No abstract

Micropores are the primary sites of methane adsorption in coal, and the heterogeneous mesostructures of coal create the non-uniform distribution of the micropores in coal. Using a thermal infrared imager, the temperature distribution on the surface of an anthracite sample during methane adsorption/desorption was tested in this paper, and a new method is advanced to calculate methane adsorption capacity in coal based on its temperature increment. The results confirm the strongly non-uniform distribution of methane adsorption in coal. A X-ray CT scan test demonstrates that the volumetric zones of coal sample with a strong methane adsorption capacity have a lower average density. In these regions, the clay minerals with developed micropores also have a strong methane adsorption capacity. During the coal skeleton deformation of methane adsorption, the high density is hard to be squeezed, while low density areas are likely to be squeezed. Therefore, the complexity density distribution in coal leads to the incompatibility of deformation, which make the direct determination of regional uptake a challenge; From the SEM micrographs of the same coal sample with different densities determined by the X-ray CT scan, the mesostructures of cell cavity pores with non-compact packing of the clay minerals appear to be the primary sites of methane adsorption in coal, and the telocollinite with fewer pores has a lower methane adsorption capacity.

This study analyzes the microstructure and deformation rule after methane adsorption on coal by scanning electron microscopy (SEM) and computed tomography (CT) scanning of microscopic coal samples. Studies have shown that coal is a natural rock composed of vitrinite coal matrix and clay mineral. After methane adsorption, coal undergoes non-uniform expansion deformation. This occurrence prompts coal density to decrease and then increase, causing the density distribution of coal to become highly concentrated. During swelling after adsorption, the effects of deformation and expansion on coal structures become stronger than that of mutual squeezing. Under low adsorption pressure, coal expansion deformation are more likely to crack the pore structure of the original coal to acquire space for expansion. When the adsorption pressure increases, compression becomes mainly concentrated in the low-density region; as adsorption pressure continues to increase, expansion deformation occur from high-density to low-density regions. The methane adsorption properties of coal are related to its pore structure. Adsorption and swelling mainly occur in the region where the pore structure is unfilled or filled with clay minerals. Expansion deformation conforms to the Langmuir equation; the region without pore structure development exhibits no swelling; the deformation degree and range of the pore structure and clay mineral mixing zone exhibit increase volatility. Overall, results reveal a microstructural change after methane adsorption.

Summary Traditional coalbed-methane (CBM) exploitation technologies, including ordinary drills, enhanced drills, water jets, and hydrofracture in the coalbed, are widely used in modern coal mining and involve high-pressure water. Because the CBM-desorption capacity decreases when using high-pressure water, the CBM output efficiency is lower than expected. To investigate the decrease in CBM desorption after water injection and the increase in CBM output after temperature is raised, an experimental system was built to simulate ideal conditions for one water-injection well per production well. The apparatus comprised a gas-liquid-injection system, a coal-sample container, a temperature-control device, a measurement system, and other auxiliary devices. Experiments on the effects of water injection and temperature on CBM desorption were carried out. The results revealed that the CBM-desorption capacity mainly depends on the water-injection pressure at constant temperature. The capacity decreased by at least 50% at less 2 MPa and became stable if the water pressure exceeded 8 MPa. Furthermore, the desorption capacity rapidly increased when the temperature was increased after water injection. Desorption reached a plateau once the temperature reached the boiling point of water, and the percentage desorption (PD) at 90°C was greater than that for free broken coal. The results demonstrate that heated hydrofracture is an effective technology with good efficiency for CBM exploitation.

A high-precision computed tomography (CT) experimental analysis system was used to perform a non-invasive CT scanning of oil shales (OS) from Daqing and Yan'an counties, Heilongjiang and Shanxi provinces, respectively, both northeastern China, in order to study changes in their internal structure at high temperatures. The formation and pattern of pores and fractures with increasing temperature was examined. Professional CT analysis software was used to statistically calculate porosities. The experimental results indicated that the thermal decomposition of organic matter at high temperatures was a major factor causing changes in the internal structure of Daqing oil shale. When the temperature exceeded 200 °C, large heterogeneous pores were formed in the originally solid material due to the expulsion of the gas and oil generated by thermal pyrolysis of organic matter at high temperature. With a further increase in temperature, the pores expanded and became interconnected by fractures. The heterogeneous thermal expansion was a major factor causing changes in the internal structure of Yan'an oil shale. After the temperature reached 200 °C, numerous fractures parallel to the primary strata were formed, which propagated and expanded with increasing temperatures. In summary, high temperatures changed the compact structure of oil shale into a porous medium with well-developed pores and fractures. As the temperature rose from 20 to 600 °C, the porosity of Daqing oil shale increased from 2.23 to 31.61% (a factor of 14.2), that of Yan'an oil shale increased from 2.70 to 8.86% (a factor of 3.3).