NUCLEAR MAGNETIC RESONANCE MEASUREMENTS IN WELL LOGGING USING AN OPTIMIZED REPHASING PULSE SEQUENCE

摘要

A pulsed NMR tool has a magnet arrangement that is used to generate a stat ic magnetic field having a substantially uniform field strength in a region of the formation surrounding the borehole. An RF coil is used to produce pulsed RF fields substantially orthogonal to the static field in the region of examination. T he nuclear spins in the formation align themselves along the externally applied static magnetic field. A pulsed RF field is applied to tip the spins into the transverse plane, resulting in a precession of the spins. The tipping pulse is followed by a series of refocusing pulses and the resulting series of pulse echoes is detected. The timing and duration of RF pulses are altered from conventional CPMG to maximize signal and minimize RF power consumption. An additional forced recovery pulse at the end of an echo train may be used to speed up the acquisition and/or provide a signal for cancelli ng the ringing artefact.