Archie’s Parameters of a Libyan Carbonate Reservoirs: Effect of Porosity Type: Case study.

الملخص

Abstract

The water saturation determinations from resistivity logs are based on the famous Archie’s equations; S_w = (ϕ^-m R_w/R_t)^1/n. The cementation factor, m and saturation exponent n, play an important role in S_w calculation. In carbonate formations, generally, m=n=2 is usually used. However, m values between 1.88 and 2.5 have been measured, and also n was found to be between 2.11 and 2.78 for a Libyan carbonate reservoir. Inaccurate m and n values would lead to large inaccuracies in the calculated hydrocarbon/water saturations. 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 four-electrodes system. Reservoir fluids are simulated using a brine and oil from the formation of interests. As a result of this study, it has been found that the Archie’s parameters are functions of pore structures. Samples mainly interparticle or intercrystalline porosity, shows m value to be less than or equal 2, while vuggy samples generate higher values of m. The resistivity/saturation correlation is well fitted by the Archie’s law except for vuggy samples, where n was found to be as high as 2.78. As a sequence, it is recommended that in order to evaluate cementation factor and saturation exponent for carbonates, representative number of samples from each zone or interval porosity type should be used rather than an average of m and n of the whole formation.