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.
Petrophysical Parameters Estimated From Visual Descriptions of Carbonate Rocks: A Field Classification of Carbonate Pore Space
The permeability, capillary properties, and m values of carbonate rocks are related to the particle size, amount of interparticle porosity, amount of separate vug porosity, and the presence or absence of touching vugs. Particle size, percent separate vug porosity, and the presence or absence of touching vugs usually can be determined visually. The amount of interparticle porosity is more difficult to determine visually and is done best by subtracting the visual estimate of separate vug porosity from the measure of total porosity obtained from wireline porosity logs or laboratory measurements. In the absence of touching vugs, the permeability, m values, and capillary properties can be estimated if the particle size, percent separate vug porosity, and total porosity are known. No acceptable method has been developed to estimate visually the permeability of touching vugs. A classification of carbonate porosity is proposed based on the data presented. This classification is intended to be used in the field or for routine laboratory description. Interparticle porosity is classified according to particle size and the dense or porous appearance of the interparticle area. Vuggy porosity is classified according to type of interconnection. Separate vugs are connected through the interparticle pore space and classified by percentmore » porosity. Touching vugs are connected to each other and classified by presence or absence.« less