Worldwide, carbonate oil-water transition zones containvast amounts of producible oil. Yet, traditional approaches toopen-hole formation evaluation often fail to predict how muchoil should flow from them, or even the location of the freewater levels. A theory applying capillary pressure scanningcurves shows how changing water saturations and variationsin levels of mixed wettability systematically control thedifferences in the pressures of the invading mud filtrate andformation oil, to result in the following unusual yet oftenobserved behavior: 1) negative pressure gradients, 2) waterlikegradients significantly above the free water level, 3)significant shifts in the measured pressure potentials betweenthe lower and upper part of the transition zone, 4) gradientsimplying an oil-density different to that which is expected.Supercharging effects are shown to be unimportant to thediscussion. Both wells drilled with water based mud and oilbased mud are considered. It is shown how it is usuallypossible to produce oil from a zone which has a water-likepressure gradient and low formation resistivity. The theory issupported by detailed analysis of examples from flowsimulations, which recreate the well known field casesreferred to above. Guidelines are presented on how to interprettraditional open hole pressure measurements in a carbonateoil-water transition zone to determine the free water level andthe locations where oil should flow, and on how to improve onthese interpretations by performing more advanced formationtesting procedures, some of which are based upon newtechnology.
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