Multi-scale x-ray computed tomography analysis of coal microstructure and permeability changes as a function of effective stress

Abstract

Gas permeability (k) and porosity (φ) are the most important parameters in CBM/ECBM and CCS in deep unmineable coal seams. k and φ depend on the coal micro structure, and k and φ significantly change with varying effective stress. However, how the coal micro structure is related to such permeability and porosity changes is only poorly understood. We thus imaged sub-bituminous coal samples at two resolutions (medium-33.7 μm and high-3.43 μm voxel size) in 3D with an x-ray microcomputed tomograph as a function of applied effective stress; and investigated how cleat morphology, k and φ are influenced by the changes in effective stress and how these parameters are interrelated. In the images, three phases were identified: micro cleats (void), a mineral phase (carbonate) and the coal matrix. When effective stress increased, the cleats became narrow and closed or disconnected. This resulted in a dramatic permeability drop with increasing effective stress, while porosity decreased only linearly.

Research on Stress Sensitivity of Fractured Carbonate Reservoirs Based on CT Technology

“…They concluded that effective stress caused closures of microcleats in the coal [19]. It was also found that with an increase in confining pressure, the permeability of natural fractures decreased exponentially, while the permeability of hydraulic fractures decreased more slowly, following cubic polynomial law [17].…”

“…We thus obtained the fitting equation: Yihuai Zhang et al studied the relationships of coal permeability and effective stress, and found that the gas permeability of coal dropped dramatically with increasing effective stress. They concluded that effective stress caused closures of microcleats in the coal [19]. It was also found that with an increase in confining pressure, the permeability of natural fractures decreased exponentially, while the permeability of hydraulic fractures decreased more slowly, following cubic polynomial law [17].…”

Swelling‐induced changes in coal microstructure due to supercritical CO2 injection

“…The small cylindrical coal plug was then mounted into a high pressure‐high temperature (HPHT) X‐ray transparent core holder, which was integrated into a HPHT micro‐CT (Xradia VersaXRM) core flooding apparatus (Figure ) [cf. Iglauer et al , ; Rahman et al , ; Zhang et al , , ]. The swelling experiments were then conducted following below steps: The core and tubing system were vacuumed for 24 h to ensure no air remained inside the system. All flood lines, fluids, and the core holder were continuously isothermally heated to 323 K (=50°C) with heat jackets and continuously circulating warm water. A confining pressure of 5 MPa (effective stress = 5 MPa) was applied, and the sample was imaged at a high resolution of 3.43 µm 3 in situ. The coal plug was then flooded with supercritical CO 2 at 10 MPa backpressure, while a confining pressure of 15 MPa was applied (i.e., the experiment was conducted at constant effective stress; 5 MPa).…”

Stress Sensitivity of Fractured and Vuggy Carbonate: An X‐Ray Computed Tomography Analysis

“…From the 2‐D grayscale images in Figure a, we noted the apparent change of pore structure shape in fractured carbonate, most of which was the fracture (Regnet et al, ; Zhang et al, ). Compared with the initial condition, marked by the yellow, the middle area of the pore space reduced significantly when the loading pressure increased to 10 MPa.…”

“…The porosity variation of vuggy carbonate as a function of confining pressure is shown in Figure c. During loading, the porosity decreased exponentially (Dong et al, ; Zhang et al, ). When the confining pressure reached 20 MPa, the porosity of vuggy carbonate reduced by 0.76% (from 4.31% down to 3.55%).…”