Conventional petrophysical models, e.g. using neutron-density cross-plot for lithology and porosity and Archie's equation for water-saturation, can be adequate for evaluating clean sands, which have null or minimal shale and clay. Moreover, routine core analysis can be enough for validating log-analysis results in clean sands. On the other hand, shaly-sand formations have shale and clay present in them and the above standard petrophysical techniques are not adequate for the accurate evaluation of shaly-sand formations. Porosity logs, e.g. neutron, density and sonic, and saturation logs, e.g. deep-resistivity, are affected by clay presence in shaly sands. Such effects need to be modeled and incorporated in the log analysis in order to obtain more accurate formation evaluation of shaly sands.
Some of the hydrocarbon-bearing formations in Saudi Arabia have shaly sands. Soft and hard data was acquired to help evaluate and describe the storage and flow capacities of these formations for optimal production developments. Density, neutron, sonic, resistivity and gamma-ray logs were routinely acquired in wells drilled in these formations. Logs like Elemental-Capture-Spectroscopy (ECS) and Nuclear-Magnetic-Resonance (NMR) were acquired in key wells. In these shaly formations, some wells were cored and core analyses were conducted to obtain formation porosity, electrical properties and clay attributes like clay-type and clay-abundance. The above soft and hard data acquired has helped in developing and applying more reliable petrophysical models and techniques in shaly sands formation evaluation.
This paper will present and discuss methods and modeling techniques for evaluating shaly sands. Applications of these techniques in some shaly formations in Saudi Arabia are included in this paper.