Hydrodynamic Traps

Movement of water through reservoir rock affects not only the amount but also the distribution of oil. An oil-water contact is usually tilted downward in the direction of flow. The pool can be displaced so far that a well drilled into the crest of an anticline might produce only water. The slope of the oil-water interface, and therefore the location of the pool, is related in a predictable way to the slope of the potentiometric surface (which is always in the direction of flow) and to the difference in density between the oil and the water. The denser the oil, the more it is displaced. Gas, the lightest petroleum fraction, tends to stay near the crest of the anticline.

Oil may accumulate hydrodynamically in a structural feature that might otherwise not trap it. The flow of water through the reservoir bed causes oil to accumulate in the tilted anticline. Were water flow to cease, the buoyancy of the oil would cause it to migrate up dip.

Water flows through a confined permeable layer at a definite volumetric rate. If the cross-sectional area varies, the volumetric flow rate remains constant, but the linear flow rate changes. In other words, to maintain the same number of gallons per hour, the water must flow faster through a narrow section than through a broad section.

In this example water flows downdip from a narrow zone into a broader section. Buoyant oil droplets can migrate upstream against the slow flow in the broad section but not against the rapid flow in the narrows. Oil therefore backs up in a pool at this bottleneck. A zone of reduced permeability can have the same effect because the cross-sectional area of the interconnected pore space is reduced.

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  • Menegilda
    What are hydrodynamic traps?
    4 months ago

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