Partial saturation of porous rock by two fluids substantially affects compressional wave propagation. In particular, partial saturation causes significant attenuation and dispersion due to wave-induced fluid flow. Such flow arises when a passing wave induces different fluid pressures in regions of rock saturated by different fluids. When partial saturation is mesoscopic, i.e. existing on a length scale much greater than pore scale but less than wavelength scale, significant attenuation can arise for frequencies 10-1000 Hz. Models for attenuation and dispersion due to mesoscale heterogeneities mostly assume fluids are distributed in a regular way. Recent experiments indicate mesoscopic heterogeneities have less idealised distributions and distribution affects attenuation/dispersion. Thus, theoretical models are required to simulate effects due to realistic fluid distributions. / The thesis focus is to model attenuation and dispersion due to realistic mesoscopic fluid distributions and fluid contrasts. First X-ray tomographic images of partially saturated rock are analysed statistically to identify spatial measures useful for describing fluid distribution patterns. The correlation function and associated correlation length for a specific fluid type are shown to be of greatest utility. Next a new model, called 3DCRM (CRM stands for continuous random media) is derived, utilizing a correlation function to describe the fluid distribution pattern. It is a random media model, is accurate for small fluid contrast and approximate for large fluid contrast. Using 3DCRM attenuation and dispersion are shown to depend on fluid distribution. / Next a general framework for partial saturation called APS (acoustics of partial saturation) is extended enabling estimation of attenuation and dispersion due to arbitrary 1D/3D fluid distributions. The intent is to construct a versatile model enabling attenuation and dispersion to be estimated for arbitrary fluid distributions, contrasts and saturations. Two crucial parameters within APS called shape and frequency scaling parameters are modified via asymptotic analysis using several random media models (which are accurate for only certain contrasts in fluid bulk moduli and percent saturation). For valid fluid contrasts and saturations, which satisfy certain random media conditions there is good correspondence between modified APS and the random media models, hence showing that APS can be utilized to model attenuation and dispersion due to more realistic fluid distributions. / Finally I devise a numerical method to test the accuracy of the analytical shape parameters for a range of fluid distributions, saturations and contrasts. In particular, the analytical shape parameter for randomly distributed spheres was shown to be accurate for a large range of saturations and fluid contrasts.

Submitted by Marcos Wanderley (marcos.wanderley@ufcg.edu.br) on 2018-07-26T20:07:51Z No. of bitstreams: 1 WELINGTON BARBOSA DE SOUSA - DISSERTAÇÃO(PPGEPM) 2017.pdf: 2079159 bytes, checksum: c91660187b1af81f4801a2dccb0a5b76 (MD5) / Made available in DSpace on 2018-07-26T20:07:51Z (GMT). No. of bitstreams: 1 WELINGTON BARBOSA DE SOUSA - DISSERTAÇÃO(PPGEPM) 2017.pdf: 2079159 bytes, checksum: c91660187b1af81f4801a2dccb0a5b76 (MD5) Previous issue date: 2017-05-26 / O conhecimento das propriedades petrofísicas é de grande importância para melhor entender o comportamento físico das rochas, especialmente quando se considera que o principal método de prospecção geofísica para alvos profundos é o método sísmico, o qual investiga a propagação de ondas elásticas em subsuperfície. O estudo das ondas sísmicas fornece informações a respeito do tipo de rocha e fluidos em subsuperfície: assim, é de grande importância o desenvolvimento de um trabalho que possibilite gerar um modelo matemático capaz de simular a propagação dessas ondas, tendo em vista sua importância para o cálculo das propriedades elásticas. Este trabalho tem por objetivo suprir essa necessidade, por meio da geração um modelo matemático (utilizando o software Comsol Multiphysics 5.1) capaz de simular a propagação de ondas cisalhantes (S) em rochas sedimentares a partir de imagens microtomográficas de raios-X de dois tipos de rocha: arenitos e carbonatos. A simulação da propagação de ondas compressionais e cisalhantes foi realizada através da aplicação do módulo solid mechanics, da sessão Structural Mechanics, que permite a análise transiente da propagação de ondas em maciços rochosos causada pela aplicação de uma carga explosiva de curta duração. Os valores obtidos pelo método objeto deste trabalho foram comparados aos valores medidos em laboratório (P e S) e aos valores obtidos utilizando o método apresentado por Apolinário (2016) para a onda P. No caso das ondas cisalhantes, os valores obtidos foram comparados apenas aos valores obtidos em laboratório. O modelo numérico desenvolvido neste trabalho apresentou uma performance satisfatória na simulação das velocidades de propagação das ondas P e S em amostras reais de arenitos e carbonatos, tendo seu desempenho sido superior ao método proposto por Apolinário (2016). Uma maior representatividade estatística dos resultados pode ser obtida pela aplicação em um maior número de amostras. / The knowledge of the petrophysical properties is of great importance to better understand the physical behavior of the rocks, especially when considering that the main method of geophysical prospecting for deep targets is the seismic method, which investigates the propagation of elastic waves in subsurface. The study of seismic waves provides information about the type of rock and subsurface fluids: thus, the development of a work that allows to generate a mathematical model capable of simulating the propagation of these waves is of great importance, considering their importance for the calculation of elastic properties. This work aims to furnish this need by generating a mathematical model (using software Comsol Multiphysics 5.1) able to simulate the propagation of shear waves (S) in sedimentary rocks from microtomographic images of X-rays of two types of rock: sandstones and carbonates. The simulation of the propagation of compressive and shear waves was carried out through the application of the solid mechanics module of the session Structural Mechanics, which allows the transient analysis of the propagation of waves in rocky masses caused by the application of a short duration explosive load. The results obtained by the object method of this work were compared to the values measured in laboratory (P and S) and the values obtained using the method presented by Apolinário (2016) for the P wave. In the case of the shear waves, the values obtained were compared only values obtained in the laboratory. The numerical model developed in this work presented a satisfactory performance in the simulation of the propagation velocities of P and S waves in real samples of sandstones and carbonates, and its performance was superior to the method proposed by Apolinário (2016). A greater statistical representativeness of the results can be obtained by the application in a greater number of samples.

Overpressure, or abnormally low effective pressures, is hazardous in drilling operations and construction of sea-bottom facilities in deepwater environments. Estimation of the locations of overpressure can improve safety in these operations and significantly reduce overall project costs. Propagation velocities of both seismic P and S wave are sensitive to bulk elastic parameters and density of the sediments, which can be related to porosity, pore fluid content, lithology, and effective pressures. Overpressured areas can be analyzed using 4C seismic reflection data, which includes P-P and P-SV reflections. In this thesis, the effects on compressional (P) and shear (S) wave velocities are investigated to estimate the magnitude and location of excess pore pressure utilizing Eaton’s approach for pressure prediction (Eaton, 1969). Eaton’s (1969) method relates changes in pore pressure to changes in seismic P-wave velocity. The underlying assumption of this method utilizes the ratio of observed P-wave velocity obtained from areas of both normal and abnormal pressure. This velocity ratio evaluated through an empirically determined exponent is then related to the ratio of effective stress under normal and abnormal pressure conditions. Effective stress in a normal pressured condition is greater than the effective stress value in abnormally overpressured conditions. Due to an increased sensitivity of variations in effective pressure to seismic interval velocity, Ebrom et al. (2003) employ a modified Eaton equation to incorporate the S-wave velocity in pore pressure prediction. The data preparation and subsequent observations of seismic P and S wave velocity estimates in this thesis represent a preliminary analysis for pore pressure prediction. Six 2D receiver gathers in the regional dip direction are extracted from six individual ocean-bottom 4C seismic recording nodes for P-P and P-SV velocity analysis. The receiver gathers employed have minimal pre-processing procedures applied. The main processing steps applied were: water bottom mute, 2D rotation of horizontal components to SV and SH orientation, deconvolution, and frequency filtering. Most the processing was performed in Matlab with a volume of scripts designed by research scientists from the University of Texas, Bureau of Economic Geology. In this thesis, fluid pressure prediction is estimated utilizing several 4C multicomponent ocean-bottom nodes in the Atlantis Field in deepwater Gulf of Mexico. Velocity analysis is performed through a ray tracing approach utilizing P-P and P-SV registration. A modified Eaton’s Algorithm is then used for pore pressure prediction using both P and S wave velocity values. I was able to successfully observe both compressional and shear wave velocities to sediment depths of approximately 800 m below the seafloor. Using Hamilton (1972, 1976) and Eberhart-Phillips et al. (1989) regressions as background depth dependent velocity values and well-log derived background effective pressure values from deepwater Gulf of Mexico, I am able to solve for predicted effective pressure for the study area. The results show that the Atlantis subsurface study area experiences a degree of overpressure. / text