The purpose of this paper is to compile and explain calibration and quality control practices for Measurement While Drilling (MUD) formation evaluation tools for current vendors. MWD logs are usually run in high-operating-cost, geologically complex, and hostile environments. They are commonly justified as "insurance logs" in case the operator can’t get a wireline log. On occasion, these insurance logs become "the only log". Uhen the MUD log exhibits either the desired or expected response, it is believed. Conversely, when an unexpected response is recorded, the MUD log is treated skeptically. Uhen the operator does not believe a log, the data may be suspected to be inaccurate or uncalibrated.
Regardless of the type of measuring device, wireline or MUD, calibrations must be understood before a log is trusted. Standards for reference for MUD logs are not always understood by operating oil company personnel. Over the past 50 years, log analysts have studied and become comfortable with the standards to which wireline tools are referenced. The understanding of calibration practices for MUD tools by oil company operators has proven to be a major hurdle for MUD service companies. Until MUD tool calibrations are understood and accepted by oil company petrophysicists, MUD tools will continue to be followed out of the hole by wireline tools. These redundant logs result in additional cost, risk, and delay in hydrocarbon production.
This paper is the product of several preparation steps. In the first phase of preparation, MWD service companies were asked to supply very general and simplified explanations of how they calibrated or verified their tools. In the second step, more detailed explanations were supplied. Then, several Ad Hoc MUD Calibration Committee meetings were held over the course of a year to develop concise explanations of calibration practices in the MUD industry.
Using a common format and vocabulary, this paper explains MUD calibration practices for gamma ray, resistivity, density, neutron porosity, and caliper tools that are now, or soon to be, commercially available. The data compiled in this paper identify the extent and frequency of calibrations for all major MUD tools. Several case histories are reviewed to illustrate some of the more common quality control concerns. MUD and corresponding wireline logs are shown that raise questions or concerns about invasion and washouts, depth control, and high rates of penetration.
Technical information on tool calibration/verification has been supplied by service companies and has not been independently verified by the authors.