Masters of Science / Petrophysical evaluation of four wells within Cretaceous gas-bearing sandstone reservoirs in blocks 4 and 5 Orange Basin, South Africa. Thierry Kamgang The present research work evaluates the petrophysical characteristics of the Cretaceous gasbearing sandstone units within Blocks 4 and 5 offshore South Africa. Data used to carry out this study include: wireline logs (LAS format), base maps, well completion reports, petrography reports, conventional core analysis report and tabulated interpretative age reports from four wells (O-A1, A-N1, P-A1 and P-F1). The zones of interest range between 1410.0m-4100.3m depending on the position of the wells. The research work is carried out in two phases: The first phase corresponds to the interpretation of reservoir lithologies based on wireline logs. This consists of evaluating the type of rocks (clean or tight sandstones) forming the reservoir intervals and their distribution in order to quantify gross zones, by relating the behavior of wireline logs signature based on horizontal routine. Extensively, a vertical routine is used to estimate their distribution by correlating the gamma-ray logs of the corresponding wells, but also to identify their depositional environments (shallow to deep marine).Sedlog software is used to digitize the results. The second phase is conducted with the help of Interactive Petrophysics (version 4) software, and results to the evaluation of eight petrophysical parameters range as follow: effective porosity (4.3% - 25.4%), bulk volume of water (2.7% – 31.8%), irreducible water saturation (0.2%-8.8%), hydrocarbon saturation (9.9% - 43.9%), predicted permeability (0.09mD – 1.60mD), volume of shale (8.4% - 33.6%), porosity (5.5% - 26.2%) and water saturation (56.1% - ii 90.1%). Three predefined petrophysical properties (volume of shale, porosity and water saturation)are used for reservoir characterization. The volume of shale is estimated in all the wells using corrected Steiber method. The porosity is determined from the density logs using the appropriate equations in wells O-A1 and P-A1, while sonic model is applied in well A-N1 and neutron-density relationship in well P-F1. Formation water resistivity (Rw) is determined through the following equation: Rw = (Rmf × Rt) / Rxo, and water saturation is calculated based on Simandoux relation. Furthermore, a predicted permeability function is obtained from the crossplot of core porosity against core permeability, and it results match best with the core permeability of well O-A1. This equation is used to predict the permeability in the other wells. The results obtained reveal that average volumes of shale decrease from the west of the field towards the east; while average porosities and water saturations increase from the south-west through the east despite the decreasing average water saturation in well P-A1. A corroboration of reference physical properties selected for reservoir characterization, with predefined cut-off values result to no net pay zones identified within the reservoir intervals studied. Consequently, it is suggested that further exploration prospects should be done between well O-A1 and A-N1.

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It is widely known in oil industry that changes in fluid flow conditions such as water breakthrough or unsteady flow due to well shut-in can lead to sand destabilization, with a possible consequent sand production. In this research, different flow situations are incorporated into stress and stability analysis for the region around a wellbore producing oil from weak or unconsolidated sands, and the analyses involve strength weakening, stress redistribution, and decrease of rock stiffness. Two main mechanisms, chemical reactions of rock with formation water and variations of rock capillary strength, are identified and analyzed to study strength weakening after water breakthrough, both qualitatively and quantitatively. Using theories from particle mechanics, rock mechanics, and interfacial science, four novel capillarity models are developed and verified to analytically capture the physical behaviors of capillary strength at the grain scale. Based on model calculations, significantly better understanding of strength behavior in two-phase fluid environments is achieved. Based on a simplified model that can conservatively but efficiently quantify capillary strength with only two input parameters (i. e. particle radius and water saturation), a verified new method that physically calculates pore pressure in a multiphase environment, and a coupled poro-inelastic stress model, the redistributions of effective stresses with water saturation around a wellbore are solved. In terms of stress changes and growth of a plastic radius defining shear-failure zone, the effects of different stability factors, including capillarity through water-oil menisci, pore pressure changes due to the variations of fluid relative permeabilities, and loss of strength through chemical reactions of water-sensitive cementation materials, are quantified and compared in order to clarify when and how they contribute to sand production after water breakthrough. The nonlinearities of rock elastic properties in stressed and biphasic fluid environments is analytically addressed, based on an improved nonlinear theory that considers both a failure-based mechanism and a confining-stress-based mechanism, the strength model, and the coupled stress model. The calculations demonstrate the redistributions of stress-dependent rock stiffness around a wellbore and its evolution with increase of water saturation, clarify the relative importance of each mechanism in reducing rock stiffness, and fundamentally explain why current predictive technologies are invalid when water appears in a flowing wellbore. To quantify the effect of well shut-down on rock stability, the redistributions of fluid pressure in reservoir are analytically solved and coupled with the stress model, while the water hammer equations provide a boundary condition for the bottom-hole pressure. This approach allows direct solution of the relationships among fluid properties, rock properties and production parameters, within the context of rock stability. The proposed new approaches and models can be applied to evaluate sand production risk in multiphase and unsteady fluid flow environment. They can also serve as points of departure to develop more sophisticated models, or to develop more useful constitutive laws for numerical solutions.

It is widely known in oil industry that changes in fluid flow conditions such as water breakthrough or unsteady flow due to well shut-in can lead to sand destabilization, with a possible consequent sand production. In this research, different flow situations are incorporated into stress and stability analysis for the region around a wellbore producing oil from weak or unconsolidated sands, and the analyses involve strength weakening, stress redistribution, and decrease of rock stiffness. Two main mechanisms, chemical reactions of rock with formation water and variations of rock capillary strength, are identified and analyzed to study strength weakening after water breakthrough, both qualitatively and quantitatively. Using theories from particle mechanics, rock mechanics, and interfacial science, four novel capillarity models are developed and verified to analytically capture the physical behaviors of capillary strength at the grain scale. Based on model calculations, significantly better understanding of strength behavior in two-phase fluid environments is achieved. Based on a simplified model that can conservatively but efficiently quantify capillary strength with only two input parameters (i. e. particle radius and water saturation), a verified new method that physically calculates pore pressure in a multiphase environment, and a coupled poro-inelastic stress model, the redistributions of effective stresses with water saturation around a wellbore are solved. In terms of stress changes and growth of a plastic radius defining shear-failure zone, the effects of different stability factors, including capillarity through water-oil menisci, pore pressure changes due to the variations of fluid relative permeabilities, and loss of strength through chemical reactions of water-sensitive cementation materials, are quantified and compared in order to clarify when and how they contribute to sand production after water breakthrough. The nonlinearities of rock elastic properties in stressed and biphasic fluid environments is analytically addressed, based on an improved nonlinear theory that considers both a failure-based mechanism and a confining-stress-based mechanism, the strength model, and the coupled stress model. The calculations demonstrate the redistributions of stress-dependent rock stiffness around a wellbore and its evolution with increase of water saturation, clarify the relative importance of each mechanism in reducing rock stiffness, and fundamentally explain why current predictive technologies are invalid when water appears in a flowing wellbore. To quantify the effect of well shut-down on rock stability, the redistributions of fluid pressure in reservoir are analytically solved and coupled with the stress model, while the water hammer equations provide a boundary condition for the bottom-hole pressure. This approach allows direct solution of the relationships among fluid properties, rock properties and production parameters, within the context of rock stability. The proposed new approaches and models can be applied to evaluate sand production risk in multiphase and unsteady fluid flow environment. They can also serve as points of departure to develop more sophisticated models, or to develop more useful constitutive laws for numerical solutions.

The aim of this thesis is to study water saturation models available in the literature and to apply a proper one to a real field case. Archie equation is the most well-known water saturation model. However, it is formulated on some assumptions and is applicable to only clean sands. Archie equation cannot be used for shaly formation. There are many shaly water saturation models that account for shale effect for water saturation estimation. In this study, 3 wells, namely Well-01, Well-02 and Well-03 are studied. These wells lie in a fractured carbonate reservoir located in Southeastern part of Turkey. From well log recordings, the production formation is seen almost clean. In other words, the shale amount of the formation is so small that it can be neglected. Thus, to calculate the water saturation in those wells, the well-known Archie water saturation equation is used. Since the formation is fractured carbonate, the cementation factor (m ) and saturation exponent (n ) of conventional value of 2 each cannot be used for the water saturation calculation. Instead, these parameters are obtained from generalized crossplot of log-derived porosity and resistivity technique. Finally, each well is divided into zones using porosity data. Zonation is conducted based on statistical method, ANOVA (analysis of variance). Well-01 and Well-02 are both divided into two zones. On the other hand, the statistical method was initially divided Well-03 into three zones. However, Well-03 is better described as a whole zone, depending on the geological analysis and engineering judgment. After the zonation, the zones are correlated from well to well. The water saturations in significantly correlated zones are examined. Also, using the same statistical method, the water saturation zones are identified. However, these zones do not coincide with the porosity zones. This difference is attributed to pore size distribution and wettability which affect saturation distribution.

>Magister Scientiae - MSc / This study has been conducted to accurately determine fluid saturation within Fula sub-basin reservoirs which is located at the Southern part of the Republic of Sudan. The area is regarded as Shaly Sand Reservoirs. Four deferent shaly sand lithofacies (A, B, C, D) have been identified. Using method based on the Artificial Neural Networks (ANN), the core surrounding facies, within Fula reservoirs were identified. An average shale volume of 0.126 within the studied reservoirs was determined using gamma ray and resistivity logs. While average porosity of 26.7% within the reservoirs was determined using density log and the average core grain density. An average water resistivity of 0.8 Ohm-m was estimated using Pickett plot method. While formation temperature was estimated using the gradient that constrained between surface and bottom hole temperature. Water saturation was determined using Archie model and four shaly sand empirical models, the calculation was constrained within each facies zone to specify a model for each facies, and another approach was used to obtain the water saturation based on Artificial Neural Networks. The net pay was identified for each reservoir by applying cut-offs on permeability 5 mD, porosity 16%, shale volume 0.33, and water saturation 0.65. The gross thickness of the reservoirs ranges from 7.62m to 19.85m and net pay intervals from 4.877m to 19.202m. The study succeeded in establishing water saturation model for the Fula sub-basin based on neural networking which was very consistent with the core data, and hence has been used for net pay determination.

Prediction of rock and fluid properties such as porosity, clay content, and water saturation is essential for exploration and development of hydrocarbon reservoirs. Rock and fluid property maps obtained from such predictions can be used for optimal selection of well locations for reservoir development and production enhancement. Seismic data are usually the only source of information available throughout a field that can be used to predict the 3D distribution of properties with appropriate spatial resolution. The main challenge in inferring properties from seismic data is the ambiguous nature of geophysical information. Therefore, any estimate of rock and fluid property maps derived from seismic data must also represent its associated uncertainty. In this study we develop a computationally efficient mathematical technique based on neural networks to integrate measured data and a priori information in order to reduce the uncertainty in rock and fluid properties in a reservoir. The post inversion (a posteriori) information about rock and fluid properties are represented by the joint probability density function (PDF) of porosity, clay content, and water saturation. In this technique the a posteriori PDF is modeled by a weighted sum of Gaussian PDF’s. A so-called mixture density network (MDN) estimates the weights, mean vector, and covariance matrix of the Gaussians given any measured data set. We solve several inverse problems with the MDN and compare results with Monte Carlo (MC) sampling solution and show that the MDN inversion technique provides good estimate of the MC sampling solution. However, the computational cost of training and using the neural network is much lower than solution found by MC sampling (more than a factor of 104 in some cases). We also discuss the design, implementation, and training procedure of the MDN, and its limitations in estimating the solution of an inverse problem. In this thesis we focus on data from a deep offshore field in Africa. Our goal is to apply the MDN inversion technique to obtain maps of petrophysical properties (i.e., porosity, clay content, water saturation), and petrophysical facies from 3D seismic data. Petrophysical facies (i.e., non-reservoir, oil- and brine-saturated reservoir facies) are defined probabilistically based on geological information and values of the petrophysical parameters. First, we investigate the relationship (i.e., petrophysical forward function) between compressional- and shear-wave velocity and petrophysical parameters. The petrophysical forward function depends on different properties of rocks and varies from one rock type to another. Therefore, after acquisition of well logs or seismic data from a geological setting the petrophysical forward function must be calibrated with data and observations. The uncertainty of the petrophysical forward function comes from uncertainty in measurements and uncertainty about the type of facies. We present a method to construct the petrophysical forward function with its associated uncertainty from the both sources above. The results show that introducing uncertainty in facies improves the accuracy of the petrophysical forward function predictions. Then, we apply the MDN inversion method to solve four different petrophysical inverse problems. In particular, we invert P- and S-wave impedance logs for the joint PDF of porosity, clay content, and water saturation using a calibrated petrophysical forward function. Results show that posterior PDF of the model parameters provides reasonable estimates of measured well logs. Errors in the posterior PDF are mainly due to errors in the petrophysical forward function. Finally, we apply the MDN inversion method to predict 3D petrophysical properties from attributes of seismic data. In this application, the inversion objective is to estimate the joint PDF of porosity, clay content, and water saturation at each point in the reservoir, from the compressional- and shear-wave-impedance obtained from the inversion of AVO seismic data. Uncertainty in the a posteriori PDF of the model parameters are due to different sources such as variations in effective pressure, bulk modulus and density of hydrocarbon, uncertainty of the petrophysical forward function, and random noise in recorded data. Results show that the standard deviations of all model parameters are reduced after inversion, which shows that the inversion process provides information about all parameters. We also applied the result of the petrophysical inversion to estimate the 3D probability maps of non-reservoir facies, brine- and oil-saturated reservoir facies. The accuracy of the predicted oil-saturated facies at the well location is good, but due to errors in the petrophysical inversion the predicted non-reservoir and brine-saturated facies are ambiguous. Although the accuracy of results may vary due to different sources of error in different applications, the fast, probabilistic method of solving non-linear inverse problems developed in this study can be applied to invert well logs and large seismic data sets for petrophysical parameters in different applications.

A água é um bem necessário a toda a humanidade, à agricultura e às indústrias. É o bem mais escasso atualmente e será muito mais nos próximos anos. A proteção dos recursos hídricos para garantia do bem-estar da humanidade torna-se essencial e com isto a restauração florestal de nascentes e áreas ciliares é alvo de projetos em todo o mundo. As áreas hidrologicamente sensíveis (AHS) dentro da bacia hidrográfica são as porções mais importantes neste contexto, no entanto, devido à saturação hídrica do solo ocasionada pela dinâmica do lençol freático, a sua recuperação é dificultada desestimulando pesquisas e ações de restauração. Diante desta lacuna, este estudo teve o objetivo de buscar estratégias para a restauração florestal de AHS. Para isso foi testado em viveiro a tolerância a diferentes níveis de saturação hídrica de 15 espécies típicas de ambientes alagados e uma espécie não típica como controle. As espécies foram avaliadas quanto ao crescimento em altura, diâmetro à altura do solo, sobrevivência, desenvolvimento de respostas morfológicas e possível associação destas com as taxas de crescimento. Foi feita uma análise de agrupamento que classificou e agrupou as espécies de acordo com cinco níveis de tolerância ao alagamento, identificando que para a maioria das espécies quanto maior o nível de saturação hídrica menor é a tolerância a este estresse. A análise de variância e o teste de Tukey identificaram quais espécies apresentaram características morfológicas (hipertrofia de lenticelas e raízes adventícias) e os indicadores morfológicos foram comparados nos grupos constituídos da análise de agrupamento que revelou que estas respostas estão associadas à adaptação quanto ao crescimento e sobrevivência das espécies. Dois experimentos em campo também foram realizados visando testar métodos de preparo de solo que mais favorecessem o estabelecimento e o crescimento inicial das espécies. Foi feita uma análise de variância e teste de Tukey para testar se o método de preparo de solo tem efeito sobre o crescimento em altura e diâmetro e o tempo de sobrevivência para as espécies e uma análise de agrupamento para definir grupos de espécies com características funcionais que favoreçam os projetos de restauração. Verificou-se que os métodos de preparo de solo não foram determinantes para o estabelecimento e desenvolvimento das espécies típicas de AHS em pequenas escalas e que a escolha das espécies mais tolerantes à saturação hídrica em diferentes níveis é o fator mais importante na restauração desses ambientes. Desta forma, em projetos de restauração de florestas brejosas, a seleção de espécies adaptadas a estas condições é mais importante do que os métodos de preparo do solo, e futuros estudos deveriam focar em identificar grupos funcionais de espécies que possam ser usadas nestes projetos. / From agriculture to the industry, water is an essential resource for humanity. It is also the scarcest resource at the present time and it will be scarcer in the next years. The protection of water resources is of most importance to ensure humanity\'s well-being. Thus, projects aiming at the restoration of springs and riparian forests are being carried out around the world. In this context, hydrologically sensitive areas (HSA) of the river basins are the most important sites for water resources provision. However, water table dynamics causes water saturation of the soil in these areas, imposing barriers to recuperation and discouraging restoration practice and research. In this context, this study aims to analyze strategies for the ecological restoration of HSA. We carried out nursery tests to assess the hydrological saturation tolerance of 15 tree species typical from wetlands and used a non-typical species as control. Species were evaluated based on height and diameter at soil height increase, survival, development of morphological responses and the possible relation of the latter with growth rates. The cluster analysis classified species in groups based on five hydrological saturation tolerance levels, pointing that, for most species, higher hydrological saturation levels results in lower tolerance to this stress. An analysis of variance and Tukey test identified which species presented morphological characteristics (lenticels hypertrophy and adventitious roots) and the morphological indicators were compared among the clustered groups; this analysis demonstrated that such morphological responses are associated to the species adaptation for both survival and growth in saturated conditions. Two field experiments were also carried out aiming to test soil preparation that could favor seedling establishment and growth. An analysis of variance and Tukey test was carried out to test the effects of soil preparation on seedling height and diameter at soil height growth and survival. Additionally, we carried out a cluster analysis to separate species into groups containing functional characteristics that may favor restoration projects. We observed that soil preparation techniques, in small scale, had no influence on the establishment and development of seedlings typical of HSA and that the selection of species that are tolerant to hydrological saturation at different levels is the most important factor for restoration of these environments. Thus, restoration projects in wetlands should give special attention for the selection of species adapted to these conditions, which is more determinant for restoration success than soil preparation. Future studies should focus in identifying species functional groups that could be used to restore these areas.

Nesta pesquisa foram estudadas morfologias e posições do relevo que apresentam solos e depósitos com regimes hídricos caracterizados pela saturação temporária ou permanente, tendo os seguintes objetivos: (i) caracterização dos solos e depósitos presentes no setor inferior de vertentes e em fundos de vale, incluindo planícies fluviais, com ênfase em suas propriedades hidromórficas; (ii) identificação de graus de hidromorfia nos solos e depósitos desses setores e morfologias, e representação de sua variabilidade espacial; (iii) elaboração de proposições a respeito do regime de saturação hídrica nos setores estudados e de sua relação com os graus de hidromorfia identificados; (iv) elaboração de proposições sobre tendências espaciais hidrodinâmicas associadas à inundação fluvial nas áreas de planície. Sob o aspecto metodológico, a pesquisa foi desenvolvida de acordo com os princípios da abordagem geossistêmica, apresentando uma análise integrada que buscou identificar as inter-relações entre as variáveis pesquisadas. Para tal estudo, foram selecionados dois perfis transversais na Alta Bacia Hidrográfica do Rio Cotia, que abrangeram o setor inferior de vertentes e fundos de vale, um deles com planície fluvial. Ao longo desses perfis transversais foram realizados os seguintes levantamentos e procedimentos: (i) levantamento morfométrico e construção de gráficos dos perfis; (ii) descrição dos atributos morfológicos dos solos e depósitos; (iii) monitoramento dos níveis dágua do lençol freático ou lençol suspenso nos pontos de descrição. A caracterização e análise detalhadas das propriedades hidromórficas revelaram gradações de intensidade nas mesmas, e, a partir dessa base, desenvolveu-se uma proposta para classificação dos solos e depósitos segundo graus de hidromorfia. Foram construídas representações gráficas bidimensionais que ilustraram a sucessão vertical e lateral de horizontes pedogenéticos e camadas com diferentes graus de hidromorfia ao longo dos setores e morfologias estudadas. Desta maneira, foram identificadas tendências gerais no que se refere à distribuição espacial dos graus de hidromorfia e à relação desses graus com o regime de saturação hídrica. Um dos padrões identificados consistiu no aumento do grau de hidromorfia dos perfis verticais de solos e depósitos com o aumento da profundidade, que está associado a um maior tempo de duração das condições de saturação hídrica nos horizontes mais profundos. Também foram observadas diferenças significativas na intensidade das propriedades hidromórficas das vertentes, quando comparadas às áreas de fundos de vale, incluindo a planície fluvial. Neste sentido, foram identificados graus de hidromorfia mais baixos nos horizontes A e B dos pontos localizados nas vertentes, e graus mais elevados nos horizontes A e B dos pontos localizados nos fundos de vale. Essas diferenças na ocorrência dos graus de hidromorfia foram atribuídas a distintos regimes de saturação hídrica, sendo que, de maneira geral, os horizontes pedogenéticos e as camadas com graus de hidromorfia elevados estão sujeitos a condições de saturação hídrica de maior duração ou permanentes, quando comparados àqueles que possuem graus de hidromorfia mais baixos, nos quais os eventos de saturação hídrica tem menor duração e/ou ocorrem com menor frequência. Com relação às proposições referentes à inundação fluvial na planície estudada, foram identificadas morfologias que recebem preferencialmente os fluxos fluviais da inundação e apresentam taxas de deposição mais elevadas, e setores com taxas de deposição mais baixas nos quais a inundação é menos frequente e há predomínio da deposição de sedimentos finos. / In this research we studied morphologies and relief positions with soils and deposits with moisture regimes characterized by temporary or permanent water saturation, with the following objectives: (i) characterization of soils and deposits located in lower slope sectors and valley bottoms, with an emphasis on their hydromorphic properties; (ii) identification of degrees of hydromorphy in the soils and deposits of these sectors and morphologies, and representation of their spatial variability; (iii) development of propositions about the regime of water saturation in the studied areas and its relationship with the degrees of hydromorphy; (iv) development of propositions about the hydrodynamics of floods in the studied floodplain. Regarding the methodological aspect, the research was conducted according to the principles of the geosystemic approach, presenting an integrated analysis which considered the inter-relations among the studied variables. For this study, we selected two cross profiles in the Cotia Drainage Basin, comprising lower slope sectors and valley bottoms, including a floodplain. Along these cross profiles, the following surveys were conducted: (i) topographic survey and construction of cross section graphics; (ii) description of the morphological attributes of soils and deposits, including detailed description of their hydromorphic properties; (iii) monitoring the water levels of the water table and perched water tables in the sites of description. The detailed characterization and analysis of the hydromorphic properties revealed gradations of intensity in them, and, based primarily on this, we developed a proposal for classification the soils and deposits of the study area according to degrees of hydromorphy. We built two-dimensional graphical representations which illustrated vertical and lateral successions of pedogenetic horizons or layers with different degrees of hydromorphy. Thus, general trends were identified in respect to the spatial distribution of the degrees of hydromorphy and in respect to its relationship with the regime of water saturation. One of the patterns identified consisted in the increase of the degree of hydromorphy in the soil profiles with increasing depth, associated with a longer duration of water saturation conditions in the deeper horizons. It were also observed significant differences in the intensity of the hydromorphic properties of the slopes, when compared to the valley bottom areas, including the floodplain. In this regard, lower degrees of hydromorphy were identified in the A and B horizons in the slope sectors, and higher degrees in the A and B horizons in the valley bottom areas. Such differences in the degree of hydromorphy were ascribed to distinct regimes of water saturation, considering that, in general, the horizons and layers with higher degrees of hydromorphy are subject to water saturation conditions of longer duration, when compared to those classified according to lower degrees of hydromorphy, in which the water saturation events have shorter duration and/or have lower frequency. Concerning the propositions about the hydrodynamics of floods in the studied floodplain, we identified morphologies that preferentially receive the overflowing river flows and have higher deposition rates, and sectors with lower deposition rates, where flooding is less frequent and there is a predominance of fine sediments deposition.

Pulsed-neutron borehole measurements involve a physical process in which a source emits energetic neutrons that lose energy upon collisions with formation nuclei, and are eventually captured by a nucleus to form a heavier, excited state. The excited nucleus decays to its ground state by the emission of gamma rays. Both thermal-neutron and gamma-ray populations decay with time at a rate defined by Sigma, which is a nuclear property that quantifies a material’s ability to capture thermal neutrons. The large contrast in Sigma between hydrocarbon and salty connate water enables calculations of water saturation directly from pulsed-neutron measurements. Sigma logs have proven useful in the assessment of thinly bedded formations because they exhibit a small volume of investigation, and have been deemed superior to resistivity logs in the petrophysical evaluation of carbonate formations. The recognized potential of Sigma logs in formation evaluation initiated the development of multi-detector Logging-While-Drilling (LWD) Sigma measurements. These measurements are acquired using one thermal-neutron and two gamma-ray detectors at different spacings from the source. Such a design is aimed at providing distinct radial depths of investigation to detect filtrate invasion in the near-wellbore zone. Despite their formation-evaluation potential, multi-detector time-decay measurements commonly remain affected by invasion, shoulder-bed, and well-deviation effects. The purpose of this dissertation is to develop a fast-forward simulation method to reproduce multi-detector time decays and combine the method with inversion techniques to improve the petrophysical interpretation of LWD Sigma measurements. First-order perturbation theory and a library of pre-calculated Monte Carlo detector-specific sensitivity functions and time decays are used to numerically simulate borehole Sigma measurements in realistic logging environments. The new simulation method is one hundred thousand times faster than rigorous Monte Carlo calculations and remains within two capture units of disparity. Next, the fast-forward simulation method is embedded within inversion algorithms to estimate layer-by-layer radial length of invasion and formation Sigma corrected for shallow invasion, shoulder-bed, and well-deviation effects. Both fast-forward and inverse modeling algorithms are benchmarked against laboratory and synthetic time decays. The improvement of formation Sigma obtained with inversion-based interpretation leads to an improvement in the estimation of Sigma-derived water saturation. Likewise, the estimated radial length of invasion is combined with neutron and density measurements to correct the latter for invasion effects. Results indicate that the inversion-based interpretation method is well suited for the evaluation of high-porosity formations invaded by salty mud filtrate. Inversion-based interpretation of field LWD time decays enables the estimation of lower values of water saturation when compared to conventional Sigma interpretation or resistivity methods. Estimated values of water saturation are as much as fifty percent lower than predicted by conventional interpretation of Sigma logs in the case of measurements affected by shoulder-bed effects, and as much as one hundred percent lower than predicted by the conventional interpretation method for measurements additionally affected by salty filtrate invasion. The key attributes of the combined petrophysical interpretation of multi-detector Sigma, neutron, and density measurements developed in this dissertation are that it explicitly enforces the physics of all nuclear measurements, honors the pressure and temperature dependency of reservoir fluid nuclear properties, and takes into account a-priori information such as mud-filtrate salinity, connate-water salinity, and bed-boundary locations. / text