I am not expert in the subject matter, however think the answer to the question is yes. Microwave (wavelengths over the range 1mm-1m) can be used to heat a region, thereby to affect the viscosity of fluid (oil here). On the basis of the known temperature dependence of the viscosity, one can deduce the porosity (and other local parameters) by measuring the flow and combining the observation with the Darcy law for fluid flow in porous media. As regards the temperature dependence of the viscosity of oil, you may wish to consult the attached page. Further, in the book Microwave-Assisted Concrete Technology: Production, Demolition and Recycling, by KC Gary Ong, and Ali Akbarnezhad (CRC Press, 2017) -- link attached, I read some applications of microwave, including local heating, thereby to determine the porosity of the concrete in conjunction with the Darcy law for fluid flow.