Publication History: This article was written especialy for "Crain's Petrophysical Handbook" by E. R. Crain, P.Eng. in 2004. Updated 2016, 2018. This webpage version is the copyrighted intellectual property of the author.

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The logging program should be carefully designed to gather all the data required today, and in the future as well. The program needs to be written, preferably in a checklist format, describing the logs to be run, the curve complement on each log, the scales for each curve, and the interval to be covered. The program forms part of the well drilling and completion program and is used by the logging engineer to guide hin as to what is required. The wellsite representative (geologist, drilling engineer, or petrophysicist) monitors the performance of the logging operation based on this program.

In the past, too few logs were run, even after the advent of many of the modern logging and analysis techniques. Thus re-evaluation of by-passed reserves, definition of geological prospects, and seismic modeling suffer today for the false economy of the past. The best procedure to follow is to run a comprehensive suite of logs. Clearly there are economic constraints, such as in shallow gas or coal bed methane projects, where a full suite of logs is not helpful. You may be required to carefully tailor the log suite to suit the circumstances. Here are my recommendations.

It has become common to just ask for a
 "triple-combo" or "quuad-conbo" and hope for the best, without thinking too much about which specific log curves or tool types will appear in the final delivery. For example, which of several resistivity or sonic log varieties do you want for this job? Is shear sonic needed for the seismic team? Are PEF and spectral GR included? Answers to these questions are vital  in unconventional reservoirs.

Here is what the generic terms mean
      Triple-combo = resistivity + density + neutron tools.
      Quad-combo = resistivity + sonic + density + neutron tools.
The gamma ray log is always included but doesn’t add to the tool count.

A standard triple-combo is resistivity curves (with GR or SP or both) plus  density – neutron curves (with GR, caliper, density correction, and PEF. Density + neutron counts as 2 of the 3 tools. Array induction or high-res array laterlog are preferred for resistivity.

A quad combo adds a sonic log to the above tool string. This includes only compressional sonic. You may want to add dipole shear sonic and spectral ganna ray. 

Some combos can have spectral GR, which adds potassium, thorium, and uranium curves.


In normal (fresh) mud systems, use:
1. array induction SP, compressional-wave sonic, and
    density neutron PEF GR Caliper (quad-combo)

The SP, gamma-ray, caliper, borehole volume integration, and sonic integration curves should be included on the induction and sonic log presentations, and the caliper, gamma-ray, density correction, cable tension, and photo electric capture cross section (PE) should be included on the density-neutron. The newest version of each tool should be used, especially in overseas, offshore, or frontier locations.

Many people eliminate the sonic log and run a triple-combo, but this means you will never be able to accurately tie seismic data or seismic inversions to this well.

Display both density porosity and neutron porosity on sandstone scale in sand-shale sequences, and limestone scale in carbonate sequences. Use dolomite scale sparingly and only in pure dolomite sequences.

Optional extras in fresh mud include:
2. dipole shear sonic if horizontal wells or frac jobs are planned
3. microlog for thin bed definition - can be run with density log in certain
4. resistivity or acoustic image for stratigraphic and structural data, and in fractured
      or unconventional reservoirs
5. nuclear magnetic resonance log for shale free connected porosity (especially in unconventional shale gas
     and shale oil plays)
6. elemental capture spectroscopy log for detailed mineral analysis

In salt mud, or high resistivity formation with fresh mud, replace or augment the induction with array or high resolution laterolog.

In fractured reservoirs, add the dipole shear sonic log with crossed dipoles and the resistivity image log to define fracture orientation and stress regime. Post-processing for fracture aperture and fracture porosity is recommended.

In radioactive reservoirs, add the gamma ray spectral log to distinguish uranium, potassium, and thorium.

In very shaly sands, or in carbonates with micritic or highly varying pore geometry, run a nuclear magnetic resonance log to determine irreducible water saturation.

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