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1. Study of pore-throat structure characteristics and fluid mobility of Chang 7 tight sandstone reservoir in Jiyuan area, Ordos Basin

Author

Zhang Quanpei, Qi Hongpeng, Huo Yong, Li Yong, Li Tao, Zhang Duoduo, Lin Kun, Yang Chen, Tong Jian, Zhao Hui, Suo Busen, Xue Yalan, and Yi Caiping

Subjects
pore-throat structure, movable fluid, jiyuan area, fluid mobility, movable fluid effective porosity, Geology, QE1-996.5
Abstract

Quantitative studies of the pore-throat structure (PTS) characteristics of tight sandstone reservoirs and their effects on fluid mobility were proposed to accurately evaluate reservoir quality and predict sweet spots for tight oil exploration. This study conducted high-pressure mercury injection (HPMI) and nuclear magnetic resonance (NMR) experiments on 14 tight sandstone samples from the Chang 7 member of the Yanchang Formation in the Jiyuan area of the Ordos Basin. The HPMI was combined with the piecewise fitting method to transform the NMR movable fluid transverse relaxation time (T 2) spectrum and quantitatively characterize the PTS characteristics and the full pore-throat size distribution (PSD). Then, movable fluid effective porosity (MFEP) was proposed to quantitatively evaluate the fluid mobility of tight sandstone reservoirs and systematically elucidate its main controlling factors. The results showed that the PTS could be divided into four types (I, II, III, and IV), which showed gradual decreases in average pore-throat radius (R a), continuous increases in the total fractal dimension (D t), and successive deterioration of reservoir fluid mobility and percolation capacity. Moreover, the full PSD (0.001–10 μm) showed unimodal and multi-fractal characteristics. According to the Swanson parameter (r apex), the reservoir space types can be divided into small and large pore-throat and the corresponding fractal dimension has a relationship where D 1 < D 2. Large pore-throat had higher permeability contribution and pore-throat heterogeneity but a lower development degree and MFEP than small pore-throat, which had a relatively uniform and regular PSD and represented the primary location of movable fluids. Moreover, the development degree and heterogeneity of small pore throat controlled the flowability of reservoir fluids. MFEP can overcome the constraints of tiny throats and clay minerals on movable fluid, quantify the movable fluid content occupying the effective reservoir space, and accurately evaluate the reservoir fluid mobility. The combination and development of various pore-throat sizes and types in tight sandstone reservoirs results in different PTS characteristics, whereas differences in the mineral composition and content of reservoirs aggravate PTS heterogeneity, which is the main factor controlling the fluid mobility.

Published
2023
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2. Combining pore structure types and reservoir forming limits to determine the grading evaluation criteria of Chang 7 tight oil reservoirs in Jiyuan Area, Ordos Basin

Author

Li, Jiangshan, Zhang, Quanpei, Li, Yong, Huo, Yong, Li, Chun, Zhang, Duoduo, Lin, Kun, Yi, Caiping, and Xue, Yalan

Abstract

In view of the common problems of difficult in characterizing pore structure, weak research on reservoir forming limit, and ambiguous meaning of grading evaluation boundary in Chang 7 tight oil reservoirs of the Jiyuan area, Ordos Basin. The pore structure types and reservoir forming limits of tight sandstone were determined by high-pressure mercury injection test and other methods, and the grading criteria were established to evaluate the reservoir quality. The results showed that the Chang 7 tight oil reservoirs have poor petrophysical properties and four types of pore structures, which correspond to different reservoir types respectively. There are three types of the reservoir forming limits in tight sandstone reservoirs, namely the theoretical lower limit, lower limit of oil accumulation, and effective seepage lower limit, with the corresponding porosity limits of 3.5%, 5.5%, and 8.0%, respectively, and the corresponding permeability limits of 0.02 × 10–3μm2, 0.05 × 10–3μm2, and 0.20 × 10–3μm2, respectively. These reservoir forming limits were in good agreement with the classification boundaries of pore structure. Combining the pore structure types with the reservoir forming limits, the Chang 7 tight oil reservoirs can be divided into four classes: I, II, III, and IV, representing easily movable tight reservoir, movable tight reservoir, oil-bearing tight reservoir, and ineffective tight reservoir, respectively. Among them, the classes I and II reservoirs were the sweet spot areas for increasing the reserves and production of tight oil. The method of combining various reservoir forming limits and pore structure types can effectively evaluate reservoir quality, and provide theoretical basis for increasing geological reserves and rolling expansion of oil fields.

Published
2023
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