Production-Induced Capillary Breakdown of Reservoir Barriers

Brown, Alton A.

Slope changes on p/z vs. cumulative production plots and other production responses have been interpreted as breakdown of reservoir barriers. Petroleum flow through the barrier can be caused by either percolation through a water-wet barrier pore system (capillary breakdown) or fracturing the barrier. This paper investigates the capillary barrier breakdown mechanism by numerical modeling of flow across barriers isolating reservoir compartments.

Capillary barrier breakdown results from increasing capillary pressure in the barrier by decreasing water pressure. Water in the barrier communicates with water in the producing compartment, so pressure drop by production reduces water pressure in the barrier. Capillary pressure increases at the barrier contact with the undrained (high petroleum pressure) compartment. When the capillary pressure reaches the barrier threshold capillary pressure, petroleum from the undrained compartment invades the barrier and quickly extends across the barrier. This is barrier breakthrough. Although the barrier is "broken", flow across the barrier is miniscule at breakthrough. Barrier breakthrough is not barrier breakdown.

With further production, capillary pressure continues to drop. Petroleum relative permeability in the barrier increases and cross-barrier petroleum flow increases. Even where the barrier petroleum relative permeability is high, petroleum flow may be negligible if the total flow resistance of the barrier is high.

Capillary barrier breakthrough is expected in any barrier separating reservoirs with production pressure differences greater than the barrier threshold pressure. In contrast, significant cross-barrier petroleum flow (barrier breakdown) requires low petroleum flow resistance combined with moderate capillary threshold pressure. This combination is rare unless the barrier is exceptionally narrow. Barrier breakdown may be rare in real compartmentalized gas reservoirs and absent in reservoirs where petroleum can flow around the barrier.

Apparent changes in p/z slope, especially those early in the production, might alternately be explained by improper average reservoir pressure estimation and gas contribution from tight facies.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013