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.