A method to determine nuclear magnetic resonanceT₂cutoff value of tight sandstone reservoir based on multifractal analysis

Abstract

Nuclear magnetic resonance (NMR) T2 cutoff value is an important parameter for pore structure evaluation. It is complicated and uneconomical to obtain T2 cutoff value by an experimental method; therefore, it is necessary to explore a prediction method of T2 cutoff value. In this paper, 10 samples of tight gas reservoirs in the eastern Ordos Basin were selected, and then saturation and centrifugal experiments of nuclear magnetic resonance were carried out. On this basis, multifractal theory was introduced to calculate the multifractal characteristics of the NMR T2 spectrum of each sample, and the relationship between multifractal parameters and T2 value was analyzed. The influencing factors of the T2 cutoff value were clarified, and the prediction model of the T2 cutoff value was constructed accordingly. The results show that the T2 spectra of sandstones in the study area can be divided into three types: single steeple peak, double steeple peak, and irregular double peak. The pore diameter of the three types is 1 nm ~ 3×104 nm, 1 nm ~ 104 nm and 1 nm ~ 4×103 nm, respectively. The T2 cutoff value ranges from 9.72 to 35.16 ms. The correlation analysis suggests that the symmetrical fractal dimension difference and symmetrical multifractal dimension ratio (Dmin−Dmax, Dmin/Dmax) shows a positive linear correlation with the T2 cutoff value. The value of T2 cutoff gradually decreases with the increase of the flow zone indicator (FZI). Therefore, three parameters, including symmetrical fractal dimension difference, symmetrical multifractal number ratio, and FZI are optimized, and the prediction model for the NMR T2 cutoff value of sandstone samples in the study area is proposed. The introduction of porosity‐related parameters compensates for the shortcomings of previous T2 cutoff value prediction models. At the same time, the prediction model is proven to be accurate and reliable by testing the measured data of the samples near the study area. The results of this paper can be used for further study of the NMR T2 cutoff value prediction of tight sandstone reservoirs in different areas.

Comparative Evaluation of Pore Structure Heterogeneity in Low-Permeability Tight Sandstones Using Different Fractal Models Based on NMR Technology: A Case Study of Benxi Formation in the Central Ordos Basin

“…It is a large multicycle cratonic basin with overall subsidence, depression migration, and simple structure, including six first-order structural units, such as Jinxi fold belt, west margin thrust belt, Yishan slope, Tianhuan depression, Yimeng uplift, and Weibei uplift. 29,46,47 In this study area, Upper Paleozoic is the main production layer of tight sandstone gas, including Benxi, Taiyuan, Shanxi, and Shihezi Formation. To compare pore structures of tight sandstone reservoirs at different depths, 22 samples of Benxi Formation collected from different wells are analyzed (Figure 1b,c).…”

“…Compared to other techniques, − nuclear magnetic resonance (NMR) has the advantage of rapidly and accurately characterizing a pore-facture system without damaging it; − it can be used to quantitatively analyze the full-scale pore size of the tight sandstone reservoir. In addition, water migration in the pore-fracture system is analyzed by centrifugal and saturation methods for one sample type; then, the fluid parameters such as irreducible, movable water saturation and T 2 cutoff values are obtained. − It has been widely adopted by many researchers to study the morphology, size, and porosity of the pore-fracture system. ,,,− …”

Fractal Characteristics and Model Applicability for Pores in Tight Gas Sandstone Reservoirs: A Case Study of the Upper Paleozoic in Ordos Basin

“…Therefore, the reservoir can be considered a typical tight sandstone reservoir with low porosity and ultra-low permeability. 51 Given the strong influence of diagenesis, the pore structure and throat type of the upper Paleozoic reservoir in the study area shows a complex feature. SEM and casting thin section observations showed that the pores are mainly intergranular pores and micropores, with a small amount of intergranular dissolved pores.…”

Advances in low-field nuclear magnetic resonance (NMR) technologies applied for characterization of pore space inside rocks: a critical review

“…As for characterization of saturation, permeability, and wettability, the determination of the T 2cutoff is on the top agenda. However, it is tough to gain a precise value as errors produced in signal collection and inversion of T 2 spectrum are inevitable, and determination of T 2cutoff strongly depends on experimenters' experience (Hu et al 2019).…”