Formation evaluation with wireline logs, for the purpose of estimating hydrocarbon pore volume (HPV), requires methods to accurately determine net-pay thickness, porosity, and hydrocarbon saturation. Traditional petrophysical algorithms, such as Archie’ s equation, need estimates of true formation resistivity (Rt) to calculate saturation and HPV, but many factors interfere with the ability to determine Rt. Resistivity modeling allows for accurate estimation of Rt from measured log data.
This chapter introduces the concepts of resistivity modeling for formation evaluation. The history of resistivity logs and modeling is reviewed briefly, and some fundamental concepts and terms, such as dimensionality of earth models and forward and inverse log modeling, are defined. The rationale for why and when resistivity modeling should be applied is introduced by a discussion of tool resolution and accuracy.