Geologic Records Of CO₂-bearing Fluids-argillaceous Rocks Interaction And Their Effects On Sealing Ability Of Cap Rocks

Geological Sequestration of CO2 released from the use of fossil fuels in industrial activities is an effective way to reduce its concentration in the atmosphere and to relieve the greenhouse effect.At present,both economic feasibility and CO2 geological storage can be achieved mainly by Carbon Capture,Utilization and Storage(CCUS)technology.CCUS technology includes three possible measures,namely,CO2–enhanced oil recovery,CO2–enhanced oil recovery and CO2–enhanced coal bed methane recovery.The main storage media include clastic rocks,volcanic rocks and coal measure strata.However,regardless of the technology used to store CO2 in any suitable medium,mudstone cap rocks with good sealing capability are necessary condition to ensure long-term geological storage of CO2.The CO2 injected into the reservoirs will rise to the bottom of cap rocks by buoyancy,and leak in the form of molecular diffusion or along the pre-existing fissures or faults.The CO2 storage mechanism of coal measure strata is special,and CO2 is mainly adsorbed in coal matrix in molecular form.In this study,the interaction between CO2-bearing fluids and argillaceous rock in the form of slow and rapid leakage was studied with argillaceous rock core samples derived from the Qingshankou–Quantou Fromations of the Central Depression and the Qingshankou Formation argillaceous outcrop in the Wangfu Depression,southern Songliao Basin,respectively.The Jurassic coal measure strata of the Yaojie coalfield,Minhe Basin,was used to study the interaction between CO2-bearing fluids and coal measure strata.In the process of slow leakage,the superimposed diagenesis of argillaceous rock caused by CO2-bearing fluids is mainly the precipitation of authigenic carbonate minerals and clay minerals,and the dissolution of feldspar minerals.The minerals in the argillaceous cap rocks and interlayer rocks with an origin from leakage of CO2-bearing fluids are ankerite,dawsonite and calcite,and the carbon source of these minerals is the mantle-derived magmatic CO2 in reservoirs of the study area.In addition,these minerals are developed in the second member of the Qingshankou Formation of the Honggang Anticline.In combination with sampling depth range,iosotope temperature range and general burial–thermal history of the Honggang Anticline,the formation of carbonate minerals related to the leakage of CO2-bearing fluids in the argillaceous cap rocks was later than that in the lower dawsonite-bearing sandstones.This phenomenon is in accordance with the characteristics of fluids leakage.The dissolution of some plagioclase and K-feldspar,and the precipitation of some authigenic illite in argillaceous rocks may be related to the leakage of CO2-bearing fluids.The content changes of main elements in argillaceous rocks are controlled by mineral composition,i.e.,the change of the Si O2 content is mainly related to the composition of the quartz and clay minerals and influences by the plagioclase content;the change of the Al2O3 content is controlled by both plagioclase and clay mineral composition.The paragenetic sequence of the argillaceous rocks affected by CO2-bearing fluids in the Honggang Anticline is the dissolution of feldspar minerals(organic acid origin)? the precipitation of authigenic illite and microcrystalline quartz(burial diagenesis origin)? the dissolution of feldspar minerals and the precipitation of authigenic illite(CO2-bearing fluids leakage origin)? the precipitation of ankerite ? the precipitation of dawsonite ? the precipitation of calcite.During the rapid leaking process,the significant signs of interaction between reducing CO2-bearing fluids and fracture-contained mudstone of the Qingshankou Fromation in the Wangfu Depression,southern Songliao Basin,are mainly the formation of fracture fillings and bleached mudstones.The fracture fillings mainly consist of calcite veins and iron concretions with mineral composition of pyrite,goethite,quartz and calcite.The CO2 in the fluids of the study area was derived from the decarboxylation of organic matter due to bacterial fermentation.The reducing components are mainly CH4,minor heavy hydrocarbons and H2 S from the lower hydrocarbon gas reservoirs.The formation of calcite veins was related to CO2 degassing from groundwater caused by pressure release after fracture opening,and the iron concretions were the products of precursor minerals(pyrite and siderite)oxidized by shallow,oxidized formation waters or microorganisms.The red surrounding argillaceous rocks of fractures were bleached by leaked reducing CO2-bearing fluids to a gray color.This process can be interpreted as the result of reducing CO2-bearing fluids slowly penetrating the mudstone by molecular diffusion and Fe2+ slowly moving out of the mudstone to fractures.The reducing CO2-bearing fluids have no significant effect on minerals within the surrounding argillaceous rocks.In terms of the geochemistry of the mudstone,only the contents of Fe2O3,U,and Mo change significantly,with the content of U increasing in the mudstone and the contents of Fe2O3 and Mo decreasing during bleaching.In the process of interaction with coal measure strata,the CO2-bearing fluids mainly leads to the precipitation of thin-film dawsonite.The dawsonite is widespread in the coal cleats and fractures of roof sandstone in coal measure strata of the study area,which is in the form of film.Most of the dawsonite film is radial in nature,with some being fasciculate or hair-like.The carbon isotope values of thin-film dawsonite was –0.1‰ to 7‰(PDB),with an average of 4.8 ‰(PDB),which is the only carbonate mineral related to magmatic CO2 and CO2 derived from the decomposition of basement marble.The thin-film dawsonite most likely nucleated and grew in the liquid-like water film formed by a wet sc CO2 phase on a solid surface,and it may be characterized by layer-by-layer crystallization.The thin-film dawsonite provides a natural analogue that may support wet sc CO2–solid phase reaction experiments.In addition,as a typical CO2-ECBM natural analogue site,the dawsonite that formed in the CO2-rich coal measures of the coalfield shows that some of the free-state injected CO2,which increases in content due to desorption of the coal matrix,can be re-stored in the form of secondary carbonate minerals,thereby reducing the risk of leakage.The leakage of CO2-bearing fluids has an impact on the sealing ability of argillaceous cap rocks.In the process of slow leakage,the effect of CO2-bearing fluids on the sealing ability of argillaceous is mainly manifested by the correlation between autogenous carbonate mineral content and breakthrough pressure,i.e.,the precipitation of ankerite cements associated with mantle-derived magmatic CO2 has the greatest contributes to the enhancement of capability,while the precipitation of dawsonite and calcite cements has a limited positive effect on capability.In the process of rapid leakage,the calcite veins formed by the interaction between CO2-bearing fluids and surrounding argillaceous rocks of fractures reduce the fracture space and effectively prevent fluid flow.