Wireline NMR
well logging measurements suffer from disadvantages of poor vertical resolution,
logging speeds less than 20 ft / min, and
power consumption in excess of 200 W. In
spite of these disadvantages, NMR
well logging is used because it is capable of providing estimates for a number of petrophysical parameters that are difficult to obtain from other
wireline data. These include estimates of the bulk volume irreducible (BVI) of fluids in the formation. The present invention targets BVI and clay
bound water (CBW) measurements.
Logging speeds of up to 60 ft / min are attainable with little or no loss of resolution. In one preferred embodiment, the tool has four sensors circumferentially distributed around the
logging tool and in contact with the borehole wall. A horseshoe like
magnet is used to generate the static
magnetic field. The
magnet poles are designed such that the
magnetic field is uniform perpendicular tool motion, as well as provide a sufficiently large extent of the
static field to provide polarization for
bound water in rock formations. The RF portion of the sensor is comprised of at least one coil configured for transmission of an RF
magnetic field into rock formations and at least two coils configured to separately receive the NMR
signal from the formation. In another embodiment a coil is wound around the pole pieces or the iron yoke for the purpose of field shifting to enable acquisition of phase-alternated measurements.