Seal Capacity, Macro-Scale Capillary Numbers and an Improved Understanding of Basin Scale Fluid Flow

Tim Matava
L&W Geosciences, Houston, TX

Seal capacity estimates and column height prediction methods balance a capillary entry pressure with the buoyant force of a hydrocarbon fluid. The physical basis for this force balance is the assumption that the capillary number, the ratio of a viscous stress to a capillary stress, is negligible. On a pore scale, the capillary number is small; however, two issues exist with estimating seal capacity based on pore scale properties. First, a capillary seal only a fraction of a meter thick should be as effective as a seal meters thick. In reality, the best reservoir seals exhibit continuity in both thickness and extent. Second, headspace gas profiles above a trap often indicate the presence of hydrocarbons at low saturation even when pore-scale capillary numbers are small. Low saturation hydrocarbons above a reservoir are generally interpreted as leakage through the seal. Leakage occurs even though pore pressure profiles indicate the presence of a high quality barrier to flow of fluids.

A macro-scale capillary number which takes into account seal thickness, extent, and structural position resolves issues not addressed using pore scale methods. Macro-scale capillary numbers are generally large indicating the viscous stress cannot be ignored when predicting seal properties. Since the viscous stress cannot be ignored, assumptions which form the basis of column height prediction methods are no longer valid. Finally, a significant macro-scale capillary number indicates that fully coupled Darcy flow equations may be simplified for basin scale flow algorithms.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005