Ricardo Leiderman

Interrogation methods for fiber Bragg grating sensors

Fiber Bragg Gratings (FBG) have been applied to a wide range of systems both in telecommunication and sensors systems. For sensors the main advantages in using FBG are the ease of multiplexing and the reliability of measurements due to spectral encoding. Although many different ways of analyzing the spectral response of a FBG have been proposed and demonstrated, most of them involve sophisticated electronics and do not take into account practical problems. We describe a simple, robust and low cost technique in which a reference grating is used as a spectral filter for the sensing grating. In our prototype a 1550 nm pig-tailed LED is used as broad band source. As the system is proposed for strain measurements, special attention is paid to field calibration and long term measurements. Two calibration methods are demonstrated. One uses the digitalized reflection spectra of sensor and filter grating and numerically predicts the sensor output. The other method is based on the application of a controlled opposite strain on the filter grating. Both methods were compared with conventional resistance strain gage and the measuring accuracy is estimated. We also point out applications where FBG strain sensor can have significant advantages over conventional electric counterparts.

Estimating the surface relaxivity as a function of pore size from NMR T2 distributions and micro-tomographic images

Abstract In the present work, we formulate and solve an inverse problem to recover the surface relaxivity as a function of pore size. The input data for our technique are the T 2 distribution measurement and the micro-tomographic image of the rock sample under investigation. We simulate the NMR relaxation signal for a given surface relaxivity function using the random walk method and rank different surface relaxivity functions according to the correlation of the resulting simulated T 2 distributions with the measured T 2 distribution. The optimization is performed using genetic algorithms and determines the surface relaxivity function whose corresponding simulated T 2 distribution best matches the measured T 2 distribution. In the proposed methodology, pore size is associated with a number of collisions in the random walk simulations. We illustrate the application of the proposed method by performing inversions from synthetic and laboratory input data and compare the obtained results with those obtained using the uniform relaxivity assumption.

PERSONAL COMPUTER-BASED DIGITAL PETROPHYSICS

ABSTRACT. In the present work, we describe our experience with digital petrophysics, enhancing our choices for performing the related tasks. The focus is on the use of ordinary personal computers. To our best knowledge, some of the information and hints we give cannot be found in the literature and we hope they may be useful to researchers that intend to work on the development of this new emerging technology. We have used micro-scale X-ray computed tomography to image the rock samples and, in that sense, we address here the issue of the corresponding image acquisition and reconstruction parameters adjustment. In addition, we discuss the imaging resolution selection and illustrate the issue of the representative volume choice with the aid of two examples. The examples corroborate the notion that it is much more challenging to define a representative volume for carbonate samples than for sandstone samples. We also discuss the image segmentation and describe in details the Finite Element computational implementation we developed to perform the numerical simulations for estimating the effective Young modulus from segmented microstructural images. We indicate the respective computational costs and show that our implementation is able to handle comfortably images of 300×300×300 voxels. We use a commercially available Finite Volume software to estimate the effective absolute directional permeability. Keywords : rock physics, micro-scale X-ray computed tomography, multi-scale homogenization, effective elastic moduli, representative volume. RESUMO. No presente trabalho descrevemos nossa experiencia com Petrofisica Digital, dando enfase as nossas escolhas para a realizacao das tarefas relacionadas. O foco e no uso de computadores pessoais e, salvo melhor juizo, algumas das informacoes e dados que apresentamos nao podem ser achados na literatura. Nos adquirimos as imagens digitais de amostras de rochas com o auxilio de microtomografia computadorizada por raio-X e, nesse sentido, discutimos aqui o ajuste dos parâmetros de aquisicao e reconstrucao de imagens. Alem disso, nos discutimos a questao da selecao do volume representativo e sua relacao com o tamanho e resolucao da imagem digital, mostrando dois exemplos ilustrativos. Os exemplos corroboram a nocao de que e muito mais dificil definir um volume representativo tratavel para carbonatos do que para arenitos. Nos tambem discutimos a segmentacao de imagens no contexto da Petrofisica Digital e descrevemos em detalhes o codigo de Elementos Finitos por nos desenvolvido para estimar o modulo de Young efetivo de amostras de rochas a partir de suas imagens microtomograficas, indicando o respectivo custo computacional. Nos mostramos que nossas escolhas levaram a uma implementacao computacional capaz de lidar confortavelmente com imagens de ate 300×300×300 voxels. Por fim, descrevemos o uso do pacote comercial de Volumes Finitos para estimar a permeabilidade absoluta efetiva das amostras de rocha. Palavras-chave : fisica de rochas, microtomografia computadorizada por raio-X, homogeneizacao multiescala, modulo de Young efetivo, volume representativo.