Investigation of the Pore Structure of Tight Sandstone Based on Multifractal Analysis from NMR Measurement: A Case from the Lower Permian Taiyuan Formation in the Southern North China Basin

Kaixuan Qu,Shaobin Guo

doi:10.3390/en13164067

Abstract

Understanding the pore structure can help us acquire a deep insight into the fluid transport properties and storage capacity of tight sandstone reservoirs. In this work, a series of methods, including X-ray diffraction (XRD) analysis, casting thin sections, scanning electron microscope (SEM), nuclear magnetic resonance (NMR) experiment and multifractal theory were employed to investigate the pore structure and multifractal characteristics of tight sandstones from the Taiyuan Formation in the southern North China Basin. The relationships between petrophysical properties, pore structure, mineral compositions and NMR multifractal parameters were also discussed. Results show that the tight sandstones are characterized by complex and heterogenous pore structure, with apparent multifractal features. The main pore types include clay-dominated micropores and inter- and intragranular dissolution pores. Multifractal parameters of sandstone samples were acquired by NMR and applied to quantitatively describe the pore heterogeneity in higher and lower probability density regions (with respect to small and large pore-scale pore system, respectively). The multifractal parameter (D−10) of lower probability density areas has better correlation with the petrophysical parameters, which is more suitable for evaluating the reservoir properties of tight sandstone. However, the multifractal parameter (D10) of higher probability density areas is more conducive to characterize the pore structure of tight sandstone. Additionally, the mineral compositions of sandstone have a complex effect on multifractal characteristics of pores in different probability density areas. The D10 increases with the decrease of quartz content and increase in clay mineral content, whereas D−10 decreases with the increase in clay minerals and decrease of authigenic quartz content and feldspar content.

Highlights

Due to their huge geological reserves and wide distributions, tight sandstone gas greatly alleviates the contradiction between the world’s increasing demand on energy and the depletion of conventional resources [1]
This study provides new insights into the microscopic pore structure and heterogeneity of tight sandstone with similar geological conditions
The results shown in the preceding section manifest that several multifractal parameters can be utilized to characterize the pore heterogeneity

Summary

Introduction

Due to their huge geological reserves and wide distributions, tight sandstone gas greatly alleviates the contradiction between the world’s increasing demand on energy and the depletion of conventional resources [1]. Energies 2020, 13, 4067 permeability as the critical parameter for the evaluation of reservoir properties, as it controls gas storage and transport mechanisms, and determines the displacement efficiency and recovery rate of hydrocarbon in the tight sandstone reservoirs [6,7,8,9]. A variety of analytical methods have been geared to qualitatively and quantitatively characterize the pore structure of tight sandstone, such as casting thin sections (CAT), scanning electron microscopy (SEM), mercury injection porosimetry (MIP), gas absorption [10,11], X-ray computed tomography (XCT) [12] and nuclear magnetic resonance (NMR). NMR is able to characterize petrophysical properties, the saturation of different types of pore fluids (movable fluids, bound fluids and hydrocarbons) and pore size distributions (PSD) of the reservoir rocks [15]

Methods

Results

Discussion

Conclusion