Stick slip: It is a topic that has been extensively studied and investigated. And there have been many approaches invented to mitigate them over the years. So, you might wonder whether there is anything new to talk about when it comes to stick slip.
Perhaps you already know that sometimes a little stick slip is good? Depending on the formation that is drilled and the bit that is used, a little stick slip could imply optimal performance. One can get “acceptable” borehole quality, high ROP, and no tool damage with some stick slip. Now “Full Stick” – that is another issue. We need to detect them and avoid them. We will cover “Full Stick” detection next week. So be sure to be on the lookout.
Now, no matter what your opinion/preference is on how to mitigate stick slip or even how much to mitigate, it is nevertheless good to keep track of what is happening downhole in real-time (using surface sensors – because – low cost). The usual metric that almost everyone uses is the variation of the torque with respect to the average torque over a time window. It is called by many names: stick slip index, stick slip alert, etc. Here we call it “Torque variation index”. See slide 2 for the equation.
To have more confidence that what we see through the torque variation index is indeed stick slip, we also track MSE, bit aggressiveness, and depth of cut trends under certain constrained conditions. While we will not expand on all the features in the model here, the network is intuitive and transparent – and that is the whole point. Building ML models that are not black boxes and something that the drillers can understand and appreciate.
Another desirable feature, is to be able to differentiate between bit (on-bottom) stick slip and drillstring (off-bottom) stick slip. This can be determined by monitoring torque oscillations as the bit goes back to bottom and quantifying the difference in oscillation with respect to on-bottom drilling (examples in this last few slides).
More information about these slides are available in SPE-212515-MS and SPE-204063-MS, as well as below: