HP/HT Sensor for Real-Time Downhole Density Measurement of Wellbore Drilling Mud

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Abstract There is a need to accurately measure the density of drilling mud and well flow rate at downhole locations in the wellbore. Mud density can be determined by measuring the differential pressure between two locations in the well. The most common manner of measuring flow rate is also through the use of a differential pressure sensor. The development of a true differential pressure (DP) sensor for downhole measurement at HP/HT conditions (500 Deg F and at 15,000 psi) was presented at the 2015 OTC (paper #25772-MS). The excellent performance of the initial two DP sensors led to the fabrication and testing of additional DP and absolute pressure cells to verify the design. Since the performance of the DP sensors showed a maximum error at each of the HP/HT extremes of less than 0.02% of the upper-range limit, their use with remote diaphragms to measure the density of the drilling mud in the wellbore was considered a possibility. If successful, this measurement could be integrated into a wired drill string to not only indicate the occurrence of a ‘kick’ during drilling, but also provide unprecedented monitoring and control of unwanted changes in mud density throughout the entire length of the drill string annulus. This would provide significantly improved early warning and control of dangerous high-pressure flow into the wellbore. Additional funding by the RPSEA 10121-4304-01 program allowed for the development of a low-differential pressure range sensor cell with a significant increase in sensitivity to a change in density of the wellbore drilling mud. Sensor performance data was obtained: (i) at ambient temperature and various common mode pressures and (ii) at ambient common mode pressure and various sensor temperatures up to 250°C. The calibration results of the HP/HT downhole DP sensor cell with remote diaphragms indicated adequate ‘kick’ detection sensitivity to enhance drilling safety in high-pressure zones. Additionally, possible methodologies to integrate the wellbore density sensor assembly into the drill string will be discussed.