Reservoir water salinity is an extremely important parameter in the analysis of open hole logs, as value of formation water resistivity (Rw) is required to calculate the saturation of fluids in the pore space of the reservoir rock. Salinity (physico-chemical analysis) information also helps to depict depositional environment.
It is believed that pore space of all the marine sediments are primarily filled up with sea water. But with geological time, salinity varies laterally as well as vertically. There are examples of fields where significant variation of salinity has been observed.
Several sources or methods are used to determine the resistivity of the formation water (Rw) as demonstrated below:
In low salinity (below 20000 ppm) environment, Archie’s equation for saturation determination may not work. All the shaly sands exhibit low resistivity due to decrease in apparent formation factor (F) as compared to the respective actual formation factor (F*). This is also known as Non-Archie effect attributed to the surface conductance phenomenon or ionic double layer effect which may or may not be related to the clay.
Deriving Formation water salinity is just an small piece of large puzzle we call :-Formation Evaluation and Reservoir Characterization.For details please click link below: