More Oil in Leaky Reservoirs?

A simple view of structural trapping of hydrocarbons is that the caprock has zero permeability or entry pressure. In this picture a virgin reservoir is in a state of static equilibrium and no external fluid flow is needed to maintain it through geological time. In reality, caprocks may have low, but finite, permeability, so that hydrocarbons may "leak" through the caprock over geological time. If the caprock leakage rate is sufficiently high to require being replenished by continued inward migration from source rocks to maintain a reservoir over geological time, then a reservoir might still be said to exist, but in a state of dynamic equilibrium . Eugene Balbinski of ECL Technology Ltd ( Eugene.Balbinski@ecltechnology.com ) presents a summary of a project being undertaken under the DTI's SHARP Programme to evaluate the Siddiqui-Lake theory of "leaky reservoirs".

The Siddiqui-Lake (S-L) theory was formulated to explain reservoirs in dynamic equilibrium [1, 2]. For the UKCS the theory is of interest both for the possibility of discovering unexpected reservoir traps and also because such reservoirs in dynamic equilibrium can, in principle, hold more hydrocarbon than those in quasi-static equilibrium. The following conclusions have been drawn from a DTI SHARP project aimed at a preliminary evaluation of this theory.

The mathematics of the S-L theory are related to Buckley-Leverett theory and there are no obvious grounds for disputing its application in this context. However, there is relatively little data as yet to demonstrate the occurrence of dynamic trapping in actual reservoirs and so verify the theory. The quantitative predictions of the S-L theory are generally conditional on three key assumptions:

• The influx rate of hydrocarbons migrating into the reservoir is sufficiently high. Although estimates have been published, more supporting data are needed.
• Counter-current relative permeabilities are similar to co-current. There are indications they may be somewhat less.
• The effect of capillary pressure on the hydrocarbon flux can be neglected. Taking account of this will increase trapping, so it is a conservative assumption.

Reservoir seals with relatively high permeabilities, e.g. 1 mD, appear to be able to trap hydrocarbons dynamically at expected migration influx rates. Typical hydrocarbon migration rates have been estimated as around 10^-9 m/s laterally in sandstones and 10^-14 m/s vertically in shales [3]. These predictions are relatively insensitive to whether the hydrocarbon is oil or gas, but sensitive to assumptions about the relative permeabilities and capillary pressure.

Figure 1: Siddiqui-Lake Theory Predicts that Dynamic Trapping is Possible at High Seal Permeabilities (Click for larger view)

There also appears to be a good probability of the persistence of dynamically trapped hydrocarbons over geological time. If hydrocarbon influx ceases, the hydrocarbon leakage time can be long, much greater than 100 million years, even for seals with relatively high permeabilities of the order of 0.1 mD.

Figure 2: Siddiqui-Lake Theory Predicts Possible Persistence of Dynamical Trapping Over Geological Time (Click for larger view)

One way in which the S-L theory might be tested is to investigate the pressure profile for virgin reservoirs in the main hydrocarbon zone. For dynamic trapping this may be intermediate between the hydrostatic water and hydrocarbon gradients, depending on the seal leakage. A correlation between such a non-standard pressure profile and evidence of hydrocarbon migration would be suggestive evidence for dynamic trapping.

References

1. A Comprehensive Dynamic Theory of Hydrocarbon Migration and Trapping , F I Siddiqui and L W Lake, SPE 38682, Presented at the 72nd Annual Technical Conference and Exhibition, San Antonio , October 1997
2. Defining Fluid Distribution and Fluid Contacts for Dynamically Charged Reservoirs , F I Siddiqui, SPE 56513, Presented at the 1999 Annual Technical Conference and Exhibition, Houston , October 1999.
3. The Movement and Entrapment of Petroleum Fluids in the Subsurface , W A England , A S Mackenzie, D M Mann and T M Quigley, Journal of the Geological Society, London , Vol 144, 1987

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