Formation porosity can be calculated from one-dimensional nuclear magnetic resonance (NMR) logging data with a fixed cutoff value and the volume model of conventional logging data. However, the porosity calculated with this method often appears to be overestimated in the enlarged hole section. In this work, the calculation methods for T2 and T1 nuclear magnetic porosity from NMR results of core samples and two-dimensional NMR logging data were investigated. Firstly, the characteristics of NMR relaxation spectra of fluids at different pore sizes were obtained and used to analyze the distribution range of pore diameter and affecting factors of the cutoff value of the drilling fluid peak. A model for estimating the cutoff value of the drilling fluid relaxation time was then established. A method for calculating the effective porosity was then proposed with variable cutoff value of clay bond water relaxation time, according to the deterministic principle about cutoff value of clay bond water relaxation time in un-enlarged and enlarged hole section. The method proposed was applied to the data from many wells, and demonstrated better accuracy than the traditional method using T1 distribution spectra. The method can effectively solve the problem of overestimated porosity in the enlarged hole section and meet the requirement for reservoir evaluation.
HOU Ke-jun
,
WU Jian-meng
,
GE Xiang
,
ZHANG Shi-mao
. Calculating Porosity From Two-Dimensional NMR Relaxation Spectra of the Leikoupo Group's 4th Section[J]. Chinese Journal of Magnetic Resonance, 2020
, 37(2)
: 162
-171
.
DOI: 10.11938/cjmr20192708
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