Abstract

As an important nuclear fuel, uranium in sandstone uranium deposits is mainly extracted by in situ leaching. The porosity of sandstone is one of the important indexes determining in situ leaching efficiency. Moreover, the microscopic pore size distribution (PSD) of the uranium-bearing layer has an important effect on porosity. It is necessary to feature the pore structure by various techniques because of the different pore types and sizes in the uranium layer. In this paper, combined with nitrogen gas adsorption, nuclear magnetic resonance techniques and scanning electron microscopy, the full-scale PSD features of uranium-bearing sandstone in the northwest of Xinjiang are effectively characterized. The results show that pores structure of uranium-bearing sandstone include dissolution pores (d ≤ 50 nm), intergranular pores (50 nm < d ≤ 200 µm) and microfractures. Intergranular pores of 60 nm and 1 µm are the significant contributors to pore volume. The effects of the pore volume of two pore types (dissolution pores and intergranular pores) on the porosity of uranium-bearing sandstone are analysed. The results show that intergranular pores have the greater influence on the porosity and are positively correlated to the porosity. Dissolution pores have little effect on the porosity, but it is one of the key factors for improving uranium recovery. Moreover, the greater the difference of PSD between sandstones, the stronger the interlayer heterogeneity of uranium-bearing sandstone. This kind of interlayer heterogeneity leads to the change of permeability in the horizontal direction of strata. It provides a basis for a reasonable setting of well type and well spacing parameters.

Highlights

  • Uranium is one of the main energy sources in the world [1,2]

  • The porosity was obtained by nuclear magnetic resonance (NMR) and the mineral composition was obtained by X-ray diffraction (XRD)

  • It can be seen from table 1 that the uranium-bearing sandstone specimens were dissolution pore intergranular pore 50 mm (c) microfractures intergranular pore 50 mm dissolution pore porosity by depth NMR

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Summary

Introduction

Uranium is one of the main energy sources in the world [1,2]. Uranium minerals are rich in sandstone uranium deposits [3]. Methods for quantitative analysis include mercury injection capillary pressure (MICP), nitrogen gas adsorption (N2GA), small-angle neutron scattering (SANS) and nuclear magnetic resonance (NMR) They are used to calculate the pore specific surface area, pore volume and PSD. SANS can provide connected pores and isolated pores of rocks, and is widely applied to pore structure research of coal seams, oil reservoirs and shale gas reservoirs [18,19,20] It has high accuracy only in the nano scale pore size range, and cannot obtain a complete PSD [21]. Combined with N2GA, NMR and SEM techniques, the complete PSD and pore type of uranium-bearing sandstone are determined. The study provides an experimental basis for the evaluation of uraniferous sandstone deposit, the effective exploration of uranium resources and the establishment of sandstone uranium deposit model

Geological setting and property of sandstone specimens
Pore structure experiments
Scanning electron microscopy
Nuclear magnetic resonance
Nitrogen gas adsorption
Pore types from SEM
Pore size distributions
Analysis of NMR T2 distributions
Determination of the full-scale PSD by combining N2GA and NMR techniques
Analysis of pore structure characteristic parameters
Analysis of heterogeneous parameter
Relative deviation
Variable coefficient
Conclusion
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