Understanding the physical properties of the formation water that will be produced along with the oil or gas is important to a proper assessment of reserves volumes, producibility, economics, and surface facilities. As a rule, it is best to have reliable laboratory measurements of the physical properties of oilfield waters. If laboratory measurements are not available, correlations may have to be used. For example, McCain has published some of the most widely used correlations for the physical properties of oilfield waters. This page discusses the resistivity, surface (interfacial) tension, viscosity, pH, and redox potential (Eh) of produced water.