| Coal reservoirs are the most important medium for buried CO2 geology. The evolution characteristics and mechanism of coal reservoirs are important parts of understanding the burial process. Using the "CO2 Enhanced Coal Bed Methane Recovery Geochemical Reaction Simulation System (SYSGEO-R30HPT)” designed by project team as research platform, select different coal rank samples from Sihe,Xinyuan, Xinjing and Yangzhuang mine, and crushed them to 4-8 mm for supercritical carbon dioxide (ScCO2) injection simulation experiment, the inductively coupled plasma mass spectrometry (ICP-MS), mercury intrusion experiments, low temperature nitrogen adsorption experiments and scanning electron microscopy (SEM)were used as research methods. The changes of pore fissure structure parameters such as density, pore volume, specific surface area and pore size distribution of coal samples before and after the reaction were discussed. The relationship and mechanism of migration of coal sample and the evolution of pore fissure structure is clarified, and the geological and geochemical model of element migration and fracture of coal reservoir pore under ScC02-H20 is established. The main results of this study are as follows:(1) Revealing the geochemical migration characteristics of elements in the coal under the influence of ScCO2-H20-coal reaction system.The types and properties of minerals determine the order and severity of their participation in the reaction. The degree of the reaction of the three main inorganic minerals such as carbonate minerals, sulfide minerals and silicate minerals and ScCO2 is decreasing in the coal, the silicate mineral reaction is the least intense, but the migration ability of the elements is stronger than that of the sulfide minerals with higher reaction intensity. The reaction rate of organic matter and ScCO2 in coal is not obvious, and the amount of element elicited with organic matter is relatively low,which shows a tendency to decrease slowly with time. It shows that the organic element will be reabsorbed after elution. And the migration characteristics of the elements are closely related to the coal rank, and the dissolution rate of the elements in the coal decreases first and then increases with the coal level, and the dissolution of the different elements is very different.(2) Revealing the evolution of fracture structure of coal reservoir under the influence of ScCO2-H20-coal reaction system.The transformation of SCCO2-H2O on the pore structure of coal and rock is mainly concentrated in the pore size range below 50nm. The main ways of the transformation of different samples are different: one is to expand the original pores in coal; second, the formation of narrow flat hole, an increase of coal connectivity; third is dissolved in the coal minerals, so that the original mineral occupied place to form a new pore hole. At the same time, ScCO2 will interact with organic matter in coal. The acidic environment formed by ScCO2-H2O into coal seam will break the connection key between the basic structural unit of coal molecule and other functional groups on the side chain, so as to improve the development of coal pore fissure. The effect of ScCO2-H20 on the pore structure of coal body is also affected by the factors such as simulated burial depth and coal rank.(3) The relationship and mechanism of mineral element migration and inorganic pore fissure structure change in coal was explored, the geochemical model of element migration and pore structure of ScCO2-H2O-coal reaction system and the prediction model of increase of pore size parameters under ScCO2-H2O were established respectively.ScCO2-H20 acidic fluids cause dissolution and dissociation of minerals in coal,forming new mineral pores, and dissolving minerals that clog the pores, increasing coal-like connectivity and accompanying the migration of elements, the dissolution characteristics of the elements in the coal can indicate the degree of reaction of the minerals in the coal, and the mineral reaction is also the direct cause of the pore fissure. The effect of different mineral reactions on the different pore sizes in the coal is different. The greater the migration of carbonate minerals, the stronger the transformation of mesopores and macropores in coal; the migration of silicate minerals and sulfides transforms the microporous pore. The reaction process is influenced by simulated burial and coal rank; which the coal rank is the intrinsic factor, the simulated burial depth is the external condition of the reaction. Based on this, the geochemical model of element migration and pore structure of ScCO2-H20-coal reaction system and the prediction model of increase of pore size parameters under ScCO2-H2O were established respectively. |
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