Abstract
Ordos Basin is an important continental shale gas exploration site in China. The micropore structure of the shale reservoir is of great importance for shale gas evaluation. The Taiyuan Formation of the lower Permian is the main exploration interval for this area. To examine the nanometer pore structures in the Taiyuan Formation shale reservoirs in the Lin-Xing area, Northern Shaanxi, the microscopic pore structure characteristics were analyzed via nitrogen adsorption experiments. The pore structure parameters, such as specific surface area, pore volume, and aperture distribution, of shale were calculated; the significance of the pore structure for shale gas storage was analyzed; and the main controlling factors of pore development were assessed. The results indicated the surface area and hole volume of the shale sample to be 0.141–2.188 m2/g and 0.001398–0.008718 cm3/g, respectively. According to the IUPAC (International Union of Pure and Applied Chemistry) classification, mesopores and macropores were dominant in the pore structure, with the presence of a certain number of micropores. The adsorption curves were similar to the standard IV (a)-type isotherm line, and the hysteresis loop type was mainly similar to H3 and H4 types, indicating that most pores are dominated by open type pores, such as parallel plate-shaped pores and wedge-shaped slit pores. The micropores and mesopores provide the vast majority of the specific surface area, functioning as the main area for the adsorption of gas in the shale. The mesopores and macropores provide the vast majority of the pore volume, functioning as the main storage areas for the gas in the shale. Total organic carbon had no notable linear correlation with the total pore volume and the specific surface area. Vitrinite reflectance (Ro) had no notable correlation with the specific surface area, but did have a low “U” curve correlation with the total pore volume. There was no relationship between the quartz content and specific surface area and total pore volume. In addition, there was no notable correlation between the clay mineral content and total specific surface area and total pore volume.
Highlights
Faced with the increasing consumption of conventional energy, such as coal, oil, and natural gas, abundant shale gas resources play a key role in easing the energy tensions caused by social development, and have substantial development potential
By comparing the adsorption and desorption curves of the Taiyuan Formation shale samples with the eight International Union of Pure and Applied Chemistry (IUPAC) standards, we can conclude that there are some differences in the adsorption and desorption curves of the different shale samples, but the overall curve shape is roughly the same, which is the reverse “s” curve with a hysteresis loop
The micropores and mesopores are the main providers of specific surface area and pore volume, which is of significant importance to shale gas adsorption and storage in the Taiyuan Formation shale samples from the Lin-Xing area
Summary
The difference in pore types has a significant influence on the storage and migration of shale gas [12] Studying both the nanopore structure of shale in gas-containing shale reservoirs and the exploration and development of shale gas is important [13,14]. An atomic force microscope (AFM) can detect the interaction between the probe and the sample atoms, so as to achieve the purpose of studying the morphology characteristics of the sample This method has good adaptability, but it is affected by the surface roughness of the sample when characterizing the pores of shale [9]. Nitrogen adsorption is a common method in gas adsorption, which can better reflect the pore structure characteristics of shale [20], and is widely used in shale reservoir evaluation in different blocks of China [21,22,23]. Lin-Xing area and examine the influence of the micropore structure on gas storage
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