The disclosure provides a system, tools and methods for estimating a property or characteristic of a fluid downhole. In one aspect, the method may include: heating the fluid at a selected or first location during a first time phase, taking temperature measurements of the fluid substantially at the selected location during a second time phase, and estimating the property of the downhole fluid using temperature measurements. Temperature measurements may also be taken at a location spaced apart from the first location and used to estimate the property of the fluid. The tool may include a device that heats the fluid during a first time phase and takes temperature measurements of the fluid during a second time phase. A processor uses the temperature measurements and a model to estimate a property of interest of the fluid.