Jadir da Conceição da Silva

Ultrasonic travel-time tomography in core plugs

This paper applies ultrasonic travel-time tomography in order to investigate the resolution (i.e. definition of dimension and shape) of velocity heterogeneities in tomograms from core plugs. Designing the tomographic algorithm involves two main steps, namely, modelling and inversion. In the modelling step, we use straight and curved ray-tracing schemes to calculate raypaths and travel times corresponding to the source–receiver configuration. The inversion step then submits a predefined norm of the residuals between calculated and measured travel times to a minimization process. The conjugate gradient method works to iteratively minimize the � 1 norm of travel-time residuals. Numerical experiments show that, in addition to the dominant frequency of the probing ultrasonic pulse, the resolution of tomograms from core plugs with high-contrast heterogeneities depends on the chosen ray-tracing scheme. In summary, if the rock sample exhibits low to moderate velocity contrasts, the tomographic algorithm using straight rays in the modelling step yields tomograms with good resolution. However, in the presence of high velocity contrasts, plausibly resolved tomograms can be obtained only by adopting a curved ray-tracing scheme. The application of ultrasonic tomography using travel-time data from real core plugs corroborates the results obtained with numerical data.

Simulação do perfil sônico a partir dos perfis de raios gama e de desistividade em poços da Bacia de Campos

The knowledge of the mechanical properties of the rocks is essential in the calculation of the stability of the oil wells and in the elaboration of drilling projects, because they help in the selection of the best equipments for each geologic scene. In these activities, the most important mechanical property is the rock compressibility, which can be calculated from the sonic geophysical log, but this one, not always available in the well data set. In order to minimize this limitation, it was developed, in this work, a methodology to simulate it through an algorithm that utilizes fuzzy logic concepts, using as input data gamma ray and resistivity logs. The basic principle of this methodology is to propose that any geophysical log, including the sonic log, can be considered function of other measured geophysical logs in the same depths. On the other hand, to test the confidence of this approach, it was compared with two others commonly used in the simulation of logs: the linear multiple regression and the neural network back-propagation, showing, our methodology, however, better results. Finally, to validate the method, it was tested using wells from Namorado Oil Field in Campos Basin, which contains gamma ray, sonic and resistivity logs.

Monte Carlo simulation of nuclear logging detection systems

The utmost challenge in nuclear logs interpretations and spectroscopy comes from the complex and dynamic structure of the radiation detectors response function. To interpret accurately such logs, the energy spectra for several dimensions of nuclear logging detectors must be satisfactorily known. In this work, different incident photon track and energies owing to events occurring into the gamma ray detector are simulated by the Monte Carlo method. The life of a particle within a NaI(Tl) scintillator crystal is computed by simulating the position, direction and energy of electrons and gamma-ray photons interaction by interaction. Four types of photon interactions are computed, namely, photoelectric absorption, pair production, and Rayleigh and Compton scattering. The specific energy loss due to ionization and excitation for electron are also computed. These pulse high spectra are determined by collecting the radiation and transforming it into current pulses. The spectral distribution of these pulses results in a matrix of detector normalized response functions for multiple and complicated source geometry linked with all gamma ray incidence normally required on borehole environment. These data are displayed in such a way that they can be readily carried out into all nuclear log modeling processes with relevant detection effects.