Some Examples of Nuclear Magnetism Logging in Three San Joaquin Valley Oil Fields
Authors: Nikias P.A. , Eyraud L.E. 
Published: December 2007

Abstract
Since Sept., 1961, nuclear magnetism logging has been used extensively by Standard Oil Co. of California for formation evaluation of development wells in the Kern Front, Kern River and Poso Creek fields in the Kern Front, Kern River and Poso Creek fields in the San Joaquin Valley. During this period, approximately 90 runs were made in these fields. Highly resistive formation waters (3 to 20 ohm-m) coupled with shaly sands in these fields make the use of induction and electric logs for quantitative or qualitative formation fluid evaluation very difficult.A special application of the nuclear magnetism log has provided a reliable qualitative solution to this problem where viscous (approximately 600-cp or above) crudes are produced. By correlating sidewall sample and production data with the nuclear magnetism log, an empirical general guide for log interpretation has been derived for the Kern river, Kern Front and Poso Creek fields.Logging examples which demonstrate the use of the nuclear magnetism log to identify wet sands, oil sands and flushed oil sands are illustrated in this paper.

Introduction
Formation evaluation utilizing the conventional induction-electric logging, sidewall sampling and formation testing techniques has not been completely satisfactory in the Poso Creek, Kern Front and Kern River fields. These fields produce heavy-gravity (11 to 14 degrees API) crude from shallow, highly porous and permeable, unconsolidated shaly sands.Differentiation between well-saturated oil sands and flushed oil sands on electric and induction logs is often impossible due to highly resistive formation waters with resistivities ranging from 3 to 20 ohm-m. Sidewall samples have been helpful in resolving this problem; however, unusually high residual oil saturations in water-swept oil sands make identification of these zones difficult. In many instances, running sand conditions preclude formation testing of prospective productive sands. A special application of the nuclear magnetism log in highly viscous crudes (approximately 600-cp or above) provides a reliable qualitative solution to this problem.

Brief Synopsis of Nuclear Magnetism Concepts
The nuclear magnetism log is designed to detect formation fluid by measuring the proton density, or number of hydrogen nuclei per unit volume. To measure this proton density, a sonde containing a coil arrangement is used to impose a polarizing field in the formation with components at right angles to the earth's magnetic field. Hydrogen nuclei, which possess spins and magnetic moments. tend to align with the magnetic field applied by the logging sonde, causing a net polarization, When placed in a magnetic field, the hydrogen nuclei precess in a manner analogous to that of a top spinning on a table under the influence of the earth's gravitational field. When the magnetic field is removed, the hydrogen nuclei undergo a precession while attempting to realign with the earth's magnetic field. Since the hydrogen nuclei possess magnetic moments, the precession generates an alternating voltage in the coil of the logging sonde. The amplitude of this voltage, at the instant precession starts, is proportional to the hydrogen nuclei concentration in the formation and is translated to free fluid index (FFI). Normally, the FFI is a measure of the total free fluid in the formation, since it cannot distinguish whether the hydrogen nuclei are due to the presence of oil or water without further information. However, the FFI does give a qualitative evaluation of free water where reservoir crudes have a viscosity of 600-cp or above, because these crudes do not give an FFI signal.No FFI is recorded because the hydrogen nuclei contained in viscous crudes return to their normal arrangement about the earth's magnetic field too quickly after the polarizing field is released to accommodate the instrumentation delay caused by switching the coils of the logging sonde from a magnetizing condition to a de-energized, or recording, position.Since no signal is received from the viscous oil in the formation, the FFI is derived solely from the formation waters.
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