Fractal Analysis of Tight Sandstone Petrophysical Properties in Unconventional Oil Reservoirs with NMR and Rate-Controlled Porosimetry

Abstract

Characterization of the pore size distribution of tight sandstone is of great significance for the effective development of tight oil resources. This paper analyzed the fractal characteristics of pore spaces of tight sandstones from the Yanchang Formation in the Ordos Basin of China with NMR and rate-controlled porosimetry (RCP). A new NMR fractal analysis method that considers the movable fluid distribution was proposed. Compared with the conventional NMR fractal analysis metrics, the fractal dimensions of the pore spaces occupied by completely movable fluid and partially movable fluid have stronger correlations with the tight sandstone petrophysical properties. This work shows that the fractal dimensions of pore spaces with partially movable and completely movable fluids are negatively correlated with the permeability and reservoir quality index (RQI), and as the fractal dimensions approach 3, the tight sandstone petrophysical properties become less favorable. The fractal dimensions of the pore spaces detected by RCP were calculated with three different fractal models, the thermodynamic model, the 3D capillary model, and the wetting phase model. The research results show that compared with the results of the other two fractal models, the fractal dimensions obtained from the 3D capillary model can be used to more accurately evaluate tight sandstone petrophysical properties, and the calculated fractal dimensions are strongly negatively correlated to the pore radius, sorting coefficient, skewness, and permeability. The fractal dimensions obtained from the thermodynamic model have no obvious relationship with the tight sandstone properties, and the wetting phase model gives opposite fractal analysis results, so these two fractal models are not recommended for fractal analysis of tight sandstone pore size distributions via RCP. Finally, the models for predicting tight sandstone permeability with NMR and RCP were evaluated and screened.

Oil utilization degree at various pore sizes via different displacement methods

“…The pores in a tight reservoir can be classified according to the transverse relaxation time (T 2 ), Peng et al (2018) reported that the pores could be divided into micro-pore (T 2 ≤ 30 ms), mesopore (30-90 ms), macropore (90-200 ms) and super macropore (T 2 > 200 ms). According to the fluid movement in the pores, the pores can be classified into completely unmovable pores, partially movable pores, and completely movable pores (Wang et al 2021). Four characteristic points in the NMR T 2 spectrum were selected, and the pores were divided into nanopores, micro-pores, meso-pores and macropores (Dai et al 2019).…”

“…The pores in a tight reservoir exhibit self-similarity. Compared with classical European geometry, the fractal dimension can describe the heterogeneity and complexity of pores (Krohn and Thompson 1986;Krohn 1988;Jin et al 2017a, b;Guo et al 2019;Wang et al 2019aWang et al , b, 2020Wang et al , 2021Wu et al 2020Wu et al , 2022Yang et al 2021). Pores with fractal dimensions below 2 and above 3 have a weak influence on the pore structure (Guo et al 2019).…”

“…Wang et al (2019b) reported that when the fractal dimension exceeded 2.586, a tight reservoir did not favor oil accumulation. If the fractal dimension was approximately 3, the porosity and permeability of the tight reservoir would decrease significantly (Wang et al 2021). The fractal dimensions of submicro-pores and nanopores commonly reflect the complexity of the pores, and the fractal dimensions of the microscale macropores feature significantly influence the permeability of tight reservoirs (Hu et al 2020).…”

“…The fractal dimensions of submicro-pores and nanopores commonly reflect the complexity of the pores, and the fractal dimensions of the microscale macropores feature significantly influence the permeability of tight reservoirs (Hu et al 2020). The fractal dimension of partially movable pores and completely movable pores contributes to enhancement of the oil displacement (Wu et al 2020;Wang et al 2021).…”

Mathematical modeling of gravity and buoyancy effect on low interfacial tension spontaneous imbibition in tight oil reservoirs

“…The capillary tube model effectively describes the spontaneous imbibition process and upscales the pore‐scale flow behavior to the core‐scale flow behavior based on the pore size distribution 11,32 . The fractal characteristics of the pore size distribution in tight sandstones have been demonstrated with mercury intrusion porosimetry, nuclear magnetic resonance, 33 and N 2 adsorption 34 . In fact, combined with the capillary tube model, the fractal theory has been widely applied to represent the pore size distribution and investigate the mechanism of spontaneous imbibition 35,36 …”