On surface gamma ray sensor for calibrating cuttings depth in underbalanced coiled tubing drilling rigs

A system for on surface calibration of cuttings depth in underbalanced coiled tubing drilling (UBCTD), the system comprising:

• a drill string configured to drill a well and collect a plurality of drill cuttings at a first point and at a second point defining a depth interval therebetween selected for calibration, wherein the depth interval comprises a predetermined interval length selected for validation of cuttings depth accuracy;
• a gamma ray device disposed at the surface configured to measure a gamma ray response for the plurality of drill cuttings at the first point and the second point of the depth interval;
• a bottom hole assembly (BHA) tool disposed on the drill string configured to measure a plurality of downhole gamma readings over a plurality of depths spanning the depth interval between the first point and the second point;
• and
• an operator located on surface configured to determine a calibration and validation assessment related to the depth interval, the calibration and validation assessment comprising a comparison between the gamma ray response at the first point and the second point and an average of the plurality of downhole gamma readings measured within the depth interval to confirm accuracy of the depth interval of the plurality of drill cuttings.