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The AAPG/Datapages Combined Publications Database

Rocky Mountain Association of Geologists


The Mountain Geologist
Vol. 45 (2008), No. 4. (October), Pages 107-119

Sub-Capillary Pressure Equilibrium Reservoirs: Their Previous HitFormationNext Hit and Effect on Oil and Gas Production

Monty Hoffman


Most reservoirs in the Western United States are not in capillary pressure equilibrium, but are under-saturated in water. This is a pressure volume temperature effect that also causes many, but not all, of these same reservoirs to be underpressured. The cause is pore expansion and water shrinkage caused by uplift and erosion of the Western United States in the late Miocene and early Pliocene. The changes to the production characteristics of the reservoirs that this causes include increasing relative permeability to hydrocarbons, making the reservoir susceptible to water imbibition with resulting Previous HitdamageNext Hit to relative permeability to hydrocarbons, and limiting reservoir connectivity to the wellbore. Preventing relative permeability Previous HitdamageTop from water imbibition requires not having the reservoir come in contact with water during drilling, completion, or production operations. Despite the fact that almost all reservoirs in the western United States are not in capillary pressure equilibrium, some still produce even after being exposed to water during operations because other reservoir characteristics limit water imbibition or flow. These include low permeability, wettability, and overpressuring. These factors allow for commercial, but not optimal completions. Water wet and intermediate wet sub-capillary pressure equilibrium reservoirs that have been drilled and completed with water-base fluids have not been properly evaluated for their true productive capacity.

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