Fluid Characterization using Nuclear Magnetic Resonance Logging

This paper presents a variety of field applications that illustrate recent advances in nuclear magnetic resonance (NMR) logging fluid characterization methods. The main concepts of NMR logging and the principles underlying NMR diffusion measurements, which form the basis for all standalone NMR fluid characterization methods, are briefly reviewed. Field examples of the MRF Magnetic Resonance Fluid characterization method are presented to demonstrate the separation of oil and water signals, saturation measurements, and oil-viscosity determination. The MRF method is also applied in conjunction with new diffusion NMR techniques to infer wettability states in a suite of partially saturated core samples. The application of two-dimensional NMR maps of relaxation times and molecular-diffusion rates to identify fluids and determine their properties in complex multi-fluid environments, including oil-base mud (OBM) filtrate, light oil, and gas is illustrated with field examples from the deepwater Gulf of Mexico and the North Sea. Manuscript received by the Editor January 8, 2004; revised manuscript received February 27, 2004. Schlumberger Oilfield Services, Sugar Land, TX USA Mark of Schlumberger ©2004 Society of Petrophysicists and Well Log Analysts. All rights reserved. mations surrounding the borehole. The hydrogen nuclei contained in the oil, gas, and brine filling the rock pore spaces behave like microscopic magnets. The magnetic moments of the hydrogen nuclei align along the direction of the applied magnetic field thus creating a net magnetization or polarization in the formation. The time required to align the hydrogen nuclei along the direction of the applied magnetic field, referred to as the longitudinal direction, is characterized by a longitudinal relaxation time denoted by T1. In practice, a distribution of T1s is required to describe the magnetization process. The distributions reflect the complex composition of crude oils and the distribution of pore sizes in sedimentary rocks. For bulk crude oils, the logarithmic mean of the T1 distribution is inversely proportional to the viscosity and can vary from a few milliseconds or less for heavy oils to several seconds for low-viscosity oils. In reservoir rocks, T1’s of non-wetting phase fluids are equal to their bulk fluid values. For the wetting phase, T1 of the fluid can be shortened by interactions of the fluid molecules with the pore surfaces. For water-saturated rocks, the surface interaction is usually dominant and provides a mechanism for estimating pore-size distributions from T1 distributions (Allen et al., 1997). The time that the hydrogen nuclei are exposed to the static magnetic field is referred to as the polarization time or wait time. Prior to the wait time, the hydrogen nuclei are randomly oriented and there is zero net magnetization in the formation. During the wait time, the nuclear magnetization grows exponentially towards its equilibrium value (Mo). The porosity and the types and volumes of fluids determine Mo. A wait time equal to three times the longest T1 produces a magnetization equal to 95% of Mo. If too short a wait time is used, NMR total porosities will underestimate true formation porosities. Long wait times, and therefore, reduced logging speeds are required in formations containing lowviscosity oil or gas (i.e., fluids with long T1’s). Following the polarization time, a train of radiofrequency (RF) pulses is applied. Between the RF pulses, the NMR signal is recorded using the same antenna used to transmit the pulses. The NMR signal observed between each pair of consecutive pulses is often called an echo because it builds up to a maximum at the midpoint between the pulses and then decays before the next pulse. In a typical NMR measurement several thousand echoes are acquired over a period of about a second. The echo amplitudes are proportional to the net magnetization in the transverse plane (transverse to the static magnetic field), which decays during the course of the measurement. It is the rate of decay of transverse magnetization that provides useful information concerning the fluids and their environment.