Fractal Analysis of Shale Pore Structure of Continental Gas Shale Reservoir in the Ordos Basin, NW China

• Published in: Energy & fuels Vol. 30; no. 6; pp. 4676 - 4689
• Main Authors: Jiang, Fujie, Chen, Di, Chen, Jian, Li, Qianwen, Liu, Ying, Shao, Xinhe, Hu, Tao, Dai, Jinxiong
• Format: Journal Article
• Language: English
• Published: American Chemical Society 16.06.2016
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/acs.energyfuels.6b00574

Describing the characteristics of shale pore structure is vital for the assessment of shale reservoir, which has significant influence on the storage and seepage mechanisms of gas shale. To profoundly understand the shale pore structure characteristics of continental shale reservoir, fractal analysis was performed on 45 continental shale samples from the Ordos Basin, NW China, via low-pressure N2 adsorption experiments. The characteristics of N2 adsorption isotherms revealed that slit-shaped shale pores are dominant among the geometric shapes of shale pores. During N2 molecules adsorption process, different characteristics were displayed at two regions where relative pressures (P/P 0) were 0–0.45 and 0.45–1. The Frenkel–Halsey–Hill (FHH) method was used to calculate fractal dimensions (D) at these two regions. In addition, the fractal exponents “(D – 3)/3” and “(D – 3)” were compared adequately. The results show creditable fractal characteristics for continental shale. Fractal exponent D – 3 is more suitable for the calculation of the fractal dimension in the study area. The surface fractal dimension (D 21) and pore structure fractal dimension (D 22) were further investigated. Results indicate that D 21, ranging from 2.04 to 2.50, was affected by shale constituents and provided a site for gas shale adsorption. D 22 reflects the irregularity and heterogeneity of the shale structure, varying from 2.20 to 2.65, and is higher overall than D 21. Furthermore, the value of D 22 negatively correlates with the average diameter of the shale. In addition, the comparisons of shale pore structure characteristics between the reservoirs Chang-7 and Chang-9 show that the shale pore structure of Chang-9 reservoir is more irregular and nonhomogeneous and is favorable for gas shale storage but unfavorable for seepage.