地质科技通报 (Jan 2023)

Analysis of the pore structure of tight sandstone by high-pressure mercury injection combined with fractal theory: A case study of the Heshui area in the Ordos Basin

  • Zhendong Lu,
  • Chenglin Liu,
  • Qibiao Zang,
  • Yuping Wu,
  • Xiya Yang,
  • Hong Yang,
  • Xiaoxiang Zeng,
  • Wenda Li

DOI
https://doi.org/10.19509/j.cnki.dzkq.tb20210203
Journal volume & issue
Vol. 42, no. 1
pp. 264 – 273

Abstract

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Pore structure is the key element of tight sandstone reservoirs, which restricts the accumulate and flow of oil and gas in reservoirs. The pore structure is one of the key and difficult point of unconventional oil and gas exploration and development. In this paper, ten dense core samples from the Upper Triassic Yanchang Group 7 section in the Heshui area of the Ordos Basin are selected to carry out experiments, including high-pressure mercury pressure and X-diffraction. Fractal theory is used to analyze the characteristics of pore throats, then the relationship between fractal dimension and reservoir physical properties, pore structure and mineral content is analyzed. The results show that according to the mercury pressure curve and pore structure, the reservoir is divided into categories Ⅰ, Ⅱ and Ⅲ, its reservoir performance and seepage capacity decrease in turn, and the microheterogeneity is enhanced. Different types of reservoirs have different fractal characteristics: according to the inflection points of fractal curves 0.05 and 0.02 μm. The pore size of class Ⅰ reservoirs is divided into large pores (> 50 nm) and medium pores (50-6 nm) by combining the actual situation with the standard of pore size division proposed by IUPAC. According to the inflection point, the reservoirs of classes Ⅱ and Ⅲ can be divided into medium pores a (50-20 nm) and mesoporous B (20-6 nm). The average fractal dimension increased, which was 2.619 3, 2.745 4 and 2.852 6, respectively, and the heterogeneity gradually increased. The main permeability is contributed by fewer large pores, and the fractal dimension mainly reflects the heterogeneity of pore size. The fractal dimension is related to some minerals. Mineral composition and content are the internal factors that determine the fractal dimension and then affect the quality and pore structure of reservoirs.

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