AUTHOR=Song Zezhang , Zhao Junyi , Zhang Yuanyin , Yang Dailin , Wang Yunlong , Hu Li , Li Chenghai , Liu Wenlei 

TITLE=Bi-Fractal Characterization of the Pore Network of Tight Sandstone

JOURNAL=Frontiers in Earth Science

VOLUME=Volume 9 - 2021

YEAR=2021

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2021.751944

DOI=10.3389/feart.2021.751944

ISSN=2296-6463

ABSTRACT=Fluid seepage performance and accumulation in tight sandstone is a critical research topic for in-depth exploration and development, closely related to the heterogeneity of the pore network. The fractal characterisation is one of the most compelling and direct ways for quantitative investigation of heterogeneity. However, in most studies, only one kind of fractal is used, and the differences and relations between different fractal dimensions are rarely discussed. This paper chose the second member of the Xujiahe Formation as the research object. Firstly, based on physical analysis and XRD analysis, we carried out a qualitative investigation on pore structure utilising thin-section and SEM. Then, detailed pore structure parameters were obtained using HPMI. Lastly, we combined two-dimensional fractal analysis on thin-section images and three-dimensional fractal analysis on HPMI data to quantitatively characterise the pore network heterogeneity. The Xu2 tight sandstone is mainly medium to fine-grained lithic feldspathic sandstone or feldspathic lithic sandstone with low porosity and permeability. Also, the Xujiahe tight sandstone is mainly composed of quartz, feldspar and clay. Quartz is the dominating mineral. The pore types of Xu2 tight sandstones are primarily intergranular pores, micro-fractures and intra- and intergranular dissolution pores. Moreover, most of the micro-fractures in gas-bearing formation are open-ended, while most are filled by clay minerals in the dry formation.  The gas-bearing formation is distinguished from the dry formation by relatively low displacement pressure (<1 MPa), coarse skewness and apparent peak in PSD. The r50 of gas-bearing and dry samples are 200 nm and 80 nm, respectively, indicating a larger pore/throat size of gas-bearing samples. The 2D fractal dimension (Ds) of gas-bearing samples is significantly larger than that of dry samples, while the 3D fractal dimension  (D1, D2) of gas-bearing samples are lower than that of dry samples. There is a strong negative correlation between D2 and gas-bearing status, permeability, quartz content and r50, but a positive correlation between Ds and these parameters. D2 represents the heterogeneity of pore space, while the Ds indicates the development of the pore network. Combining 2D and 3D fractal analysis could give a more in-depth investigation of pore structure.