Hydrocarbon Migration and Its Mechanisms
Secondary migration of oil and gas: refers to the process of oil and gas conducting through reservoirs with higher porosity and permeability, or large fractures and unconformities, after they have been released from the source rock. This includes the re-migration of accumulated oil and gas due to changes in external conditions.
Phase: After oil and gas are initially transported from the source rock into permeable rocks, they begin secondary migration. Due to the change in the medium environment of secondary migration, which is mainly a permeable porous medium with large pore spaces and permeability, capillary pressure decreases and permeability increases, facilitating the movement of pore fluids (including water, oil, and gas). Therefore, during secondary migration, oil and gas generally migrate as a continuous free phase, and should be regarded as seepage action within porous media.
The Mechanism of Oil and Gas Secondary Migration: The secondary migration of oil and gas depends on whether the migration conditions are met. Firstly, it must have a certain degree of oil and gas saturation; only when the oil and gas saturation exceeds the critical saturation, there will be relative permeability and effective permeability. Secondly, the oil column must be greater than the critical height, with sufficient buoyancy and hydrodynamic force to overcome capillary resistance. Under static water conditions, the condition for the oil to rise is that the buoyancy force Fr should be greater than the capillary resistance difference Pc; under dynamic conditions, the migration condition for the oil is that the vector sum of the buoyancy force Fr and the hydrodynamic force Fo, Eo, should be greater than the capillary resistance difference Pc; when they are equal, oil and gas accumulation occurs.
Therefore, as water flows from a higher potential area to a lower potential area, oil and gas also spontaneously seep from their higher potential areas to lower potential areas under the influence of their field intensity. The existence of potential differences in oil and gas is the driving force for secondary migration.