Fundamentals of Fluid Flow in Porous Media

Chapter 2

Porosity:

Well Logging

Well logging, also known as borehole logging is the practice of making a detailed record (a well log) of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface (geological logs) or on physical measurements made by instruments lowered into the wellbore (geophysical logs). Well logging is done during all phases of a well’s development; drilling, completing, producing and abandoning. Logging measurements are quite sophisticated. The prime target is the measurement of various geophysical properties of the subsurface rock formations. Of particular interest is porosity. Logging tools provide measurements that allow for the mathematical interpretation of porosity. There are different types of well logging that used to estimate the porosity of the formation around the well, such as:

CNL (compensated neutron) logs: also called neutron logs, determine porosity by assuming that the reservoir pore spaces are filled with either water or oil and then measuring the amount of hydrogen atoms (neutrons) in the pores. These logs underestimate the porosity of rocks that contain gas.

FDC (formation density compensated) logs: also called density logs, is a porosity log that measures electron density of a formation and determine porosity by evaluating the density of the rocks. Because these logs overestimate the porosity of rocks that contain gas they result in “crossover” of the log curves when paired with Neutron logs.

NMR (nuclear magnetic resonance) logs: may be the well logs of the future. These logs measure the magnetic response of fluids present in the pore spaces of the reservoir rocks. In so doing, these logs measure porosity and permeability, as well as the types of fluids present in the pore spaces.