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The AAPG/Datapages Combined Publications Database

CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 22 (1974), No. 3. (September), Pages 305-324

Water Above the Transition Zone in Carbonate Oil Reservoirs

N. C. Wardlaw

ABSTRACT

Reliable estimation of water content above the transition zone in oil reservoirs is of fundamental importance in calculating reserves. A major objective of this study is to record the amounts of water in several carbonate oil reservoirs and to evaluate possible factors causing variations in water content. Over 2000 measurements from oil-base cores are used.

Water above the transition zone, expressed as a percentage of rock bulk-volume (SBVW) decreases with porosity decrease for carbonate samples with less than about 7 per cent porosity. Limestone samples tend to contain more water than dolomite samples of equivalent porosity. In contrast with the water-saturation data for carbonate rocks, SBVW appears to increase with porosity decrease for a suite of Cretaceous clastic rocks.

Capillary pressure measurements and visual observation of pore casts from carbonate samples show that pore size and number, and therefore also surface area, tend to decrease as porosity decreases. Decrease of surface area with decrease of porosity also is shown to be the most probable relationship from theoretical considerations of pore geometry. Little is known about the form or distribution of water in pores above the transition zone; however, for several models considered, it seems probable that water content would correlate positively with surface area or with the number of pores per unit rock volume, which would correlate with surface area.

If water content is related to surface area, the decrease in surface area with decrease in porosity, below about 7 per cent, is consistent with the observed tendency for SBVW to decrease as porosity decreases. Dolomites have less water than limestones of equivalent porosity because large pores or vugs, having small area-to-volume ratios, are more abundant in dolomite samples. Similarly, samples with over 7 per cent porosity tend to have large vugs in greater abundance, and water content does not increase proportionately with porosity increase because area-to-volume ratios are less.

Increasing water (SBVW) with decreasing porosity in the clastic sediments may result from the larger surface area associated with more numerous small pores in finer-grained, lower-porosity samples.


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