Abstract
Water saturation determinations from resistivity logs are based on Archie’s equation: Sw=(ϕ-m Rw /Rt)1/n. The cementation factor (m), and saturation exponent (n) play an important role in (Sw) calculation. In carbonate formations, (m = n = 2) is usually used. However, (m) values between 1.88 and 2.5 have been measured, and (n) was found to be between 2.11 and 2.78 for a Libyan carbonate reservoir. Incorrect (m) and (n) would lead to large inaccuracies in the calculated hydrocarbon/water saturation.
An experimental study was carried out to measure (m) and (n) for carbonate samples with a wide range of porosity and permeability. Ten samples were collected from a formation having vuggy and interparticle porosity types. The resistivity measurements were performed using the porous plate method, with a four-electrode system. Reservoir fluids were simulated using a brine and oil of the formation of interest. As a result of this study, it has been found that Archie’s parameters are functions of pore structures. Samples, mainly with interparticle or intercrystalline porosity, show (m)-value to be less than or equal to 2, while vuggy samples generate high values of (m). The resistivity/saturation correlation is well fitted by Archie’s law except for vuggy samples, where (n) was found to be as high as 2.78. As a consequence, it is recommended that in order to evaluate cementation factors and saturation exponent for carbonates, representative numbers of samples from each zone or interval porosity type should be used rather than an average of (m) and (n) of the whole formation.