Masters of Science / The Bredasdorp basin is situated off the south coast of the Republic of South Africa, southeast of Cape Town and west-south-west of Port Elizabeth. It covers approximately 18,000 sq. km beneath the Indian Ocean along the southern coast of South Africa, which is in the southwest of Mosselbay. Bredasdorp basin contains South Africa’s only oil and gas production facilities and has been the main focus for oil and gas exploration in South Africa. It is one of the largest hydrocarbon producing block in South Africa, rich in gas and oil prone marine source rocks of kimmeridgian to berriasian age. The wells of interest for this study are located within block 9 which is made up of 13 wells but for this study the focus is only on 3 wells, which are well F-01,F-02 and F-03. The goal of this study is to predict as accurately as possible the areas within and around the sandstone reservoir intervals of these wells with abnormal pressure, using well logs and production test data. Abnormal pore pressure which is a major problem for drillers in the oil industry can cause serious drilling incidents and increase greatly drilling non-production time if the abnormal pressures are not predicted accurately before and while drilling. Petrophysics log analysis was done to evaluate the reservoirs. The intervals of the reservoir are the area of interest.Pore pressure gradient, fracture gradient, pore pressure and fracture pressure model were run. Pressures of about 6078.8psi were predicted around the zone of interest in well F-01, 7861 psi for well F-02 and 8330psi for well F-03. Well F-03 was the most pressured of the three wells. Abnormal pressures were identified mostly at zones above and below the area of interest and predicted pressure values were compared to actual pressure values to check for accuracy.

I estimate overpressure in a seismic cross-section along a 12km traverse associated with a 2D/4C OBC line in the Columbus Basin, Trinidad, West Indies, where shallow gas reduces both data quality and apparent seismic velocity for P-waves, using a modified Eaton's equation for PS-waves. Pore pressure prediction using the modified Eaton's method involves velocity analysis, conversion of the stacking velocities to interval velocities via the Dix's equation, converting the interval velocity trends to pressures and mudweights, and comparison of the predictions to 3 wells surrounding the seismic line. In the presence of shallow gas, the magnitude and areal extent of seismically derived P-wave and PS-wave velocity deviates from regional trends along the seismic line. PS-waves show a more accurate areal extent of velocity deviation due to overpressure than the P-wave, which is also affected by the presence of shallow gas. I verify my derived velocities and predicted-pressure values using sonic log data and observed pressure from 3 wells. Direct comparison between the sonic-derived velocities and the seismic-derived velocities shows that shallow gas reduces P-wave velocity, and that PS-wave velocity is less affected. Pressure prediction is verified using mudweights and formation tests from well logs and drilling reports. I find pressure predictions associated with P-waves, especially in areas of shallow gas are less reliable than for pressure predictions using PS-waves. I conclude that PS-wave velocity provides a superior map of overpressure in this region in areas with shallow gas clouds. / text

There are many complications associated with abnormally high fluid pressures in overpressured formations. Pore pressure can directly influence all parts of operations including drilling, geological studies, completion, and production. Accurate predictions of pore pressure and fracture pressure are vital aspects to the production and completion of safe, time efficient, and cost efficient projects. Knowledge of pressure distribution in the formation can greatly reduce complexities associated with drilling and completing a well. A three-method pore pressure and fracture pressure study was performed on two prospect deepwater wells located in the Gulf of Mexico. More than thirty offset wells in the greater region were initially analyzed for similarities with the two prospect wells. In the final analysis, only six wells were used to create pore pressure and fracture pressure models due to inconsistencies in similarities or lack of usable data in many of the offset wells. Pore pressure and fracture pressure models were constructed for the offset wells, and then applied and calibrated for the two prospect wells using drilling data such as mud weights, MDTs (Modular Dynamic Testing), and LOTs (Leak-off Test). Three types of pore pressure and fracture pressure models were used in the study: Eaton’s deep resistivity method; Eaton’s acoustic sonic method; and Bower’s interval seismic velocity method. Pore pressure and fracture pressure prediction was complicated by abnormal pressure in the formation due to undercompaction and seals. Both prospects were located in a deep subsalt environment. Low permeability and traps prevents fluid from escaping as rapidly as pore space compacts thus creating overpressure. Drilling through salt in deep water is expensive and risky. Elevated pore pressure and reduced fracture pressure underneath salt seals can create very tight mud weight windows and cause many drilling problems, as seen in the results of the offset wells’ pore pressure and fracture pressure models. Results indicate very small pore pressure and fracture pressure windows, or mud weight windows, because of overpressures in the formation caused by such a deep subsalt environment. Many casing points were needed in the final casing design of prospect wells to accommodate the smaller mud weight windows. Pore pressure has the most significant increase immediately below the salt, while the mud weight window remained constant or decreased with depth. The average mud weight window ranged between 1 to 2 pounds per gallon below the salt. / text

An investigation was conducted as a preliminary effort to develop a methodology to predict pore pressure in a rock formation during drilling, for all types of rocks and situations. Specifically, it was investigated whether or not the virgin pore pressure (the pore pressure of the undisturbed rock) can be determined at the drill bit from drilling and environmental parameters, as well as solid and pore fluid properties. Several drilling situations were analyzed to develop models relating pore pressure to drilling and environmental parameters, as well as solid and pore fluid properties. Three approaches to the modeling of such drilling situations were considered, which were used to predict pore pressure and compare the predictions to actual drilling data. The first approach used the concept of the effective stress in conjunction to the Mohr-Coulomb failure criterion. The second approach used the concept of the mechanical specific energy. The third approach made use of basic principles to relating virgin pore pressure to drilling and environmental parameters, as well as solid and pore fluid properties. This third approach resulted in the proposal of a more fundamental way of viewing mechanical specific energy (MSE) and the use of Biot's poroelasticity theory to describe the cutting process of rock. The first approach did not provide an adequate prediction of virgin pore pressure for all types of rocks and situations. The second approach showed promising results with limited actual drilling data. A sensitivity analysis of the model resulting from the third approach indicated that pore pressure, type of rock, and back rake angle of the cutter are the most significant factors affecting the energy required to break the rock. Moreover, rate of cutting stress, depth of cut, and type of pore fluid become significant factors of the cutting process only when a low-porosity, low-permeability rock is considered. It was concluded that there exists a relationship among pore pressure, drilling and environmental parameters, as well as solid and pore fluid properties. Therefore, it is possible in principle to determine the virgin pore pressure at the drill bit from drilling parameters, environmental parameters, and material properties. However, further work is required to establish a quantitative relationship among the significant parameters before a methodology to predict virgin pore pressure for all types of rocks and situations can be developed.

[pt] Os mecanismos geradores de pressão de poros em subsuperfície têm sido alvo de diversas pesquisas ao longo dos anos. A identificação prévia de zonas de pressão anômalas auxilia na redução do tempo não produtivo (NPT), evitando paralisações, bem como na prevenção de acidentes durante a etapa de perfuração do poço de petróleo. Nesse contexto, este trabalho estuda métodos de previsão de pressão de poros a partir de perfis petrofísicos e de parâmetros de perfuração. Com este objetivo, é apresentada uma revisão bibliográfica abordando tanto os conceitos fundamentais quanto os métodos de previsão de pressão de poros existentes. De modo geral, as estimativas de pressão de poros podem ser divididas em três etapas: (1) antes da perfuração, onde são utilizados perfis petrofísicos de poços de correlação para estimar a pressão de poros; (2) durante a etapa de execução do poço, na qual geralmente são usados os parâmetros de perfuração, além dos eventos de perfuração identificados, e quando disponíveis, as ferramentas de perfilagem em tempo real e dados de pressão; e (3) finalmente na etapa pós-perfuração, onde são utilizados os perfis a cabo, os possíveis perfis de Logging While Drilling (LWD), os eventos de perfuração e os dados de tomadas de pressão na retroanálise. Esses dados são usados para calibrar a pressão de poros estimada previamente para o poço e alimentar a base de dados de poços de petróleo perfurados em uma certa região. Neste trabalho foram realizados estudos de caso, com a realização de estimativas de curvas de pressão de poros, que foram comparadas aos valores de testes de pressão de poços, quando disponíveis. Além dos testes de pressão, também podem ser utilizados eventos indicadores de pressão de poros elevada na calibração, por exemplo o torque, arraste, cavings estilhados ao longo de formações argilosas, influxos em trechos permeáveis, entre outros indicativos. Foi constatado que os gradientes de pressão estimados pelo Método de Bowers apresentam resultados com maior oscilação dos valores quando comparados aos estimados pelo Método de Eaton. A boa aplicabilidade do Método de Eaton é devido ao fato de que na bacia em questão, o mecanismo de subcompactação mostrou-se predominante. Notou-se também que na comparação entre os métodos do expoente d e DEMSE, o resultado do gradiente de pressão de poros obtido através do expoente d está sujeito a menos interferências nas estimativas com relação ao método DEMSE. / [en] The mechanisms that generate pore pressure in subsurface have been the subject of several research over the years. The prior identification of anomalous pressure zones helps to reduce non-productive time (NPT), avoiding stoppages, as well as preventing accidents during the drilling stage of the oil well. In this context, this work studies pore pressure prediction methods from petrophysical profiles and drilling parameters. With this objective, a literature review is presented covering both the fundamental concepts and the existing pore pressure prediction methods. Generally speaking, pore pressure estimates can be divided into three steps: (1) before drilling, where petrophysical profiles from correlation wells are used to estimate pore pressure; (2) during the well execution stage, in which drilling parameters are generally used, in addition to identified drilling events, and when available, real-time logging tools and pressure data; and (3) finally in the post-drilling stage, where the cable profiles, the possible Logging While Drilling (LWD) profiles, the drilling events and the pressure tapping data in the back analysis are used. This data is used to calibrate the previously estimated pore pressure for the well and feed the database of oil wells drilled in a certain region. In this work, case studies were carried out, with estimates of pore pressure curves, which were compared to the values of well pressure tests, when available. In addition to pressure tests, events that indicate high pore pressure can also be used in calibration, for example torque, drag, chipped cavings along clayey formations, inflows in permeable stretches, among other indicators. It was found that the pressure gradients estimated by the Bowers Method shows greater oscillation of the values when compared to those estimated by the Eaton Method. The good applicability of the Eaton Method is related to the subcompaction mechanism predominant in the studied basin. It was also noticed that in the comparison between the Exponent d and DEMSE methods, the result of the pore pressure gradient obtained through the Exponent d is subject to less interference in the estimates in relation to the DEMSE method.

[pt] Nos últimos 45 anos foram publicados muitos artigos referentes à previsão da pressão de poros em folhelhos, como resultado da necessidade de otimizar o processo construtivo de poços de petróleo. Neste trabalho se apresenta um panorama dos métodos de previsão de pressão de poros existentes, com suas vantagens e desvantagens, com seus pontos fortes e suas críticas, com seus acertos e não acertos; procurando explicar os motivos das diferenças entre o previsto e o real. Em geral são descritos 12 métodos de previsão de pressão de poros, além do conceito do Centróide, 3 técnicas para detectar descarga de tensões efetivas numa formação rochosa, e uma descrição do uso da sísmica na previsão da pressão de poros. Foram aplicados os métodos de Eaton (1975) e Bowers (1995) com o objetivo de fazer uma discussão sobre as incertezas presentes nos parâmetros de cada modelo, complementado o estudo com uma análise de sensibilidade. Como resultado das incertezas existentes se aplicou uma análise probabilística baseada na simulação de Monte Carlo e usando o método de Eaton, com o objetivo de apresentar resultados dentro de intervalos de confiança e permitir planos de contingência durante o projeto de construção do poço. Finalmente são avaliados os resultados de uma análise 3D de previsão de pressão de poros utilizando o modelo de Eaton e o Trend de Bowers. Os cubos de dados foram obtidos por interpolação espacial ponderada partindo de registros de poços. Os resultados mostram que este tipo de análise pode ser usado com fins qualitativos, obtendo cubos de gradientes de pressão de poros aonde se observam as zonas de maior e menor risco. / [en] In the last 45 years were published many articles referring to shale pore pressure prediction, due to the necessity of optimizing the constructive process of petroleum wells. In this work shows up a view of the pore pressure prediction methods with its advantages and disadvantages, with its strong points and its critics, with its hits and failures, trying to explain the causes of the differences between the predicted values and the real ones. As result of the bibliographical revision, we obtained a historical of the pore pressure prediction, furthermore gathering the principal mechanisms of generation of pore pressures and mechanisms of lateral variation of the same. Also, were described 12 methods of pore pressure prediction, the Centroid concept, 3 techniques to detect unloading of effective tensions in a rock formation, and a description of the use of the seismic in the pore pressure prediction. The Eaton (1975) and the Bowers (1995) methods were applied with the objective to discuss the uncertainties in the parameters of each model, this was complemented with sensibility analysis. As result of the existent uncertainties, we applied a probabilistic analyze, based on the Monte Carlo simulation and using the Eaton´s method, with the aim to present results within confidence intervals and to allow contingency plans during the well construction project. Finally, the results of a 3D pore pressure prediction using the Eaton model and the Bowers Trend, were assessed. The data cubes were obtained by weighted space interpolation using well logs at the same basin. We concluded that the results from this type of analysis can be used such as qualitative purposes, obtaining pore pressure gradients cubes, where can be observed bigger and lesser risk zones.

[pt] O tempo não produtivo (NPT) durante a perfuração de poços de petróleo pode ser responsável pela perda de milhões de dólares em atividades offshore. A má previsão da pressão de poros pode ser uma das responsáveis pelo NPT de um poço ou mesmo sua perda definitiva em campos geologicamente complexos, como em bacias evaporíticas. Nesses campos complexos, os métodos de previsão de pressão de poros convencionais nem sempre são capazes de prever bem a distribuição de pressão de poros, mesmo após a perfuração de número considerável de poços. Este trabalho estuda técnicas alternativas que atendam ao problema de previsão de pressão para esses casos. Para fundamentar os estudos, é apresentada uma revisão sobre os riscos associados à perfuração em bacias evaporíticas e sobre os métodos de previsão de pressão existentes (métodos convencionais, sísmicos, modelagem geológica geomecânica 3D, modelagem pelo método dos elementos finitos e modelagem de bacias). Avaliando os problemas de perfuração nestes campos e as dificuldades de previsão dos métodos convencionais, nota-se que a complexidade imposta pelas consequências da presença do sal pode ser reduzida pelo uso de métodos que considerem a geologia local de forma mais abrangente em seu fluxo de trabalho. Concluiu-se que a modelagem de bacias e a modelagem geológica geomecânica 3D têm forte potencial de aplicação para estes casos. As técnicas, contudo, não tem a previsão de pressão de poros por objetivo principal, mas podem ser aplicadas ou adaptadas para tal fim. Este estudo apresenta adaptações de metodologia e/ou aplicações direcionadas de ambas para fins de previsão de pressão de poros. Para validar as propostas apresentadas, estudos de caso foram desenvolvidos e apresentaram resultados considerados bastante satisfatórios. / [en] The non-productive time (NPT) while drilling oil and gas wells may be responsible for losing millions of dollars, especially in offshore activities. Bad pore pressure predictions may be responsible for large NPT or even the definitive loss of well in geologically complex fields, such as evaporate basins. On these complex fields, the conventional pore pressure prediction methods sometimes are not capable of providing good predictions, even if a considerable number of wells has been already drilled. This thesis studies alternative techniques which may attend for pore pressure prediction in these cases. In order to develop a consistent knowledge about the case, a literature review has been conducted in two ways: to understand what are the risks associated to drilling in evaporate basins; to review what are the available methods for pore pressure prediction (conventional methods, seismic methods, 3D geological and geomechanical modeling, finite element methods and basin modeling). During analyzing geomechanical drilling risks in these sites, and the difficulties found by conventional methods to predict it, it was noted that the complexity imposed by the presence of salt bodies can be reduced by using methods that make a strong use of geological knowledge on their workflow. It has been concluded that basin modeling and 3D geological and geomechanical modeling have a good potential to be applied for this goal. The techniques, nevertheless, do not have pore pressure prediction as their main goal, but can be applied to or adapted for such finality. This work presents some methodology adaptations and/or applications of both of techniques directed to pore pressure prediction goals. In order to validate the presented proposals, case studies has been developed, and their results were considered satisfactory.

[pt] Esta tese propõe uma metodologia de estimativa de pressão de poros em rochasreservatório através dos atributos sísmicos velocidade compressional V(p) e velocidade cisalhante V(s). Na metodologia, os atributos são encarados como observações realizadas sobre um sistema físico, cujo comportamento depende de um determinado número de grandezas não observáveis, dentre as quais a pressão de poros é apenas uma delas. Para estimar a pressão de poros, adota-se uma abordagem Bayesiana de inversão. Através de uma função de verossimilhança, estabelecida através de um modelo de física de rochas calibrável para a região, e do teorema de Bayes, combina- se as informações pré-existentes sobre os parâmetros de rocha, fluido e estado de tensões com os atributos sísmicos observados, inferindo probabilisticamente a pressão de poros. Devido a não linearidade do problema e ao interesse de se realizar uma rigorosa análise de incertezas, um algoritmo baseado em simulações de Monte Carlo (um caso especial do algoritmo de Metropolis- Hastings) é utilizado para realizar a inversão. Exemplos de aplicação da metodologia proposta são simulados em reservatórios criados sinteticamente. Através dos exemplos, demonstra-se que o sucesso da previsão de pressão de poros depende da combinação de diferentes fatores, como o grau de conhecimento prévio sobre os parâmetros de rocha e fluido, a sensibilidade da rocha perante a variação de pressões diferenciais e a qualidade dos atributos sísmicos. Visto que os métodos existentes para previsão de pressão de poros utilizam somente o atributo V(p) , a contribuição do atributo V(s) na previsão é avaliada. Em um cenário de rochas pouco consolidadas (ou em areias), demonstra-se que o atributo V(s) pode contribuir significativamente na previsão, mesmo apresentando grandes incertezas associadas. Já para um cenário de rochas consolidadas, demonstra-se que as incertezas associadas às pressões previstas são maiores, e que a contribuição do atributo V(s) na previsão não é tão significativa quanto nos casos de rochas pouco consolidadas. / [en] This work proposes a method for pore pressure prediction in reservoir rocks through compressional- and shear-velocity data (seismic attributes). In the method, the attributes are considered observations of a physic system, which behavior depends on a several not-observable parameters, where the pore pressure is only one of these parameters. To estimate the pore pressure, a Bayesian inversion approach is adopted. Through the use of a likelihood function, settled through a calibrated rock physics model, and through the Bayes theorem, the a priori information about the not-observable parameters (fluid and rock parameters and stress state) is combined with the seismic attributes, inferring probabilistically the pore pressure. Due the non-linearity of the problem, and due the uncertainties analysis demanding, an algorithm based on Monte Carlo simulations (a special case of the Metropolis- Hastings algorithm) is used to solve the inverse problem. The application of the proposed method is simulated through some synthetic examples. It is shown that a successfully pore pressure prediction in reservoir rocks depends on a set of factors, as how sensitive are the rock velocities to pore pressure changes, the a priori information about rock and fluid parameters and the uncertainties associates to the seismic attributes. Since the current methods for pore pressure prediction use exclusively the attribute compressional velocity V(p), the contribution of the attribute shear velocity V(s) on prediction is evaluated. In a poorly consolidated rock scenario (or in sands), the V(s) data, even with great uncertainties associated, can significantly contribute to a better pore pressure prediction. In a consolidated rock scenario, the uncertainties associated to pore pressure estimates are higher, and the s V data does not contribute to pore pressure prediction as it contributes in a poorly consolidated rock scenario.