Three-Phase Secondary Migration of Hydrocarbon

The objective of this work is to study the migration of hydrocarbons from a source rock into possible reservoirs. In particular, we consider simultaneous migration of gas and oil through a water-filled carrier bed and how this flow might result in characteristics that could be interpreted by nontraditional exploration methods. In the modeling, one-dimensional, immiscible, isothermal flow is assumed, and capillary effects are excluded. The seal is modeled as a layer of very low permeability. Our analysis is based on the method of characteristics and simple wave theory. Waves are changes in saturation(s) that move through a reservoir or carrier bed at a characteristic velocity. The analysis does not require capillary forces to trap hydrocarbons, but relies on nonlinear wave phenomena to explain migration and entrapment. No doubt, capillary forces are important in secondary migration and entrapment. Our treatment can be viewed as being part of a more complete theory on secondary migration, considering hydrodynamic effects here. We demonstrate that capillary forces are not the only mechanism that can cause trapping of hydrocarbons. We focus on the role of gas in the trapping of oil. If the seal is slightly permeable, some oil will flow through it. With gas present, the efficiency of the seal can increase, and oil can be completely trapped by a structure that would otherwise be permeable. For example, gas, being much more mobile than oil, can form a bank at the interface between two layers that have a modest permeability contrast. This gas bank will have a relatively large gas saturation. This will decrease the relative permeability to oil and completely trap subsequently migrated oil. Oil can also be trapped even when no gas bank is formed at the permeable seal. Features of this problem that might affect a seismic signal are (1) the existence of gas above (caused by a stow leakage of gas) and below a modest seal, (2) the gas leakage itself, (3) modest saturations in the oil column, and (4) overpressuring in the gas column.