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AAPG Bulletin, V.
Capillary effects on fault-fill sealing
11603 Waterview Drive, Richardson, Texas, 75080; email: altonabrownyahoo.com
Alton Brown worked as a research geologist at ARCO's Plano, Texas, Research Center from 1980 until ARCO's merger with BP. Since then, he has been an independent consultant. His research interests include petroleum migration, carbonate sedimentology and diagenesis, basin analysis, and gas geochemistry.
I thank Russell Davies for constructive comments early in manuscript preparation and for later reviews and suggestions. I also thank AAPG reviewers Quentin Fisher and Jim Handschy for additional comments and suggestions.
Capillary-pressure models and concepts were used to evaluate the effects of excess pressure, capillary hysteresis, and relative permeability on fault-fill sealing. Overpressured fault fill (fault water pressure higher than reservoir water pressure) always increases the height of the sealed petroleum column. The sealing interface moves into the overpressured fault fill where water flows from the fault into the reservoir. Underpressured fault fill decreases petroleum column height only where cross-fault water flow is absent. If water flows across the fault, column height is unaffected. Water cannot flow across faults where the reservoir is at irreducible water saturation.
Relative permeability smoothes the transition from membrane sealing to leakage, and thus, hydraulic-resistance sealing is possible after membrane-seal failure. The height of membrane sealing by homogeneous, water-wet fault fill exceeds the height of hydraulic-resistance sealing at geological leakage rates. Hydraulic-resistance sealing becomes more significant when charge and leakage are both high, when trap life is short, and during production.
Trap leakage rate through a water-wet, fault-fill pore network cannot exceed trap charge rate during initial trap charging. If charging slows, leakage exceeds charge until a new equilibrium column height develops. If charge stops, the seal continues to leak until the petroleum column height is reduced substantially below its original height. Membrane sealing is reestablished at low capillary pressure. Theoretically, restored seal capacity is close to the original capacity.
Cross-fault pressure and petroleum column height cannot be converted to seal capacities because charge history and seal type influence sealing. Cross-fault pressure data should be analyzed in light of the charge and pressure history so the different controls on fault-fill sealing can be assessed.
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