Improved Petrophysical Methods and Techniques for Shaly Sands Evaluation

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractClean sands have null or minimal shale and clay, and can adequately be evaluated using petrophysical techniques like neutron-density cross-plot and Archie's equation. Routine core analysis can be enough for validating log-analysis results in clean sands. On the other hand, shaly sands have considerable shale and clay; consequently the above petrophysical evaluation techniques are not adequate. Porosity logs, e.g. neutron, density and sonic, and saturation logs, i.e. deepresistivity, are affected by the shale and clay present in shaly sands. Additionally, shale and clay affect the electrical properties, the capillary pressure and the transmissibility in shaly sands. The effects of shale and clay, on logs and loganalysis, need to be modeled and incorporated in the petrophysical evaluation in order to obtain a more accurate assessment of shaly sands.Soft and hard data was acquired in some shaly sands, in Saudi Arabia, to evaluate their storage and flow capacity. Density, neutron, sonic, resistivity and gamma-ray logs were routinely acquired in wells drilled in these formations. Logs like Elemental-Capture-Spectroscopy (ECS) andNuclear-Magnetic-Resonance (NMR) were acquired in key wells. In some key wells, the core was cut and core analysis was conducted to obtain formation porosity, electrical properties and clay attributes, e.g. clay-type and clay-abundance. The above data was exploited in improving shaly sands petrophysical evaluation, which is discussed and demonstrated in this paper.

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“…Where 𝜌 𝑏 is bulk density (i.e the input density log), 𝜌 𝑚𝑎 is density of rock matrix and 𝜌 𝑓 is fluid density given the values 2.67g/cm 3 and 0.8 g/cm 3 respectively [16]. The reservoir effective porosity was then obtained by applying shale correction [26];…”

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“…For a shaly sand model, the relationship between total porosity and effective porosity can be represented as: ………………………. (6) Figure 9-The porosity model for a shaly sand reservoir, after [20].…”