Production-Induced Capillary Breakdown of Reservoir Barriers*

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. 0.00001 0.0001 0.001 0.01 0.001 0.01 0.1 1 frac STOOIP Produced PI ,Q /P ,b bl s/ d/ ps i “Dip” caused by displacement front moving through barrier Barrier breakdown 1 cp 10 cp Stable PI Stable PI Petroleum flows into a water-wet barriers where capillary pressure exceeds the threshold pressure of the barrier. Cross-barrier petroleum flow is significant if barrier flow resistance to petroleum (L/(kakr) is low. These two criteria are met where the barrier is thin and relatively permeable. The barrier must first develop partial petroleum saturation before petroleum can cross the barrier. This is the barrier breakdown process. As production reduces the oi l pressure in the left compartment, barrier water pressure drops, thus increasing the capillary pressure on the right side of the barrier. Oil invades the barrier once the capillary pressure exceeds the threshold pressure (Figure 3, top). Water is displaced into the left compartment, slightly increasing water saturation adjacent to the barrier (Figure 3, middle). Once the barrier has broken down, oil flows across the barrier, oil pressure in the right compartment drops, and a displacement front forms at the interface with the aquifer (Figure 3, middle). The displacement front then moves across the model with relatively small effects from the barrier except for higher flow resistance across the model (Figure 3, bottom). Effects of the barrier can be tracked by changes to the production index (PI). PI drops when the barrier breaks down because the barrier adds to flow resistance across the model (Figure 4). When the displacement front reaches the barrier, PI decreases even more. Sum of phase permeabilities is lowest at the displacement front, so the displacement front adds to the total flow resistance of the model. When the displacement front crosses the low permeability barrier, the total flow resistance of the model increases and the PI drops. After the displacement front crosses the barrier, PI increases due to higher water cut (Figure 4). Shape of the PI vs. cumulative production curve will change with relative permeability assignment. Sw @ 0 d