When most people think of oil & gas drilling, they think of a rig punching vertical holes in the ground, and for most of history, that was the case. Now drilling commonly involves complex geometry including builds, turns, and tangents in order to construct the desired well. This is achieved by directional drilling, which is the subject of this Drilling Minute.
So how do they do it? While there are multiple tools aimed at directional drilling, the most common is a directional motor. Drilling motors use hydraulic energy in the mud flow to turn the bit. Directional motors have a bend which points the bit in a direction.
Rotating vs. Orienting
During “normal” drilling the rig rotates the entire drill string from surface. This mode of drilling is called “rotating.” While rotating, the bit is turning at the surface RPM plus the motor RPM; this is known as the total or bit RPM. But in order to drill in a specific direction, the bend in the motor has to be consistently pointed in that direction. To achieve this, the directional driller will stop the surface rotation and orient the motor in the desired direction; this process is called “orienting.”
Sliding, Building, Turning
Once the assembly is oriented in the right direction, weight is applied and the bit turns only by the power of the motor. Drilling in this mode is called “sliding,” because the entire drill string has to slide against the bore hole wall. During an interval of sliding, or a slide, for short, angle is accrued in the well bore. Steering away from vertical is referred to as “build;” steering side-to-side is called “turn.”
How are we doing down there?
There is a directional plan – what we want to do – but how do we know what was actually drilled down hole? Surveying tells us the trajectory the well has taken. It is done periodically throughout the well, sending back information about where the well bore has been drilled. In the well, the maximum angle built is known as the “dog leg severity,” while the average angle change is the “build up rate.”
Factors such as the extra time it takes to orient, the ability to control the direction while drilling (called “tool face control”) and the effectiveness of weight transfer to the bit all affect ROP during sliding. In general, rotating is much faster than sliding, which is the topic of the next segment.
Tags: Drilling Minute