Technical Paper: Field Test Results of the New Combinable Magnetic Resonance Autotune Logging Tool

Society: SPE
Paper Number: 96759
Presentation Date: 2005
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A new combinable magnetic resonance tool that automatically tunes and corrects the operating frequency for changes in the static magnetic field, Bo, has been developed. These changes can occur when metallic debris adheres to the tool's permanent magnet. The Bo field directly affects the Larmor frequency, which is required for the tool to generate correct porosity values. Traditionally, the Larmor frequency is determined during a Larmor frequency search task (LFST) prior to logging. The procedure during logging is to monitor changes in Bo (Delta Bo), and to perform a new LFST whenever Delta Bo exceeds a given threshold.

In many situations, such as deviated or deep-water wells, removal of metallic debris by ditch magnets is ineffective. Excessive debris causes the Bo field to drift, resulting in frequent tuning stops, which can add considerable rig time to the logging job. Tuning might even be impossible when heavy metallic debris prevents the tool from being tuned, resulting in non-productive rig time or total loss of nuclear magnetic resonance (NMR) data acquisition. Other difficult NMR tuning conditions include the following:

  • Low measured signal amplitude when formation porosity is less than 5 p.u.
  • High bottomhole temperatures (above 135° C), where tuning stops expose the tool to long periods of high temperature.
  • Conductive muds, such as when the mud resistivity Rm is less than 0.05 omega–m. In these cases, the high-gain-corrected noise amplitudes are similar to the LFST signal amplitudes.

Intuitive as it may seem, we demonstrate that the detuning information is contained in the echo itself. Theoretically, the shape of the echo is distorted if the tool is not operated at the Larmor frequency. The distortion can be quantified by measuring the phase difference between the two halves of the echo. Deviation from the Larmor frequency can then be monitored continuously and a feedback loop automatically adjusts the operating frequency. For optimal radio frequency (RF) transmission and echo reception, a second feedback loop maintains the resonance condition of the antenna. An additional feature of the new tool is the capability to acquire up to 16,000 echoes with wait time as long as 26 sec, thereby increasing the application range of complex fluid evaluation modes using diffusion editing (DE) pulse sequences.

Extensive testing has taken place in a wide range of environments in North America, Africa, the Middle East, and Asia. Examples presented in this paper illustrate rig-time savings, data acquisition in environments where acquisition was previously deemed impossible, and improved data quality because of the tool's ability to maintain a proper tuning. In addition, automatic tuning has simplified the new tool's data acquisition and log quality-control (LQC) procedures for field operations. New operating guidelines are included in the paper.