Отправлено: 18.12.14 13:37. Заголовок: Determination of nuclear magnetic resonance T2 cutoff value based on multifractal theory

Determination of nuclear magnetic resonance T2 cutoff value based on multifractal theory — An application in sandstone with complex pore structure
Xinmin Ge1, Yiren Fan1, Xuejuan Zhu2, Yiguo Chen3, and Runze Li4

ABSTRACT
The cutoff value of nuclear magnetic resonance (NMR)
transversal relaxation time T2 is vital for pore structure characterization,
permeability prediction, and irreducible water saturation
calculation. Conventional default values often lead to
inaccurate results for rocks with complex pore structure. Based
on NMR experiments and multifractal theory, we have developed
an effective statistical method to predict T2 cutoff values
without other petrophysical information. The method is based
on multifractal theory to analyze the NMR T2 spectrum with
the assumption that the T2 spectrum is an indicator of pore size
distribution. Multifractal parameters, such as multifractal dimension,
singularity strength, and mass exponent, are calculated
to investigate the multifractal behavior of T2 spectrum via NMR
experiments and a dyadic scaling-down algorithm. To obtain the
optimal T2 cutoff value, the rotation speed and time of centrifugation
are enlarged increasingly to optimal centrifugal state. A
predicating model for T2 cutoff value based on multiple linear
regressions of multifractal parameters was proposed after studying
the influential factors. On the basis of the multifractal analysis
of NMR T2 spectrum, a reasonable predication model for T2
cutoff value was rendered. Upon testing, the predicted results
were highly consistent with the experimental results.

Geophysics 2015; 80 (1)