Here are a few ramblings to give my take on the bit/torque reaction thing,
When you approach bottom just before touching bottom is the best time to get a “benchmark” for the system (less bit torque).
As the bit touches bottom, first of all the most important variable is string RPM as this translates to cutter speed. Any cutter needs a dwell time to apply weight, fracture the formation and start the beginning of the “furrow” into the formation. The slower the speed (generally) the more time to develop the formation fail and the deeper the furrow. I think bit people would call this crack initiation and crack propagation.
Once the initial contact is made, then Weight on Bit becomes important as this is the parameter that governs depth of cut. Deeper cut exposes more of the “wall” ahead of the cutter and so requires more torque to progress. Now the fancy bit design elements such as chip breakers, rake angles etc come into play and the subject of cutter efficiency becomes a whole new topic of discussion.
So in a nutshell, I would say that the torque demand from a given bit in a given situation is dependent upon a host of variables. When I model in Torque and Drag programs I normally include a bit torque of 1000 ft/lbs. There is no magic formula behind this figure, it merely gives me a figure to start with. Once actual drilling for a specific bit/formation is underway the daily reports give me actual data and allow me to calibrate the model more realistically (notice the use of the word realistically as opposed to accurately, as in any given situation there will be a certain amount of masking effect from the drill-string dynamics).
As for reaming, this also demands torque but the cutters here are doing work that is out with their initial design spec. For this reason when reaming, I always like to maximise on flow (to maximise the cooling effects of the cutters), and reduce RPM (minimise friction effects of a higher cutter speed).
Again in a reaming situation the torque demand is the product of a lot of variables. A hard formation, for example, may tend to fracture (due to its brittle nature) and fail ahead of the PDC cutter.
Reaming would typically be less torque than drilling (simply because the volume of rock being encountered is less), but the torque “signature” could be completely different with reaming producing more cyclic and erratic torque from the bit.
For further reading, I would suggest;
JCPT paper 2007-082Cheers............Chris