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Thc Ainrrlcan Associalion ot Pelriilcum Geoiogists BulleiinV 76, Ni ) 6 (June 1912). P 840-850, 7 Figs.. 6 Tables

Geologica l Appl icat ions of Capil lary Pressure:A ReviewlCharles L. Vavra,zJohn G. Kaldi,3 and Robert M. Sneidefi

Capillary pressure concepts can be used to evaluatereservoir rock quality, expec ted reservoir fluid satura-tions and depths of fluid contacts, thickness of transi-tion zone, sea1 capacity, and pay versus nonpay, and toapprox imate recovery efficiency. Mercury-injectioncapillary pressure is typically favored for geologicalapplications, such as inferring the size and sorting ofpore throats. The differences between mercury injec-tion and withdrawal curves can provide information onrecovery efficiency. The height above free water levelcan be determined by comparing capillary pressuredata to hydrocarbon shows and measured fluid satura-tions. Capillary pressure data can also be used to distin-guish reservoir from nonreservoir rocks and pay fromnonpay on the basis of nonwetting-phase saturations.Other applications of capillary pressure data includerelating capillary pressure to absolute and relative per-meabilities, and using porosirnetry to investigate pore-level heterogeneity. This paper reviews geologicalapplications and interpretation of capillary pressure inreservoir stiidies.

INTRODUCTIONCapillary pressure concepts can be used by geolo-

gists. petrophysicists and petroleum engineers to evalu-ate reservoir rock quality, pay versus nonpay, expectedreservoir fluid saturations, sea1 capacity (Le., height ofhydrocarbon column a sea1 can hold before it leaks;

OCopyright1992.The Arnerican Association of Petroleum Geologists. Allrights re se ~e d.'Manuscript received August 19, 1991; revised manuscript receivedJanuary 31,1992; final acceptance February 10, 1992.2ARC0 Exploration and Production Technology, 2300 West PlanoParkway, Plano, Texas 75075.3ARCO Exploration and Production Technology, 2300 West PlanoParkway, Plano, Texas 75075. Present address: Atlantic Richfield Indonesia,Inc., P.O. Box 260888, Plano, Texas 75026.%obe~-tM. Sneider Exploration, Inc., 11767 Katy Freeway, Houston,Texas 77079.We thank M. Scheihing, G. Jerauld, F. Smith, J. Rathrnell, W. Ebanks,Jr.. and C. Atkinson for their comrnents and discussions. The rnanuscriptwas reviewed and improved by cornments of D. Powley, D. Spain, and N.Wardlaw. We thank ARCO Exploration and Production Technology forperrniscion o publish.

throughout this paper, "seal" refers to membrane sealsto hydrocarbon in water-wet rocks), depths of thereservoir fluid contact, and thickness of the transitionzone, and to approximate recovery efficiency duringprimary or secondary recovery. Evaluating capillarypressure of potential hydrocarbon reservoir and sealrocks is important because capillarity controls the staticdistribution of fluids in the reservoir prior to produc-tion and the remaining hydrocarbons after primary pro-duction.Capillary pressure results from interactions of forcesacting within and between liquids and their boundingsolids. These include both cohesive (liquid-liquid)forces and adhesive (liquid-solid) forces. Where adhe-sive forces are greater than cohesive forces, the liquidis said to b e "wetting" (Figure 1) ; if cohesive forcesexceed adhesive forces, the liquid is "nonwetting." Therelative wettability of the fluids is described by the con-tact angle ( O ) , which is the angle between the solid andthe fluid-fluid interface as measured through the denserfluid (Figure 1).

If the end of a narrow capillary tube is placed in awetting fluid, net adhesive forces draw the fluid intothe tube (Figure 2). The wetting phase rises in the cap-illary above the original interface or free water leveluntil adhesive and gravitational forces are balanced.Because the wetting and nonwetting fluids have differ-ent densities, they have different pressure gradients(Figure 2). Capillary pressure (P,) is defined as the dif-ference in pressure across the meniscus in the capillarytube. Put another way, capillary pressure is the amountof extra pressure required to force the nonwettingphase to displace the wetting phase in the capillary.Capillary pressure can be calculated by

20 cos ePc.=-cwhere p , nd p are the specific gravities of the wet-ting and nonwetting fluids, respectively; g is the gravi-tational constant; h is the height above the free waterlevel; O is interfacial tension; O is the contact angle

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