Nuclear Magnetic Resonance Experimental Study on Pore Structure Effect of Deep Shale Under Liquid Nitrogen Freezing

Abstract

ABSTRACT The freezing of liquid nitrogen (LN2) can affect the rock's structure, which can be used in the extraction of natural gas from deep shale formations. However, its effect on the pore structure of the shale remains unclear. To address this issue, this study subjected three different types of deep shale: Carbonaceous (C) shale, siliceous (Si) shale and calcareous (Ca) shale. The mineral composition was determined by X-ray diffraction (XRD). And the physical properties of deep shale were analyzed by nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM). The evolution of porosity of shale samples under LN2 cooling by different lithology, rock temperature and cyclic cooling were compared to explore the effect of LN2 freezing on the pore structure. The results of this study indicated that LN2 fluid would be a promising source of longitudinal and bedding fractures in carbonaceous shales. And the resulting fractures would increase the permeability of the shale. In addition, repeated freezing also lead to the formation of conductive fractures in the shale. In conclusion, since LN2 has low-temperature properties, has a freeze-damaging effect on reservoir rocks and promotes microfracture development, it is important to inject LN2 as fracturing fluid during the reservoir process to improve the effect of fracturing transformation. INTRODUCTION As an important component of unconventional oil and gas resources, shale gas has played an important role in recent years in meeting the growing global demand for fossil energy (Alfi et al., 2019; Qu, Hu, et al., 2022; Wu et al., 2017). As shale gas reservoirs are characterized by low pore permeability and large burial depth, corresponding production enhancement measures need to be implemented to achieve effective extraction of shale gas. In response to the problems of water scarcity and environmental pollution caused by hydraulic fracturing, the liquid nitrogen (LN2) waterless fracturing proposed in recent years can be one of the key technologies for effective development of deep shale gas and alleviating water scarcity (Qu, Tang, et al., 2022). The cold impact effect of LN2 on the rock is used to reduce the difficulty of rock fracturing, thus improving the fracturing effect of the reservoir (Qu et al., 2023). Therefore, it is important to clarify the study of the fracturing mechanism of low-temperature LN2 on deep shale rocks and explore the feasibility of LN2 fracturing in deep shale rocks for the guidance of shale gas development (Qu, Liu, et al., 2022; Wu et al., 2019; Yang et al., 2021).