Mud motor drilling with mill-tooth or insert-type bits in the surface upper hole can often result in increased rates of penetration and less hole deviation -- two factors that can often mean cost savings to the operator.
Most people associate mud motors with two different types of drilling -- controlled directional and long-interval straight-hole drilling. Using mud motors is now the norm rather than the exception in drilling a directional section, and in the last five years, mud motors, when used with diamond or PCD type bits, have gained wide acceptance in many long-interval drilling applications. The reasons for the increased use of mud motors in straight-hole drilling are higher rates of penetration (ROP) than traditional methods, less hole deviation, and much improved tool performance, resulting in savings for the operator.
Recent case histories in the Rocky Mountain Region show that these same reasons apply to a section of the hole not often associated with mud motors, but nevertheless where mud motors are now providing a viable alternative to rotary drilling -- the surface or upper hole. Drilling a large (12¼″, 17½″, 26″) surface hole with a mud motor and mill-tooth or insert-type bits can, in many cases, 1) increase ROP, and 2) maintain a straighter upper hole which helps minimize subsequent problems as the hole depth increases. In some cases, rates of penetration have been improved even when drilling single pass large diameter hole, thus also eliminating the need for hole opening operations, and adding to cost savings.