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Hydraulic Fracturing: an Overview and a Geomechanical Approach

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Hydraulic Fracturing: an Overview and a Geomechanical Approach Hydraulic Fracturing: An overview and a Geomechanical approach Miguel Teixeira Luís Fialho Medinas Supervisor: Prof. Teresa Maria Bodas de Araújo Freitas Thesis to obtain the Master of Science Degree in Civil Engineering Examination Committee Chairperson: Prof. Jaime Alberto dos Santos Supervisor: Prof. Teresa Maria Bodas de Araújo Freitas Members of the Committee: Prof. Manuel Francisco Costa Pereira June 2015 Acknowledgements To my family and friends . In particular to my grandparents; Avô Zeca, Avó Deda, Avô Medinas and Avó Graziela. i Resumo A exploração de reservatórios não convencionais, através da estimulação com recurso à técnica de fracturação hidráulica e aplicação de perfuração direcional horizontal, define uma nova era no paradigma energético mundial. Os custos elevados destas operações e as particularidades dos reservatórios alvo obrigam a indústria a investir fortemente no desenvolvimento de ferramentas fiáveis para o seu pré- dimensionamento. Para aumentar a capacidade de previsão do comportamento das operações são frequentemente usadas perfurações orientadas. Contudo a determinação das tensões in- situ e das direções principais é complexa, pelo que se torna importante conhecer o efeito da orientação da perfuração no comportamento das fracturas induzidas, ao nível da sua iniciação e propagação, para garantir o sucesso das operações de fracturação hidráulica e a viabilidade económica do campo a explorar. Esta tese apresenta um estudo numérico pelo método XFEM (Extended Finite Element Method), utilizando o programa Abaqus desenvolvido pela Dassault Systémes, em que se analisa o comportamento mecânico de fracturas induzidas, nomeadamente as condições de iniciação e propagação, quando se recorre à utilização de perfurações com diferentes orientações. Tendo em vista a validação da ferramenta de cálculo, procedeu-se à simulação de uma série de ensaios experimentais descritos por Abass et al. (1994), efetuados em laboratório sob provetes de gesso, consolidados em câmara de triaxial verdadeiro, em que após a execução de perfurações se provocou a formação de fracturas. Apesar das simplificações introduzidas no modelo numérico, em particular o facto da ação do líquido de injecção ser modelada como uma pressão aplicada nas paredes do poço, os resultados numéricos apresentam boa concordância com os dados experimentais. O mesmo modelo é utilizado para desenvolver um estudo paramétrico em que se analisa o efeito de um conjunto de parâmetros relevantes, nomeadamente o comprimento da perfuração, anisotropia de tensões, raio do furo e desalinhamento da perfuração, na iniciação e propagação de fracturas quando se utilizam perfurações orientadas. Os resultados obtidos permitem concluir que a pressão que provoca a iniciação da fractura é fortemente dependente das tensões tangenciais na ponta da fractura, enquanto a reorientação é controlada pela anisotropia de tensões, sendo que ambos os aspectos são determinados pelo estado de tensão na proximidade do furo. Quando ocorre desalinhamento da perfuração em relação ao plano preferencial de fracturação, verifica-se que uma maior densidade de perfurações não afecta a pressão de iniciação da primeira fractura, mas dificulta a iniciação das restantes. Palavras-Chave: Fracturação hidráulica, perfurações orientadas, fractura, Abaqus, XFEM, geomecânica. ii Abstract The exploitation of unconventional reservoirs, made possible by stimulation using the hydraulic fracturing technique and application of horizontal directional drilling, defines a new era in the global energetic paradigm. Due to the costs of these operations and the specificities of unconventional reservoirs, the industry is required to invest in the development of reliable predictive tools to pre-design these operations. To improve the predictions of the operation outcome oriented perforations are often used. However, the determination of the in-situ stresses and the principal directions is complex; therefore it is important to understand the effect of the perforation orientation on the behaviour of induced fractures, in terms of their initiation and propagation, to ensure the success of hydraulic fracturing operations and that the exploration of these fields is economically viable. Using the XFEM method (Extended Finite Element Method) available in Abaqus developed by Dassault Systémes, this thesis presents a numerical study on the mechanical behaviour of induced fractures that make use of perforations with different orientations, in particular the conditions of fracture initiation and propagation. To validate the numerical tools, numerical simulations of a series of laboratory tests that reproduce hydraulic fracturing on rectangular blocks of gypsum cement (Abass H. et al, 1994) were carried out. Despite the simplifications of the numerical model, in particular the fact that the fracturing fluid was modelled as a pressure applied in the wellbore, the numerical results provide a good match to the experimental observations. The same numerical model was employed to carry out a parametric study on the effect of a number of relevant parameters on fracture initiation and propagation, namely perforation length, stress anisotropy, wellbore radius and phasing misalignment. The results of this study show that the breakdown pressure is strongly dependent on the tangential stresses at the crack tip, while reorientation is controlled by the principal stress direction anisotropy at the fracture tip, and thus both aspects are determined by the stress state in the near-wellbore zone. In addition, it is found that, when there is perforation misalignment relative to the preferred fracture plane, the density of perforations does not influence the breakdown pressure, but the initiation of the remaining fractures becomes more difficult as the density increases. Keywords: Hydraulic fracturing, oriented perforations, fracture, Abaqus, XFEM, geomechanics. iii Contents Acknowledgements ........................................................................................................................ i Resumo .......................................................................................................................................... ii Abstract ........................................................................................................................................ iii I - Index of Figures ....................................................................................................................... viii II - Tables ....................................................................................................................................... xi III - List of abbreviations ............................................................................................................... xii IV - List of symbols/nomenclature .............................................................................................. xiii 1. Introduction .......................................................................................................................... 1 1.1 Motivation and aims ........................................................................................................... 1 1.2 Structure .............................................................................................................................. 2 Theoretical framework .......................................................................................................... 3 Mechanics ............................................................................................................................. 3 Modelling - Validation ........................................................................................................... 3 Modelling - Parametric study ................................................................................................ 4 Conclusions ........................................................................................................................... 4 2. Geological and petrophysical introduction ........................................................................... 5 2.1 Oil and gas generation process ........................................................................................... 5 2.2 Oil and gas migration process ............................................................................................. 5 2.3 Importance of traps for oil and gas accumulation .............................................................. 6 2.4 Reservoir rock ...................................................................................................................... 7 2.4.1 Conventional Reservoir Rock ....................................................................................... 8 2.4.2 Unconventional Reservoirs .......................................................................................... 8 3. Horizontal Directional Drilling ............................................................................................. 13 3.1 Introduction....................................................................................................................... 13 3.2 Applications of the technique ........................................................................................... 13 3.3 HDD application - Advantages and limitations.................................................................. 15 3.4 Comparison between horizontal directional and vertical drilling ..................................... 16 iv 3.5 Well Planning Trajectory
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