Water saturation on Albian carbonates reservoirs-ancient Brazilian oil fields

Abstract

One of the main objectives for petroleum exploration is to quantify the reservoir's oil in place; usually this process is based on a petrophysical evaluation that, among other parameters, the reservoir water saturation (S-w) must be calculated, because it represents the main factor for determining hydrocarbon saturation (S-h). Classical petrophysical analysis includes the use of the water saturation's resistivity (R-w) and two main constants (for Archie saturation), as well as cementation (m) and tortuosity (a); both of which are obtained from laboratory plug/cores measurements. However, in the case of ancient Albian carbonate reservoirs (Brazil), this kind of data is not available for a fully water saturated reservoir, and as a consequence, we proposed to apply a logarithmic graphic solution of the main Archie's equations to determine these main factors using a linear regression based on wire logs. As result, an R-w was confirmed at 127 Kppm of NaCl for Quissama Formation, m = 1.071 and a = 0.87. Finally, the value of 'm' is more affected by the pore and size configuration, and the 'a' value appears to be a logical answer for carbonates reservoir with high porosity.