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Petrophysical analysis is a factor in a reservoir study because it provides primary input data for characterization of subsurface formations and evaluation of resources. Common petrophysical input data to a reservoir study include porosity, water saturation, permeability, and mineral or rock volumes. These reservoir variables are typically not directly measured by well-logging tools; instead, they are commonly derived through multiple processes, including acquisition, processing, interpretation, and calibration. As each of these steps involves uncertainty, the resultant petrophysical data will have uncertainty and limitations. Common uncertainties in interpreting the most important petrophysical variables from well logs are discussed in this chapter. Uncertainties from well-log and petrophysical interpretations should be analyzed, quantified, and explicitly communicated to the integration team to be accounted for in reservoir characterization and modeling.

Petrophysical analysis is critical in a reservoir study; it provides a primary source of input data for reservoir characterization and modeling. To characterize subsurface resources and gain knowledge of the reservoir, it is necessary to acquire data. Wire-line logs provide continuous recordings over the rock formations and give geoscientists insights on the rock properties. These measurements supply critical information for both single well formation evaluations and fieldwide subsurface resource evaluations.

Since the first electrical log was recorded in 1927 in a well in a small oil field in France (Schlumberger, 1999), the use of wire-line logging has grown greatly. Today, a dozen or more types of logging data and measurements are typically acquired for formation evaluations.

These commonly include gamma-ray (GR), spontaneous potential, density, neutron, sonic, magnetic resonance, and various resistivity (RT) logs. Well-log data are commonly the main source of information for subsurface evaluation, especially in areas where core and/or seismic data are sparse or unavailable. It is, however, important to understand that logging data have uncertainties and limitations that, if not accounted for, can lead to an incorrect picture of the area around the wellbore and ultimately of the reservoir.

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