System for geosteering and formation evaluation utilizing near-bit sensors

A method of measuring a formation surrounding a wellbore, comprising:

• deploying a downhole drilling tool into the wellbore, wherein the downhole drilling tool comprises:
• a downhole motor comprising a rotor and a stator;
• a universal-joint assembly comprising an upper universal joint and a lower universal joint and a connecting rod connecting the upper universal joint and the lower universal joint;
• a drive shaft assembly comprising a drive shaft;
• a drill bit coupled to a distal end of the drive shaft;
• and
• a measurement sub,
• wherein a distal end of the downhole motor is coupled to the upper universal-joint, the drive shaft assembly is coupled to the lower universal-joint, and
• wherein the connecting rod extends through the measurement sub in a longitudinal direction of the measurement sub, and
• wherein the stator in the downhole motor comprises a conduit built in the elastomer liner that receives an electric wire or a data cable, and
• wherein the measurement sub comprises:
• a resistivity measurement tool for measuring a formation resistivity, the resistivity measurement tool having a transmitter, a first receiver, and a second receiver;
• at least one gamma ray detectors disposed on a surface of the measurement sub;
• at least one accelerometer for measuring an inclination of the wellbore;
• at least one ultrasonic probe for measuring a caliper of the wellbore;
• and a communication module for transmitting data through wired connection or wirelessly;
• rotating the measurement sub with the downhole drilling tool;
• emitting electromagnetic waves at two or more frequencies using the transmitter in the resistivity measurement tool;
• receiving azimuthal electromagnetic signals at the first receiver and the second receiver in the resistivity measurement tool;
• calculating a formation boundary information based on measured amplitude ratios and measured phase shifts between electromagnetic fields from two azimuthal sections having a 180° phase difference;
• obtaining an apparent resistivity based on the measured amplitude ratios, the measured phase shifts, or both the measured amplitude ratios and the measured phase shifts between electromagnetic signals received by the first receiver and by the second receiver in the resistivity measurement tool;
• adjusting the apparent resistivity for a downhole temperature effect;
• measuring the wellbore caliper using the at least one ultrasonic probes;
• obtaining a formation resistivity by adjusting the apparent resistivity for an effect of the wellbore caliper;
• obtaining gamma ray readings using the at least one gamma ray detectors disposed on the surface of the measurement sub;
• and constructing a gamma ray image of the formation.