RECOMMENDED LOGGING PROGRAM
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.

 

Crain's Recommended Logging 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 The following list will cover all near and long term technical needs for most situations.

In normal fresh or invert mud systems, use:
  1. Array Induction 2 ft bed resolution, with SP, GR
      Use Array Laterolog in salt mud or resistivity > 600 ohm-m
  2. Array Sonic  2 ft bed resolution, with SP, GR, Caliper
      Use Dipole Shear Sonic for unconventional reservoirs or stressed regimes
  3. Density Neutron PE combination, with SP, GR, Caliper, Density Correction
  4. ADD Spectral GR in unconventional, frontier, or radioactive sands
  5. ADD Nuclear Magnetic Resonance  in unconventional wells.

As needed:
  6. Resistivity Image for stratigraphic, structural data, fractures
  7. Other specialty tools

Not all “Quad Combo” tool sets are equal;, some omit the sonic log, some “forget” to turn on the PE curve. Carefully specify what you want  and don’t accept less just because thw quote “looks good”. If what you want is not specifically on the quote, it won’t magically get run at the wellsite. YOU must be proactive and CHECK

Other needed logs:
  After cement job, run ultrasonic cement / pipe integrity log.
  After stimulation, run tracer log.

In cased hole with limited or no open hole logs,
run compensated neutron, and pulsed neutron (RST) for porosity and water saturation, with gamma ray for shale calculations and correlation. Modern sonic logs can be run in casing if there is a good cement job. Cased hole resistivity and density are available in some areas.

These recommendations provide sufficient redundancy to provide fail-safe log analysis except in the case of massive and numerous tool failures, or extremely bad (large) hole condition. A reasonable estimate of shale content, porosity, water saturation, rock velocity, rock density, lithology, and zone productivity can be made from this data.

 Log quality control is an important feature of log analysis, both in the field when the log is run, and in the office when the log is used. All quality checks are relative - the major concern is to determine in the field that the log can ultimately be used for all its intended purposes without misleading the user.

Few logs are perfect. Deviations from perfection should be noted, and corrected if possible, but many errors or faults will not invalidate the data presented. Some errors or faults do invalidate the log, so these should be re-run immediately without the error (usually a new or different logging tool is needed). If any fault can obviously be remedied by computer manipulation this should be done, since the computer time is cheaper than the rig time (and it is also done at the service company's expense). If the redundancy built into the suggested logging program is lost due to a tool failure, then steps should be taken to verify that the logs you do have can provide at least the minimum data required from the logs.

These requirements are usually met on a routine basis by most logging companies. If you want better results, or more redundancy for safety, then you or your representative will have to go to the well site and monitor quality and performance of the logging operation. 

Logging Tool Bed Resolution
The description of logging tool resolution can be broken down into three components - horizontal resolution (depth of investigation), vertical resolution (bed definition), and tool accuracy (possible error in the actual measurement).

The bed resolution is the bed thickness needed for the tool to read the true formation value, unaffected by the adjacent or surrounding beds. The depth of investigation is the depth of rock penetrated by the measuring system, which returns about 90% or more of the measured signal. This is defined in the table for the optimum case; that is, the borehole, fluid and rock properties are correct for the tool described.

  Logging Tool Accuracy
Tool accuracy defines the possible error in the measured value. This may vary as the measured value varies and is a function of tool design. Additional errors caused by the borehole environment are not included in this accuracy figure. Table 2 shows tool specifications and measurement accuracy for the Integrated Porosity Log (IPL), a modern version of the density neutron log. If the size of the possible error is of interest to you, you will need to obtain this data for each tool from each service company. This sample will give you an idea of the range of accuracy available for a typical modern tool of today. Older tools are worse.

Sample Tool Specs and Accuracy, from a service company catalog

 Logging Tool Borehole Requirements
Your choice of logging tools dictates the borehole environment required , and vice versa; the borehole environment dictates which tools can be run. The table below suggests some of these limitations.