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Core-Log Integrated Approach: A Solution to the Petrophysical Characterisation of the Eagle Ford Shale


Petrophysical analysis of shale reservoirs aims the evaluation of properties like TOC, total porosity, fluid-saturation and min-eral compositions. Estimation of fluid saturation can be misleading using conventional methodology due to differences among various commercial labs in evaluating hydrocarbon filled porosity and saturations. Additional errors are related with the presumption of Archie's cementation and saturation exponent i.e. ‘m’ and ‘n’, since obtaining these parameters from fluid-impervious matrices is difficult on core samples. Core derived empirical equations are more consistent for determination of basic parameters like TOC, porosity and Water saturation. However, Petrophysical characterisation of shale plays requires volumetric estimation of minerals as shale being anisotropic, consists of ductile and brittle clay minerals which determine the fracability of the formation. Increase in radioactivity in the organic rich shale is due to increased uranium concentrations which vary within a fixed volume of organic matter as uranium adsorption depends largely on the type, amount, degree of maturity; and pH/Eh conditions of the organic matter. Therefore, using spectral gamma-ray log as a tool to evaluate TOC and clay volume can be erroneous. Estimating kerogen volume and assigning input parameter in the stochastic model involves uncertainty as no definite petrophysical measurement of kerogen is available for respective shale plays. To overcome these limitations, simple petrophysical techniques are used, built on nearly 800 rotary side wall cores retrieved in 36 core-lab wells of Eagle ford Shale, South Texas. Apparent in-situ values of Archie's exponent of bound water volumes i.e. ‘mn’ have been derived by combining the in-situ measurement of resistivity and bulk volume of water (BVW) from core measurements. Mineral composition has been precisely computed and calibrated with XRD measurements, using basic open hole wireline logs along with three external input curves, volume of kerogen, pyrite and quartz which were derived through regression analysis of core data. This paper focuses on the maximum utilization of core data along with the open hole logs to determine the shale's petrophysical characteristics and thus to build a locally validated petrophysical model that can be applied to future wells with basic wireline data and no core data.