Methods and apparatus for evaluating downhole conditions with rfid mems sensors

A system comprising:

• a casing string having a communication assembly supported by the casing string, wherein the communication assembly is configured to obtain information associated with radio frequency identification (RFID) tags in an annulus surrounding the casing when the casing is in place within a borehole;
• and
• a first plurality of RFID tags placed within a first fluid pumped into the annulus, each of the first plurality of RFID tags including,
• a die attached to an inductive-capacitive (LC) circuit, the LC circuit having a first resonant frequency, and
• a coating material disposed around the die to form an outer surface, the coating material having a thickness to permit a conductivity property of the first fluid to affect the first resonant frequency such that the first resonant frequency shifts to a shifted resonant frequency;
• and
• a processor configured to receive data indicative of the shifted resonant frequency and to determine a property of the fluid based on the shifted resonant frequency.

A method for monitoring wellbore conditions, the method comprising:

• associating a sensor assembly with the exterior of a casing string being placed in a borehole, wherein the sensor assembly is configured to communicate with radio frequency identification device (RFID) tags in an annulus surrounding the casing when the casing is in place within the borehole;
• pumping a fluid into the annulus, the fluid containing a plurality of RFID tags, each of the plurality of RFID tags having a first inductive-capacitive (LC) circuit with a first intrinsic resonant frequency, the plurality of RFID tags having an associated set of shifted resonant frequencies to which the first intrinsic resonant frequency shifts responsive to properties of the fluid;
• providing, by the sensor assembly, a plurality of interrogation frequencies, the plurality of interrogation frequencies including each shifted resonant frequency of the set of shifted resonant frequencies and the first intrinsic resonant frequency of each of the plurality of RFID tags;
• and
• detecting a property of the fluid upon receiving a signal with a first shifted resonant frequency, responsive to providing an interrogation frequency of the plurality of interrogation frequencies, from an RFID tag of the plurality of RFID tags.

An RFID tag, comprising,

• an induction-capacitance circuit, including,
• an inductor in the general form of a two dimensional helix expanding from a central region, the inductor having a first inner end and a second outer end, the first inner end extending from the central region,
• a capacitor formed entirely within the central region, and having a first and second electrodes, the first capacitor electrode coupled to the first inner end of the inductor, and
• a conductive link coupling the second outer end of the conductor to the second capacitor electrode;
• an encapsulant surrounding the induction-capacitance circuit.