136-7 A Comparative Study of Differences in Pore Structure and Connectivity Between Deep Coalbed Methane and Shale Gas

Session: Micro-Nano Scale Pore-Fracture Architecture and Fluid Dynamics in Shale and Coal Reservoirs

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Abstract:

 In reference to shale gas typical development techniques such as horizontal well drilling and volumetric fracturing, significant breakthroughs have been made in deep coalbed methane (CBM) extraction (>1500 m depth, also termed coal-rock gas). The annual production of deep CBM in 2024 reached 2.4 billion cubic meters.Deep coalbed reservoirs are characterized by low water saturation and high free gas content, showing certain similarities with shale reservoirs. However, coal seams exhibit distinct features such as higher organic matter content, more developed micropores (<2 nm), stronger plasticity, and well-developed cleats. These characteristics result in significant differences in pore structures and connectivity compared to shale formations.      As a consequence, coalbed methane extraction cannot simply replicate shale gas extraction technologies. In this paper, the marine shale samples and deep coal rock cores were selected respectively from southern Sichuan Basin and the Ordos Basin. These core samples were used for comprehensive characterization in pore structure and connectivity qualitatively and quantitatively by means of CO₂ physisorption, high-pressure mercury  intrusion porosimetry (MIP), focused ion beam-scanning electron microscopy (FIB-SEM), helium ion microscopy (HIM), Wood's metal injection porosimetry and so on. The findings were presented as follows: based on CO2 adsorption analysis, unlike shale, coal rocks exhibited a remarkably high proportion of micropores (>70%), serving as the primary storage space for adsorbed gas. However, Wood's metal injection and SEM imaging revealed that slit-shaped pores (>10 nm) were well developed but isolatedly distributed with poor connectivity in deep coal rocks. It distinctly contrasted with shale pore structure dominated by organic-hosted circular pores. In addition, the coal rocks were indicated by Helium ion microscopy (HIM) to contain a minor proportion of densely clustered circular pores with good interconnectivity, though these constituted a relatively small fraction of the total pore system. The fluid flow capacity in coal reservoirs was predominantly governed by the interconnectivity between cleat systems and slit-shaped pore systems, leading to a broad permeability distribution ranging from 0.0001 mD to 1 mD. In the further studies, the cleat network existence and its contribution to permeability need to be emphasized, which will provide theoretical support for gas productivity assessment and extraction technology optimization in coalbed methane reservoirs.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8426

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