Borehole measurement of NMR characteristics of earth formations and interpretations thereof

摘要

Borehole NMR logging apparatus and methods, and methods for the interpretation thereof. A logging tool is provided which produces a strong, static and homogeneous magnetic field B0 in a Volume of an adjacent formation on one side of the tool to measure nuclear magnetic resonance characteristics thereof. In the preferred embodiment, the tool has an RF antenna mounted on the outside of the metal body of the tool, directing focussed oscillating magnetic fields B1 at said Volume to polarize or tip the magnetic moments of hydrogen nuclei of fluids within rock pores. The same antenna can be used to receive signals of proton precession in the Volume of interest immediately after transmission of the RF polarizing field B1. Extremely rapid damping of the antenna between the transmitting and receiving modes of operation is accomplished by a Q-switch disclosed herein. The invention provides for the direct measurement of NMR decay having transverse relaxation time T2 behavior, and further provides for the fast repetition of pulsed measurements from within a borehole. An additional magnet array may be mounted offset from the first magnet configuration to prepolarize a formation before it is measured in order to pre-align a larger number of protons than the single magnet configuration could do by itself. Additional features of the invention are disclosed which increase the Signal/Noise ratio of the measured data, and improve the quality and quantity of borehole NMR measurements, per unit of time spent. Disclosed interpretation methods determine fluid flow permeability and longitudinal relaxation time T1 -type parameters by directly comparing the measured decay signals (such as T2 or T2* type decay) to a representation which responds to both the decay time tdec and the imposed polarization period prior to such decay, tpol. The parameters of amplitude and T1 are determined and combined with certain preferred methods to generate robust values of formation characteristics such as fluid flow permeability. Other related methods are disclosed.