Methods and apparatus for evaluating downhole conditions through fluid sensing

A method using a sensor 1400 associated with the exterior of a casing string comprising providing an excitation signal to a first electrode of the sensor, receiving a signal at a second electrode of the sensor, and using the received signal to determine a frequency dependent dielectric property of a fluid in an annulus between the casing 20 and formation walls 1414. The sensor may also include a third electrode for receiving a second received signal. The second and third electrodes may be different distances from the first electrode. The electrodes may measure impedance to determine the dielectric properties of the fluid. The electrodes may be spaced longitudinally or laterally around the casing and may be a distinct component or formed from a coating. Also disclosed is a system for detecting downhole micro-electro-mechanical systems (MEMS) sensors and radio frequency identification (RFID) tags.