Heat-Induced Pore Structure Evolution in the Triassic Chang 7 Shale, Ordos Basin, China: Experimental Simulation of In Situ Conversion Process

• Published in: Journal of marine science and engineering Vol. 11; no. 7; p. 1363
• Main Authors: Zhao, Zhongying, Hou, Lianhua, Luo, Xia, Chi, Yaao, Pang, Zhenglian, Lin, Senhu, Zhang, Lijun, Liu, Bo
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01-07-2023
• Subjects: Analysis; Chang 7 shale; Clay minerals; Composition; Connectivity; Conversion; Crack initiation; Diffraction; Electron microscopy; Experiments; Feldspars; Heating; Heating rate; High pressure; High temperature; ICP; Kerogen; Mathematical models; Membrane permeability; Minerals; Numerical models; Oil shale; Organic matter; Permeability; permeability evolution; Petroleum industry; Physical properties; pore–fracture network; Porosity; porosity evolution; Pyrite; Pyrolysis; Reflectance; Reservoirs; Scanning electron microscopy; Sedimentary rocks; Shale; Shale oils; Shales; Siderite; Simulation; Temperature; Thermal stress; Triassic; X-ray diffraction; X-rays; China; United States > US; Ordos Basin
• ISSN: 2077-1312; 2077-1312
• DOI: 10.3390/jmse11071363

The reservoir properties of low–medium-maturity shale undergo complex changes during the in situ conversion process (ICP). The experiments were performed at high temperature (up to 450 °C), high pressure (30 MPa), and a low heating rate (0.4 °C/h) on low–medium-maturity shale samples of the Chang 7 Member shale in the southern Ordos Basin. The changes in the shale composition, pore structure, and reservoir properties during the ICP were quantitatively characterized by X-ray diffraction (XRD), microscopic observation, vitrinite reflectance (Ro), scanning electron microscopy (SEM), and reservoir physical property measurements. The results showed that a sharp change occurred in mineral and maceral composition, pore structure, porosity, and permeability at a temperature threshold of 350 °C. In the case of a temperature > 350 °C, pyrite, K-feldspar, ankerite, and siderite were almost completely decomposed, and organic matter (OM) was cracked into large quantities of oil and gas. Furthermore, a three-scale millimeter–micrometer–nanometer pore–fracture network was formed along the shale bedding, between OM and mineral particles and within OM, respectively. During the ICP, porosity and permeability showed a substantial improvement, with porosity increasing by approximately 10-times and permeability by 2- to 4-orders of magnitude. Kerogen pyrolysis, clay–mineral transformation, unstable mineral dissolution, and thermal stress were the main mechanisms for the substantial improvement in the reservoir’s physical properties. This study is expected to provide a basis for formulating a heating procedure and constructing a numerical model of reservoir properties for the ICP field pilot in the Chang 7 shale of the Ordos Basin.