Influence Of Minor Faults Within The Reservoir On CO₂Spatial Distribution And Storage

Carbon Dioxide (CO2) Geological Sequestration (CGS) is one of theinternationally recognized techniques to efficiently reduce CO2discharge.The minorfaults within the reservoir are usually not taken in consideration, based on theformation characteristics and the requirement of the model simplified. Although theminor faults do not cut through the cap rock or lead to a large-scale leakage, localheterogeneity caused by fault would have an influence on CO2spatial distribution andfinal storage after CO2is injected into the reservoir.Based on the above reasons, the study of this paper simulates and predicts theinfluence of fault through TOUGH2, TOUGHREACT and TOUGHVISUALsoftware.The physical and hydrogeochemical data of simulation models come fromShiqianfeng Formation of CGS demonstration projects in Ordos basin.In the effects of physical heterogeneity of the fault, referring to common physicalparameters range of fault, this article designs10simulation cases, which havedifferent porosity and permeability, to explore the influence of physical heterogeneityof fault on CO2spatial distribution and storage amount. The study found that the faultwithin the reservoir had a significant influence on CO2spatial distribution and storage.In terms of CO2storage: when the permeability of the fault changes in the commonscope, high permeable fault (whose permeability increases to100~1000times of theoriginal formations) increased the CO2storage amount in the reservoir observably,which is1.22~1.49times of that under the situation of no faults contained in thereservoir; low permeable fault (whose permeability decreases to1/10~1/1000of theoriginal formations) decreased the CO2storage amount in the reservoir significantly,which reduce by14.35%compared to storage amount in reservoir without a fault. Interms of CO2distribution and migration distance: because of the high permeable fault,the CO2migration distance increased in sandstone interlayers connecting by the fault,which also homogenized the CO2spatial distribution, compared to the CO2migrationdistance and spatial distribution in reservoir without a fault; low permeable faultdecreased the rate of CO2migrating from the injection well to the fault significantly,and led to CO2gathered in the front of the fault. At the end of simulation process (at100thyear), CO2did not migrate through the fault, this is mainly due to the lowporosity and low permeability properties of the original formation and the existence ofthe low permeable fault further decreases the local permeability. In the effects of reverse fault on CO2spatial distribution and storage amount inthe reservoir, this article designs8geochemical simulation cases under theconsideration of hydrogeochemical reaction, based on measured hydrogeochemicaldata of Shiqianfeng Formation. Through numerical simulation results, it is found thatthe secondary carbonate minerals generating at1000thyear includes calcite, ankerite,siderite and magnesite. When there is no fault in reservoir, carbonate minerals mainlydistribute in the surface of mudstone interlayers above and below the sandstoneinterlayers; when the fault exists in the reservoir, carbonate minerals still distribute inthe surface of mudstone interlayers above and below the sandstone interlayers, but thehorizontal distribution scope of carbonate minerals decreases in the area affected byfault. From the total amount of CO2storage of the whole Shiqianfeng Formation, theexistence or inexistence of reverse fault (the thickness of interlayers verticallyruptured by the fault is about half of the Shiqianfeng Formation) has no significanteffect on it. At1000thyear, mineral sequestration amount of CO2account for morethan97%of the total injection amount. In the interlayer reservoirs affected by thefault, at1000thyear, mineral sequestration amount of CO2decrease more than10%ofthat under the situation without a fault.Shiqianfeng Group is an interlayer caprock-reservoir formation system,composed of a set of mudstone and sandstone interbed. And the system caprock isalso a system composed of a set of mudstone and sandstone interbed. The mudstoneinterlayers of Shiqianfeng Group have similarities with system caprock in thestratigraphic structure, thickness, permeability and other physical parameters, and thegeochemical characteristics. Therefore, this paper designs six sets of simulation cases,through changing the physical parameters of mudstone and sandstone in the fault ofShiqianfeng Group, and evaluates the CO2sealing safty of system caprock based onthe simulation results with analogy method. Results show that the regional caprockhas a good sealing ability and higher CO2sequestration security, because of itsinterbed structure composed of many classes of mudstone and sandstone, physicalproperties of low porosity and low permeability, and mineral compositioncharacteristics of strata which is beneficial to mineral sequestration of CO2.To sum up, the minor fault within the reservoir can make the CO2spatialdistribution and storage obviously different from the situation in homogeneousreservoir. Therefore, if the influence of minor faults within the reservoir can be takeninto consideration during calculating CGS simulation models, the CO2migrating pathand storage distribution will be predicted more accurately.