Faults as Hydrocarbon Phase Filters and Dynamic Seals

Øyvind Sylta
Migris AS, Trondheim, Norway (email: [email protected])

Fault-sealed traps depend on barriers other than four-way closure to provide seals for migration of oil and gas. The assessment of seal potential of such barriers may therefore be an important aspect of exploration risk assessments. The assessment can typically address three questions: Is the barrier a seal? (1); how large columns of oil and/or gas can be sealed by the barrier? (2); what are the uncertainties? (3). In this paper we address question (2).

From the perspective of multi-phase fluid flow, the sealing potential of faults can be divided into three parts: a geometric seal, an entry (threshold) seal and a dynamic seal. For a fault that only exhibits a geometric seal, migration of oil and gas will act as a classical spill-point process.

Entry (threshold) and dynamic seals are results of capillary flow through barriers to fluid flow. As long as there is an entry pressure that resists cross-barrier migration, the liquid and vapour hydrocarbon phases will react slightly differently to it. If a trap has been filled with oil and gas starts to replace oil, then gas will easily push oil out of the trap. However, there are scenarios where oil columns will be protected by the sealing barriers of faults because gas will not fill below the geometric leak point.

A dynamic seal occurs when the effective permeabilities of the leak zones are too low to accommodate the flow-rates created by migrating volumes of oil and gas into the trap. The amounts of dynamic leakage through fault barriers are quantified for traps with realistic filling rates. The calculations suggest that there is a strong positive correlation between the entry column of a fault seal and it’s dynamic column. The fraction of dynamic seal versus entry seal increases as the entry seal increases. We conclude that dynamic seals need to be accounted for in geologic scenarios where large quantities of oil and/or gas flow through fault seal traps over fairly short geologic times.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands