Date Published: March 18, 2016
Publisher: Public Library of Science
Author(s): Wenjie Li, Changcheng Wang, Zejin Shi, Yi Wei, Huailai Zhou, Kun Deng, David A Lightfoot.
Shale has been considered as good gas reservoir due to its abundant interior nanoscale pores. Thus, the study of the pore structure of shale is of great significance for the evaluation and development of shale oil and gas. To date, the most widely used approaches for studying the shale pore structure include image analysis, radiation and fluid invasion methods. The detailed pore structures can be studied intuitively by image analysis and radiation methods, but the results obtained are quite sensitive to sample preparation, equipment performance and experimental operation. In contrast, the fluid invasion method can be used to obtain information on pore size distribution and pore structure, but the relative simple parameters derived cannot be used to evaluate the pore structure of shale comprehensively and quantitatively. To characterize the nanoscale pore structure of shale reservoir more effectively and expand the current research techniques, we proposed a new method based on gas adsorption experimental data and the method of moments to describe the pore structure parameters of shale reservoir. Combined with the geological mixture empirical distribution and the method of moments estimation principle, the new method calculates the characteristic parameters of shale, including the mean pore size (x¯), standard deviation (σ), skewness (Sk) and variation coefficient (c). These values are found by reconstructing the grouping intervals of observation values and optimizing algorithms for eigenvalues. This approach assures a more effective description of the characteristics of nanoscale pore structures. Finally, the new method has been applied to analyze the Yanchang shale in the Ordos Basin (China) and Longmaxi shale from the Sichuan Basin (China). The results obtained well reveal the pore characteristics of shale, indicating the feasibility of this new method in the study of the pore structure of shale reservoir.
Shale has attracted increasing attention recently due to its nanoscale pores containing abundant oil and gas resources and has become a focus of global unconventional oil and gas exploration and development. Compared to conventional reservoirs, shale is a tight reservoir with a complex origin and strong heterogeneity [1–2]. Their pores have a complex geometry at the nanoscale and have low porosity and ultra-low permeability [3–6]. Previous studies have investigated the pore size and distribution of different shale formations. The Barnett shale in North America has various types of pores [6–8], with an organic nanopore distribution between 5 and 750 nm, a median size of approximately 100 nm . The pore size of the marine shale in South China is between 2 and 900 nm, with an average of approximately 3 to 40 nm [9–14]. The pores of the continental shale of North China have a distribution between 2 and 35 nm [15–18]. The abundant nanoscale pores of shale provide good spaces for gas accumulation, playing an important role for gas storage and migration [1,6,8,19–21]. Thus, the study of pore structure features is of great significance for shale oil and gas evaluation as well as for exploration and development.
Based on extensive investigations Luo and Wang (1981) discovered that the pore throat distribution of limestone and carbonate rocks did not comply with the normal distribution proposed by Chilingar (1972). Instead, the distribution was a combination of various pore throat distributions, which should be caused by multiple factors of diagenesis and epidiagenesis. For a mixed distribution, the eigenvalues can be determined by the method of moments according to the numerical characteristics of the empirical geological distribution . Using the method of moments, Luo (1981) and Wang (2003) extensively studied the low permeability reservoirs of the oil and gas bearing basins in China, such as the Ordos Basin, Sichuan Basin, and Bohai Bay Basin. The characteristic parameters obtained revealed the pore structure characteristics of reservoir rocks and provided constructive guidance for the exploration and development of oil and gas fields [22,39].
N2 isotherms demonstrate a wide range in adsorption and show a hysteresis pattern for all samples (Fig 1). The shape of the hysteresis loop reflects the pore geometry [20,35,44]. The adsorption isotherms show an upward trend in areas of high pressure, implying the existence of mesopores and macropores [35,45]. In the medium-pressure area, the adsorption curve does not coincide with the desorption curve, forming a hysteresis loop, indicating a capillary condensation phenomenon [35,40]. A capillary condensation phenomenon indicates the existence of open pores, e.g., ink bottle-shaped pores, slit-shaped pores and conical pores [35,44]. According to IUPAC , the hysteresis loop of thirty-seven samples mainly falls into two types. Samples Y-1 to Y-13 and Y-23 to Y-31 can be classified as type H2 (Fig 2; ink bottle-shaped pores), and samples Y-14 to Y-22 and Y-32 to Y-37 can be classified as type H3 (Fig 2; slit-shaped pores).
According to the calculated characteristic parameters in Table 3, the relationship between the mean pore size (Φ) and standard deviation (sorting coefficient), variation coefficient and skewness were established (Fig 3). As seen from Fig 3A, the mean pore size (Φ) and standard deviation of Yanchang shale has a certain correlation (R = 0.467, R2 = 0.218): a smaller Φ value is associated with a larger pore size (D) and poorer pores sorting, it reflects the pores of larger pore size (D) has a large proportion of the total pores. In contrast, a larger Φ value, is associated with a smaller size (D) and better pores sorting, i.e., a large proportion of pores of smaller pore size (D). The mean pore size (Φ) of Yanchang shale shows a significant relation to the variation coefficient (R = 0.730, R2 = 0.533), as shown in Fig 3B. A small Φ value, corresponds to a large pore size (D), which is favorable for oil and gas storage and migration, suggesting a good pore structure (large variation coefficient). In contrast, if the Φ value is large, the pore size (D) is small, which is not favorable for oil and gas storage and migration, suggesting a poor pore structure (small variation coefficient). The skewness shown in Fig 3C extends from -1.930 to 0.302, which is dominated by the negative contribution, indicating that the pore size (D) is smaller than the mean value (D), i.e., the pore size distribution of most samples is inclined to fine skewness.
(1) The method of moments considers the influence of diagenesis and epidiagenesis on the shale pore structure, which is consistent with the geological regularity of pore size distribution and can be used to describe pore structure characteristics through various parameters quantitatively and accurately. Thus, the method of moments can be used to study the pore structure of shale.