speed were varied under the two later mentioned lubricant. Following the running in method, the results depicted a decrease in friction coefficient as load or contact pressure was increased. This was observed both when the steel/steel contact (simulating drill string/casing contact) and when steel/sandstone rock (simulating drill string/formation contact) were simulated. Increasing speed increase perturbations in the coefficients of friction, and has no direct effect on the mechanical friction in comparison to observations in the case when the when load was incremented at low speed ranges. However, as the speed increases the friction coefficient for steel to steel contact reduced significantly. The latter invalidates the universality third law of friction, which state that friction is independent of speed. Offset of the steel pin, meant that the pin encountered new and fresh/hard asperities and hence increases the vibration. Thus to better control this was concluded that load allowance and measurements thereof is to be made when designing drilling strings for deep downhole drilling. In addition the incorporation of the bentonite particles into the drilling mud increased the friction for steel/steel contact due to two-body abrasion which becomes the prevailing mechanism on the entrainment of the bentonite particles between steel pin and steel disc. For steel/sandstone contact the friction decreased on the entrainment of bentonite particle. This was attributed to the microstructure and porosity of the sandstone disc. Furthermore and important for lubricant design, the water to bentonite ratio was tested. The result for the latter testing showed that friction increased with increased water concentration in the bentonite mud. This means that the overall performance of pure water lubricant in comparison with bentonite mud was found to be poor. Thus addition of bentonite mud at certain concentration for specific drilling contact, can either increase friction or decrease friction and consequently it was found advantageous to use bentonite particles under drill string/formation contact depending on the hardness of the formation. The porosity seemed to reduce the tribofilm, making it difficult for smooth and good friction reduction since the charged lubricant is lost in filling the pores. Upon visual observation the tribofilm thickness for when sandstone was lubricated with bentonite was slightly thicker than in the case when water as a lubricant was used. Though for different sandstones the rock porosity increased friction, it was assumed that with time the worn material and the bentonite particles quickly works to fill the pores that were initially taken to be gas filled. The microstructures of the contacting materials and their abrasiveness strong influence the drilling friction.
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    Friction and wear of selected metal ceramic and polycrystalline diamond sliding couples
    (2017) Damn, Oliver Frank Rudolf August
    This thesis describes a study of the friction and wear characteristics of a range of oxide and silicon-based ceramics sliding against AlSI 440C stainless steel, as well as various sliding combinations of two types of De Beers polycrystalline diamond (PCD), namely Syndite and Syndax, To facilitate the former work, a high-speed reciprocating sliding test machine with computerized data acquisition and control was developed. It was confirmed that under water-lubricated sliding, the oxide ceramics (alumina, PSZ, 3YTZP, and Ce-TZP) showed higher friction coefficients and Weal' rates than the silicon-based ceramics (Sialon and silicon nitride). This was related to different levels of adhesion and the formation of metallic transfer films. For the zirconia ceramics, increased transformation toughening was associated with increased surface fracture damage and metallic film formation. In general, the metallic transfer films were beneficial, protecting the underlying ceramic and dominating the friction and wear behaviour. The superior performance of the silicon-based ceramics was related to the formation of lubricious tribofilms containing silicon oxides and hydroxides. Experiments with synthetic mine water as lubricant demonstrated that the presence of significant amounts of chloride and sulphate in the water generally reduced friction and wear. This was tentatively explained in terms of reduced adhesion and the promotion of iron oxide and hydroxide formation. It is suggested that the influence of sulphate may be more important in thi'3 regard than that of chloride. The tribological behaviour of self-mated Syndite PCD sliding couples is dominated by the formation of Co-rich trlbofilms, which nrc associated with increased friction coefficients and reduced load carrying capacity (LCC). Syndax, which employs silicon as the binder phase, shows lower friction coefficients anti higher LCe under both dry and water-lubricated sliding conditions. Mixed Syndax/Syndite couples show superior performance to self-mated Synditc tinder dry sliding conditions, but 110 improvements in the presence of water. The former effect is related to the preferential removal of Co 11'0111 the Syndite surface to the Syndax