Abstract
Pore structure determines the gas occurrence and storage properties of gas shale and is a vital element for reservoir evaluation and shale gas resources assessment. Field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion porosimetry (HMIP), and low-pressure N2/CO2 adsorption were used to qualitatively and quantitatively characterize full-scale pore structure of Longmaxi (LM) shale from the southern Sichuan Basin. Fractal dimension and its controlling factors were also discussed in our study. Longmaxi shale mainly developed organic matter (OM) pores, interparticle pores, intraparticle pores, and microfracture, of which the OM pores dominated the pore system. The pore diameters are mainly distributed in the ranges of 0.4–0.7 nm, 2–20 nm and 40–200 μm. Micro-, meso- and macropores contribute 24%, 57% and 19% of the total pore volume (PV), respectively, and 64.5%, 34.6%, and 0.9% of the total specific surface area (SSA). Organic matter and clay minerals have a positive contribution to pore development. While high brittle mineral content can inhibit shale pore development. The fractal dimensions D1 and D2 which represents the roughness of the shale surface and irregularity of the space structure, respectively, are calculated based on N2 desorption data. The value of D1 is in the range of 2.6480–2.7334 (average of 2.6857), D2 is in the range of 2.8924–2.9439 (average of 2.9229), which indicates that Longmaxi shales have a rather irregular pore morphology as well as complex pore structure. Both PV and SSA positively correlated with fractal dimensions D1 and D2. The fractal dimension D1 decreases with increasing average pore diameter, while D2 is on the contrary. These results suggest that the small pores have a higher roughness surface, while the larger pores have a more complex spatial structure. The fractal dimensions of shale are jointly controlled by OM, clays and brittle minerals. The TOC content is the key factor which has a positive correlation with the fractal dimension. Clay minerals have a negative influence on fractal dimension D1, and positive influence D2, while brittle minerals show an opposite effect compared with clay minerals.
Highlights
Shale gas has become an important unconventional natural gas resource after its rapid development in North America [1,2], and it has become the focus of exploration in China, Australia, and other countries [3,4]
The organic matter abundance, thermal maturity degree and mineral composition of shale have an important influence on shale pore structure [13,32,63,64]
The analysis of shale geochemical characteristics and mineral composition is the basis for pore structure investigation
Summary
Shale gas has become an important unconventional natural gas resource after its rapid development in North America [1,2], and it has become the focus of exploration in China, Australia, and other countries [3,4]. Research works associated with pore system mainly focus on shale pore observation, the classification of pores, the characterization of pore size distribution, factors influencing pore development, and its effect on gas storage and enrichment [13,14,15,16]. Due to the variety and complexity of shale pores, several pore system classification schemes have been proposed by researchers to describe various pore types in shale [13,17,18,19]. These pore system classification schemes are mainly based on pore shape, size, and the relationship between pores and particles. According to the International Union of Pure and Applied Chemistry (IUPAC) [20], shale pores are generally classified into micropores (50 nm), which is another widely used classification scheme
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