In order to obtain a better understanding of hydrate plug formation mechanism in natural gas pipelines, formation and growth of hydrate layer within a rectangular channel formed by brass bottom and top surfaces and an insulated inner and an outer surface of transparent polycarbonate tube was studied experimentally. A gas mixture of 90 % methane balanced with propane was supplied at specified flow rates while the humidity and temperature of the supply gas was controlled at desired values using bubble type saturators and heat exchangers placed in series. Hydrate formation occurred along the top and bottom brass surfaces maintained at temperatures below equilibrium hydrate formation temperature, while the transparent tube served as window for visual observation. A series of carefully controlled laboratory experiments were performed to reveal the shape of porous hydrate layer under different combinations of under-cooling and moisture concentrations. The observed transient characteristics of hydrate layer profiles will provide important data that can be used for validation of numerical models to predict hydrate plugging of natural gas pipelines.

[pt] Um dos problemas encontrados na indústria do petróleo no que diz respeito a garantia de escoamento durante produção de óleo cru trata-se da deposição das frações parafínicas no interior dos tubos.Gastos significantes estão associados ao bloqueio parcial ou total das linhas de produção. O procedimento mais aplicado para remoção mecânica destes depósitos é a passagem de pigs. Uma das forças de contato que atua no pig durante o procedimento de remoção deste depósito é exercida pela camada de parafína depositada na parede do tubo. À medida que se processa a remoção mecânica e se estabelece o deslocamento do pig e da parafina removida outras forças de contato se caracterizam. O conhecimento da força necessária para quebra e remoção de um depósito específico é um ponto chave para o planejamento seguro e eficiente de uma operação de passagem de pig. Este trabalho propõe-se a estudar experimentalmente a força de contato deste depósito controlando as variáveis que influenciam esta força, analisando os resultados de acordo com os parâmetros envolvidos e os comparando com os resultados obtidos com modelos disponíveis na literatura. / [en] Wax deposition inside transportation pipelines is a relevant problem for the petroleum industry. Significant capital and operatinal cost are associated with the total or partial blockage of production lines. Pigging is the most widely used technique for removing wax deposits. The knowledge of the forces necessary to break and remove a particular wax deposit are key to the safe and a efficient design of a pigging operation. The present work presents an experimental laboratory study of the forces necessary to remove wax deposits inside a duct. The results obtained with the experiments are used to validate predicting models avaiable in the litErature.

[pt] Pigs ainda são a ferramenta mais utilizada pela indústria para remover depósitos de parafina em linhas submarinas de petróleo. A passagem de Pigs é considerada uma operação de risco, pois existe a possibilidade do gradiente de pressão disponível para deslocar o Pig não ser suficiente para vencer as forças de contato entre o Pig e a parede do tubo e as forças necessárias para remoção do depósito de parafina. Neste trabalho realizou-se um estudo experimental visando o levantamento de dados de laboratório sobre as forças de contato Pig-tubo para alguns tipos de Pigs comumente usados em operações de campo. Além disso, foram também determinadas forças para remoção de depósitos de parafina fabricados no laboratório sob condições controladas. Foi projetada e construída uma seção de testes onde os Pigs ensaiados eram puxados com velocidade constante através de trechos de tubo, sendo a força trativa de puxada monitorada continuamente por células de carga. Os resultados de força de contato Pig-tubo sem depósito de parafina foram comparados com modelos teóricos disponíveis na literatura. Para o caso dos experimentos onde depósitos estavam presentes, as forças necessárias para a quebra dos depósitos foram também comparadas com o único modelo disponível na literatura. Os resultados obtidos mostraram que, para Pigs de disco e de espuma os modelos disponíveis prevêem as forças de contato Pig-tubo com incerteza aceitável. Para o caso de remoção de parafina a comparação com os experimentos desenvolvidos mostrou que o modelo disponível consegue estimar a força inicial para a quebra do depósito dentro de uma faixa de incerteza de ± 30% para os Pigs de disco e de espuma. / [en] Pigging is still the most widely used method to remove wax deposits in subsea pipelines. The passage of a pig is still considered a risky operation, since the pressure gradient available for driving the pig might not be sufficient to overcome the contact forces between the pig and the wall, as well as the forces required to remove the wax deposit. If this is the case, the pig will be stuck, and costly remediation procedures will have to be implemented. In the present work, an experimental study was carried out aimed at collecting reliable data on the contact forces developed between pigs and the pipe wall for some types of pigs commonly used in field operations. Also, as part of the work, the forces required to remove wax deposits prepared under controlled conditions were recorded. To this end, a test section was specially designed and constructed. In the tests, the pigs were pulled through a set of pipes at constant speed by a winch, while the pulling force was continuously recorded by load cells. The results obtained for the contact forces between disc, foam and conical cup pigs and the pipe wall without wax deposits were compared with models available in the literature. Good level of agreement was obtained. Experimental results obtained for the cases where the test pipes had controlled wax deposits where compared with the only model available in the literature for predicting the forces required to break the wax deposits. The comparisons demonstrated that the measured forces required for breaking the wax deposit can be estimated by the available models within an uncertainty of ±30%.

Heavy oil production has drawn more and more attention in petroleum industry. The amount of heavy oil in the world is twice more than the conventional oil (low viscosity), which has been consumed rapidly from the past. The understanding of flow patterns and pressure losses in multiphase flow with high viscosity oil are vital to assist the design of transportation pipeline. This thesis involves experimental investigation of two phase and three phase flows under high oil viscosity conditions (up to 17000cP) in horizontal pipelines. The multiphase (oil/water/solid/gas) facility was designed and constructed at Cranfield University and consists of 6m long horizontal pipeline of 0.026m diameter along with instrumentations. The principal objectives of the work were to study the effect of viscosity, water cut, temperature variance, and flow conditions on flow patterns and pressure drops for (oil/gas and oil/water) two phase flows; to compare the measured flow parameters and phase distribution with those predicted from models found in the literature for two phase flows; and to conduct an experimental study of gas injection effect on pressure gradient in (oil/water/gas) three phase flow. Due to the nature of heavy oil reservoirs, sand is associated with oil/water mixture when extracted; therefore sand concentration effect on pressure drop in (oil/water/sand) three phase flow is also examined. For oil-air flow, a smooth oil coating was observed in the film region of slug flow, while a ripple structure of oil coating film was found at higher superficial air velocity for slug flow regime and annular flow regime. The ripple structure was believed to increase the effective roughness of the pipe wall, which resulted in higher pressure gradients. The pressure drop correlations from Beggs and Brill (1973) and Dukler et al. (1964) were used to compare with experimental pressure gradients for oil/air flow. It was found that these correlations failed to predict the pressure gradients for heavy oil/air flows in this work. Several new heavy oil/water flow patterns were named and categorized based on observations. Though the heavy oil viscosity is an essential parameter for oil continuous phase flow on pressure drop, it had no significant effect beyond Water Assist Flow (WAF) condition, as a threshold was found for water cut with fixed superficial oil velocity. The transition criterion by McKibben et al. (2000b) for WAF was found to be able to predict this threshold reasonably well. Core Annular Flow (CAF) models were found to greatly under predict the pressure gradients mainly due to the coating (oil fouling) effect associated with this study. A new coating coefficient was introduced to models presented by Bannwart (2001) and Rodriguez et al (2009). The addition of solid in the mixed flow led to minor increase in the pressure gradient when the particles were moving with the flow. However, higher sand concentration in the system led to higher pressure gradient values. The addition of gaseous phase to the oil/water flow was more complex. The gaseous injection was beneficial toward reducing the pressure gradient when introduced in oil continuous phase only at very low water cuts.

During the production and transportation of oil and gas, flow assurance issues may occur due to the solid deposits that are formed and carried by the flowing fluid. Solid deposition may cause serious damage and possible failure to production equipment in the flow lines. The major flow assurance problems that are faced in the fields are concerned with asphaltene, wax and scale deposition, as well as hydrate formations. Hydrates, wax and asphaltene deposition are mostly addressed in deep-water environments, where fluid flows through a long path with a wide range of pressure and temperature variations (Hydrates are generated at high pressure and low temperature conditions). In fact, a large change in the thermodynamic condition of the fluid yields phase instability and triggers solid deposit formations. In contrast, scales are formed in aqueous phase when some incompatible ions are mixed. Among the different flow assurance issues in hydrocarbon reservoirs, asphaltenes are the most complicated one. In fact, the difference in the nature of these molecules with respect to other hydrocarbon components makes this distinction. Asphaltene molecules are the heaviest and the most polar compounds in the crude oils, being insoluble in light n-alkenes and readily soluble in aromatic solvents. Asphaltene is attached to similarly structured molecules, resins, to become stable in the crude oils. Changing the crude oil composition and increasing the light component fractions destabilize asphaltene molecules. For instance, in some field situations, CO₂ flooding for the purpose of enhanced oil recovery destabilizes asphaltene. Other potential parameters that promote asphaltene precipitation in the crude oil streams are significant pressure and temperature variation. In fact, in such situations the entrainment of solid particulates in the flowing fluid and deposition on different zones of the flow line yields serious operational challenges and an overall decrease in production efficiency. The loss of productivity leads to a large number of costly remediation work during a well life cycle. In some cases up to $5 Million per year is the estimated cost of removing the blockage plus the production losses during downtimes. Furthermore, some of the oil and gas fields may be left abandoned prematurely, because of the significance of the damage which may cause loss about $100 Million. In this dissertation, we developed a robust wellbore model which is coupled to our in-house developed compositional reservoir model (UTCOMP). The coupled wellbore/reservoir simulator can address flow restrictions in the wellbore as well as the near-wellbore area. This simulator can be a tool not only to diagnose the potential flow assurance problems in the developments of new fields, but also as a tool to study and design an optimum solution for the reservoir development with different types of flow assurance problems. In addition, the predictive capability of this simulator can prescribe a production schedule for the wells that can never survive from flow assurance problems. In our wellbore simulator, different numerical methods such as, semi-implicit, nearly implicit, and fully implicit schemes along with blackoil and Equation-of-State compositional models are considered. The Equation-of-State is used as state relations for updating the properties and the equilibrium calculation among all the phases (oil, gas, wax, asphaltene). To handle the aqueous phase reaction for possible scales formation in the wellbore a geochemical software package (PHREEQC) is coupled to our simulator as well. The governing equations for the wellbore/reservoir model comprise mass conservation of each phase and each component, momentum conservation of liquid, and gas phase, energy conservation of mixture of fluids and fugacity equations between three phases and wax or asphaltene. The governing equations are solved using finite difference discretization methods. Our simulation results show that scale deposition is mostly initiated from the bottom of the wellbore and near-wellbore where it can extend to the upper part of the well, asphaltene deposition can start in the middle of the well and the wax deposition begins in the colder part of the well near the wellhead. In addition, our simulation studies show that asphaltene deposition is significantly affected by CO₂ and the location of deposition is changed to the lower part of the well in the presence of CO₂. Finally, we applied the developed model for the mechanical remediation and prevention procedures and our simulation results reveal that there is a possibility to reduce the asphaltene deposition in the wellbore by adjusting the well operation condition. / text

Dissociation of gas hydrates has been the primary concern of the oil and gas industry for flow assurance, mainly in an offshore environment. There is also a growing interest in the rapid formation of gas hydrates for gas storage, transport of natural gas and carbon sequestration. In this thesis, we experimentally measure the kinetics of formation and dissociation of methane hydrates and the effect of various anionic and cationic surfactants such as sodium dodecyl sulfate (SDS), cetyl trimethylammonium bromide (CTAB) and alpha olefin sulfonate (AOS) on the association/dissociation rate constants. The importance and necessity of micelle formation in these surfactants has been studied. The effect of foam generation on the rate of formation of these hydrates has also been measured. SDS was found to significantly decrease the induction time for hydrate formation. There was an added decrease in the induction time when a foamed mixture of water and SDS was used. On the other hand CTAB and AOS had an inhibiting effect. The contribution of micelles towards promoting hydrate formation was demonstrated with a series of experiments using SDS. The micelles formed by these surfactants appear to serve as nucleation sites for the association of hydrates. New experimental data is presented to show that some surfactants and the use of foam can significantly increase the rate of hydrate formation. Other surfactants are shown to act as inhibitors. A new experimental setup is presented that allows us to distinguish between surfactants that act as promoters and inhibitors for hydrate formation. / text

[pt] A produção de óleo e gás através de sistemas submarinos de produção vem sendo testada e realizada em diversos campos. As etapas iniciais do processamento primário, que eram realizadas nas Unidades Estacionárias de Produção (UEPs), vêm sendo deslocadas para o leito marinho, reduzindo os problemas de garantia de escoamento e aumentando a área disponível nas UEPs para processamento do óleo. Em primeiro lugar, o presente trabalho descreve os principais projetos de separação submarina já instalados e as motivações para suas aplicações. Adicionalmente, o presente trabalho apresenta os benefícios dos sistemas submarinos de separação água-óleo por meio dos resultados obtidos com um simulador de escoamento multifásico elaborado ao longo desta dissertação. Baseado no método de Beggs e Brill, este simulador foi desenvolvido na base computacional do Matlab e é capaz de avaliar a perda de carga no escoamento multifásico da produção em diversos arranjos submarinos. Por meio do simulador, foram feitas análises de sensibilidade para avaliar os efeitos das alterações nos principais parâmetros que influenciam o escoamento: razão água-óleo, razão gás-óleo, vazão de produção e grau API. Ao final do trabalho, são apresentados os cálculos das perdas de carga de dois arranjos submarinos hipotéticos que apresentam desafios relacionados à garantia de escoamento e a solução destes problemas com a instalação de sistemas submarinos de separação água-óleo. / [en] The production of oil and gas by subsea production systems has been tested and performed for several fields. The first steps of the primary oil and gas processing, which were held in Stationary Production Units (SPU), have been shifted to the seabed, reducing flow assurance problems and increasing the available area in SPUs for oil processing. On top to describing the main subsea separation projects and the reasons for their applications, the objective of this work is to present the benefits of subsea oil-water separation systems by means the results obtained with a multiphase flow simulator developed along this masters dissertation. Based on the method of Beggs and Brill, this simulator has been developed on Matlab platform and is able to evaluate the pressure drop of multiphase flows in various subsea production arrangements. With the simulator, a number of sensitivity analyzes is performed by changing the main parameters that affect the flow pressure drop: water-oil ratio, gas-oil ratio, production flow rate and API gravity. At the end of this work, the simulation results of two hypothetical subsea scenarios that have flow assurance problems and the solution of these problems with the installation of oil-water subsea separation systems are presented.

[pt] A deposição de parafina é um fenômeno presente nos sistemas de produção de petróleo (principalmente em águas profundas devido às baixas temperaturas), consistindo na aderência de frações sólidas de hidrocarbonetos nas colunas e linhas, conduzindo à redução da área aberta ao fluxo até o eventual bloqueio. A compreensão dos mecanismos que influenciam na deposição ainda não foi totalmente alcançada. Dada a relevância deste tipo de sistema para o desenvolvimento de novos campos e a ausência de uma teoria consolidada que seja capaz de explicar a evolução e as características do depósito, a limitação de produção por este fenômeno é um dos principais problemas de garantia de escoamento. Visando a aumentar o conhecimento acerca dos fenômenos existentes no processo de deposição, e identificar os mecanismos dominantes, diferentes modelos matemáticos podem ser confrontados com dados experimentais. Geralmente, os escoamentos encontrados ao longo das linhas de produção encontram-se no regime turbulento. Dessa forma, no presente trabalho, desenvolveu-se um modelo de turbulência de duas equações k–omega, acoplado com o modelo entalpia-porosidade, no qual o depósito é considerado um meio poroso. A partir de um equilíbrio termodinâmico determinam-se as espécies que saem de solução e a sua distribuição é determinada pela equação de conservação molar. As equações de conservação foram resolvidas pelo método de volumes finitos, utilizando o esquema Power-law e Euler implícito para as discretizações espacial e temporal. Comparações com dados experimentais em um duto anular foram realizadas, apresentando boa concordância para o regime permanente, mas superestimando a espessura do depósito durante o regime transiente. Constatou-se redução de espessura do depósito com o aumento do número de Reynolds. / [en] Wax deposition is a phenomenon present in oil production systems (mainly in deep water due to the low temperatures), which consists in the adhesion of solids fractions of hydrocarbon to tubing and lines, reducing the area opened to flow until be completely blocked. The comprehension of the mechanisms that influences in the deposition has not yet been fully achieved. Given the relevance of this kind of system in new fields development and the absence of a theory able to explain the deposit s evolution and characteristics, the production limitation caused by this phenomenon is one of the main issues in flow assurance. Aiming to expand the knowledge about the phenomena that exist in deposition process and identify dominant mechanisms, different mathematical models can be compared with experimental data. The flow regime in production lines is usually turbulent. Thus, in this work, a two equation k-omega turbulence model coupled to the enthalpy-porosity model, where the deposit is a porous media, was developed. From a thermodynamic equilibrium, the species that comes out of solution are determined while their distribution are determined by each molar conservation equation. The conservations equations were solved with the finite volume method, employing the Power-law and implicit Euler schemes to handle the spatial and temporal discretization. Comparisons with experimental data in an annular duct were realized, showing good agreement in the steady state. The deposit thickness, howeve, was overestimated during the transient. The deposit thickness reduction with the Reynold number increase was verified.

[pt] Uma combinação de fatores geológicos e econômicos exige que as empresas produzam petróleo e gás em campos com profundidades de água cada vez maiores. Muitas das vezes não é econômico, ou no pior dos casos impraticável, instalar uma plataforma sobre os cabeçotes dos poços, por isso acaba se tornando comum transportar petróleo e gás através de amarras submarinas que podem ser de até 145km ou mais. Geralmente isso significa que as temperaturas são baixas o bastante e as pressões altas o suficiente para tornar aquele ambiente dentro do que chamamos de envelope de formação de hidrato e ações deverão ser tomadas afim de evitar os plugs de hidrato. Como resultado, a indústria foi forçada a intensificar sua pesquisa em químicos e sistemas que evitasse a formação da estrutura cristalina. Uma dessas pesquisas em estudo é a avaliação de um fluido modelo, emulsão A/O, analisando suas principais características e verificando as propriedades reológicas da estrutura cristalina em formação. Para tornar a pesquisa viável, este hidrato é formado a pressão atmosférica utilizando moléculas hóspedes que proporcionam essa formação em tal pressão e baixa temperatura. Logo, é utilizada uma substância líquida chamada ciclopentano, que substituirá o gás natural e irá proporcionar a formação do hidrato nestas novas condições. Dessa forma, este trabalho apresentou diferentes emulsões A/O, de acordo com a porcentagem de água, e reologia do hidrato formado para cada uma delas. / [en] A combination of geological and economic factors requires companies to produce oil and gas in fields with increasing water depths. It is often impractical to install a platform over the heads of the wells, so it is becoming common to transport oil and gas through underwater moorings that can be up to 145 km or more. Usually this means that the temperatures are low enough and the pressures high enough to make that environment into what we call a hydrate formation envelope and actions should be taken to avoid the hydrate plugs. As a result, the industry was forced to intensify its research into chemicals and systems that prevented the formation of the crystalline structure. One of these researches is the evaluation of a model fluid, A / O emulsion, analyzing its main characteristics and checking the rheological properties of the crystalline structure in formation. To make the search feasible, this hydrate is formed at atmospheric pressure using guest molecules that provide such formation at such pressure and low temperature. Therefore, a liquid substance called cyclopentane is used, which will replace the natural gas and will provide the formation of the hydrate under these new conditions. In this way, this work presented different A / O emulsions, according to the percentage of water, and rheology of the hydrate formed for each of them.

Gas hydrate formation and corrosion can cause serious safety and flow assurance problems in subsea environments. One aspect that has been given less attention is the corrosion behavior of materials in gas hydrate formation environment (GHFE). This work introduces a new technique/method for corrosion testing of materials exposed to low temperatures GHFEs. This technique allows pH monitoring, and control of test conditions like temperature. In this work, GHFE is defined as an environment that includes water, methanol and its degraded products in the presence of corrosive agents like CO2 and chloride salt at gas hydrate formation temperatures (GHFT). After 20 hrs immersion in CO2-saturated salinity environment at GHFT, as-deposited Ni-Mo alloy coating has the highest corrosion resistance of 33.28 kΩ cm2. The corrosion resistance dropped to 14.36 kΩ cm2 and 11.11 kΩ cm2 in the sweet low-salinity and sweet high-salinity test solutions respectively. The combined results of SEM/EDX showed that the Ni-Mo coating oxide layer broke down quicker in sweet high-salinity environment than sweet low-salinity environment. When carbon steel was immersed in a CO2-saturated high salinity environment at GHFT, there was slight overall change in corrosion rate (CR) as salt concentration increase from 3 wt% to 25 wt%. In degraded methanol environment, methanol showed an inhibitive effect on the corrosion of carbon steel. Higher methanol content (up to 50 vol. %) increased the corrosion rate of carbon steel at gas hydrate formation temperature, however, the corrosion rates were lower with methanol contents between 10 to 20 vol%.