Fractal Analysis of Pore Network in Tight Gas Sandstones Using NMR Method: A Case Study from the Ordos Basin, China

Abstract

To characterize the pore structure and quantify fractal dimensions of tight gas sandstones, a case study is performed on the Lower-Middle Permian tight sandstones in the Ordos Basin in China by conducting a series of experiments including X-ray diffraction (XRD) analysis, routine petrophysical measurements, thin section and scanning electronic microscope (SEM) observations, and nuclear magnetic resonance (NMR) experiment. The studied tight sandstones mainly consist of quartz and clay minerals, and pore types include primary intergranular pores, inter-and intragranular dissolution pores, as well as micropores associated with clay aggregates; T 2 spectra reflect three types of pore size distributions in the studied samples, indicating a rather irregular pore distribution pattern in tight sandstones; NMR can estimate porosity of tight sandstones accurately, and movable-fluid porosity from NMR can better reflect the permeability of tight sandstones than total porosity. Two fractal dimensions, D bnd (with respect to bound-fluid pores) and D mov (with respect to movable-fluid pores), are calculated to be 1.1135−1.8116 (average 1.4750) and 2.6816−2.9932 (average 2.8921), respectively. D bnd increases with the decrease of detrital quartz content and the increase of clay mineral content, whereas D mov increases with the increase in authigenic quartz content and the decrease of detrital quartz content; fractal dimensions can reflect the physical properties of tight sandstones, as large D bnd and D mov values typically result in low movable-fluid porosity and permeability; the pore network of tight sandstones can be considered as a dual-scale pore system based on fractal theory, whereas D bnd and D mov can reveal the roughness of bound-fluid pore surface and the distribution of movable-fluid pores, respectively. This study shows that NMR fractal dimension can be employed as an effective indicator to characterize the pore network of tight sandstones.

Fractal Characterization of Tight Oil Reservoir Pore Structure Using Nuclear Magnetic Resonance and Mercury Intrusion Porosimetry

“…17 With the development of unconventional oil and gas, NMR is gradually applied to the study of the pore characteristics of unconventional reservoirs. [18][19][20] Yao et al applied the NMR to study the pore characteristics of coals and the strong correlation between transverse relaxation time T 2 and pore structures of coals were found; they used the T 2cutoff point to divide NMR T 2 spectrum and pore distribution models were established combining with MIP. 21 They also compared MIP, constant-rate-controlled mercury porosimetry, micro-CT and NMR for analyzing pore size distribution and found that NMR can better evaluate pore size distribution of coal under nondestructive conditions.…”

Multi-Scale Quantitative Characterization of Pore Distribution Networks in Tight Sandstone by integrating FE-SEM, HPMI, and NMR with the Constrained Least Squares Algorithm

“…The fractal dimensions have been widely employed to quantitatively characterize the complexity and heterogeneity of tight sandstone reservoirs [50]. Based on recent studies [27,44,51], the number of pores with sizes greater than r o (N r ) can be obtained as: where r is the pore radius (µm); r max is the maximum pore radius (µm); B is the fractal factor; f(r) is the density function of the pore radii and D is the fractal dimension of the pore space.…”

A novel productivity evaluation approach based on the morphological analysis and fuzzy mathematics: insights from the tight sandstone gas reservoir in the Ordos Basin, China

“…Tight gas reservoirs are widely present in China; there are more than ten basins including Ordos, Sichuan, Shanxi, Songliao, and Tarim with extremely rich resource potential (Wu et al 2017a, b). However, it is also accompanied by the complex geological conditions, poor reservoir physical properties and strong reservoir heterogeneity (Shao et al 2017). Hydraulic fracturing must be implemented to obtain commercial gas production.…”