Characterization of the Full-Sized Pore Structure of Coal-Bearing Shales and Its Effect on Shale Gas Content

Abstract

The characterization of the pore structure and shale gas content provides useful information for shale gas reservoir assessment and evaluation and guides the exploration and development of shale gas. Fresh core samples obtained from three different basin formations in China were analyzed by field-emission scanning electron microscopy, low-pressure CO 2 and N 2 gas adsorption−desorption, high-pressure mercury intrusion, and methane adsorption experiments to clarify the pore structure characteristics of coal-bearing shales and their effects on shale gas content. The inter-and intraparticle pores, organic matter pores, and microfractures were well developed in coal-bearing shales. These pores had different geneses, morphologies, and sizes with main diameters of <6.5 and 80−200 nm and the main shape of slit, taper, and ink bottle. Pores with diameters <10 nm dominated the shale pore networks. Shale gas content was directly influenced by shale pores, and small pores had a large surface area, which resulted in the high adsorption capacity of shale gas. Clay mineral and total organic carbon contents positively controlled the pore structures and shale gas adsorption, whereas brittle minerals were counterproductive. Coalbearing shale gas content was lower than marine shale gas content, with an adsorption gas content percentage of 50−85%. The proportion of adsorbed gas decreased with the increase of pore size diameters, whereas the proportion of free gas increased. When the pore size diameter reached approximately 3.5 nm, the free and adsorption gases reached dynamic balance. The adsorption gas content would be slightly low with pore size ranges of >6.5 nm, whereas the free gas content would be stable and merely increase in the range of 100−300 nm.

Effect of the water injection pressure on coal permeability based on the pore‐fracture fractal characteristics: An experimental study

“…The fractal dimension can be used to quantitatively describe the irregularity of a set. Previous studies have shown that the pore structure of coal rock has obvious fractal characteristics 39 . To investigate the scale characteristics of the pore‐fracture structure in the coal samples, the MIP results of the water‐injected samples were analyzed based on fractal theory.…”

Characterization of the Full-Sized Pore Structure and Controlling Factors of the Coal-Bearing Shale in the Wuxiang Block, South-Central Qinshui Basin, China

“…The pore structure of shales has strong heterogeneity, such as the PSD in the shale ranges from nanometer to micrometer scales; macropores, mesopores, and micropores have different contributions to the PV and SSA, respectively (Zhang J. et al, 2017;Zhang J. et al, 2019). The pore is the main place to adsorb and gather gas.…”

“…The increased energy and environmental demands promoted rapid development of the shale gas industry (Sun et al, 2017;Zhang J. et al, 2019;Xie et al, 2021). Unlike conventional reservoirs, shale gas is mainly stored in adsorbed and free states in complex pore systems (Ding et al, 2013;Wang et al, 2015;Chen et al, 2016;Sun et al, 2021;Tang et al, 2021).…”

Classification Evaluation of Gas Shales Based on High-Pressure Mercury Injection: A Case Study on Wufeng and Longmaxi Formations in Southeast Sichuan, China

“…), ray methods ,, (such as small-angle X-ray scattering (SAXS), (super) small-angle neutron scattering, etc. ), and fluid injection methods − (such as MICP, LT-NA/D, and carbon dioxide gas adsorption (LPA), low-field nuclear magnetic resonance (NMR), etc.). With regard to shale gas extraction, only connected pores will affect the migration and diffusion of shale gas, thereby controlling shale gas production to a certain extent.…”