I became a drilling engineer back in 2010. Truly, the first time I ever even saw a downhole motor “spec sheet” wasn’t until a few years into the position. It was jam-packed with info to sort out. Lots of specs, lobe counts, power curves, etc…
I’ll be honest, analyzing and comparing all that motor information was definitely confusing at first. I’d typically do my best to narrow it down to a handful of options, then zero in on the max differential pressure (DP) rating. I mean, the more DP the better, right?
However, as with many complex systems, trying to reduce power sections down to a single variable can create blind spots and a false sense of understanding. Since then, my experience as an Applications Engineer at PV Fluid Products has taught me that there is much more to differential pressure than meets the eye. Let’s take a closer look.
First off, we have to understand what differential pressure is and why it matters.
When most people talk about differential pressure, or “motor diff”, they’re referring to the difference between rig circulating pressure while off-bottom versus on-bottom.
Assuming the flow rates and mud properties are held constant, what drives this change?
When the BHA goes to bottom, the bit engages with the rock, resulting in reactive torque. This torque is transmitted to the power section, and the result is an increase in differential pressure.
One common misconception is that differential pressure can be adjusted, as needed, to deliver more torque to the bit. However, the increase in motor differential pressure is actually a response to the increased torque load from the bit. If the bit has transmitted a torque load that the motor can’t overcome, the motor will stall. In other words, if the required DP exceeds what the motor can deliver, you’re out of luck. More on this in a future post.
The biggest determining factor of a power section’s maximum differential pressure is its geometry. In general, the more stages and/or lobes a power section has, the more cavities it will contain, hence the greater differential pressure capacity.
However, there is a second key variable when determining the differential pressure (DP) capability – the individual pressure holding capability of the elastomer.
Each elastomer has its own unique properties, one of which is its pressure sealing capacity (pressure/cavity). So, you could have two power sections with identical geometry (length, stages, lobes, etc.), but one with significantly higher DP capability, based on a different pressure holding capacity of the elastomer.
This can be a big deal, as shown in the graphs below.
In the graphs above, graphs of RPM drop curves, max torque, and horsepower capability are shown for two identical power sections with different elastomers. In this case, the motor with HS88 elastomer would deliver 54% higher max DP. This is due to the fact that HS88 can hold more pressure in its cavities than the other elastomer.
I’d like to add one more point about something which is often overlooked.
Just like any other hydraulic system, power sections have frictional losses that must be overcome before torque can be generated. This is known as off-bottom pressure or OFB.
As OFB increases, the amount of pressure left over to generate torque decreases. The OFB shown on most spec sheets is based on pumping water through the motor at a given flow rate. However, OFB can be increased by several factors, including pumping higher density, more viscous drilling fluids through the motor, increasing flow rate and by other components the power section is being forced to turn (ex. motor and/or RSS).
As seen in the graph above, changing fluid properties and flow rates can produce in excess of a 400% increase in OFB as compared to the minimum recommended flow with water.
PV’s Applications Engineers intend on posting more articles like this in the future, so you’ll hear from other authors in the coming months as we explore related topics. We’re passionate about performance and want to grow the general knowledge around power sections. The more our customers know, the faster and more reliably they will drill. This can add up to some serious money saved on a drilling project.
Please feel free to further the discussion by leaving a comment or question on the articles we post. Or you can also reach out to us directly.
