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
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.
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