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Permeability of the Eagle Ford Shale: Organic Matter “Cement,” Matrix Storage, Limestone Fractures and the Importance of Choke Management


Permeability measurements were obtained on 24 intact plugs from two wells of different thermal maturity in the Eagle Ford in South Texas. Ten plugs were taken from a low thermal maturity (Ro = 0.62) well, and 14 from a high thermal maturity (Ro = 1.45) well. Thin sections and x-ray diffraction data were obtained for all samples from plug end-trims. The permeability of the marls (defined as having <35% clay and 35–65% carbonate by volume) which is on the order of 1 to 100 nD, was observed to increase with increasing calcite volume in laminations, but no fractures were observed in any of the marl samples. The high permeability (>200 nD) of the limestones (defined as having >65% carbonate by volume) was also seen to increase with increasing calcite volume, reflecting an increasing volume of fractures. Scanning electron microscope (SEM) microscopy of the plugs used for permeability measurement and lower-maturity outcrop samples (Ro = 0.4) shows that all of the intergranular pores in the Eagle Ford, regardless of TOC, mineralogy or facies, contain hydrocarbon. The lowest maturity outcrop samples contain viscous bitumen migrating through pores, while thermally mature samples are filled with solid hydrocarbon, identified by visual kerogen analysis and solvent extraction as both bitumen and porous pyrobitumen. This solid organic matter effectively occludes primary pores like a diagenetic cement. The thermally-mature organic-matter cement is porous, but permeability is not directly related to the total organic carbon content. Most of the fractures in the limestone are the result of coring (do not contain either mineralization or solid hydrocarbons) but nonetheless illustrate the presence of a connected pore system once fractures form during the process of hydraulic fracturing. Fractures that are present in situ (contain mineralization and solid hydrocarbon) would be activated by hydraulic fracturing. The Eagle Ford is therefore a dual-porosity system, with matrix storage feeding a network of progressively larger natural and induced fractures that carry hydrocarbons to the wellbore. The importance of choke management in the Eagle Ford, in which both matrix and natural fractures have stress-dependent properties, including permeability, is illustrated.