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Comparison of Reservoir Quality from La Luna, Gacheta, and Eagle Ford Shale Formations


A large fraction of all whole core samples recovered today come from shale (mudstone) reservoirs. The primary reason is that 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/exceeded by the uncertainty observed in traditional methods of analyzing core samples. High-quality organic-rich shales usually have permeability lower than 0.001 mD. This 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 (DRP). - DRP merges 3 key technologies: 1)High-resolution diagnostic imaging method that permits detailed examination of the internal structure of rock samples over a wide range of scales. 2)Advanced numerical methods for simulating complex physical phenomenon. 3)High speed, massive parallel computation using powerful graphical processing units (GPU's) originally developed for computer graphics and animation. - This presentation describes an integrated DRP process for analyzing rock properties of shale reservoirs at multiple scales. The process begins with whole core samples, progresses to smaller plug size samples, then ultimately to very high-res 3D imaging of the pore space. This imaging, combined with unique and proprietary fluid flow algorithms, allows us to compute shale reservoir properties and provide clear 3D renderings of the pore structure. - Core samples were available from 4 wells in the Eagle Ford formation of SW Texas. Results from these wells are compared to La Luna and Gacheta formation cores from the Litoteca Nacional Bernardo Taborda, Colombia. Results show that in many important characteristics (porosity, TOC, permeability) the samples from Colombia are comparable to or better than those from the Eagle Ford. - Summary: a multi-scale DRP process was applied to the analysis of shale reservoir properties in the La Luna and Gatcheta formations. Key results: Porosity and permeability are comparable to the Eagle Ford formation; Permeability in horizontal plane is 10 to 100 times greater than in the vertical direction; Porosity associated with organic matter varies from 100% of total to less than 10% of total depending on specific formation/depth; The transformation ratio for kerogen to porosity ranges from about 20 to 40%.