Open Hole Well Logging
Authors: Timur Aytekin 
Published: December 2007

ABSTRACT
Borehole measurements of geophysical parameters involve downhole sensors, means for transmitting data to the surface, and recording and processing systems. As demand for accurate formation evaluation increases, not only the number of well logging sensors but also the amount of data acquired by each has been increasing at an accelerating rate. To meet this challenge of data explosion, logging tools have been equipped with downhole computers for digital transmission of data to the surface, where other computers are used for recording and real time processing.
Open-hole evaluations of porosity, hydrocarbon saturations, and permeability are conducted through a broad spectrum of borehole geophysical measurements, including electromagnetic and elastic wave propagation, and nuclear radiation. Modern suites of electric logs such as focused resistivity and conductivity, and mi-croresistivity allow accurate determination of resistivities of the invaded zone and of the formation for borehole environments varying from fresh-water to saltwater muds and high- to low-resistivity formations. Measurements of the properties of electromagnetic wave propagation yield the dielectric constant of the formation, which in turn allows determination of hydrocarbon saturation independent of the water salinity.
Digital recording of the acoustic full waveforms has facilitated the extraction of both shear and com-pressional wave data. Compressional and shear wave velocities are used for porosity and lithology interpretation as well as identification of gas zones; and their amplitudes for fracture evaluation. With the recent development of image enhancement technology, borehole televiewers are finding greater use in evaluation of fractured reservoirs.
In radioactivity logging, natural gamma-ray spectroscopy measures the contributions of the uranium series, the thorium series, and potassium nuclei. These provide information regarding clay type and cation exchange capacity (CEC), and fracture evaluation. Borehole compensated density logs and the dual-detector thermal and epithermal neutron logs provide information on porosity, lithology, and gas saturation. The recent density-lithology log, in addition to electron density, also measures photoelectric absorption cross-section, which provides information on rock mineralogy.
Pulsed neutron capture logs provide hydrocarbon saturations in both open and cased holes. Inelastic and capture spectroscopy provide various elemental determinations, including C/O and Si/Ca logs. These are especially useful in determining hydrocarbon saturations in reservoirs containing waters with unknown or very low salinities.
Another highly useful measurement in open holes is the nuclear magnetic resonance (NML) which yields free-fluid index and permeability information in sand-shale sequences and porosity in carbonates.
Downhole and wellsite computers have made possible the combining of many of these measurements into a single tool, which, in one run, provides a complete suite of logs for formation evaluation. Wellsite computers provide quick-look processing of these log data for immediate completion decisions. They also allow transmission via satellites or phone lines to central computer facilities, for more complete analyses.
Core analysis provides significant input into open-hole evaluation. Petrophysical parameters such as porosity, permeability, formation factor, and resistivity index are determined by measurements on core samples under simulated subsurface conditions. Rock samples are studied by scanning electron microscopy (SEM), X-ray analysis, gamma-ray spectroscopy, neutron activation analysis, and cation exchange capacity (CEC) measurements. The repeat formation tester provides essential data on reservoir pressures and fluid samples for laboratory analyses.
None of the above techniques alone is sufficient to provide the basis for costly decisions. They can most effectively be made through an integrated evaluation of well log, rock and fluid analyses, and reservoir data.