The Concept of Specific Energy in Rock Drilling, by R. Teale
This paper was published in International Journal of Rock Mechanics and Mining Sciences in 1965.
That was 55 years ago.
That concept, known today as Mechanical Specific Energy, has had a huge impact on the improvement of drilling performance in many basins around the world.
Why Specific Energy?
Teale explained in a concise manner in the abstract (underline mine):
The fundamental problem in rock working is the breakage of fragments out of the face of a solid wall of rock. Mechanically this can be done only by forcing a tool into the rock surface, after the manner of an ‘indenter’ such as is commonly used for testing surface hardness. Since the process breaks rather than cuts solid rock into small fragments of assorted sizes it can be regarded as essentially one of crushing. As in crushing processes generally, energy/volume relationships are therefore of interest.
Drilling seeks first and foremost to break the rock into fragments so they can be removed (excavated).
What we want is to create a passage (hole) all the way to the targeted reservoir.
It does not seek to grind down the rock into smaller grains than it is actually composed of. That would take disproportionately more energy. Think grinding wheat into flour vs just scoop out wheat grains.
Teale thus realised that it would be interesting to study the energy/volume relationship in the rock-working process and came up with the specific energy as an index to evaluate the mechanical efficiency of such process.
What is Specific Energy?
Teale defined it as the energy required to excavate unit volume of rock. He used the following equation to calculate it.
This equation can be formulated with drilling parameters familiar to drillers:
Fundamental Understanding of Rotary Drilling
R. Teale gave the best description of rotary drilling that I wish I had been told when I started out as a junior drilling engineer. Here’s what he wrote (the format is my own arrangement):
“Rotary drilling may be regarded as a combination of two distinct actions :
– ‘indentation’, by which the cutting edges of the bit are continuously pushed into the rock to give them a bite;
– ‘cutting’, by which the bit is given a lateral movement to break out fragments of rock.
Though these actions are in practice virtually simultaneous, it is possible to conceive of them being applied separately and alternately.”
In short, all bits cut the rock the same way: they indent and slide. This is also what Professor Fred Dupriest taught in his High Performance Drilling Course, on the topic of basic drilling mechanics.
More specifically, the weight on bit (WOB) causes the indentation, and it then takes torque to slide and generate the cuttings, thus furthering the hole-making process.
Now why is this significant?
Because it provides a clear understanding of the fundamental phenomena of rock drilling in rotary mode. All the equipment, tools and procedures that are designed and used serve to improve the efficiency and effectiveness of these two actions.
Read The Classic, Critical Papers
At the start of my career as a drilling engineer back in 2008, I remember going through a scanned PDF copy of the classic textbook, Applied Drilling Engineering.
Chapter 1 was called “Rotary Drilling Process” and it dived straight into a plethora of knowledge points:
- Typical drilling rig orgainizations,
- Different types of rigs,
- Rig power system,
- Hoisting system,
- Circulating system,
…the whole nine yard.
It was very comprehensive for sure. But it was also overwhelming for someone who is new to drilling engineering. I couldn’t tell which part was the most critical or where to start first, apart from stressing myself to memorize everything, which was really impossible.
I wish someone had sit down with me and talked me through half a dozen critical papers such as this classic one from R. Teale, so that I could first grasp what drilling is all about and how it works at the fundamental level, and how to assess the efficiency thereof.
Fred’s paper on maximizing drill rate with real-time surveillance of MSE would have definitely been illuminating as well. It had already been published in 2008 when I started in TOTAL. It still amazes me how I got my hands on that paper only in 2019, almost 15 years after its publication!
If you’re just starting out as a junior drilling engineer, I sure hope it won’t take you that long to get there. I’m quite hopeful about it in this age of Google and LinkedIn. The cost of getting accurate information keeps coming down. I hope you will take advantage of it and grasp the fundamentals as quickly as possible.
Teale’s paper is a great place to start and to add to your collection of technical papers. You can find it here.
If there’s a critical paper that has helped you with your drilling career, feel free to share in the comment and link a review if you have written one.
If there’s a topic of drilling on which you’d like to deepen your understanding, don’t hesitate to leave a comment below. I will research and share what I find.