Quantitative Discrimination of Effective Porosity Using Digital Image Analysis - Implications for Porosity-Permeability Transforms
Five relevant digital image analysis parameters for fluid flow are obtained from 2-D image analysis of carbonate rocks: 2-D image porosity, amount of pores, pore shape, total perimeter per area and dominant pore size range. Micro-porosity leads to low permeable, highly porous rocks. 2-D image porosity represents the macro-porosity because the resolution subtracts the small pores from the porosity. Compared to the total porosity, it improves the prediction of permeability by an order of magnitude in the high porosity range. Pore shape factor γ reduces the uncertainty in permeability prediction to 2 orders of magnitude. Pore shape factor analysis is restricted to samples with more than 4000 pores/cm. CT scans of plugs reveals that the pore shape factor γ is a relatively constant measure of the pore shape (+/0.2) throughout the sample plug. The larger the average pore size, the higher the velocity at a given porosity. The average pore size can be quantified by the upper limit of the dominant pore size range.
ESTIMATIVA DE PARÂMETROS HIDRÁULICOS DO AQUÍFERO BARREIRAS-RN A PARTIR DE ANÁLISE COMPUTACIONAL DE IMAGENS DE LÂMINAS DELGADAS ESTIMATING HYDRAULIC PARAMETERS OF BARREIRAS AQUIFER-RN FROM DIGITAL IMAGES ANALYSIS OF THIN SECTIONS
The attainment of hydraulic parameters derive primarily from the interpretation of aquifer tests, practice that usually involves a fairly complex logistics in terms of equipment and staff required. Thus, the digital processing and analysis of two-dimensional micrographs of aquifer rocks samples is presented as an alternative tool for the characterization of hydraulic parameters, and constitutes a promising procedure accordingly. The methodology is based on a flowchart involving (1) sampling and preparation of thin sections and their respective images, (2) seg- mentation and geometric characterization, (3) three-dimensional reconstruction and flow simulation. Taking as a target for the present study The Barreiras Aquifer System, in the eastern coast of Rio Grande of Norte state (north- eastern Brazil).The computational image analysis of thin sections of its rocks showed reservoir storage coefficients, associated in this context with effective porosities (free hydraulic character), ranging from 0.035 to 0.12, with aver- ages of 0.076; its hydrogeological carbonate substrate showed effective porosities on the order of 2 %. Relative to the transport of fluids, it was found that the method presented results smaller than those found in bibliographic data (related to hydraulic conductivity), probably due to the size of the scale under study and the heterogeneity of the investigated medium.
Evolution of porosity and seismic structure of upper oceanic crust: Importance of aspect ratios
Seismic properties of the uppermost igneous crust of the oceans are dominated by porosity effects, that is, the size, concentration, and shape of void spaces. Porosity is initially determined by the physics of extrusion (does an eruption form breccia, pillows, or massive flows?) but is very rapidly modified by alteration and hydrothermal deposition. Laboratory data provide insight into compressional wave velocity-porosity behavior of basalts at a hand sample scale, while well logs provide data at outcrop scale. Relating observations at all scales to porosity structure and extrapolating to seismic scale requires application of rock physics theory. Using information from ophiolites and deep ocean cores, we have defined rock physics parameters for two simple models of upper oceanic crust. The models approximate different levels of void filling by alteration products by differing in the amount of crack (low aspect ratio) porosity they contain. From the models we compute theoretical compressional wave velocity and porosity profiles. Calculated profiles agree well with both well logs and seismic data and illustrate that the increase in seismic velocities measured seismically in the upper crust need not be accompanied by large changes in total porosity.