Estimating the in situ mechanical properties of sediments containing gas hydrates from seismic or log data is essential for evaluating the risks posed by mechanical failure during drilling, completions, and producing operations. In this paper, a method is presented for constructing correlations between the mechanical properties of gas hydrate bearing sediments and geophysical data. A theory based on micromechanics models was used to guide the selection of parameters that govern the physical behavior of sediments. A set of nondimensionalized relations between elastoplastic properties and those that could be inferred from log or seismic data was derived. Using these relations, a correlation for the Young’s modulus was constructed for sands with methane and THF hydrate using data from a wide variety of sources. It was observed that the correlation did not fit data obtained from samples with high THF hydrate saturations, due possibly to the existence of cohesive mechanisms that operate in such regimes.

Monitoring of the seafloor for gas hydrate dissociation around boreholes during hydrocarbon production is likely to involve seismic methods because of the strong sensitivity of P-wave velocity to gas in sediment pores. Here, based on geomechanical models, we apply commonly used rock physics modeling to predict the seismic response to gas hydrate dissociation with a focus on P-impedance and performed sensitivity tests. For a given initial gas hydrate saturation, the mode of gas hydrate distribution (cementation, frame-bearing, or pore-filling) has the strongest effect on P-impedance, followed by the mesoscopic distribution of gas bubbles (evenly distributed in pores or “patchy”), gas saturation, and pore pressure. Of these, the distribution of gas is likely to be most challenging to predict. Conceptual 2-D FD wave-propagation modeling shows that it could be possible to detect gas hydrate dissociation after a few days.

Gas hydrates in near-mudline subsea sediments present significant challenges in the production of underlying hydrocarbons, impacting wellbore integrity and placement of subsea equipment. As the fluids of an underlying reservoir flow to the mudline, heat carried by the fluids warms nearwell sediments and dissociates hydrates, which releases gas that can displace and fracture near well soil. This gas release may be calculated with numerical simulations that model heat and mass transfer in hydrate-bearing sediments. The nature and distribution of hydrates within the sediments, the melting behavior of the hydrates, the thermal and mechanical properties of these shallow sediments, and the amount of hydrates contained in the sediments are required for the model simulations. Such information can be costly to acquire and characterize with certainty for an offshore development. In this information environment, it is critical to understand what information, processes, and calculations are required in order to ensure safe, robust systems, that are not overly conservative, to produce the hydrocarbon reservoirs far below the hydrates.

Every year, many wells are subject to well intervention operations for a variety of different reasons, such as Plug and Abandon (P&A) operations or well control situations. Wells that are not properly plugged, in addition becoming an inherent blowout threat, can act as a preferential pathway for surface contaminants to reach and impair ground water quality, and could cause injury to livestock, wildlife, or humans. Hence, federal code (or state code if in state waters) states that the wells must be plugged according to regulations. If attempts with a surface intervention operation fail, a relief type subsurface intervention project is deemed appropriate. A relief well type of intersection into each target wellbore will create a hydraulic flow path suitable for plugging operations. The plugging operation will require the placement of permanent plugging fluids into the Target Well (TW) to meet Mineral Management Services (MMS), or other regulatory agency, approved plugging criteria. Evidently, there is a need to design a method to insure that the scenarios are accurately defined, analyzed and the results can be effectively implemented to complete the plug and abandonment operations. A software package, coupled with the skill of a hydraulic modeling specialist, could provide final resolution to and better understanding of the problem. However, considering uncertainties in some input information, there is a need to develop a multi-purpose package which enables the user to manipulate dynamically a wide range of input data in order to obtain the best fit. Therefore, the decision was made to develop a software package specifically built and designed to address the common problems encountered during well intervention projects. The well intervention toolkit will be used to investigate the plugging and abandonment scenarios. The well intervention toolkit not only provides the critical input parameters to other commercial software but would also be a means to analyze and simulate the well intervention hydraulics

[pt] As descobertas no pré-sal estão entre as mais importantes no setor de óleo e gás em todo o mundo na última década. As operações de perfuração na área do pré-sal brasileiro são complexas e desafiadoras em função de fatores como águas ultra profundas, efeito do creep na seção evaporítica (fluência do sal), geologia estrutural complexa, presença de cavernas e poros de diferentes tamanhos geradores de perda de circulação. Estima-se que os custos associados à instabilidade de poços representem cerca de 5 a 10 por cento dos custos de perfuração nas fases de exploração e produção, implicando, a nível mundial, custos de centenas de milhões de dólares por ano. No presente estudo, foi desenvolvido um modelo de estabilidade de poços na área do pré-sal brasileiro utilizando-se correlações analíticas para a obtenção da janela operacional de perfuração. A janela operacional auxilia na otimização do peso de fluido que deverá ser utilizado na perfuração e é definida por um limite inferior (pressão de poros ou de colapso) e por um limite superior (gradiente de fratura). Para a construção do modelo geomecânico, foram considerados perfis de oito poços de correlação, medições diretas de pressão, provas de integridade de formação, temperatura das formações e dados públicos do world stress map. Foram realizadas análises de sensibilidade de três parâmetros para lidar com as suas respectivas incertezas e estimar suas variações: resistência uniaxial não confinada (UCS), do coeficiente de Biot e magnitude da tensão horizontal máxima (SHmax). Os resultados do estudo indicam que o gradiente de sobrecarga na profundidade dos carbonatos da área estudada (5000 até 6140m – profundidade vertical (TVD)) variaram de 13,6 a 14,3ppg (lb/gal). A análise de pressão de poros (PP) indicou que na região existem dois cenários envolvendo esse parâmetro: uma área com pressão de poros levemente alta (9,2ppg) e em outra que apresenta pressão de poros anormalmente pressurizada (13,0ppg). A análise dos resultados das provas de integridade de formação (Leak-offs tests – LOTs) possibilitou a estimativa da Razão de Tensão Horizontal Efetiva mínima (ESRmin) como sendo uma constante de 0,65 no trecho dos carbonatos e, com isso, estimou-se a tensão horizontal mínima em 12,4ppg para a região com pressão de poros levemente alta e 13,8ppg para a região de alta pressão. Os valores médios de resistência de rocha foram de 43MPa e foram comparados resultados de ensaios de resistência de uma rocha análoga ao pré-sal. A direção das tensões horizontais máximas foi estimada em cerca de 95NE e a magnitude foi estimada através do polígono de tensões, onde considerou-se diferentes cenários nos quais envolveram a variação de UCS, coeficiente de Biot e abertura de breakout. Os resultados de SHmax encontrados foram utilizados para a estimativa da Razão de Tensão Horizontal Efetiva máxima (ESRmax) que variou entre 0,95 e 1,8 em função dos cenários avaliados e, com isso, concluiu-se que o campo de falhamento na área estudada estaria entre o normal e o transcorrente. Estimou-se que a janela operacional nos carbonatos na região com PP levemente pressurizada apresenta como limite inferior valores entre 10,5 e 11,1ppg e como limite superior 12,3 a 12,6ppg. Já na região com PP anormalmente pressurizada a janela operacional para a fase dos carbonatos compreende valores entre 13,3 a 13,8ppg. A presente pesquisa apresenta também as discussões relativas às incertezas associadas à ausência de dados e o modelo constitutivo adotado. / [en] The discoveries in the pre-salt are one of the most important ones in the oil and gas industry around the world in the last decade. Drilling operations in the Brazilian pre-salt area are complex and challenging due to factors such as ultra-deep water, effect of creep in evaporite section, complex geological structure, presence of caves and vugs generating total circulation losses etc. It is estimated that the costs associated with wellbore instability problems represent about 5 to 10 percent of the drilling costs in the stages of exploration and production, generating costs around hundreds of millions of dollars a year. In this study, a wellbore stability model was built of the pre-salt area using analytical correlations to obtain a mud window. The operational window assists in optimizing the fluid weight to be used in drilling and is defined by a lower limit (pore or collapse pressure) and an upper limit (fracture gradient). In order to build the geomechanical model, eight offset well with log data, direct pressure measurements, formation integrity tests, formation temperature, and data from the world stress map were used. Sensitivity analyzes were made from uniaxial unconfined strength (UCS), Biot coefficient and magnitude of the maximum horizontal stress to evaluate model s uncertainties and estimate its variations. The geomechanical model indicates that the overburden in the carbonates of the studied area (5000 until 6140m – True Vertical Depth (TVD)) ranges from 13,6 to 14,3ppg (lb/gal). Pore pressure indicates two different behavior, presenting slightly over-pressurized (around 9,2ppg) and over-pressurized in another area (13,0ppg). The results of the formation integrity test (Leak-off tests, LOTs) were used to estimate the minimum effective stress ratio (ESRmin), a constant value around 0,65 with a minimum horizontal stress equal to 12,4ppg in the slightly overpressured zone and 13,8ppg in overpressure zone. The average values of uniaxial compressive strength (UCS) were 43MPa. The azimuth of maximum horizontal stress was estimated to be around 95NE obtained from World Stress Map. The effective maximum horizontal stress ratio (ESRmax) is ranging between 0,95 and 1,8 according to the presented scenarios and it can be concluded that the faulting stress regime is between normal and strike slip. It is estimated due to the slightly over pressured PP that the operational windows on the carbonates have as lower limits values from 10,5 to 11,1ppg and as higher limits values from 12,3 to 12,6ppg. In the area with PP abnormally pressurized the operational window to the carbonates phase is between 13,3 to 13,8ppg. The present research also discusses the uncertainties associated to the absence of available data and the constitutive models used.

[pt] Os problemas de instabilidade de poços durante a perfuração, têm sido um tema de estudo e de interesse muito relevante na indústria, devido às consequências com as quais estão relacionados, por exemplo, o tempo perdido na operação e o consequente aumento de custo do projeto. Nos estudos de análise de estabilidade, com intuito de otimizar a perfuração do poço, normalmente eram assumidas propriedades isotrópicas do meio. Com o desenvolvimento da indústria do petróleo e as novas fronteiras exploratórias, faz-se necessário um estudo mais realista e aprofundado de estabilidade de poços. Este trabalho teve como objetivo a otimização da janela operacional, esta que por sua vez define os limites admissíveis de peso do fluido de perfuração, para que se mantenha a estabilidade do poço. Desta forma, foi desenvolvido um estudo numérico da distribuição de tensões e uma análise analítica para a verificação de falha ao redor do poço, no qual são consideradas as propriedades elásticas anisotrópicas além de critérios de falha na rocha intacta e no plano de fraqueza. Os softwares ABAQUS e MATLAB foram utilizados para a realização dos cálculos necessários no estudo. Os resultados das avaliações realizadas, mostram que a consideração da anisotropia (e de características do meio, como planos de fraqueza) é necessária em estudos deste tipo, pois dependendo do conjunto de propriedades analisadas, estas mostraram que a anisotropia possui um efeito significativo sobre os limites da janela operacional. / [en] Wells stability problems during drilling processes have been a subject of study and interest in the oil and gas industry due to its consequences, such as nonproductive times (NPTs), formation damage, Wells integrity and economic impacts. Isotropic properties in the formation usually had been assumed, however, it is necessary to define more realistic models to represent well stability. In this research, a numerical stress distribution and an analytical analysis have been proposed in order to calculate rock failure around the wellbore and optimize operative mud window, considering anisotropic elastic properties and failure criteria in the intact rock and in the plane of weakness. ABAQUS and MATLAB software were used to represent and solve the numerical-analytical model. The results presented in the assessment proved that the anisotropy consideration (including characteristics of the formation that can induce anisotropy, as plane of weakness) is necessary to be taken in count in this type of investigation because depending of the set of analyzed properties, the range of the operating mud weight window could significantly change.

[pt] Grande parte dos problemas de instabilidade de poços de petróleo ocorre quando rochas argilosas, como os folhelhos, são perfuradas. Tais problemas são creditados, em geral, à interação físico-química entre os fluidos de perfuração e as referidas rochas. Esta dissertação se foca no estudo experimental do comportamento de folhelhos expostos a diferentes soluções salinas, que simulam os fluidos de perfuração a base de água. O objetivo é estimar os parâmetros de transporte de massa (permeabilidade absoluta, coeficiente de reflexão e difusão) necessários para as análises de estabilidade de poços. São utilizadas um conjunto de células de difusão capazes de simular as condições de tensões in-situ e aplicar gradientes hidráulicos e químicos a corpos de prova de folhelhos. Foram caracterizados dois tipos de folhelhos típicos de plataformas offshore (BC-01) e onshore (Pierre01) provenientes, respectivamente, da Bacia de Campos (Rio de Janeiro, Brasil) e de Salt Lake City (Utah, Estados Unidos). Ensaios de porosimetria indicam que o folhelho BC-01 apresenta maiores diâmetros de poros quando comparados com o folhelho Pierre01. Resultados dos ensaios nas células de difusão sugerem que a direção dos planos de acamamento presentes nos corpos de prova afetam a permeabilidade e, por conseguinte, o coeficiente de reflexão que o folhelho possui. Os resultados sugerem que a composição mineralógica não tem influencia nos parâmetros de transporte de massa. / [en] The majority of the problems related to wellbore stability occur when argillaceous rocks, such as shales, are drilled. Such problems are believed, in general, to be caused by the physical-chemical interaction between drilling fluids and the referred rocks. This dissertation focuses on the experimental study of the behavior of shales exposed to different saline solutions, which simulated water-based drilling fluids. The objective is to estimate the mass transporting parameters (absolute permeability, reflection coefficient and diffusion) necessary for the analysis of well stability. Groups of diffusion cells are used, being capable of simulating in-situ stress conditions and apply hydraulic gradients and chemicals upon shale samples. Two types of shales were characterized; representative of offshore (BC-01) and onshore (Pierre01) platforms derived, respectively, from Bacia de Campos (Rio de Janeiro, Brazil) and Salt Lake City, Utah, United States. Porosimetry tests indicate that BC-01 shales present larger pore diameters when comparing with Pierre01 Shales. Experimental results from diffusion cells suggest that the direction of foliation planes present in the samples defines its permeability and, therefore the coefficient of reflection that the shales possess. The results suggest that mineral composition does not influence the mass transporting parameters.

Effective solvent extraction incorporating electromagnetic heating is a relatively new concept that relies on Radio Frequency heating and solvents to replace steam in current thermal processes for the purpose of extracting bitumen from oil rich sands. The work presented here will further the understanding of the near wellbore flow of this two phase system in order to better predict solvent vaporization dynamics and heat rates delivered to the pay zone. This numerical study details the aspects of phase change of immiscible, two component, liquid/vapor systems confined in porous media heated by electromagnetic radiation, approximated by a spatially dependent volumetric heat source term in the energy equation. The objective of this work is to utilize the numerical methodology presented herein to predict maximum solvent delivery rates to a heated isotropic porous matrix to avoid the over-saturation of the heated pay zone. The total liquid mass content and mean temperature in the domain are monitored to assess whether the liquid phase is fully vaporized prior to flowing across the numerical domain boundary. The distribution of the volumetric heat generation rate used to emulate the physics of electromagnetic heating in the domain decays away from the well bore. Some of the heat generated acts to superheat the already vaporized solvent away from the interface, requiring heat delivery rates that are many times greater than the energy required to turn the liquid solvent to vapor determined by an energy balance. Results of the parametric study from the pay zone simulations demonstrate the importance of the Darcian flow resistance forces added by the porous media to stabilize the flow being pulled away from the wellbore in the presence of gravity. For all cases involving an increase in solvent delivery rate with a constant heat rate, the permeability range required for full vaporization must decrease in order to balance the gravitational forces pulling the solvent from the heated region. For all conditions of permeability and solvent delivery rates, sufficiently increasing the heat rate results in complete vaporization of the liquid solvent. For the case of decreasing solvent delivery rate, a wider range of higher permeabilities for a given heat rate can be utilized while achieving full vaporization. A three dimensional surface outlining the transition from partially vaporized to fully vaporized regimes is constructed relating the solvent delivery rate, the permeability of the porous near wellbore zone and the heat rate supplied to the domain. For the range of permeabilities ~3000mD observed in these types of well bores, low solvent delivery rates and high heat rates must be utilized in order to achieve full vaporization.

[pt] O fator fundamental que leva um poço de petróleo ao colapso é o processo transiente de elevação da pressão de poros ao seu redor, devido à penetração da lama de perfuração no interior da formação. Entre as propriedades da rocha, a permeabilidade é responsável pela determinação da velocidade de penetração, bem como da profundidade atingida. Quando são perfurados folhelhos, rochas de baixa permeabilidade, uma região de dano mecânico se desenvolve ao redor do poço. Esta região é caracterizada pela alteração das propriedades mecânicas e hidráulicas da rocha. O material aumenta de volume acompanhado do surgimento de fissuras, como resultado do descarregamento e subsequente carregamento desviador. Este comportamento do material foi representado neste trabalho através de um modelo de plasticidade não-associada. Este trabalho discute a influência das tensões e da escavação sobre a permeabilidade das rochas, testando a aplicabilidade de duas relações de permeabilidade ao problema da perfuração de poços de petróleo. Estas relações foram implementadas num simulador numérico baseado no método dos elementos finitos, capaz de modelar o processo acoplado de fluxo monofásico através de um meio poroso deformável. Os resultados obtidos mostram que a consideração da permeabilidade da região circunvizinha às paredes do poço como constante, durante e após a perfuração, é uma simplificação que pode levar a erros significativos. As relações de permeabilidade adotadas, embora ainda necessitem de comprovação experimental ou de campo, contribuirão para o melhor entendimento do estudo da estabilidade de poços. / [en] The underlying factor that leads the wellbore collapse is the porepressure transient process of elevation around the wellbore due to the penetration of the drilling mud. Among the rock properties, the permeability is responsible for determining penetration speed, as well as the reached depth. When drilling shales, wich are rocks of low permeability, a zone of mechanical damage is developed around of the wellbore. This zone is characterized by the alteration of the mechanical and hydraulic rock properties. There is a material volume increase, accompanied by microcracking, as a result of the unloading and subsequent deviatoric stresses. This behavior of the material was represented in this work by a model of non-associated plasticity. Here is discussed the stresses and excavation influence on rock permeability, by testing the application of two permeability relationships to drilling problem. These relationships were implemented in a numeric simulator finite element based, wich can model the coupled process of the single-phase flow through porous media.The obtained results show that the consideration of constant permeability on zone close to the wellbore as constant, during and after the drilling, is a simplification that can lead to significant mistakes. The permeability relationships adopted, although still need experimental or field validation, will contribute to a best understanding of the study of the wellbores stability.

[pt] Frente à crescente complexidade dos cenarios de exploração de petróleo, as análises de estabilidade convencionais tornam-se insuficientes para determinar as condições reais dos poços. Assim, ciente destas limitações, a indústria do petróleo vem aplicando com mais frequência novos métodos como o modelo geomecânico denominado Mechanical Earth Model (MEM), pois permite gerar uma previsão da estabilidade do poço e ajuda a reduzir os riscos de perfuração. Neste sentido, o presente trabalho apresenta uma metodologia para estimar as condições da estabilidade de poços com ênfase nas formações de folhelhos, através da identificação e análise de problemas e eventos que revelem sinais de instabilidade geomecânica levantados nos dados de perfuração disponíveis. Boletins diários de perfuração e perfis elétricos de poços são as fontes de dados para análise de problemas de estabilidade que são os responsáveis pela maior parte dos tempos não produtivos, e consequentemente, de custos extras de perfuração. Por tanto, o estudo e o entendimento destes problemas contribuirá para a otimização do processo de perfuração, melhorando assim as práticas ou mitigando os efeitos severos das anormalidades. / [en] Facing the increasing complexity of scenarios for oil exploration, the conventional stability analysis became insufficient to determine the actual condition of the wells. Aware of these limitations, the oil industry has been applying new methods such as the geomechanical model named Mechanical Earth Model – MEM, which has been applied on the prediction of wellbore stability and drilling risks mitigation. In this sense, this work presents a methodology for estimating the wellbore stability conditions of wells with special emphasis on shale formations, through the identification and assessment of events which indicate geomechanical instability during drilling. These data are available from daily drilling reports and electric logs. Well Stability problems are responsible for most non-productive time, and consequently, the extra drilling costs. Therefore, the study and understanding of these problems contribute to the drilling optimization, thus improving the practices or mitigating the effects of severe abnormalities.