## 4 Discussion  
In 2019, the Perfobore company conducted a series of field trials (FT) of special assemblies with centering elements placed on a developed small-sized sectional PDM in the Arlanskoye oil field of LLC Bashneft-Dobycha. One of their test objectives was to verify compliance with the calculated (planned) and the actual trajectories. As part of the FT, two different geophysical instruments were lowered into the drilled channels with the help of TS to eliminate the errors associated with the possibly of incorrect operation of the inclinometers. One inclinometer was a small-sized magnetometric inclinometer, \"KYARTS-36\", and the second one was a small-sized gyroscopic inclinometer \"TWIN GYRO\". Both inclinometers were autonomous.  
Figure 6: Typical characteristics and points of characteristic modes of small-sized PDM.  
Based on the developed methodology for choosing the optimal variant of the power section of a downhole drilling motor, a rational configuration of a universal downhole drilling motor was selected for drilling a network of branched channels using the TS Perfobore. It has been established that the use of PS with a kinematic ratio of 5:6 does not meet the basic technical requirements needed in terms of geometric, tribological indicators. A screw pair with a kinematic ratio of 9:10 is recommended as the optimal version of a small-sized PDM. The PDM with a diameter of 48 mm with a kinematic ratio of 9:10 was adopted for implementation, since its manufacture (according to a special technical assignment) is economically justified. This engine with installed centralizers allows drilling with a radius of curvature of less than 8 m, which meets the objectives of the technology (completion of thin layers). Moreover, the choice of the PDM with a diameter of 48 mm originated from the possibility of using a bit with a larger diameter than that used when drilling the PDM with a diameter of 43 mm, which has a positive effect on the effective filtration area of the drilled channel.  
A PDM with a diameter of 43 mm and a kinematic ratio of 9:10 is expected to be produced in the future, since such a PDM will reduce the value of the radius of curvature and, as a result, expand the field of application of the technology. However, from a technological point of view, the manufacturing of such a PDM is a more laborious process.  
The use of the developed universal downhole drilling motor will allow milling and drilling of channels using one drilling tool for a round trip, which optimizes equipment manufacturing costs, reduces the final cost of the service for the customer, and expands the scope of application of the developed innovative technology of perforating drilling, both in high-rate and low-rate wells, including fields that are currently considered to have hard-to-recover reserves.  
[PERSON] stated the problem of the research, the objective and the strategy of problem solving. [PERSON] was responsible for the collection of initial data and comparative analysis of the principal technical indicators for each variant of the PS. [PERSON] wrote the original draft. [PERSON]. ([PERSON]) was the project administrator and leader of the research. [PERSON]. ([PERSON]) was responsible for conceiving and designed the analysis. He reviewed and edited the original draft. [PERSON]. ([PERSON]) was responsible for the calculation in VINT software, creation of the figures and translated the paper into English. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.