ESTIMATING MOLECULAR SIZE DISTRIBUTIONS IN FORMATION FLUID SAMPLES USING A DOWNHOLE NMR FLUID ANALYZER

摘要

A method for estimating a property of subsurface material includes extracting a sample of the material using a downhole formation tester and performing a plurality of nuclear magnetic resonance (NMR) measurements on a sensitive volume in the sample where each measurement in the plurality is performed in a static homogeneous magnetic field with a pulsed magnetic field gradient that is different in magnitude from other NMR measurements to provide a waveform signal. The method further includes transforming each received waveform signal from a time domain into a frequency domain and comparing the frequency domain signal to a reference to provide proton chemical-shift information related to a chemical property of one or more molecules in the sample and transforming the frequency domain signals into a complex number domain that quantifies waveform signal amplitude changes to provide one or more diffusion rates with each diffusion rate being associated with a corresponding frequency.

主权项

1. A method for estimating a property of subsurface material, the method comprising:
conveying a carrier through a borehole penetrating the subsurface material; extracting a sample of the subsurface material using a formation tester disposed at the carrier; performing a plurality of nuclear magnetic resonance (NMR) measurements on a sensitive volume in the sample using an NMR instrument, each measurement in the plurality of NMR measurements providing an electromagnetic waveform signal received with an antenna, each NMR measurement in the plurality of NMR measurements being performed (a) in a static homogeneous magnetic field, (b) with a sequence of pulses of radio-frequency (RF) electromagnetic energy transmitted from a transmitter antenna, and (c) with a series of pulsed magnetic field gradients being applied to the sample, the series being superimposed over the static homogeneous magnetic field and having a magnitude that is different from the magnitude in other NMR measurements; transforming each received waveform signal from a time domain into a frequency domain to provide a frequency domain signal that is a function of frequency and comparing the frequency domain signal to a reference to provide proton chemical-shift information related to a chemical property of one or more molecules in the sample; transforming the frequency domain signals having the same frequency as a function of the magnitude of the pulsed field gradient into a complex number domain that quantifies waveform signal amplitude changes to provide one or more diffusion rates with each diffusion rate being associated with a corresponding frequency; and estimating the property using the proton chemical-shift information and the one or more diffusion rates.