Shaly sands have numerous challenges that hinder the accuracy of their petrophysical evaluation. In shaly sands, shale, clay and silt have peculiar effects on their core and log measurements. Accurate estimates of such effects are required for accurate evaluation of shaly sands and their hydrocarbon contents. In clean sands, shale, clay and silt are null or minimal. Henceforth, conventional logs, e.g., density, neutron and formation resistivity are sufficient for accurate description of clean sands. For accurate description of shaly sands, conventional logs are necessary but not sufficient and advanced logs, e.g., Elemental-Capture-Spectroscopy (ECS and Nuclear-Magnetic-Resonance (NMR are additionally needed. Standard petrophysical models, e.g., neutron-density cross-plot and Archie's equation, are adequate for accurate evaluation of clean sands from logs. Sophisticated petrophysical models, which can quantify shale, clay and silt as well as their effects on logs, are required for obtaining accurate shaly sands evaluation from logs. In clean-sand core, accurate measures of porosity, permeability and electrical properties are obtainable and can be sufficient for evaluating clean sands from core. In shaly-sand core, special measurements and procedures are required for obtaining effective-porosity, total-porosity, electrical properties clay-abundance and clay-type, which are necessary for integral evaluation of shaly sands.
Clay has affinity to bind water, i.e., clay-bound-water. The clay has influential effects on porosity and resistivity measurements. Novel petrophysical methods have been developed for quantifying clay and its effects on porosity and resistivity measurements. Furthermore, accurate calculation of effective porosity and water saturation have been enabled and tested in applications.
Some of the shaly sands in Saudi Arabia have soft and hard data, which helped in characterizing and evaluating them for optimal production developments. Density, neutron, sonic, resistivity and gamma-ray logs were routinely acquired through wells drilled in these sands. Elemental-capture-spectroscopy (ECS and Nuclear-Magnetic-Resonance (NMR logs were acquired in key wells. Some wells were cored and core analyses have been conducted to obtain porosity, permeability, electrical properties, clay-type and clay-abundance across these sands. The above soft and hard data acquired were exploited in testing the petrophysical methods developed. Comparison to core and well-testing data, revealed that the developed methods are highly accurate and within international accuracy standards.
This paper will present and discuss the developed petrophysical methods and will demonstrate their applications in some shaly sands in Saudi Arabia.