Carbonates contain several different types of porosity and often have complex pore size distributions. It is important to understand reservoir producibility from a rock-textural perspective at the pore scales, which range from less than 0.5 microns to over 5 microns. Determining the correct lithology (limestone, dolomite, or a combination of minerals) is a key step in carbonate reservoir evaluation.
Different measurements are needed to build a picture of the petrophysical properties of a carbonate formation. The standard resistivity and porosity measurements of a triple-combo logging suite (e.g., Platform Express wireline logging tool or EcoScope logging while drilling) are often not sufficient to resolve changes in pore size and texture, so additional measurements are required. Measurements using nuclear magnetic resonance (NMR) provide information on porosity types and pore size distributions, and can also provide permeability estimates. Formation testing using the MDT modular formation dynamics tester can be used to calibrate permeability estimates made from log data. Acoustic measurements, as acquired using the Sonic Scanner acoustic scanning platform, help to identify fractures and vugs that obscure porosity and permeability measurements.
Elemental thermal neutron capture spectroscopy measurements are essential to understand complex lithologies, identify dispersed anhydrites, and determine mineralogical concentrations in the carbonate rocks. The ECS elemental capture spectroscopy and the EcoScope tools offer this type of measurement.