CO₂-H₂O-Coal （Rock） Geochemical Interaction In Coal Seam And Its Effects On Caprock Sealing Ability

Injection of CO₂ into deep unminable coal seams, with or without enhanced coal bed methane recovery（CO₂-ECBM）, is one method which needs further research for sequestering CO₂ greenhouse gas emissions in the subsurface. Researches on gas adsorption thermodynamics and kinetics have been relatively mature, and large amounts of achievements have been achieved. However, researches involves the interaction between CO₂-water-coal geochemistry were carried out later, with the aid of water-rock interaction mechanism, coal structure can be modified due to the interaction between CO₂ and minerals in coal, also changed the porosity, permeability and pore distribution of coal, these changes will, in turn, affect adsorption ability and CO₂ storage process. In addition, in the process of CO₂ sequestration in coal seams, dissolution of the minerals in coal brings migrate of the related elements, especially toxic trace elements may cause harm to the shallow aquifer environment. Different occurrence state of the elements will migrate and enrichment in their different ways, migration characteristics and mechanism of elements is an important aspect for us to understand the geochemical process in CO₂ storage in coal seams. Also CO₂ can react with coal bed roof and gangue in the interlayer, cause mineral dissolution and precipitation, porosity and permeability of caprock can be changed, especially when fractures exist in coal bed roof, this will be a great influnce on the seal security.Coal measures strata of Shanxi Group in southeast of Qinshui Basin was the target layer in this study, combined of experiment and numerical simulation methods to study CO₂-water-coal and rock interaction after CO₂ injected into coal seams, the influence on caprock sealing ability evolution, modification of coal and rock structure and element migration, and verified the experiment and simulation results with the typical natural CO₂ gas reservoir analogue, provided a scientific basis for deep coal seam CO₂ storage safety assessment and CO₂-ECBM projects site selection in the future.Coal and coal seam water was selected in north section of Shizhuang in Qinshui Basin, according to the actual formation conditions, eight batch experiments were conducted at 25 oC、40 oC、55 oC and 5 MPa、8 MPa、11 MPa respectly in different reaction time, an experiment with N₂ was also conducted as a compare. Results showed that the injected CO₂ can modify the structure of coal. At the beginning of the reaction, specific surface area of coal decreased due to coal swelling, with the increase of reaction temperature and time, after 30 dayes reaction at 55 oC, minerals in coal dissolved aggravation, the closed or semi-closed pores in coal matrix were opened gradually, the specific surface area and true density reached maximum value, then tend to be stable. There was much more influnce on micropore in anthracite coal than mesopore and macropore. Therefore, coal matrix had a larger space for gas storage, adsorption ability of coal was also enhanced. In the reaction process, metal elements in coal could release and migration, especially the release of harmful trace elements in coal, bring certain negative impacts on water quality in shallow strata, damage to the ecological environment and human health. But from the results of the experiments, most of the trace element content in the reaction liquid is very low, far lower than that in raw coal, only Se and Mn beyond the value of the drinking water quality, which therefore will not be enough to serious impacts on shallow strata water quality. But from the long term geological storage period, it is need for continuous monitoring.Rock samples were selected from coal roof and interlayer gangue in No.3 coal seam, nine batch experiments were conducted on lithic sandstone and calcareous mudstone respectively, and a mineral saturation index simulation calculation was carried out. Experimental results showed that in the process of CO₂-ECBM, cap rock actively participated in the geochemical reaction. Chemical composition of fluid and solid rock mineral alteration reflects the CO₂-water-rock interaction process. Significant changes have taken place in the composition and content of minerals, lithic sandstone in silicate minerals showed obvious dissolution, chlorite and illite precipitated at the same time. In comparison, the reaction of calcareous mudstone is more active, there were mainly the dissolution of calcite and I/S, as well as calcite, siderite, illite and chlorite precipitated, CO₂ was fixed in the form of carbonate minerals, increased the safety of CO₂ storage in coal seams. Compared saturation index simulation with experimental results can be concluded that the simulation results were basically consistent with the experimental results, smectite-na and kaolinite have higher saturation index, kaolinite and smectite-na is also observed in the experiments. But chlorite was unsaturated in simulation results, not completely consistent with the experiments results, may be due to the heterogeneity of the sample, it also showed that the simulation can reflect the result of the experiment to a certain extent, but not completely consistent with the experimental results.Long term safety of CO₂ sequestration in deep coal seams is also needed to pay special attention, in view of the existing natural or hydraulic fractures in the roof of coal seams, selected mineral composition and CO₂ gas saturation as two indexs, a total of four 2-D models were constructed, to study mineral dissolution and precipitation, and the change of pore structure during the leakage process of CO₂ along fracture, to explore the security of sequestration. Numerical simulation results showed that when the initial Sg=0.2 in coal seams is relatively small, gas-water two phase CO₂ was mainly distributed in the bottom of cap rock, the leakage along fracture is also less. When the initial Sg=0.8, gas-water two phase CO₂ was mainly distributed in the bottom of cap rock and the upper of fracture, and vertical and horizontal penetration distance was farther. When cap rock contains k-feldspar and calcite, the main precipitation minerals were quartz, kaolinite, smectite-ca, calcite, dissolved minerals were oligoclase and illite. When cap rock contains chlorite, the main precipitation minerals were secondary carbonate such as siderite and dawsonite besides quartz, sodium smectite-na and calcite. In both two types of cap rock, porosity showed a trend of decrease, and the bigger initial gas saturation, the greater porosity changed. And within the fracture, porosity showed a trend of increase in cap rock contains k-feldspar and calcite, while decrease in cap rock contains chlorite.Finally, to validate the results of experiments and simulations, the Haishiwan coalfield and Jixi Basin were selected as typical natural analogue. Dawsonite distribution in coal cleats and gangue fractures in Haishiwan coalfield, is carbon sequestration mineral after inorganic CO₂ migrated along fault F19. Calcite and dolomite in the lower interface of sandstone cap rock of No.14 coal seam in Jixi Basin were gradually dissolved, then the sandstone cap rock from down to up gradually densification, confirmed the safety of CO₂-ECBM projects in coal seams with sandstone cap rock. These phenomena provided evidence for the reliability of the experiment and numerical simulation results in this study.