A majority of the whole core samples recovered in the US today come from shale (mudstone) reservoirs. A primary reason for so much shale coring is that shale petrophysical models require rigorous core calibration to provide reliable data for reservoir quality, hydrocarbon-in-place, and hydraulic fracturing potential. However, the uncertainty in interpreting shale well log data is sometimes matched or exceeded by the uncertainty observed in traditional methods of analyzing core samples. Most commercial core analysis methods in use today were developed decades ago for sandstones and carbonates exceeding 1 millidarcy in permeability. High quality organic-rich shale on the other hand is usually lower than 0.001 millidarcy. This extreme low permeability creates substantial challenges for existing methods and has contributed to the rapid rise of a new approach to reservoir evaluation called Digital Rock Physics or DRP.
DRP merges three key technologies that have evolved rapidly over the last decade. One is a high-resolution diagnostic imaging method that permits detailed examination of the internal structure of rock samples over a wide range of scales. The second is advanced numerical methods for simulating complex physical phenomenon and the third is high speed, massively parallel computation using powerful graphical processing units (GPU’s) that were originally developed for computer gaming and animation.
Based on pore-scale images from a wide range of organic shales, it can be seen that organic material is present in a variety of forms. Three primary forms, non-porous, spongy, and pendular are commonly observed. Non-porous organic components fill all of the available non-mineral space leaving virtually no porosity or fluid flow path. Porous or "spongy" organic material is commonly encountered in thermally mature gas shales. Pendular organic material appears to fill the small inter-granular and grain contact regions, leaving open pore space in the larger voids. These pore types are largely controlled by kerogen type and thermal maturity, and they exert large influence on the porosity, permeability, and overall shale reservoir quality.