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 characterization is one of the most compelling and direct ways for quantitative investigation of heterogeneity. However, only one kind of fractal is used in most studies, and the differences and relations between different fractal dimensions are rarely discussed. This paper chose one of the most representative tight sandstone formations in China, the second member of the Xujiahe Formation, as the research object. First, based on physical analysis and XRD analysis, we carried out a qualitative investigation on pore structure utilizing thin-section and scanning electron microscopy. Then, detailed pore structure parameters were obtained using high-pressure mercury intrusion (HPMI). Lastly, we combined two-dimensional fractal analysis on thin-section images and three-dimensional fractal analysis on HPMI data to characterize the pore network heterogeneity quantitatively. 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. 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 r50 (median pore radius) is the most sensitive parameter to seepage capability (permeability) and gas-bearing status. 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 is 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. Tectonic movements that generate micro-fractures and clay cementation that blocks the seepage channels are the two main controlling factors on fractal dimensions. Combining 2D and 3D fractal analysis could give a more in-depth investigation of pore structure.
Highlights
After about 10 years of rapid development of unconventional hydrocarbon, tight gas plays a vital role in the energy framework of China
This study aims to combine 2D and 3D fractal analysis for the pore structure characterization of tight sandstone, comparing the advantages and disadvantages of 2D and 3D fractal characterization and investigating their relations
The physical parameters further confirm that the Xujiahe formation is a typical tight sandstone formation
Summary
After about 10 years of rapid development of unconventional hydrocarbon, tight gas plays a vital role in the energy framework of China. In 2019, the increases of the world proved gas reserves and gas demand were both driven by China, reaching as high as 3 Tcm (trillion cubic meters) and 24 bcm (billion cubic meters), respectively [BP 2019 energy review, (Ersoy et al, 2019)]. Compared with shale gas (Xu et al, 2020a; Xu et al, 2020b), tight sandstone gas has a more significant role in China’s natural gas industry, accounting for 39.2 and 24.6% of China’s total gas reserves and annual natural gas production as early as 2010 (Dai et al, 2012), respectively. Tight sandstone gas is one of the most developed unconventional gas resources in the world. Comprehensive and in-depth reservoir characterization is the core for successfully solving the problems mentioned above in tight gas exploration
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