Wednesday, July 25, 2012

Can a Trap Spill and Leak at the Same Time ?

There are several scenarios when a trap can both spill and leak from a structure trap. Understanding the physics may help us explain the distribution of oil and gas in a petroleum system and predict what may happen in a prospect. 

1) Spill oil and leak gas at the same time: 

In the scenario of the figure below, when the buoyancy force of the combined column reaches the seal capacity (Pc), additional gas will leak, because increasing gas column will increase buoyancy pressure and cause it to leak. But additional oil will spill as increasing oil column by reducing gas column will reduce buoyancy to below seal capacity and cause oil to spill.

leak and spill at the same time
This happens as long as oil and gas densities are different, and the seal capacity is able to hold if the column is entirely oil, but not if the column is entirely gas. ie.
As you can see, this can happen in a wide range of capillary pressures. For a 200 meter structure closure (H), and the typical subsurface oil density of 0.7g/cc and gas density of 0.3g/cc, any Pc between 199 psi and 85 psi will satisfy the condition.  The range is larger with a heavier oil and drier gas.

2) Leak and spill single phase: 

This maybe a bit controversial, but if leaking through the seal is governed by Darcy flow, the rate of leakage could be slower than rage of charge as seal permeability is orders of magnitude lower than that of the carrier bed. Leakage may happen from a smaller area at the crest of the structure, while charge may come from a larger area around the trap. If the charge rate is greater than the leakage rate, the column can build up and the excess fluid may spill. The charge rate may be limited by the generation rate in the kitchen. We can quantify these using reasonable assumptions of seal permeability, but I will spare you the details here.   

3) Changes through time: 

Many shallow (some are Pleistocene in age) oil fields exist in Tertiary basins, indicating charging can begin at very shallow depth. When the shallow traps begin to receive charge, the seal may be very weak. In the first 1 kilometer of burial, typical mud stone seal capacity is less than 50 meters of oil column, and the seal capacity increases with burial due to compaction and diagenesis, reaching two to three hundred meters at the depth of 3 kilometers

As the burial depth increases, seal capacity may become greater than the trap closure, and and additional change will result in spilling. 

4) Other situations: 

Structure tilting can reduce trap closure, causing a trap that was filled to seal capacity to start spilling. Faulting can also affect the seal. The densities (therefore the buoyancy) of fluids migrating into the trap change through the maturation process and affect seal capacity. Seal capacity is not only a function of the seal's pore throat size, and buoyancy, but is also a function of the interfacial tension between the fluids.  Although lighter fluids mean higher buoyancy pressure, their interfacial tension with water is also higher, meaning higher capillary pressure. Secondary effects, such as biodegradation, water washing, and PT changes can also change fluid properties. 

There is even a chance, that the oil under a gas cap in the above figure can leak at the oil/gas contact directly through the seal on the flank of the structure. This is because oil-water interfacial tension is weaker than gas-water, so it can leak with less buoyancy.

In the subsurface, thermogenic gas always contains some condensate liquid, and oil always contain some dissolved gas. When the leaked or spilled gas migrates to shallower depth, the condensate may drop out and form an oil rim. When the oil migrates up dip to a shallower trap at lower pressure, the dissolved gas may effervesce and form a gas cap.