Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination

Abstract

ABSTRACT The paper describes the first spin echo Magnetic Resonance Imaging Logging (MRIL*) system. The system makes measurements while logging and does not require special borehole treatment. The tool employs static and radio-frequency magnetic fields to perform downhole the spin-echo magnetic resonance measurements with short â??dead timeâ?¿ that previously have been possible only in the laboratory. Logs of lithology independent porosity, irreducible water saturation and fluid diffusion coefficient are provided from real-time analysis of the spin-spin relaxation time (T2) decay of the fluids contained within the pore space of the formation. Spin-lattice relaxation time (T1) behavior is directly measured by the MRIL system. Engineering prototypes have undergone several field tests, including two tests at the Conoco Borehole Test Facility well in Newkirk, Oklahoma. Field data, including the demonstration of accuracy and repeatability are discussed. The magnetic resonance logs correlate well with measurements of porosity and irreducible water saturation made on core samples and with porosity data from standard density and neutron logging tools. Such data, from magnetic resonance logs, can be applied to the determination of porosity and irreducible water saturation, and thus to the estimation of permeability. INTRODUCTION Nuclear Magnetic Resonance (NMR) is an important spectroscopic tool gaining broad usage as a laboratory technique in the oil industry to determine core and fluid properties. The Magnetic Resonance Imaging Log (MRIL) offers in situ measurements of these same properties in a moving, non destructive downhole logging system that does not require special borehole preparation. This results, for the first time, in a measurement of lithology independent porosity and irreducible water saturation, and multiple relaxation components of spin-spin relaxation time (T2) decay of the fluids contained in the formation. Also for the first time, downhole logs of fluid diffusion coefficients have been made with the MRIL system and preliminary results show consistent agreement with laboratory fluid diffusion coefficients. This paper describes the underlying NMR theories on which the MRIL system was developed, the calibration procedure, and how the NMR measurements made by the system are related to the petrophysical conditions encountered in the various rock formations downhole.