Modelagem dos efeitos do processo de fechamento de fraturas sobre a permeabilidade de rochas reservatório
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Data
2018-07-30
Autores
Gaiotto Junior, Aldo Theodoro
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Editor
Universidade Estadual Paulista (Unesp)
Resumo
O estudo acerca dos efeitos provocados pelos processos de exploração de fontes de hidrocarbonetos em rochas reservatório está cada vez mais presente nos processos investigativos que visam mitigar e remediar os impactos ambientais originários desta prática, de modo que se possa explorá-los por mais tempo e com menos danos ao meio ambiente. Visando o desenvolvimento de novas técnicas que possibilitem compreender melhor estes efeitos, o presente trabalho estuda as potencialidades de um novo modelo constitutivo, baseado no modelo de fechamento de juntas proposto por Barton e na teoria de dano contínuo, denominado “Modelo de Contato Progressivo”. Desenvolvido para simular o processo de fechamento mecânico das fraturas existentes em rochas em uma condição de contato entre interfaces, o modelo possui uma formulação adaptada à situação pretendida. O processo de fechamento das fraturas ocorre de acordo com a variação da poropressão da rocha. À medida que as paredes das fraturas se aproximam, a área de contato entre elas aumenta, causando um aumento na rigidez compressiva da fratura, segundo a relação hiperbólica dada por Barton-Bandis entre a componente normal da tensão e o deslocamento normal relativo. Por meio de uma abordagem 2D em elementos finitos é analisado o comportamento da poropressão em fraturas submetidas a tensões de compressão. Para representar explicitamente a fratura são utilizados elementos finitos com elevada razão de aspecto, estrategicamente posicionados. São realizados testes computacionais via códigos de programação próprios para validar o modelo. Os testes mostram que os elementos de interface são apropriados para representar as fraturas naturais e que o modelo de contato progressivo é capaz de reproduzir o efeito de fechamento de fraturas. O modelo pode ser empregado em reservatórios naturalmente fraturados para relacionar a tensão normal e a deformação no plano da fratura, além de fornecer valores de permeabilidade equivalente.
The study of the effects caused by the processes of hydrocarbon sources exploration in reservoir rocks is becoming widely present in investigative processes aimed at mitigating and remedying the environmental impacts originating from this practice, so that it can be explored for more time and with less damage to the environment. Aiming the development of new techniques to better understand these effects, the present work studies the potentialities of a new constitutive model, based on Barton's closure model and the continuous damage theory, called "Progressive Contact Model". Developed to simulate the process of fractures mechanical closure in rocks with contact between interfaces condition, the model has a formulation adapted to the intended situation. The fractures closure process occurs according to rock porepressure variation. As the walls of the fractures approach each other, the contact area between the walls increases, causing a growth in the compression stiffness of the fracture, according to the hyperbolic relationship given by Barton-Bandis between the normal component of the stress and the relative normal displacement. Using a 2D approach in finite elements, the porepressure in fractures submitted to compression is analyzed. To represent explicitly the fracture, finite elements with a high aspect ratio, strategically positioned, are used. Computational tests via own programming codes are performed to validate the model. The tests show that the interface elements are appropriate to represent the natural fractures and the progressive contact model is able to reproduce the fracture closure effect. The model can be used in naturally fractured reservoirs to relate normal stress and deformation to the fracture plane, besides to provide equivalent permeability values.
The study of the effects caused by the processes of hydrocarbon sources exploration in reservoir rocks is becoming widely present in investigative processes aimed at mitigating and remedying the environmental impacts originating from this practice, so that it can be explored for more time and with less damage to the environment. Aiming the development of new techniques to better understand these effects, the present work studies the potentialities of a new constitutive model, based on Barton's closure model and the continuous damage theory, called "Progressive Contact Model". Developed to simulate the process of fractures mechanical closure in rocks with contact between interfaces condition, the model has a formulation adapted to the intended situation. The fractures closure process occurs according to rock porepressure variation. As the walls of the fractures approach each other, the contact area between the walls increases, causing a growth in the compression stiffness of the fracture, according to the hyperbolic relationship given by Barton-Bandis between the normal component of the stress and the relative normal displacement. Using a 2D approach in finite elements, the porepressure in fractures submitted to compression is analyzed. To represent explicitly the fracture, finite elements with a high aspect ratio, strategically positioned, are used. Computational tests via own programming codes are performed to validate the model. The tests show that the interface elements are appropriate to represent the natural fractures and the progressive contact model is able to reproduce the fracture closure effect. The model can be used in naturally fractured reservoirs to relate normal stress and deformation to the fracture plane, besides to provide equivalent permeability values.
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Palavras-chave
Rochas naturalmente fraturadas, Fechamento de fraturas, Modelo de contato progressivo, Simulação computacional, Impactos ambientais, Naturally fractured rocks, Fractures closure, Progressive contact model, Computational simulation, Environmental impacts