Petrographic Image Analysis (PIA) is an emerging technology used to estimate the properties of pore systems that occur worldwide in sandstone, carbonate and conglomerate reservoir rock samples. It provides quantitative evaluations from standard core plugs as well as from material unsuitable for conventional core analysis such as core rubble and drill cuttings. PIA technology also supplements routine core analysis by estimating other parameters like the specific pore size interval that regulates fluid flow. The method described here is based on a simulated capillary pressure curve. It has been used routinely to estimate pore and pore-throat size distributions, porosity, permeability, air-mercury capillary pressure, irreducible water saturation, effective porosity, microporosity, seal capacity, and the minimum saturation of the non-wetting phase at which it becomes mobile throughout the pore system. These and other new relationships being developed and tested can be mapped to help delineate trends in reservoir quality. PIA procedures and relationships are discussed and illustrated with an example application from a large gas reservoir in the Western Canadian Sedimentary Basin.
Rocks in thin section under a microscope display great complexity in pore sizes and shapes. It is difficult to visualize key properties of the pore system, like a capillary pressure curve. We have found that, using an integrated system of electronic devices that includes a scanning electron microscope (SEM) equipped with noise reduction software, an image analyzer, desk top computer and other peripheral devices, we are able to estimate key properties of the pore system. In practice, the SEM systematically acquires high resolution images of a rock pore system represented by the too surface of a polished thin section, while an image analyzer detects, measures, and counts individual pore elements. The pore element size distribution information thus acquired is processed using PC software to simulate petrophysical properties of the pore system, such as an air-mercury capillary pressure curve. Thomeer developed a hyperbolic curve fitting technique for evaluating air-mercury capillary pressure curves produced in the laboratory by a standard, mercury displacement procedure. His method determines values for three parameters used in an equation to estimate permeability (Thomeer). Swanson described an alternative method for estimating permeability from laboratory derived air-mercury capillary pressure curves, which is based on determining the co-ordinates of a unique point on the capillary pressure curve, commonly referred to as the "A" point. Given the usefulness of capillary pressure curves in evaluating oil and gas accumulations, we developed our method incorporating the fundamental relationships identified by Swanson and Thomeer into our own quantitative petrographic image analysis technology. Others are also investigating the use of petrographic image analysis in rock characterization. A significant advantage of our technology, based on capillary pressure curve analysis, is that it is calibrated to rock catalogues, which contain comprehensive core analyses with corresponding thin sections acquired from a wide variety of rock types. These high quality, in-house rock catalogues and a commercially available, international Rock Catalogue provide the underlying framework.