Through-tubing (TT) drilling was used to drill Upper Permian Zechstein carbonates in Germany. The project was partially set up as an experimental performance comparison between a positive displacement motor (PDM) and a turbodrill bottomhole assembly (BHA) equipped with real-time logging tools. This would be a world-first drilling run for this size of turbodrill as well as the first turbodrill run on coiled tubing (CT) using this real-time data feedback.
Through-tubing coiled tubing (TT-CT) drilling incurs significantly lower cost per drilled footage than conventional rotary drilling methods. A dedicated candidate selection process emphasized the geometry and extent of the pay zone, borehole stability, tubing geometry, and well integrity, which are all crucial for the application of TT-CT drilling. As a result of this screening process, well Sh Z1a, a horizontal sour gas production well was selected. Dedicated BHA designs were used for both the PDM and the turbodrill to allow for a direct performance comparison whilst drilling the section.
TT-CT drilling was used to deepen the well. Initially, 410 ft [125 m] were drilled with a 2.283-in. [58.0-mm] impregnated bit and a 2.125-in. [54.0-mm] PDM using 1.750-in. [44.5-mm] CT suitable for sour service. An additional 148 ft [45 m] were drilled with a 2.283-in. [58.0-mm] impregnated diamond bit with polycrystalline diamond compact (PDC) cone cutters. This bit was run with a 2.125-in. [54.0-mm] turbodrill BHA equipped with real-time logging tools using a sour-service-suitable 1.750-in. [44.5-mm] CT with a fiber-optic cable. The section was drilled under total losses and through multiple natural fractures. While drilling with the real-time logging tool, internal and external downhole pressures, torque, and tension and compression forces were recorded to assist in the drilling process. Direct comparison of the PDM and the turbodrill BHA was made for rate of penetration (ROP), weights and pressures. The turbodrill BHA produced more than double the ROP than was possible with the conventional PDM BHA using less weight on bit and showed overall smoother drilling mechanics.
The well is currently on production. The project showed that TT-CT drilling is a viable technology for production enhancement in depleted gas reservoirs and that new turbodrill technology generates significant performance improvements over conventional drilling tools.