FACTORS OF GAS MIGRATION WITHIN THE MT. SIMON AND EAU CLAIRE FORMATIONS IN THE UNDERGROUND GAS STORAGE FIELD AT TROY GROVE, ILLINOIS

At the Troy Grove gas storage facility, gas migration has been a factor since the early development of the reservoir. The underground gas storage field was originally intended to be within the Mt. Simon formation only, but faults were revealed when the gas started to migrate to the formation just above, the Eau Claire. The natural gas tends to migrate to the two uppermost sand units of the Eau Claire, even though the majority of the natural gas is injected into the Mt. Simon Formation. In addition, gas from the uppermost sand unit of the Eau Claire, the A Sand, has to be recycled into the lower units to help control gas pressures, in turn gas migration out of the reservoir. Also, varying amounts of shale within sandstones in the underground gas storage field may help determine a source of the migration. The amount and occurrence of heterogeneity due to discontinuous shale layers within the units and formations may be a factor in which unit or units are most likely to capture the most gas during its migration upwards. By reevaluating gamma ray logs, trends in the amounts of shale within each unit and patterns of occurrence of discontinuous laminar shale layers may help determine the migration pathways and best gas destinations. Also, a certain amount of gas migrates to the surface and/or other units. The heterogeneity of shale within the sandstones may be a limiting factor or it may be allowing the migration.