A Dynamic Theory of Hydrocarbon Migration and Trapping

Understanding hydrocarbon migration and trapping is important, since it can mean the difference between success and failure in exploration projects. Our current understanding is that a capillary pressure change between the seal and carrier bed is the main factor responsible for the trapping. This theory uses capillary pressure gradients under static (no flow) conditions to define the maximum amounts of hydrocarbon that can be trapped under a particular seal. It assumes that at the very low flow rates encountered in secondary migration viscous pressure drops are negligible. This research shows that even at low flow rates the viscous pressure drops may not be neglected. Trapping at a boundary is a result of complex interplay among buoyancy, viscous, and capillary forces. A theory that includes the effect of all of these forces on the trapping of hydrocarbons is presented. First, the effects of viscous forces are isolated by neglecting capillary pressure and solving by the method of characteristics. The results show that viscous forces alone can .cause trapping of hydrocarbons at a heterogeneity boundary. The effects of capillary pressure are then added and UTCHEM simulator is used to study the combined effect of viscous and capillary forces. The results.show that, even at very low flow rates, viscous pressure drops are not negligible and that pressure gradients within phases can be substantially different from the static gradients. This difference is the result of countercurrent flow and relative permeabilities at extreme saturations. The results also show that neglecting viscous pressure drops may lead to underestimating the capacity of a seal. The conditions at which a particular seal will leak are also defined. The method of characteristics solution is extended to include the effects of capillary pressures. The results of this research show that it is possible to classify seals into static seals and dynamic seals based on the flux rate from the source rock, the capillary pressure curves for the reservoir and the seal, and the flux-saturation relationship for the reservoir and the seal. Static seals conform to the conventional theory. Dynamic seals, on the other hand, can show substantially more trapping than static seals. In both cases, it is possible to associate a time scale to the accumulation and indicate explanations to several other features commonly observed in secondary migration.