Review of Solid Earth article MS# 2021-63,

The manuscript of Lipus et al., “Dynamic motion monitoring of a 3.6 km long steel rod in a borehole during cold-water injection with distributed fiber-optic sensing” is an article concerning the observation of thermally induced stick-slip events.  Cold water pumped into a well causes the rod inserted into the well to contract, with shaking from the stick-slip events recorded on DAS.  The results are compared against DAS and DTS data to build a picture of where and why these events are occurring in the well.  I think that the results of the paper could be interesting the readership of Solid Earth.

            For this paper, I have only a few small critiques that should be addressed before acceptance with Solid Earth.  In general, my comments revolve around better explaining some of the arguments the authors are trying to make.  A more detailed list of my thoughts follows below:

1. In the conclusion, the acronym FO is used for the first time. I’m assuming it means fibre optic.  I’d recommend removing it, as it’s only ever listed here.
2. From the results of this paper, it should be possible to get a rough estimate of what the coefficient of friction is between the well and the rod. Would be interesting to get a back-of-the-envelope sense of what that value is.
3. In Section 4.1 the authors talk a bit about errors that apply in the measured and expected strains. I’m curious as to what sorts of errors could be introduced from the DAS data based on the response spectrum of the fibre optic cable.  Are we potentially attenuating frequencies that could contribute significantly to the measured strain?
4. The following page lists minor corrections and typos to be fixed.

Thanks,

-Ryan

Near Lines 372 & 377 & elsewhere: “extend” should be “extent”

Figure 9: Why do events only seem to occur at the top of the casing liner after ~16 minutes?

This manuscript reports interesting observations in a borehole with DTS and DAS, which includes sucker rod contraction and sudden contraction events. The sudden contraction events on the DAS records during the first 72 minutes of cold-water fluid injection are reported. A friction force model was proposed to explain the mechanism of vibration events instead of the microseismic event. General speaking, it provides very useful information. The following is my comments.

Section 2.

Eq 1 should be in “2.1 derivation of strain from Distributed Temperature Sensing” rather than “2.3 Deformation balance …”.

“2.2 Direct measurement of strain via DAS”. iDAS measures the strain rate instead of strain. Was any high pass filtered applied to the raw data?

“2.3 Deformation balance from DTS and DAS measurements”. This section includes how to compute strain from DTS and obtain strain from strain rate records, which is not strongly related to the “balance”. It may be better to merge with section 2.1 & 2.2.

Eq. 5, the applied force Fapp is used in this study instead of the stress.

“2.4 Event detection and picking” looks not related to the other sections.

Section 3.

Line 290-295, the difference between strain_DTS and strain_DAS looks relative to the inclination angle. It may be worth to make a figure showing this difference and inclination angle. Adding some discussions about this phenomenon is also useful. Another interesting observation is that difference at 01:18 is larger than the one at 02:08, especially between ~700m and 2800m.

On the 2^nd subplot of the Figure 4, the differences between strain_DTS and strain_DAS below the 3100m MD are quite different at 01:18 and 02:08. At 02:08, the strain_DTS is positive while the strain_DAS is close to zero. Such difference is not observed on the data at 01:48. Any clue?

Since both section 3.2 and 3.3 reported sudden contraction events, it is possible to merge together.

Line 320. It is not easy to see precursors and successors on the Figure 6. Add marks on the Figure 6 ?

As shown in Figure 8, some STA/LTA detections are outliers. How to determinate the origin time of each event and the origin depth? Another interesting parameter is the strength of event. Does the stronger event have stronger spatial extend?