Migration of Petroleum

Petroleum migrates from low permeability source rocks into high permeability reservoir rocks from which the petroleum can be produced (Fig. 1.2b). The main driving force for petroleum migration is buoyancy because it is less dense than water. The forces acting against migration are the capillary forces and the resistance to flow though rocks with low permeabilities Migration of oil and gas will therefore nearly always have an upwards component.

We distinguish between primary migration, which is the flow of petroleum out of the source rock and secondary migration, which is the continued flow from the source rock to the reservoir rock or up to the surface (Fig. 1.4). Oil and gas may also migrate (leak) from the reservoir to a higher trap or to the surface. Hydrocarbons are relatively insoluble in water and will therefore migrate as a separate phase. Solubility varies from as little as 24 ppm for methane to 1,800 ppm for benzene. Other compounds, such as pentane, are even less soluble (2–3 ppm). However, solubility increases markedly with pressure.

Many hydrocarbons have solubilities of less than 1 ppm in water It is difficult to envisage oil being dissolved in water and transported in an aqueous solution, both because of the solubility and the low flow rates. It would also be difficult to explain how the oil would come out of solution in the reservoirs (traps). Gas, in particularly methane, has a fairly high solubility in water, especially under high pressure. If methane-saturated water rises to lower pressures, large quantities of methane can bubble out of a solution. It is therefore necessary to assume that oil is mostly transported as a separate phase. Oil is lighter than

water, and oil droplets would be able to move through the pores in the rocks but the caplliary restance is high for separate oil drops in a water-wet rock (Fig. 1.5). In order to pass through the narrow passage between pores (pore throat), the oil droplets must overcome the capillary forces. When the pores are sufficiently small in a fine-grained sediment, these forces will act as a barrier to further migration of oil. The small gas molecules, however, can diffuse through extremely small pores and thus escape from shales which form tight seals for oil.

Oil can therefore not migrate as small discrete droplets, but moves as a continuous string of oil where most of the pores are filled with oil rather than water (highly oil-saturated). The pressure in the oil phase at the top is then a function of the height of the oilsaturated column (string) and the density difference between oil and water.