Petrophysical Evaluations

Abstract

Formation evaluation is one of the most important parts of a reservoir study. This process is mainly performed using wire-line logs. These data are usually available from all wells and reservoir intervals. Various formulas and parameters are used for calculating porosity, water saturation and lithology from logs. Reservoir heterogeneities may considerably change these formulas and their parameters. Matrix responses, pore types, permeabilities and other petrophysical properties are different for each unit or rock type. Therefore, applying one formula for all samples yields erroneous results. These results are used for calculating the final amount of oil in place. The results of these calculations change future development plans and investments in the field. After partitioning the reservoir into different rock types or reservoir units, the appropriate parameters are used in the relevant calculations. These include the log responses for various lithologies of different units as well as special petrophysical characters (such as Archie’s cementation factor) for the unit. In deterministic approach, these are different for each step. In other words, unique parameters are used for each formula in different units or rock types. Then, the results of the previous stage are used for the next calculation. In a stochastic approach, different mathematical models are developed for calculating the reservoir properties in each homogeneous unit. Parameters are different for each model. For example, if pore types control the distribution of water saturation in a reservoir, the reservoir is classified into various parts according to its pore types. Then, a separate model is developed for each part and laboratory-derived data for each rock type are combined with wire-line logs for calculating the water saturation of the entire reservoir interval.