Calculation Method of Bound Water Saturation in Unconventional Reservoirs Using Fractal Theory

The irreducible water saturation of reservoirs seriously restricts the efficient drainage of unconventional energy sources. NMR logging can be used to determine parameters such as total porosity, effective porosity, irreducible water saturation, and permeability, which play an important role in oil and gas identification. T2 cut off value identification using the NMR T2 spectrum is the key to clarifying the irreducible water saturation of unconventional reservoirs. In this paper, saturation and centrifugal T2 spectra of sandstone and coal samples are used to study and calculate the T2 cut off value, with methods including single fractal dimension, multi-fractal dimension, and spectrum morphological discrimination; in addition, the applicability of these three methods in characterizing T2 cut off is discussed. According to the morphological difference of the saturated T2 spectrum, relationships between morphological parameters and the T2 cut off of four types of sample are described. The parameters related to T2 cut off can be divided into two types: (1) the first type includes morphological parameters main peak position (TM) and smaller-pore volume percentage (SPVP); with an increase of T2 cut off, TM increases linearly and SPVP decreases exponentially, and the correlation between SPVP and T2 cut off is stronger than that of TM. (2) The other type includes fractal parameters D2 (fractal dimension of larger pore), D−10 – D10, and D−10/D10; with the increase of T2 cut off, single and multi-fractal dimensions all increase linearly, and the correlation between D2 and T2 cut off is stronger than that of the multi-fractal dimension. When calculating the T2 cut off of samples with macro-pores developed, spectrum morphological methods should be used preferentially, while the fractal dimension discrimination methods need be used for the T2 cut off of samples with developed micro-pores. Then, the T2 cut off value prediction and evaluation system are described. The overall results of this work can provide a theoretical basis for the inversion of bound water content in the original formation.