Stringer Detection and Drilling System Significantly Reduces the Invisible Lost Time (ILT) Caused by Reaming

Drilling hard stringers that are erratically distributed in an underlying rather soft formation is challenging from different perspectives. An unforeseen change of the drilled formation from soft to hard and dense rock can cause impact damage to the bit, deflect the bottom-hole assembly (BHA), result in high bending loads, increase vibration, and cause wear/tear on BHA components. If not properly managed, this leads to non-productive time (NPT) and increased maintenance costs. Further, a deflection caused by a stringer away from the planned well path that is detected late results in high local doglegs (HLD) and requires time-consuming correction through reaming with invisible lost time (ILT). Recently, a stringer detection method based on vibrations, namely high-frequency torsional oscillations (HFTO), has been presented. A case study with 21 sections in the North Sea based on this solution is shown that demonstrates a reduction in ILT by 80%. The system is based on a timely and reliable detection of stringers, an optimized mud pulse telemetry scheme, and an automated advisory system. The downhole algorithm embedded in a measurement while drilling tool is consistently interpreting HFTO based on tangential acceleration and dynamic torsional torque measurement. By defining thresholds for the amplitude and the localization with respect to frequency content of HFTO, the algorithm results are translated into a binary value with 1 – stringer currently drilled or 0 - no stringer is drilled. The low bandwidth consuming 1-bit value and downhole measured bending moment are sent in 10 to 15 second intervals to the surface by mud pulse telemetry. Once the stringer is detected, the bending moment data is closely monitored to react correctly and efficiently to a stringer in different scenarios. This solution is discussed in a case study in Norway covering 21 sections with and without the system deployed. The offshore application is challenged by frequently occurring stringer layers and nodules of different geometry. Based on the stringer content, the reaming time has been typically high in this application. The system, however, enabled a timely detection of the stringers and an optimal stringer drilling enabled by the frequently sent bending moment information. Therefore, stringer drilling was done without having to pull off-bottom frequently and ream the transition area between soft and hard formation thereby saving time and reducing wear on the BHA and drill pipe, ultimately ending up with a smoother/straighter wellbore. By using the system, a faster reaction to any stringer and the use of appropriate parameters to avoid costly HLDs are achieved. The case study demonstrates a significant and consistent improvement in ILT. The reaming hours per 1000 m as a benchmark have been reduced from 2-5 hours without to 0.3-0.6 hours with the system resulting in an average saving of 12 hours per reservoir section.