Experimental Study On Mineral Geochemical Reaction And Its Inducing The Reservoir Structure Response With The Injection Of ScCO₂ Into The Coal Seam

CO₂-ECBM in deep anthracite reservoirs of Qinshui Basin,Shanxi Province,China,possesses the broad application prospects.For deep coal reservoirs,the injected high-pressure CO₂ will be in a supercritical state（ScCO₂）.Under the dual effects of ScCO₂ and formation water,the physical-chemical structure of coal reservoirs will be reformed due to the changes of occurrence characteristics relating to the organic composition and inorganic mineral in coal,which thus affects the effectiveness of CO₂-ECBM.Taking the high rank coal of Qinshui Basin as the research object,the temperature-pressure conditions of the deep anthracite reservoirs as the experimental basis and the“geochemical reaction simulation system on CO₂injection and enhanced drainage of coal bed methane”as the main platform,the geochemical mechanism with injecting Sc CO₂ into deep coal seam was experimentally stimulated.Then,based on the experimental methods of X-ray diffraction（XRD）,X-ray fluorescence（XRF）,scanning electron microscopy（SEM）,field emission scanning electron microscopy（FESEM）,inductively coupled plasma Emission spectrometer（ICP-OES）,mercury intrusion porosimetry（MIP）,liquid nitrogen absorption method,CO₂ adsorption,nuclear magnetic resonance（NMR）and computerized tomography（CT）,the geochemical reaction mechanism of mineral in coal and the variation of coal reservoir structure were analyzed.Finally,according to the FESEM observation and quantitative characterization of the same minerals,pores and fractures before and after the reaction the manner and scope of the geochemical effect on the pores and fractures of coal reservoir were dicussed.The main research results obtained in this study are as follows:（1）The characteristics of mineral dissolution-element migration-mineral precipitation and its mechanism in the reaction system of ScCO₂-H₂O-coal were revealed.After the interaction of ScCO₂-H₂O,the elements migrate from the solid to the solution with the dissolution of minerals.During the short reaction time,the mobility rate of the elements depends on the mineral dissolution rate and the degree of metal ions exchanges in the clay,regardless of the mineral solubility.The environment under high-temperature and high-pressure increases the solubility and ion exchange rate of CO₂ in coal seam water,As a result,the deeper the burial depth,the higher the mobility of elements.The reaction on carbonate in coal is the strongest,forming corrosion pit,corrosion crystal cone,dissolution ditch,etc.The congruent dissolution mode leads the element Ca and Mg shows the highest mobility.The second is aluminum silicate which crystallinity and morphology of the reaction are changed.The incongruent dissolution mode of aluminosilicate minerals leads to the migration of more cations in the interlayer than in the octahedral lattice;and the mixed elements of the isomorphism are more likely to migrate than the intrinsic elements in the crystal lattice.Accordingly,the mobility of elements K and Na is relatively high while the mobility of the elements Al and Si is comparatively low.（2）The variation characteristics of pores and fractures in the reaction system of ScCO₂-H₂O-coal rocks were clarified.The interaction of ScCO₂-H₂O can increase or enlarge pores and fractures,leading to an increase in the porosity,pore volume,specific surface area and permeability,which will augment along with the increase of burial depth.The pore diameter distribution of the coal does not alter,but the volume of micro pores shows the greatest growth while macro pores exhibits the largest growth rate（which can reach as high as 1057.86%）.ScCO₂-H₂O has a weak effect on the overall connectivity of microscopic coal particle samples,but strong on the transformation of local connectivity.And the connected pore is developed along the vertical direction of the original micro-fracture.Nevertheless,for the coal core samples,the overall permeability of macroscopic is obviously enhanced.The parallel bedding samples have the larger permeability than the vertical bedding samples.（3）The manner and scope of the effects of different geochemical processes on the pores and fractures of coal reservoirs were exposed.After ScCO₂ injection,the dissolution of carbonate minerals contributes significantly to the pore volume,and the swelling of the coal body has a remarkable effect on the connectivity of coal reservoirs.Totally and partially dissolved carbonate minerals in coal are the major contributor to the macroporous volume.The change of mesoporous and macroporous volume is mainly due to the erosion cones and dissolution trenches on the surface of remaining carbonate particles and dissolution pores of other minerals,as well as the intragranular pores of the expansion and exfoliation of clay minerals.On the other hand,the microporous volume is primarily contributed by the intermolecular pores formed by small molecules during ScCO₂extraction,and the hydration and swelling of pores in the clay crystal layer.The swelling of the coal leads to the increase of micro-crack and differential deformation joint-enlarging,which enlarges the macroporous volume and local connectivity;the increase of bedding and cleavage is the main reason for the improvement of macroscopic permeability.