Anisotropic Adsorption Swelling and Permeability Characteristics with Injecting CO2 in Coal

作者：Niu, Qinghe; Cao, Liwen^*; Sang, Shuxun^*; Zhou, Xiaozhi; Wang, Zhenzhi

来源：Energy & Fuels, 2018, 32(2): 1979-1991.

DOI：10.1021/acs.energyfuels.7b03087

• LOW-RANK COAL
• CARBON-DIOXIDE
• GAS-PERMEABILITY
• METHANE RECOVERY
• BITUMINOUS COAL
• PORE STRUCTURE
• QINSHUI BASIN
• CLOSED PORES
• FIB-SEM
• CLEAT

The changes of anisotropic adsorption swelling and permeability with injecting CO2 in coal influence the CO2 injectivity during CO2-ECBM or CGS (ECBM = enhancing coal bed methane; CGS = CO2 geological sequestration). To strengthen the understanding of this issue, two special-made cubic coal samples were adopted to test the porosity, swelling, and permeability in parallel face cleat and bedding plane direction, parallel butt cleat and bedding plane direction, and vertical bedding plane direction. To quantitatively characterize the anisotropic porosity, anisotropic swelling, and anisotropic permeability, an anisotropy index was introduced in this work. The results show that porosity anisotropy reflects the pore connectivity in different directions, which fall in the order of parallel face cleat and bedding plane direction > parallel butt cleat and bedding plane direction > vertical bedding plane direction. The porosity varieties can be owed to the compaction effect, thermal evolution effect, banded structure, and cleat distribution in coal seams. The maximum swelling ratios of the vertical bedding plane direction to the parallel bedding plane direction are 2.30 in sample 1 and 1.89 in sample 2. However, the ratios of parallel face cleat to parallel butt cleat are 1.28 in sample 1 and 1.30 in sample 2. The inhomogeneity of matter composition in the vertical bedding direction and the difference of cleat distribution in various coal bands mainly cause the anisotropic swelling. Both injecting CO2 in coal and raising its temperature increase the anisotropy swelling index, but the effect of thermal swelling is quite weak. Adsorbing CO2 especially for supercritical CO2 will enhance the permeability anisotropy of coal. This is because the low-permeability cleat possesses higher permeability adsorption sensitivity and the bedding plane fracture with higher permeability instead does not produce a pronounced permeability drop because of its lower permeability adsorption sensitivity. Cleats that are easily affected by adsorption swelling always serve as throats between fractures and the coal matrix in a highanisotropic coal, which will restrain CO2 flow in coal pores. Accordingly, cleat seepage and corresponding potential enhanced permeability measures deserve being paid enough attention to in future research. This work clarifies the understanding and offers some implications for CO2 injecting into coal seams from the perspective of anistropic properties of coal.

• 出版日期2018-2
• 单位河南理工大学; 中国矿业大学（北京）