Petroleum Trap Leakage Processes Revisited

Michael Erdmann¹ and Christian Zwach^2
1Research Center, Hydro Oil & Energy, 5020 Bergen, Norway
²Global Exploration, Hydro Oil & Energy, 0240 Oslo, Norway

Capillary leakage of trapped petroleum occurs ubiquitous and over a wide range of physical properties of sealing rocks. The competing infill (Petroleum charge) rate is critical when a capillary barrier is broken.

We are revisiting the fundamental process involved in capillary leakage from traps using North Sea examples and interpret experimental results simulating the leakage process over geologic time.

Experimental results showed depletion of trapped petroleum when charging stopped and capillary leakage continued. The leakage rate is, among other things, controlled by the permeability of the cap rock and possibly also by the seal thickness.

The leakage rate competes with the charge rate in case of an active charge which may lead to an increased column height even though the accumulation is leaking. When leakage occurs but an active charge is missing the accumulation will be depleted until reaching a so called snap-off pressure. As a consequence, it is assumed that leaked accumulations will always maintain a residual column and that previously filled column will maintain a residual saturation as the case of a residual oil zone beneath the productive oil zone of Troll demonstrates.

The revisited hydrocarbon leakage problem demonstrates that the key to understanding hydrocarbon trap leakage lies in understanding the charge history and the trap volume history. The results are in contrast to the model of using a fixed snap-off pressure to determine the column height after leakage occurred. Experimental results also suggest that permeability and possibly seal thickness play a major role for the magnitude of leakage rates. We propose that the leakage process is more complex and much more charge-rate depending as previously assumed.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas