Identification of Potential Vertical Gas Migration Pathways above Gas Storage Reservoirs

Abstract

Natural gas is stored underground in geologic structures throughout the United States. However, complexities associated with these geologic structures may provide vertical pathways for gas migration, and thus gas loss. Possible upward migration (loss) of natural gas in an underground gas storage field in stimulated this investigation that aims to identify potential migration pathways. Spatial analysis of volume of shale (Vsh) and formation porosity (n) values were conducted in conjunction with high-resolution shallow seismic surveys to identify potential vertical pathways. Surficial gas accumulations within glacial deposits were confirmed by the seismic surveys. These gas pockets accumulated by migration along steeply inclined structural discontinuities, most likely faults and fracture zones within the underlying shallow bedrock units. With the seismic technique limited to a depth of 100 m, Vsh data were used to assess the reservoir’s seal rocks ability to limit vertical gas migration. The Vsh data indicate that the seal rocks are best classified as heterogeneous siltstones. Spatial analysis highlights an aligned pattern of low Vsh values in both the reservoir rocks and the seal rocks. The low Vsh values lie beneath the faults and fracture zones and the documented surficial gas deposit. Higher n values for the sandstones in the reservoir area as opposed to other areas suggest porosity enhancement associated with deformation. Structural deformation, faults and fracture zones, appears to provide a pathway for vertical gas migration. However, stratigraphic (lateral) heterogeneities associated with the reservoir and seal rocks may provide additional vertical pathways.