A drilling operation leads to thermal disturbances in the near-wellbore stress, which is an important cause of many undesired incidents in well drilling. A major cause of this thermal disturbance is the temperature difference between the drilling fluid and the downhole formation. It is critical for drilling engineers to understand this thermal impact to optimize their drilling plans. This thesis develops a numerical model using partially coupled thermoporoelasticity to study the effects of the temperature difference between the drilling fluid and formation in a drilling operation. This study focuses on the thermal impacts at the bit/formation interface. The model applies the finite-difference method for the pore pressure and temperature solutions, and the finite-element method for the deformation and stress solutions. However, the model also provides the thermoporoelastic effects at the wellbore wall, which involves wellbore fractures and wellbore instability. The simulation results show pronounced effects of the drilling fluid temperature on near-wellbore stresses. At the bottomhole area, a cool drilling fluid reduces the radial and tangential effective stresses in formation, whereas the vertical effective stress increases. The outcome is a possible enhancement in the drilling rate of the drill bit. At the wellbore wall, the cool drilling fluid reduces the vertical and tangential effective stresses but raises the radial effective stress. The result is a lower wellbore fracture gradient; however, it benefits formation stability and prevents wellbore collapse. Conversely, the simulation gives opposite induced stress results to the cooling cases when the drilling fluid is hotter than the formation.

There are two big challenges which restrict the extensive application of fully coupled geomechanics-reservoir modeling. The first challenge is computational effort. Consider a 3-D simulation combining pressure and heat diffusion, elastoplastic mechanical response, and saturation changes; each node has at least 5 degrees of freedom, each leading to a separate equation. Furthermore, regions of large p, T and σ′ gradients require small-scale discretization for accurate solutions, greatly increasing the number of equations. When the rock mass surrounding the reservoir region is included, it is represented by many elements or nodes. These factors mean that accurate analysis of realistic 3-D problems is challenging, and will so remain as we seek to solve larger and larger coupled problems involving nonlinear responses. To overcome the first challenge, the displacement discontinuity method is introduced wherein a large-scale 3-D case is divided into a reservoir region where Δp, ΔT and non-linear effects are critical and analyzed using FEM, and an outside region in which the reservoir is encased where Δp and ΔT effects are inconsequential and the rock may be treated as elastic, analyzed with a 3D displacement discontinuity formulation. This scheme leads to a tremendous reduction in the degrees of freedom, yet allows for reasonably rigorous incorporation of the reactions of the surrounding rock. The second challenge arises from some forms of numerical instability. There are actually two types of sharp gradients implied in the transient advection-diffusion problem: one is caused by the high Peclet numbers, the other by the sharp gradient which appears during the small time steps due to the transient solution. The way to eliminate the spurious oscillations is different when the sharp gradients are induced by the transient evolution than when they are produced by the advective terms, and existing literature focuses mainly on eliminating the spurious spatial temperature oscillations caused by advection-dominated flow. To overcome the second challenge, numerical instability sources are addressed by introducing a new stabilized finite element method, the subgrid scale/gradient subgrid scale (SGS/GSGS) method.

There are two big challenges which restrict the extensive application of fully coupled geomechanics-reservoir modeling. The first challenge is computational effort. Consider a 3-D simulation combining pressure and heat diffusion, elastoplastic mechanical response, and saturation changes; each node has at least 5 degrees of freedom, each leading to a separate equation. Furthermore, regions of large p, T and σ′ gradients require small-scale discretization for accurate solutions, greatly increasing the number of equations. When the rock mass surrounding the reservoir region is included, it is represented by many elements or nodes. These factors mean that accurate analysis of realistic 3-D problems is challenging, and will so remain as we seek to solve larger and larger coupled problems involving nonlinear responses. To overcome the first challenge, the displacement discontinuity method is introduced wherein a large-scale 3-D case is divided into a reservoir region where Δp, ΔT and non-linear effects are critical and analyzed using FEM, and an outside region in which the reservoir is encased where Δp and ΔT effects are inconsequential and the rock may be treated as elastic, analyzed with a 3D displacement discontinuity formulation. This scheme leads to a tremendous reduction in the degrees of freedom, yet allows for reasonably rigorous incorporation of the reactions of the surrounding rock. The second challenge arises from some forms of numerical instability. There are actually two types of sharp gradients implied in the transient advection-diffusion problem: one is caused by the high Peclet numbers, the other by the sharp gradient which appears during the small time steps due to the transient solution. The way to eliminate the spurious oscillations is different when the sharp gradients are induced by the transient evolution than when they are produced by the advective terms, and existing literature focuses mainly on eliminating the spurious spatial temperature oscillations caused by advection-dominated flow. To overcome the second challenge, numerical instability sources are addressed by introducing a new stabilized finite element method, the subgrid scale/gradient subgrid scale (SGS/GSGS) method.

[pt] As falhas geológicas são estruturas tipicamente interpretadas em duas dimensões, como superfícies, nos dados sísmicos e da mesma maneira são representadas em modelos geológicos de reservatórios de petróleo. Entretanto, as falhas são zonas tridimensionalmente complexas que representam regiões de fraquezas que concentram fraturas e rochas altamente e heterogeneamente deformadas. Portanto, a representação adequada destas zonas é importante para o gerenciamento e avaliação econômica de um campo de petróleo, com impacto nas áreas de perfuração, completação e locação de poços, estratégias para aumento de fator de recuperação e até na estimativa da reserva recuperável. Devido a grande importância dos reservatórios carbonáticos fraturados, mais de 60 por cento das reservas provadas de óleo e 40 por cento das reservas de gás no mundo [1] estão presentes nesses reservatórios, o trabalho proposto tem como objetivo a modelagem geológica estrutural de uma falha em rochas carbonáticas do reservatório de Gawar da Arábia Saudita a partir de parâmetros de deformabilidade obtidos por Ameen et al. [2]. O trabalho também aborda os impactos da resolução espacial dos dados sísmicos na intepretação destas estruturas, através da simulação da imagem sísmica da falha. Os resultados mostram que o método de elemento discreto é uma ferramenta adequada para modelagem realística de falhas geológicas, entretanto, alguns modelos obtiveram resultados não realísticos devido à dificuldade da manutenção da tensão confinante durante a produção da falha. Os estudos mostraram que apesar da interpretação volumétrica destas estruturas através das metodologias de interpretação baseadas em atributos sísmicos serem possíveis, existe uma considerável limitação devido a resolução espacial e na dificuldade dos algoritmos em formar a imagem sísmica da zona de falha, onde há contraste lateral de propriedades acústicas. / [en] Faults are structures typically interpreted in two dimensions, such assurfaces, in seismic data and are similarly represented in geological models of oil reservoirs. However, faults are three-dimensionally complex zones that represent regions of weakness that concentrate fractures and highly heterogeneously deformed rocks. Therefore, the adequate representation of these zonesis important for the management and economic evaluation of an oil field, withan impact on the areas of drilling, completion and location of wells, strategies for increasing the recovery factor and even on estimating the recoverable reserve. Due to the great importance of fractured carbonate reservoirs, more than 60 percent of the proven oil reserves and 40 percent of the gas reserves in the world[1] are present in these reservoirs, the proposed work aims at the geomechanical modeling of a geological fault in carbonate rocks of Saudi Arabia s Gawar reservoir from deformability parameters obtained by Ameen et al. [2]. The work also addresses the impacts of the spatial resolution of seismic data on the interpretation of these structures, through the simulation of the fault seismic image. The results show that the discrete element method is an adequate tool for realistic modeling of geological faults, however, some models obtained unrealistic results due to the difficulty of maintaining the confining stress during fault production. The studies showed that although the volumetric interpretation of these structures through interpretation methodologies based on seismic attributes are possible, there is a considerable limitation due to the spatial resolution and the inadequacy of the seismic data to adequately deal with the lateral contrast of acoustic properties present in areas close to the damage zones.