THE EFFECTS OF SALT TYPE AND SALINITY ON FORMATION WATER VISCOSITY AND NMR RESPONSES

Reservoir water salinity and viscosity are important parameters for formation evaluation and production. For log analysts, salinity is critical to resistivity-based saturation estimation. It is well understood that water salinity affects its viscosity and diffusivity, which, in turn, affects Nuclear Magnetic Resonance (NMR) relaxation time estimation and NMR-based log interpretation. For production engineers, the ability to measure the formation water viscosity is important for enhanced oil recovery because water viscosity affects the CO2 injection via the effective permeability. The formation water viscosity in sedimentary rocks can vary by one order of magnitude for different types and concentrations of salts. Previous studies on formation water properties have focused on NaCl and KCl, the two most common brines in connate water and in water-based drilling mud. The common practice in formation evaluation is to treat the physical properties of non-NaCl brines by a phenomelogical NaCl-equivalent quantity. While this approach may be reasonable for the estimation of brine resistivity, the viscosities of NaCl and KCl follow a different salinity trend. In fact, viscosity versus salinity behaves differently for different saline types. Three factors contribute to the viscosity of the ionic solution: Brownian motion, DebyeHuckel interaction (electrostatic potential due to all other ions surrounded), and structural temperature effect (the structural tightening or loosening due to hydrated or unhydrated ions, respectively). The Brownian motion and the Debye-Huckel interaction always contribute positively to the viscosity of any brine. The structural temperature effect, however, alters the viscosity of brine either positively or negatively depending on the type of salts. Until now, this effect has not been recognized for brines of interest within the formation evaluation community. We found the solute composition of formation water plays an important role in determining its viscosity and diffusivity with varying salt concentration. By considering the structural temperature effect of brine components and using our experimental data on various brines, we are able to predict the viscosity and diffusivity of a large suite of brine types over a selected temperature range that more accurately compares with the previous correlations and does not consider the structural temperature effect. Furthermore, our