Abstract
The proposed method in this paper estimates FWL based on the relationship derived between resistivity, capillary pressure, water saturation and permeability. Since porous plate experiment provides simultaneous resistivity index (RI) and capillary pressure at identical saturation state, one can combine these two pieces of information (Pc-vs-Sw and RI-vs-Sw) and produce a new Pc/Rt-vs-Sw plot. Interestingly, Pc/Rt-vs-Sw exhibits an exponential behavior which can be described using an exponential function with fitting coefficient a, b, c and d. Once these fitting coefficients are obtained for each individual plug, they are correlated to petrophysical property such as permeability. After all the relationships are established on core plugs, the log-based capillary pressure (Pc, laboratory condition) can be estimated from the resistivity, water saturation and permeability logs. For application in oilfields, this laboratory condition Pc has to be converted to reservoir condition Pc. The reservoir condition Pc is interpreted as reservoir condition buoyancy pressure (Pb). We then subtract this reservoir condition Pb from the hydrocarbon pressure established from the formation tester. Finally, a water line with a known gradient (eg: 0.433 psi/ft) can be drawn across the predicted log-based buoyancy pressure. FWL is located at the intersection between this ‘pseudo’ water line and hydrocarbon line established from formation tester.
