Many drilling engineers still associate the use of impregnated bits for hard and abrasive formations with the use of hydrodynamic turbines. In fact, turbines were the first high speed direct bit drive systems on the marketplace - providing maximum bit speed, long life on bottom and, due to the lack of elastomers in the power section, could be used in high temperature environments. However, turbines offer low drilling efficiency, especially in combination with low flow rates, and are very expensive to build and maintain. Positive displacement mud motors are more cost effective and show much better efficiencies, even at lower flow rates, but used to survive fewer hours on bottom and tended to fail at higher bottomhole temperatures. Current manufacturing technologies, especially the capability to manufacture pre-contoured stator tubes covered with a thin layer of elastomer, have helped to overcome the above mentioned problems. Today's latest generation high speed mud motors combine the industry's highest bit speeds with unmatched torque - resulting in a power output of up to 1000 HP. These motors can be used in temperatures up to 160 C and have survived numerous runs consisting of hundreds of hours of on-bottom drilling time without failure. The paper describes latest developments in mud motor design and shows case histories of motor runs from different vertical and directional wells of various hole sizes across multiple continents. Premium drilling performance is not only dependent on the availability on latest mud motor technology, but requires a detailed understanding of the entire drilling process as well as the optimization of the drill rig, the drill string, the BHA and the bit. Special attention is therefore given to an ongoing optimization project, which has taken place in close coordination between a service company and an operator in North Germany. Drilling the very hard and abrasive Bunter Sandstone used to be a time consuming and costly enterprise. Within few years the ROP could be doubled and the distance drilled per BHA could be tripled.
The very hard, abrasive Middle and Lower Bunter Sandstones in Northwest Germany is very difficult to drill. Economic success is only achievable with impregnated diamond bits which possess a very small depth of cut, as they work like sand paper. Based on several case studies from the Middle East and Italy, it is typically assumed that there is a linear relationship between the rotating speed and the penetration rate of an impregnated bit (i.e., if bit speed is doubled then penetration rates also double). The same rule applies for weight on bit (WOB). Maximum penetration rates therefore require both high WOB and high bit RPM.
The situation is more complex in the North German Bunter Sandstone. A series of drill-off tests indicated that high penetration rates could only be achieved at very high mud flow rates. Using the test data, optimal flow rates were extrapolated. However, these rates clearly exceeded the technical limits for the existing rigs, drillstrings, and BHA's - especially in the commonly used hole size 12 1/4".
In recent years, some dedicated drilling rigs were equipped with stronger mud pumps and larger diameter drill pipes to enhance drilling performance in these applications and the higher flow rates did increase the penetration rate. Additional improvements were made by introducing impregnated bits with an interrupted cutting structure. However, optimum conditions could still not be achieved as the system's hydraulic "bottleneck" shifted from the rig equipment and bit to the downhole BHA - specifically the high-speed motor.
The paper describes latest developments and features of a new, custom-engineered 9 1/2" high-speed motor for drilling the Bunter Sandstone. This high-performance motor provided a 25% increase in flow rates vs. previously used motors. Due to its extreme bit speed and unmatched torque capabilities, the motor is the first commercial downhole motor in the field breaking the 1000 horsepower (HP) limit. The combination with a customized drill bit led to another step change in downhole performance. Case histories from Germany are used to demonstrate, how ongoing combined efforts from an operator and a service company lead to doubled penetration rates and tripled the footage of continuous BHA runs in the hard and abrasive Bunter Sandstone.