Applications of Nuclear Magnetism Logging to Formation Evaluation

Abstract

Summary The nuclear magnetism log (NML) indicates the portion of fluids in porous rock that are free to flow. Thus, estimation of permeability has been a major area of NML application. The log has other uses, however. In this paper, principles are reviewed and examples of three nonpermeability applications are shown. The NML can be used (1) to measure residual oil after the near-wellbore region investigated by the log is flushed by treated mud filtrate, (2) to distinguish producible water from heavy oil where water salinities are too low for confident resistivity log interpretation, and (3) to reduce ambiguity resulting from shaliness in identification of gas- bearing zones by use of the density/neutron log combination. Accuracy and confidence in NML interpretation can be enhanced by reprocessing the data that are digitally recorded when the log is run. This reprocessing is particularly important to distinguish low-level signals from noise or to interpret accurately signals that decay rapidly. The basis for the reprocessing is reviewed, and resulting signal displays are shown for each of the applications discussed. Introduction The concept and basic design principles for nuclear magnetism logging were published first in 1960. During the 1960's, two competitive NML tools were introduced to commercial service. Both were applied successfully to some of the formation evaluation problems discussed in this paper, but neither was developed sufficiently for sustained routine use. Both use and maintenance of the tools declined in the later 1960's and early 1970's. In the early 1970's, Schlumberger Well Services began efforts that led to the recent introduction of a second-generation NML. This tool is more reliable and performs much better than earlier tools. Digital recording of signals provides data that represent fully the nuclear magnetism signal at every measurement depth. At typical logging speeds [750 ft/hr (230 m/h)], signals are recorded digitally every 0.5 ft (0. 152 m). The primary measurement from the NML is the free fluid index (FFI), presented in porosity units. For most reservoir conditions, the FFI represents the part of the pore-filling fluids that contribute to permeability of porous rocks. Because the FFI and the time required for signal buildup or decay have been shown to indicate the permeability of porous sandstones, this application has received the widest attention. However, the log has additional value in formation evaluation. In this paper we review the fundamentals and show examples of three other NML applications. Each application reflects the concept that log response differs with different pore-filling fluids. Interpretation is based on assumed similarities or differences in the components of recorded signals associated with different fluids and fluid environments. Confidence in log interpretation is enhanced by processing and displaying the digitally recorded data that represent the nuclear magnetism signals. The basis for processing the 1,024 members that represent detected signals is reviewed in the Appendix. Display of signals in different forms contributes to confidence in good signals and diagnosis or enhancement of marginal signals. The only display form discussed here shows the logarithm of the amplitude of detected signal vs. time after the Polarizing pulse. JPT P. 2853^