Hydrodynamic invalidation of synformal traps for dissolved CO2

Abstract

Abstract Reservoirs with the geometry of structural lows trap negatively buoyant fluids, mirroring the way structural highs trap positively buoyant fluids. In both scenarios, lateral flow of reservoir water alters the trapping geometry by causing fluid contacts to tilt. Tilt increases in proportion to hydraulic head gradient increases and the density contrast between the flowing and trapped fluids decreases. Positively buoyant, immiscible supercritical CO2 is at least 20% lighter than typical saline formation waters at CO2 subsurface storage pressures and temperatures, and would experience relatively low tilts, <1°, similar to hydrocarbon fields. On the other hand, CO2-saturated brines are within 1% of the density of CO2-free equivalent brine. This relatively low density contrast creates tilted contacts at much higher angles even at the low hydraulic head gradients that typify deep saline aquifers. For example, a fluid contact with 0.5% density contrast exceeds 2° tilt for hydraulic head gradients of only 15 cm/km. Many large and basin-scale synformal traps are formed with structural dips of just a few degrees and therefore cannot trap CO2 solutions under hydrodynamic conditions. This problem could be overcome by utilization of hydrodynamic traps or appropriately configured structural-stratigraphic traps for CO2 sequestration.