New Experimental Approach of Saturation Exponent Measurements by Continuous Injection Technique

Abstract

Water saturation determination from wireline resistivity log depends on the measurements of resistivity index on core samples. Resistivity index measurements are carried out in the laboratory using Archie’s equation:  I_R = R_t / R₀ = s_w^-n . Errors in the saturation exponent ( n ) can give rise to serious errors in the estimation of hydrocarbon saturations. One of the major sources of uncertainty is the method of measurements. The continuous injection technique is used routinely in the oil industry. In this technique, one fluid phase (brine) is continuously displaced by another phase (oil) at a constant injection rate. In this paper, a new experimental approach is proposed in which repeated resistivity index measurements are carried out on the same samples at various injection rates. The effect of injection rate on saturation exponent for core samples can be investigated. Four carbonate core samples with a wide range of porosity and permeability were tested to demonstrate resistivity index measurements at low and high injection rates. Use of continuous injection technique at different rates has illustrated that for the water-wet samples a uniform  I_R / S_W correlation was established over a wide range of ( S_W ) when low rate is used. It is concluded that the injection rate selection has an important role on the I_R / S_W relation. It is recommended as a result of this work that for resistivity index measurements, various injection rates should be used to obtain the best I_R / S_W  relation and consequently the proper saturation exponent could be obtained.