Comparative Studies of Three Nonfractured Unconventional Sandstone Reservoirs with Superlow Permeability: Examples of the Upper Triassic Yanchang Formation in the Ordos Basin, China

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• 作者：Ruifei Wang ; Guangjian Xu ; Xuguang Wu ; Zhongqun Liu ; Yungang Chi
• 刊名：Energy & Fuels
• 出版年：2017
• 出版时间：January 19, 2017
• 年：2017
• 卷：31
• 期：1
• 页码：107-118
• 全文大小：704K
• ISSN：1520-5029

文摘

Unconventional sandstone reservoirs with ultralow permeability (1 mD < K_air < 10 mD, where K_air represents the permeability in air) and superlow permeability (0.1 mD < K_air < 1 mD) in China are widely distributed in major basins and have significant economic potential. (Here, D is a permeability unit: 1D = 1000mD = 1 μm².) A comparative study has been performed here on three Upper Triassic sandstone reservoirs with superlow permeability in the southern Ordos Basin to understand the characteristics of each reservoir, the differences in reservoir porosity and permeability, and their causes. The study shows that pore type is the main factor that controls the physical property of nonfractured, superlow permeability sandstone reservoirs in the area of study. The main pore type observed in all three reservoirs is dissolution-enhanced intergranular, which comprises a maximum of 70% of the total porosity. Remnant intergranular and intragranular dissolution pores are also observed in these reservoirs. The pore throat size of dissolution-enhanced intergranular pores is generally 2–3 times larger than that of the intragranular dissolution type. Therefore, the reservoir with the greatest quantity of dissolution-enhanced intergranular pores has a poorer sorting of pore throat sizes and stronger microscopic heterogeneity. The dissolution-enchanced intergranular pores, on the other hand, also contribute to higher surface porosity, larger average pore diameter, higher pore throat coordination value, and, most importantly, higher permeability. This latter result contrasts with observations of conventional sandstone reservoirs, in which better pore throat sorting is favored for permeability.