The Study Of Permeability Change In Mineral Chemical Reaction Afer CO₂ Injecting To Coal

That coal reservoir storage carbon dioxide to improve coalbed methane recovery（CO₂-ECBM） technology has a duel economic and environment benefits. The migration ability of reservoir fluid, the output of coalbed methane and the velocity of CO₂ sequestration are decided by the permeability of reservoir. The dynamic change of permeability of coal reservoir is very complex and therefore has become a hot research topic in the current research. The further study in coal and CO₂, the interaction between water are shallow. Fingering out the mechanism of coal and CO₂, the interaction among water could supply scientific evidence for recovery technology of deep coal seam injection CO₂ to enhanced coalbed methane. This paper makes a great use of theoretical knowledge of coalbed gas geology, coal petrology, coal geology, physics and chemistry and groundwater geochemistry, thus systematically studying the permeability influence laws caused by chemical factors of CO₂ injection enhanced coalbed methane. Through the chemical reaction simulation experiment between coal and CO₂ and water, using scanning electron microscopy, energy spectrum analysis, mineral composition analysis, the reaction solution composition, liquid nitrogen adsorption experiments and permeability testing method and so on, the reaction mechanism and influence factors between coal and water and CO₂ could be discussed, and the chemical reaction effect on the change of the structure of coal and permeability change mechanism would be revealed. Establishing the change of permeability mathematical model of dissolved filling minerals on coal reservoir pore, combined with the injected CO₂ field test data of CBM wells group（SX006 well group）in the block of Shizhuang, Qinshui Basin, the ion component variation of drainage well, the permeability change of coal reservoir and its effect on gas production could be analyzed.Taking the middle rank coal sample of Malan and Shaqu as the experimental research object, the CO₂ chemical reaction simulation experiment was carried out under normal temperature and pressure. By scanning electron microscope, the fracture structure of coal surface was observed before and after the reaction of coal sample and carbonate solution. The pore and intergranular pore of Malan and Shaqu were developed, and the pore size was 0.1-2μm. The main mineral in intercrystalline is kaolinite, and other pores were filled with calcite and kaolinite. The pore connectivity is large usually. Some observation areas formed honeycomb porosity and thus the good connectivity. The tissue holes developed respectively and generally disconnected. Most of the pores are not connected to each other. Intergranular hole developed only in the better minerals, and there is no obvious connection among various types of pores. In addition, cracks and polycondensation fracture aslso existed in a small amount. Polycondensation fractures also have kaolinite fillings and tensile fractures are not found with mineral filling phenomenon. After the reaction between coal sample and carbonate solution, some filling minerals in fractures were dissolved, and the pores was enlarged. Then the phenomenon of mineral recrystallization occurs.According to the mineral composition test of coal sample before and after the reaction, this research found that the relative content of carbonate minerals（calcite and dolomite） in coal decreases gradually. But the clay content（kaolinite, illite, chlorite, montmorillonite） relatively increases. The relative content of potassium feldspar reduces, and the variation at the beginning of the interaction is larger. Due to the dissolution of carbonate minerals in carbonate solution, and clay minerals stability is strong in the acid. The ion exchange capacity is low, in which montmorillonite is swelling, and chlorite has acid sensitivity. The potash feldspar will produce kaolinite and quartz after reaction, resulting in the different change rule of different mineral composition content after the reaction. The content of clay minerals is higher in Malan coal samples, and carbonate mineral content is relatively high in Shaqu coal samples, which leading the different change rule of its mineral composition content.By testing for ion content in the reaction solution, this paper found that all kinds of ions in the solution were increased after the reaction of coal samples and carbonate solution. Under the same conditions, the lower the p H of solution, the larger ion change amount in the solution, and the greater the ion exchange capacity of minerals and the fluid. Potassium ion and sodium ion are gradually increased with the increase of reaction time, and reaction solution type does not affect the change law. Calcium ions and magnesium ions in acidic condition increases with reaction time, and in alkaline conditions gradually settle into sediment which leading to the decrease of ion content. In these three types of coal sample, ion exchange capacity of the sample of Shaqu minerals is larger, and ion exchange capacity is decided by the mineral composition of sample and the pore-fracture system.Through the analysis of experimental data of the coal sample mineral composition and reaction solution ion composition, reaction principle and type of the interaction of CO₂, H2 O and mineral were discussed. CO₂ dissolved in the groundwater, which made the fluid in weak acid, decomposing of H+ and HCO3-.Weak acid fluids reacted with minerals in coal, and different minerals in coal showed different reaction types in carbonate solution. Among them, the carbonate minerals, including calcite, dolomite, reacted with carbonate solution; Potassium feldspar and chlorite formed kaolinite and quartz precipitation after the reaction; Montmorillonite and illite were in better stability in acid, easy expansion under the influence of water, and side chains fell off from the main chain; Chlorite with strong acid treated with acid, formed precipitation with iron chemical compound.Through the specific surface area and pore size analyzer, the size of hole before and specific surface of coal reacted with carbonate solution were tested. Piece of the nitrogen adsorption-stripping curve coal sample of Malan and Shaqu was without loop or loop was very small, reflecting the coal sample’s pore system mainly with the tightness of hole closed at one end, including the cylindrical hole closed at one end, the parallel plate hole closed at one end and closed at one end and closed at one end tapered wedge hole hole. BET specific surface area was 0.432 m2/g of Malan coal sample before reaction, BJH total pore volume was 0.0022 cm3/g, micro hole, middle hole, macro hole accounted for 2%, 30.4% and 30.4% respectively, for the specific surface area, middle pore in contribution was the largest, which was 63.7%, porous and macro pore specific surface area percentage were 19.7% and 17% respectively; While BET specific surface area of Shaqu coal sample before reaction was 0.531 m2/g, and BJH total pore volume was 0.0018 cm3/g. Micro hole, middle hole, macro hole accounted for 0.5%, 32.4% and 32.4% respectively, percentage of middle hole in the pore surface area was 66.2%, and the macro pore specific surface area of the percentage was 26%, the pore specific surface area of micro hole was smaller, at 8.3%. Malan and Shaqu coal sample was the macro hole mainly before acidification. The number of micro holes and middle hole increased after acidification. Through analyzing the experimental data, it could be found that Malan and Shaqv coal sample piece of pore volume and specific surface area increased first with reaction time presents, then reduced. Shaqu coal samples gained a larger pore volume, specific surface area, respectively, which has increased 86.4% and 59.3% compared with the initial amount, while Malan coal sample’s pore volume and specific surface area increased by 5.9% and 5.9% than the original amount. This is mainly controlled by the initial physical characteristics, mineral type and content and drainage impact after acidification.The permeability of the coal samples before and after the experiments was tested by the permeability overburden pressure gauge that we get the permeability changed law with the time and analyze the impact mechanism. The coal seam showed different characteristics in different stages of reaction after injected CO₂. Permeability of coal samples showed after then the trend of decrease with the time going. Permeability of the Malan coal samples reach maximum value by the time 7.23 months average, and the maximum value average is 0.37×10-3μm2. Permeability of the Shaqu coal samples reach maximum value by the time 7.46 months average, and the maximum value average is 0.86×10-3μm2. The paper found the permeability obvious improvement in 1 to 3 months. The law of permeability change showed different characteristics after chemical reaction because of the different samples initial permeability. The paper that the effect of the weak acid solution improved the permeability is not well in the coal sample of the low permeability and high permeability, and the effect is well in the permeability value between 0.2×10-3μm2 to 0.5×10-3μm2.The model of the samples initial permeability was established according to the volume distribution function, Poiseuille law and Darcy law. The mathematical model of the permeability change caused by the mineral which filled in the coal reservoir fracture dissolution was established according to the pore size distribution of coal, mineral content, the way of Minerals filling in the fissure and the different minerals solubility in carbonate solution. The numerical model was verified Combinating with laboratory test data. The paper achieved permeability changed law in the different filling aperture and permeability changed law with the time going. The paper analyzed the parameter that include the pore size distribution of coal, mineral content, the way of Minerals filling effect on the permeability changed by the model validation results. The model was proved correctly and reasonably by contrast with the results ofmodel calculation and the data of experimental measured.Through analyzing the data of water composition of SX-006-3 well after injection CO₂ with time in north Shizhuang Block in Qinshui Basin. Sodium, potassium, calcium, magnesium, iron ions content of output water of SX006 well group were increased gradually, but the p H of the water are well group output in 7.5 above, therefore, it was thought that CO₂ might not migrat to SX006-3. Gas production of SX006-3 well was improved obviously after injection of CO₂. the visible increase in output affected by CO₂ injection.