Petrographic Image Analysis As A Tool To Quantify Porosity And Cement Distribution
Nejedlik, John J
Degree of Master of Science, 2001
University of Adelaide
Porosity style and distribution for reservoir simulation are obtained by petrography through a combination of techniques including transmitted light microscopy, petrographic image analysis (PIA) and routine core analysis (RCA). The research focuses into the ability of PIA to map and quantify heterogeneity of porosity within a clean quartz arenite from the Hutton Sandstone that is commonly considered homogeneous, with the intention of developing a set of rules for the use of PIA to effectively measure porosity at optimum magnification with an optimum number of fields of view. Porosity descriptions involved analyses of the size, shape and distribution of pore spaces. The data sets produced provide partial coverage, along with full coverage of porosity across thin sections from the Hutton Sandstone. Statistical analysis was then applied to the data with the aim of understanding the spatial correlation by developing statistical models that mimic the pore distribution.
Semi-variograms obtained from statistical analysis imply that porosity shows very little correlation within the Hutton Sandstone at a microscale. This means that a set of rules to model porosity distribution involve determining the range of porosity values in the measured areas, then randomly distributing the range of values within the cross-bed set or flow unit to which those values apply. Spatial displays produced showed that PIA could be used to identify some of the factors that control porosity in sandstones, such as silt layering. Unfortunately, an extremely large number of readings are required for these to be recognised.
Through the use of a random number generator, it was determined that between 30 and 40 readings were required to calculate average porosity within one standard deviation of the true mean porosity. The use of pore casting and mercury injection capillary pressure (MICP), in association with SEM, provided a three-dimensional (3D) visual assessment of the connectivity between the pores and pore throats.
A series of image processing techniques available through PIA led to the recognition that large pores appeared grouped around apparent concretionary centers. Concretionary centers were a combination of quartz, quartz cement and kaolinite. The identification of these concretions suggest that the Hutton Sandstone should not be considered a series of sand-sized grains separating intergranular pores, but rather, should be considered a series of concretions separating interconnected porous zones.