Predicting irreducible water saturation of unconventional reservoirs by using NMR T2 spectra: Methods of morphological division and fractal models

Abstract

AbstractIrreducible water saturation restricts an efficient petroleum drainage in unconventional reservoirs. T2 cutoff value identification by using NMR T2 spectrum is the key to clarifying irreducible water saturation. In this paper, saturation and centrifugal T2 spectra of sandstone and coal samples were carried out to calculate T2 cutoff value, with methods of single fractal dimension, multi‐fractal dimension, and spectrum morphological discrimination; In addition, the applicability of these three methods in characterizing T2 cutoff was discussed. The parameters related to T2 cutoff can be divided into two types: (1) The first type is morphological parameters including main peak position ™ and smaller pore volume percentage (SPVP); with an increase of T2 cutoff, TM increases linearly and SPVP decreases exponentially, and the correlation between SPVP and T2 cutoff is stronger than that of TM; (2) Second type is fractal parameters of D2 (larger pore), D−10‐ D10 and D−10/D10; with the increase of T2 cutoff, all single and multi‐fractal dimensions increase linearly, and correlation between D2 and T2 cutoff is stronger than that of multi‐fractal dimension. When calculating the T2cutoff of samples with developed macro‐pores, spectrum morphological methods should be preferentially used, while the fractal dimension discrimination methods need to be used for T2cutoff of samples with developed micro‐pores.