In Russia oil and gas wells are mainly drilled with downhole hydraulic motors (DHM). In the second half of the 20th century turbodrills, which are high-speed downhole motors, found wide application in large-scale drilling in Volga-Ural region, and further in Western Siberia. The most commonly used type of rock-cutting tool was a tricone bit with "open" bearings working in high-speed mode. Also, diamond bits were used to penetrate deep hard rocks. More than 80% of wells were drilled with turbodrills at that time. Putting into practice tricone bits with sealed oil-filled bearings required application of low-speed downhole motors, so downhole screw motors now dominated as a bit drive in large-scale drilling, which was also boosted by the introduction of high performance PDC bits. Compared to high-speed turbodrills, downhole screw motors are low-speed motors with increased torque, they are also known as Positive- displacement motors (PDM). Comparison of turbodrill and PDM energy characteristics shows that turbodrill has a so-called "soft" torque dependence on rotation speed, whereas in PDM it is "hard". That is why with PDM, drilling mode parameters are more stable. On the other hand, in the process of drilling and downhole motor wear turbodrill energy characteristic does not change much, whereas PDM demonstrates significant decrease of energy parameters. Furthermore, the level of lateral vibrations generated by PDM is also high, which has a negative effect on the wellbore quality. Turbodrill and PDM differ essentially, each having specific advantages and drawbacks. However, they are technical means of one and the same drilling method. Today, each type of DHM has its own area of effective application.
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