Multiple echo train inversion

Abstract

A method for inversion of multiple echo trains with different wait times uses a cutoff times for each of the echo trains for full polarization. Simultaneous inversion is carried out for T2 bins where full polarization exists

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] This invention is related to methods for acquiring nuclear magnetic resonance (NMR) measurements for determination of petrophysical properties of formations and properties of fluids therein. Specifically, the invention deals with simultaneous inversion of multiple echo trains for processing and interpreting Nuclear Magnetic Resonance (NMR) log data that exhibit relaxation and / or polarization. [0003] 2. Description of the Related Art [0004] Nuclear magnetic resonance is used in the oil industry, among others, and particularly in certain oil well logging tools. NMR instruments may be used for determining, among other things, the fractional volume of pore space and the fractional volume of mobile fluid filling the pore space of earth formations. Methods of using NMR measurements for determining the fractional volume of pore space and the fractional volume of mobile fluids are described, for example, in “Spin Echo Magne...

AI Technical Summary

Benefits of technology

[0016] Another embodiment of the present invention is a machine readable medium for use with an apparatus for characterizing an earth formation having a fluid therein. The medium includes instructions that enable a nuclear magnetic resonance (NMR) tool conveyed in a borehole to acquire a plurality of echo trains. The echo trains result from pulsing of the earth formation by an associated pulse sequence that has a wait time. The instructions also enable determination from the plurality of echo trains a value associated with each of a plurality of bins of a T2 distribution. The determination is based at least in part on cutoff times for full polarization associated with each of the plurality of echo trains. The instructions enable the processor to process a fully polarized echo train, a CBW echo train, and a BVI echo train. The instructions further enable performing performing a least squares inversion. The machine readable medium may be a ROM, an EPROM, an EAROMs, a Flash Memory, or an optical disk. The instructions may further enable determination of a characteristic of the formation such as clay bound water, bulk volume irreducible, or porosity.

Problems solved by technology

The reasons for the distortions to occur are mainly due to poor signal-to-noise ratio (SNR) and poor resolution in the time domain of the NMR data.

Rig time is expensive, so that the general objective in wireline logging is to obtain interpretable data within as short a time as possible.

Although proven successful in wells where these contrasts are large, the NMR-based techniques are challenging when the contrasts are small.

This difficulty is exacerbated by the small difference in the diffusivities of water and very light oil.

The method taught therein is difficult to extend to multiple echo trains, and the specific problem of small contrasts is not addressed.

The model itself is limited in scope.

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