Very hard and abrasive formations are often drilled with impregnated bits and high speed turbines. Typical application areas include the Middle East, onshore Germany, the North Sea, West Venezuela, and Italy. As opposed to positive displacement motors, turbines do not need elastomers in the power section, which can result in better durability in high temperature environments.
Due to its hydrodynamic function principle the power output of a turbine is not linear with the flow rate. A 20% reduction in mud flow reduces the power output by 50%, which drastically reduces ROP in all deep wells, slim holes or lost circulation situations, where it is not possible to operate the BHA with the full flow rate. Furthermore, the use of hydrodynamic power sections does not allow a clear indication of the bit RPM for the optimum operating point or bit stalling via standpipe pressure readings.
In order to overcome these operational difficulties a special high speed positive displacement motor (PDM) was developed. Due to the new design and manufacturing processes this motor combines the advantages of turbines (high bit speed, low sensitivity with regards to temperature) with the advantages of PDMs (RPM proportional to flow rate and nearly independent of the loading). With the superior torque capability the power output of this new downhole motor is more than double as compared to turbines, resulting in the possibility of using more aggressive bits and increasing ROP.
The paper discusses case histories from Oman, Germany and Italy and directly compares the performance of turbines with the latest design high speed PDMs. The case histories also demonstrate that the use of additional performance tools (i.e. downhole thrusters) can not only significantly improve the steering behaviour of the drilling assembly, but also extend the maximum possible horizontal displacement of a well, and minimizes BHA failures.
It is considered that PDC bits can be used in formations of compressive strengths up to 25,000 to 30,000 PSI. In harder formations roller cone bits or impregnated bits are used. Steering with impregnated bits is nearly as easy as with roller cone bits, but as impregnated bits do not have bearings or moving parts they can be kept in the hole much longer. Impregnated bits drill sand, shales and some limestones with same or higher ROPs than roller cone bits.
Due to their low cutting depth impregnated bits require very high bit RPM to achieve good penetration rates. For many years turbines have been able to generate much greater bit speeds compared to mud motors and have been the preferred drive mechanism for impregnated bits. However, already in the early Eighties high speed positive displacement motors have been developed which operate at rotational speeds comparable to turbines 1. These motors have now been introduced into the market utilizing the reliable components especially developed for modern extended length drilling motors 2.
Certain application areas require high RPM at the bit and a certain bit technology for optimized drilling as indicated above. It is a common practice to use a hydraulic downhole motor to generate the mechanical power at high speeds by conversion of hydraulic energy from the mud system. The advantage is that the drill string itself can be rotated at moderate speed and depending of the motor system technology directional drilling is possible without drill string rotation.
Two system are commonly used:
the "hydrodynamic" turbine based on the impact of the fluid on the turbine wheels and3
the "hydrostatic" positive displacement motor also known as a progressive cavity system, or Moineau motor2.