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Abstract
To understand the pore structure and fractal characteristics of tight gas reservoirs, thin sections, nuclear magnetic resonance, rate-controlled mercury injection, microcomputed tomography scanning, and field emission scanning electron microscopy investigations under laboratory conditions were conducted on a suite of core samples from the Middle Permian Shihezi Formation of Sulige area in the Ordos Basin, China. The investigated tight gas sandstones comprise three types of pores, i.e. residual intergranular pore, secondary dissolution pore, and micropore. The pore–throat size distribution is extremely wide and multiscale (10 nm–400 μm) co-existing in tight gas reservoirs. The submicron- and micron-scale pore–throats with radius above 0.05 μm, which are characterized by combining rate-controlled mercury injection with nuclear magnetic resonance, are considered to be the effective pores and throats that dominated the reservoirs flow capacity. Tight gas sandstones have stage fractal characteristics, and the intrusion pressure of approximately 1 MPa is regarded as an inflection point. Fractal dimension is negatively correlated with permeability, average throat radius and mainstream throat radius, positively correlated with heterogeneous coefficient, while there are no obvious relationships with porosity and average pore radius. Additionally, the percolation characteristics of tight gas reservoirs can be characterized by fractal structure. When the pore structure does not follow the fractal structure (i.e. intrusion pressure is lower than 1 MPa), the mercury intrusion saturation is dominated by pores; in contrast, the mercury intrusion saturation is almost solely dominated by throats. This research sheds light on the pore–throat size distribution of tight gas reservoirs by identifying the role of multiple techniques and the relationships between the pore structure parameters and percolation characteristics of tight gas reservoirs and fractal dimension.
Highlights
Tight sandstone gas is considered an important and the most realistic alternative resource to be developed on a large scale in China (Yu et al, 2014; Jia et al, 2012; Zou et al, 2015)
The P2h8 sandstones pertain to the typical tight gas reservoirs with extremely low porosity and permeability
The Dpt was less than Dt except for the sample Slg-14-1 that may be caused by the ultra-lower permeability with ultrafine and complexed pore structure, indicating that the throat size distribution (TSD) of tight sandstone samples was more complicated
Summary
Tight sandstone gas is considered an important and the most realistic alternative resource to be developed on a large scale in China (Yu et al, 2014; Jia et al, 2012; Zou et al, 2015). As with other unconventional oil and gas reservoirs (Clarkson et al, 2012; Ghanizadeh et al, 2015; Ross and Bustin, 2009; Zhang et al, 2016), tight sandstone gas reservoirs typically exhibit a wide pore–throat size distribution (PSD), and the conventional methods for characterizing the pore structure are restricted. A combination of various methods is utilized to describe the morphology, size, and other characteristic parameters of pore, throat, and cracks in tight sandstones (Bustin et al, 2008; Clarkson et al, 2012; Gao and Andy Li, 2016). Nuclear magnetic resonance (NMR) (Gao and Andy Li, 2016; Li et al, 2015), microcomputed tomography (micro-CT) scanning (Bai et al, 2013; Zou et al, 2015), and field emission scanning electron microscope (FE-SEM) methods (Jiao et al, 2014; Zhao et al, 2015; Zou et al, 2011) are widely applied to investigate pore structure of tight reservoirs especially for the identification of submicron and nanoscale pores, and achieved good results
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