As a driller, holding the brake handle or sitting in your cyber-chair, you have watched it plotted on your EDR screen.
As a drilling supervisor, you have watched it with the driller and maybe jotted it down in your tally-book with other notes and punched it (or have the rig engineer punch it for you) into the DDR (Daily Drilling Report) before “sending it to town” at 06h00.
As a drilling engineer, you have seen it when you clicked through the DDR on one of your widescreen monitors while sipping your morning coffee. And maybe you also plot them in a spreadsheet to compare with previous wells or some other kind of benchmarks.
We all get what it means, right? Rate of penetration, or drill rate, tells you how fast you’re making a hole. What more can be said about that?
Turns out, it is the most fundamental concept about drilling and holds the roadmap to extreme drilling performance IF you take a closer look at the physical level.
Thus the #1 thing you need to understand about ROP is this:
ROP is physically generated by the WOB and RPM applied at bit against a given type of rock.
The schematic at the top of this article gives a clear illustration: as weight is applied at the bit, the cutting structure bites into the rock (“indentation”, literally means to put teeth into) and with the rotation, the cutters slide and remove what they have bitten. Moment by moment.
Imagine yourself biting an apple. It’s pretty much the same principle on a physical level. Your teeth are like the cutters, and the apple is the rock you’re cutting. Simple as that.
What makes this physical description really interesting though, is that the relationship between WOB/RPM and ROP is linear IF your bit is cutting the rock efficiently.
In other words, you double the ROP when you double the WOB. You can also 3x the ROP if you 3x the RPM. As long as the bit is cutting efficiently (no bit dysfunction, such as bit-balling, vibrations, etc.), you can independently increase the ROP by increasing WOB and/or RPM at the bit, proportionately.
This is significant because:
a) Our brain really likes linear things. It’s easy to track and identify linear responses. That means our drillers on the rig can easily track this linear relationship and when that relationship stops being linear, they know something is wrong and can take actions. (You need a workflow to make that happen, but that would be for another article)
b) It brings us clarity, freedom, and focus:
- Clarity because we now know the only two things that generate ROP, not a dozen factors;
- Freedom because we no longer need to confine ourselves to the regional ROP benchmarks (experience-based, but we don’t have all the information to understand the causes of other people’s experience);
- Focus because we can now put our energy on the one thing that is preventing us from raising WOB and/or RPM. The ROP is the fruit that would naturally come once the current constraint is removed.
I’m sure there are many things that can be said about ROP, but I believe an understanding on the physical level is the very first thing you need to grasp. That understanding will become your key to understand many other situations and to answer many other questions.
Some ending thoughts
A statement from Professor Dupriest came to mind:
What is limiting us from drilling all of our 12.25″ sections at 1000 ft/h (about 300 m/h)? I’ll tell you that, worldwide, not much. —- Fred Dupriest
A question like that has never occurred to me in my past experience, because I was engineering for feasibility, calculating kick tolerances, casing load cases, torque & drag and buckling, etc., all for avoiding downhole problems rather than maximizing ROP.
But maximizing ROP is not contradictory to avoiding downhole problems or safe operations once you really understand how it works. Deep and true knowledge actually brings safety and light.
In fact, the question “what’s limiting my ROP right now?” will lead you precisely to the specific risk that you have not yet dealt with and reveal the real reason your driller is hesitating to raise the WOB or RPM.
So what’s limiting your ROP right now?
This is part of the epiphany I got since I discovered Professor Fred Dupriest’s work last month. I wrote an article here to recap the broader framework of physics-based drilling that aims to do one thing and one thing only: remove what is limiting you from going faster.
Other resources on physics-based drilling practices:
- All the SPE papers by Professor Fred Dupriest, you can find them all here on Onepetro.
- The chapter of “Drilling Practices” in the 12th edition of IADC Drilling Manual