| Injection of CO2captured from the atmosphere into deep, suitable geologicalstructure (CGS) is currently the most effective way to reduce the concentration of CO2from the atmosphere. Leakage of CO2is critical to carrying out and promoting CGS,successfully. Caprock covering on the reservoir is considered to be an effective sealinglayer and an important part of the whole CGS, and could hamper the leakage of CO2from the reservoir, effectively. After intrusion of CO2physical properties of the caprock,such as permeability and porosity, could be changed sharply if there are fractures orfaults within the caprock, which are pre-existing or reopened. At the geological timescale, the caprock whose integrity and stability were weakened due to the presence ofCO2is likely to be a natural pathway for CO2leakage. By employing a combinationmethod of experiment and numerical simulation, this study systematically investigatedprocesses of CO2-brine-rock interaction after CO2invasion, and influences of theinteraction on the evolution of caprock sealing quality. Finally, the reliability ofconclusion made in this study was tentatively justified by comparing the results ofsimulation with natural CO2analogy.High temperature/pressure experiments in terms of CO2-brine-rock interactionwere conducted on the mudstone sample taken from the Shiqianfeng Formation inOrdos Basin. Results of two parallel experiments at T=56℃and70℃with P=170barindicate that plagioclase, K-feldspar and illite dissolved to some extent, and quartz,kaolinite experienced secondary enlargement. The amount of mineral alteration of thecase with T=56℃is smaller than that of the case with T=70℃, however, the alterationtype are the same. No carbonate minerals were detected through the two parallelexperiments. The change in the concentration of Ca2+is very limited during experimentperiod, which dictates the plagioclase is mainly consist of albite. According to the sizeof high temperature and pressure reactor, a model with the same size was set up. A reactive solute transport simulator TOUGHREACT was used to reproduce the processof experiment. Results suggest that except pH andH O, concentrations of other ionsoutput by the code fit well with these detected through experiments. Additionally,precipitations of dawsonite and muscovite were predicted during simulation but weren’tfigured out in the two experiments. The accuracy of instrument and existence ofuncertainty during picking up samples for inspection might be responsible for missingthese two minerals. At the same time, making use of resulting solution, the kineticparameter (rate constant k25, and activation energy Ea) of some minerals were calibrated.On the basis of1D and2D geometric models, this paper employed the numericalsimulation method to systemically study the influence of CO2-brine-rock interaction onthe evolution of caprock sealing quality after intrusion of CO2from ShiqianfengFormation with the actions of molecular diffusion+advection caused by capillarypressure, only molecular diffusion (capillary advection for short), moleculardiffusion+capillary advection+pressure gradient, and through fractures existing in thecaprock, respectively. On the basis of1D model, three scenarios, specific to a casetaking place at the front of CO2plume (Sg=0.3), within the plume (Sg=0.5) and close tothe injection well (Sg=0.9), were considered. Results reveal that under the specificconditions of reservoir and caprock of Shiqianfeng Formation, the distance penetratedby CO2within the caprock after invasion due to the force of molecular, capillaryadvection and pressure gradient is the biggest, the change in porosity is the largest,simultaneously. Comparison of three case with different initial reservoir Sgimplies thatthe distance penetrated by CO2in the case with Sg=0.9is the farthest, however, themaximum porosity change occurs in the case with Sg=0.3. Until500years, thedissolution of calcite is responsible for the increment of porosity. After then thedissolution of oligoclase, chlorite and illite results in the enhancement of porosity. Inthe fault scenario (2D model), after invasion into the fracture, gaseous CO2and fluidmigrate upward due to the buoyancy and pressure gradient, concomitantly penetrateinto the caprock matrix laterally. The difference of initial reservoir Sg(0.3and0.99)leads to different patterns of fluid flow within the caprock, which control the positionand distance of CO2invasion. After1000years, in the initial reservoir Sg=0.3case, CO2penetrates the caprock at the bottom, and the distance exceeds5m. However, for thereservoir Sg=0.99case, positions where CO2enter the caprock distribute along with thefracture, and the distance from fracture only reaches about0.4m after1000years. The tendencies of porosity change in the two scenarios express tiny differences, i.e., in thecase with the reservoir Sg=0.3the porosity of fracture and caprock both increase,whereas, for the reservoir Sg=0.99case the porosity of fracture increases but theporosity of caprock matrix decreases. This could be explained by the alteration ofminerals. In the reservoir Sg=0.99case, pH is higher than that in the reservoir Sg=0.3case, which causes a bigger precipitation amount of ankerite in the case with thereservoir Sg=0.99than the case with reservoir Sg=0.3.By comparing the responses of Shiqianfeng Formation, Heshanggou Formationand Zhifang Formation after CO2intrusion, this study concluded the effects of initialmineralogical assemblage on the evolution of caprock sealing efficiency. To avoid theinfluence of reservoir water, which possesses a lower pH caused by the dissolution ofCO2, injection method was introduced. Results suggest that the distance migrated byCO2in Shiqianfeng Formation is the biggest, and the smallest distance occurs inHeshanggou Formation. We interpreted this to be the different ranges of change inporosity. Although porosities of these three formations all experienced decrementsduring the simulation, the biggest decrement occurs in the Hesahgngou Formation withthe value of-0.002after1000years, and followed by Zhifangzu Formation, about-0.0016. The porosity of Shiqianfengzu Formation is reduced by-0.0011at the end ofsimulation. The initially different mineral compositions are the reason resulting invarious ranges of change in porosity. The dissolution of dolomite supplies sufficientCa2+and Mg2+for the precipitation of ankerite in the Heshanggouzu and ZhifangFormations. Comparison of results suggests that the Heshanggou Formation is moresuitable to be used as caprock due to the mineral composition.The random field theory was employed to generate heterogeneous distributions ofpermeability and porosity within the caprock. On the basis of models withheterogeneous permeability and porosity, the effects of heterogeneity in geologicalparameters (permeability and porosity) on the alteration of minerals, the evolution ofcaprock sealing quality and the ability of containing CO2were investigated. Meanwhile,the relative effect of heterogeneity only in porosity on the evolution of caprock sealingefficiency was also discussed. Results indicate that the heterogeneity of permeabilityand porosity facilitate the vertical migration of intruded CO2, but the enhancementmitigates with time. However, the flooding area of CO2is somewhat hindered byheterogeneities due to the difference of intruding patterns for all three models. The heterogeneous distribution of geological parameters leads to a “bypassingphenomenon”, which results in a quicker velocity of CO2migration and circulation ofaqueous species within permeable zones. The CO2-brine-raock interaction andalteration of minerals are enhanced by the frequently updates of solutes. For the twoheterogeneous cases, the incorporation of porosity heterogeneity increases the contactarea between mineral and solution, which is favor of the interaction of CO2-brine-rock,increases the amount of mineral alteration. Additionally, we also found that thedissolution mainly occurs in initially permeable zone and the precipitation takes placein initially less-permeable zones. The range of porosity change is enhanced due to thepromotion of CO2-brine-rock interaction. Although the ranges of porosity change in theheterogeneous cases are larger than that in the homogeneous base case, the band-shapezones where the porosity were reduced retards the migration of CO2compared to theheterogeneous distribution of zones with decrement in porosity in the heterogeneouscases. Bypassing process results in bigger distances in heterogeneous cases than inhomogeneous case. Similar to the precipitation/dissolution of minerals, theenhancement in porosity mainly occurs in the initially permeable zones, and thereduction of porosity prevailingly takes place in initially less-permeable zones, whichmeans that the heterogeneous distribution of permeability and porosity enhances theheterogeneity in permeability and porosity of the caprock.Finally, to validate the results of numerical simulation, the Lishui Sag of DonghaiBasin and the Huazijing Terraces of Songliao Basin were selected as typical CO2reservoirs. Combining the real conditions of geological storage system carried out inOrdos Basin, the simulation results were verified in terms of the evolution of caprocksealing ability and mineral alterations. Results indicate that without effects of reservoirwater, the simulations and natural CO2analogue both predict enhancements of caprocksealing ability. Comparison of the alterations among14minerals reveals that, exceptthat Ca-smectite and Na-smectite can’t be specified clearly in natural CO2analogue,evidences of other mineral alterations were found in the Lishui Sag or HuazijingTerraces natural CO2reservoir, which means that the conclusions obtained usingnumerical simulations in this study are reasonable and reliable. |
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