In situ stress database of the greater Ruhr region (Germany) derived from hydrofracturing tests and borehole logs

Kruszewski, Michal; Klee, Gerd; Niederhuber, Thomas; Heidbach, Oliver

doi:https://doi.org/10.5194/essd-2022-196

Preprints

https://doi.org/10.5194/essd-2022-196

Preprints

21 Jun 2022

Status: this preprint is currently under review for the journal ESSD.

In situ stress database of the greater Ruhr region (Germany) derived from hydrofracturing tests and borehole logs

Michal Kruszewski^1,2, Gerd Klee³, Thomas Niederhuber⁴, and Oliver Heidbach^5,6 Michal Kruszewski et al.

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¹Fraunhofer IEG, Fraunhofer Research Institution for Energy Infrastructures and Geothermal Systems IEG, Am Hochschulcampus 1 IEG, 44801 Bochum, Germany
²Institute of Geology, Mineralogy, and Geophysics, Ruhr-University Bochum, Universitätsstraße 150, 44801 Bochum, Germany
³Solexperts GmbH (former MeSy GmbH), Meesmannstraße 49, 44807 Bochum, Germany
⁴Karlsruhe Institute of Technology, Institute of Applied Geosciences Division of Technical Petrophysics Campus Süd, Adenauerring 20b, Geb. 50.40, 76131 Karlsruhe
⁵GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
⁶Technical University Berlin, Institute for Applied Geosciences, Ernst-Reuter Platz 1, 10587 Berlin, Germany

Received: 08 Jun 2022 – Discussion started: 21 Jun 2022

Abstract. Between 1986 and 1995 429 hydrofracturing tests have been carried out in six, now abandoned, coal mines and two coal bed methane boreholes at depths between 600 and 1950 m within the greater Ruhr region in western Germany. From these tests, stress magnitudes and orientations of the stress tensor are derived. The majority of hydrofracturing tests were carried out from mine galleries away from mine workings in a relatively undisturbed rock mass. These data along with detailed information have been disclosed recently. In combination with already published material, we provide the first comprehensive stress database of the greater Ruhr region. Our study summarizes results of the extensive in situ stress test campaign and assigns quality to each data record using the established quality ranking schemes of the World Stress Map project. The stress magnitudes suggest predominantly strike-slip stress regime, where the magnitude of the minimum horizontal stress, S_hmin, is half of the magnitude of the maximum horizontal stress, S_Hmax, implying that the horizontal differential stress is high. We observe no particular change in the stress gradient at depth throughout the Carboniferous layers and no significant difference between tests carried out in coal mines and deep boreholes. The mean S_Hmax orientation varies between 133 ± 13° in the easternmost located Friedrich-Heinrich coal mine and 168 ± 23° in the westernmost located Westfalen coal mine. The mean S_Hmax orientation, based on 87 data records from this as well as already published studies, of 161 ± 43° is in good agreement with the regional stress orientation observed in Northwestern Europe. The presented public database provides in situ stress magnitude and stress orientation data records that are essential for the calibration of geomechanical numerical models on regional and/or reservoir scales for, among others, assessing stability issues of borehole trajectories, caverns, and georeservoirs in general. For an application example of this database, we estimate slip and dilation tendencies of major geological discontinuities, discovered during the 700-year-long coal mining activities in the region. The result shows that the discontinuities striking in the N-S and NW-SE directions have a higher slip tendency compared to the ones striking ENE-WSW and NNW-SSE, whereas a high dilation tendency is observed for discontinuities striking NNW-SSE and a low dilation tendency for the ones striking ENE-WSW.

Michal Kruszewski et al.

Status: final response (author comments only)

CC1:
'Comment on essd-2022-196 - Slip Tendencies', Daniel Bücken, 09 Aug 2022

When going through the paper a few questions opened up to me - I’d appreciate your input there!

The stress polygon in Figure 6 only displays absolute stress values, correct? The stresses come from Table 3 (which gives absolute stresses), and result in absolute stress ratios. I am asking because I am accustomed to just use stresses for the stress polygon, not stress ratios (I know that the latter is often used). In your plot the stresses all land below the µ =0.6 line, but that’s for absolute stresses if I understand it right. I was thus wondering where your stresses would plot when using effective stresses, based on (S1-Pp)/(S3-Pp) = [(µ²+1)^1/2 + µ]². I isolated µ in this equation to calculate the µ at which a fault would fail for a given S1, S3 and Pp (i.e. the maximum theoretical slip tendency), and used the stresses you proposed in your paper (1200 m depth, Fig. 9). This would result in a slip tendency of 0.88 for most critically oriented fault, which is quite a lot (µ would have to be higher to prevent fault slip). Plotted it in a stress polygon, for µ=0.8 and 1200m depth, this would mean that the stresses you provide would be unstable (also for a µ = 0.85 as proposed by Hinzen (2003, doi:10.1016/j.tecto.2003.10.004) for depths <9km). However, the value of 0.88 matches well with the slip tendencies in your Figure 9, which appears to use effective stresses.
My main question now is how you can explain the very high “effective" slip tendencies (which are higher than most typical friction coefficients), and how you compare them to the slip tendencies below µ=0.6 in your absolute-stress stress polygon in Figure 6. A possible solution would be to reduce the pore pressure gradient, which would lead to stable conditions (max. slip tendency = 0.75 for grad.Pp = 10 MPa/km).
Best wishes, Daniel

Citation: https://doi.org/10.5194/essd-2022-196-CC1
- AC1: 'Reply to the comment by Daniel Bücken', Michal Kruszewski, 24 Aug 2022
  
  Dear Daniel,
  thank you for your questions. We are coming back with answers.
  In the end, the present-day in situ stress state of the greater Ruhr region is far from simple. Due to more than 700 years of coal mining activities across the region (i.e., rock withdrawal and pore pressure reduction due to pumping out of the water from the mines, as well as recent mine flooding operations), it is expected that the in situ stress state "situation" has changed significantly. The hydrofracturing tests we have analysed, although being located around 40 meters away from the mine infrastructure, can be still considered to be located within the mine’s local sphere of influence. Thus, we expect that the derived stress magnitude values besides general measurement errors have also systematic errors (assuming that the undisturbed stress components are being measured). These uncertainties as well as the ones resulting from fault geometry assumptions and pore pressure values (see below) need a sound assessment to derive uncertainties and mean values of the slip tendency. The value that we present is only from a single set of parameters and should be interpreted with caution and we will make this clearer in the paper to avoid conclusions by the reader that are not robust.
  Regarding the pore pressure the hydrofracturing tests recorded very low or zero values. These negligiblepore pressures could be explained by i) impermeable rock mass (mentioned in the manuscript and exemplified with low permeability values registered during in situ testing) and ii) long-term mine water drainage (ongoing until this day) which led to significantly low pore pressure values within the mine and in its close vicinity. Additionally, fluid withdrawal in the Ruhr region could lead to a significant poro-elastic stress changes induced by the reservoir compaction (Segall & Fitzgerald, 1998; https://doi.org/10.1016/S0040-1951(97)00311-9). It has been also observed in the Ruhr region that the mine flooding operations produce asignificant amount of microseismicity, potentially proving the criticality of the geological structures in the areaand a significant amount of stress changes created by the mining activities (see: https://doi.org/10.23689/fidgeo-4002 and https://doi.org/10.23689/fidgeo-5401). We have, anyway, used the stress measurements from mines to construct total stress gradients of the area. Figure 6 presents a normalized stress polygon based on the assumption of no pore pressure with values of stress registered in coal mines and coal bed methane wells.
  For the computation of the slip and dilation tendencies, we assumed that the pore pressure is present in the area at 1.2 km depth. This is due to the fact the pore pressure reduction by dewatering activities is a local phenomenon and areas located away from coal mines (or dewatering stations) are expected to remainunaffected. In the end, the pore pressure field of the greater Ruhr region remains relatively unknown. From the recent mine flooding operations in the area, it is known that in some coal mines the water level is locatedat depths between 0.6 to 1.2 km (see Fig. 1). Such water table reduction would mean a decrease in pore pressure values between approx. 6 and 12 MPa and showcase that local pore pressure differences will besignificant in the region. Such pore pressure changes will drastically change (i.e., decrease) the slip tendencies of major faults in the region. We could agree that the assumption of the abnormally high pore pressure, we did in the manuscript, could be seen as a bit of an exaggeration. The abnormally high pore pressure values were assumed from the recent study by Kruszewski et al., 2021 (https://doi.org/10.1007/s00603-021-02636-3), where they established that elevated pore pressure gradients could be observed in the Ruhr region (based on a high density of water samples in the coal mines and recorded drilling fluid densities from the exploration drilling campaigns in the region). For simplicity, we agree on amending the pore pressure values in the computation of slip and dilation tendencies to a hydrostatic pressure of cold water starting from the surface (i.e., with a pore fluid density of 1000 kg/m3 the pore pressure at 1.2 km depth will amount to approx. 11.8 MPa). In this way, as you pointed out, the slip tendencies will stay below 0.75 (please see Fig. 2 for a new map of slip tendency).
  The main scope of our study was, however, to present in situ stress magnitude and orientation data compilation. The slip and dilation tendencies presented in the manuscript were just one of the possible applications of this data set. The effects of pore pressure influence should be, however, investigated in follow-up studies using our database as a reference. Additionally, we would recommend performing in situ measurements in areas not affected by the coal mines (e.g., south of the Ruhr region) to compare with the ones performed in coal mines. I hope that cleared out your doubts and please do not hesitate to reply to this reply.
  Please see figures to this reply in the .zip file in the attachement.
  Best Wishes,
  Authors
  
  Citation: https://doi.org/10.5194/essd-2022-196-AC1
RC1:
'Comment on essd-2022-196', Jochem Kück, 09 Aug 2022

This data publication presents a long overdue collection, evaluation and synopsis of the available data on the stress field of the Ruhr area in Germany. Congrats!

It is important and necessary for diverse applications and future technical challenges in this highly industrialized agglomeration in the middle of Europe.

The stringent and transparent structure of the manuscript in general makes it easy to follow the process of data collection and quality assessment.

Nevertheless, for a better understanding of the workflow, I tried to reproduce the process of generating a few data points in this data compilation containing my own data (Haus Aden, 940 m bottom , Kück, 1988). I must admit, however, that I could not really reproduce the values that are ultimately shown in Table 2, lines 23 to 26 of this publication.

For one, the depth is given as 998 m but actually it was the 940 m level (Hauptquerschlag der 940 m Sohle).

Could it be the stress relocation effect of the cavity of the gallery was not considered, because it seems always the highest stress values were chosen no matter at which distance to the gallery wall these were measured?

In line 23 (vertical borehole B2V) I can recognize the average Shmin of 14.2 MPa and the SHmax of 25.4 MPa, where always the maximum occurring value given in the Diplomarbeit is 23-26 MPa was used.

In line 25 (vertical borehole B4V) the SHmax = 21.3 MPa, but in the Diplomarbeit the highest SHmax value is 14 MPa only.

Long story short: I assume that the values given in the publication were correctly determined by a procedure that I simply cannot resolve.

Some very few more corrections I noted in the attached PDF file and I also added some doi links that were missing.

Citation: https://doi.org/10.5194/essd-2022-196-RC1
- AC2:
  'Reply to the review by Jochem Kück', Michal Kruszewski, 24 Aug 2022
  Dear Jochem,
  Thank you for your comments regarding our manuscript. Please see our replies below. The comments included in the continuous text of the manuscript, and not mentioned below, were accepted, and are already implemented into the new version of the manuscript. If you would have some additional comments to our answers or any helpful feedback, please feel free to submit more questions.
  With Best Regards,
  Authors
  _____________________________
  
  Nevertheless, for a better understanding of the workflow, I tried to reproduce the process of generating a few data points in this data compilation containing my own data (Haus Aden, 940 m bottom , Kück, 1988). I must admit, however, that I could not really reproduce the values that are ultimately shown in Table 2, lines 23 to 26 of this publication.
  
  Please be aware that the values in Table 2 (from the manuscript) are mean values from all the tests carried out at a specific location. To see all the measurements from each of the test locations please refer to the data set of all stress magnitudes (https://fordatis.fraunhofer.de/handle/fordatis/272).
  For one, the depth is given as 998 m but actually it was the 940 m level (Hauptquerschlag der 940 m Sohle).
  
  From the mining reports the NHN (or then the NN) level is 938 m, whereas the actual depth below the surface was 998 m. We select the latter to compute stress gradients etc. and for further analysis of stress data. To avoid misunderstanding and clarify the difference in numbers we will explicitly mention this in the manuscript text.
  Could it be the stress relocation effect of the cavity of the gallery was not considered, because it seems always the highest stress values were chosen no matter at which distance to the gallery wall these were measured?
  
  Yes, this is correct. We have not assessed the individual data records (measurements) if they are potentially influenced by excavations or other man-made effects. This should be, however, the task for further studies.The key goal of this study was to make all available data public without any interpretation in the first place. We will add this information more prominent in the manuscript and state more clearly that individual data records may be influenced by mining activities and, thus, may not represent the undisturbed in situ stress state.
  In line 23 (vertical borehole B2V) I can recognize the average Shmin of 14.2 MPa and the SHmax of 25.4 MPa, where always the maximum occurring value given in the Diplomarbeit is 23-26 MPa was used.
  
  We assume that you refer to Table 2 from our manuscript and as stated before the values given there are mean values from a given test location. For the specific location that you refer to we have six stress measurements, where the values of Shmin from these tests were 9, 11.5, 17, 18, 15.5, and 14 MPa (with an average Shmin of 14.2 MPa and a SD of 3.4 MPa), whereas the computed values of SHmax are 17, 18.5, 29.3, 30.5, 30, 27 MPa (with an average SHmax of 25.4 MPa and SD of 6.0 MPa). However, we have decided to recompute the SHmax values based on the Shmin, mean Pr, and assumption of no pore pressure rather than taking values from the report. As a result, values from the hydrofracturing report presented in Table 1 (see attachment) and values in our manuscript differ slightly. In the end, we find the recomputed SHmax values from the hydrofracturing report (Table 1; see attachment) as questionable as the SHmax magnitude in some cases seems to be lower than the Shmin magnitude. However, if you have more insights here, we would be very happy to discuss this issue in more in-depth. We will extend the text accordingly to make the reader aware why there are differences in numbers.
  In line 25 (vertical borehole B4V) the SHmax = 21.3 MPa, but in the Diplomarbeit the highest SHmax value is 14 MPa only. Long story short: I assume that the values given in the publication were correctly determined by a procedure that I simply cannot resolve.
  
  In the well B4V, the same principle as before (i.e., in the well B2V) was applied. The values of SHmax were recomputed from the three stress measurements, within this test location, based on the Shmin magnitude, mean Pr, and assumption of no pore pressure rather than taking values from the report (see Table 2 in the attachment) and resulted in 22, 22, and 19.8 MPa (with an average of 21.3 MPa and SD of 1.3 MPa). Again, some of the predicted values of SHmax in the report seem to be below Shmin, which we consider as questionable. However, if you have more insights here, we would be very happy to discuss this issue in more in-depth.
  Here also some discrepancy between results presented in Kück (1988), where results from 4 hydrofracturing tests were presented and the report from MeSy GmbH from 1994, where only 3 tests were presented, was observed.
  Reply to comments from the manuscript:
  Iserlohn is not visible on the map
  
  The figure we are referencing is Figure 1c, Iserlohn is on this figure (see on the right side of the figure).
  Shouldn’t the plot ‘d’ be left of ‘b’ chronologically and in the same sense as in plot ‘a’
  
  We created this figure, so it goes chronologically with what is mentioned in the text of the manuscript. Thus, we think that this is appropriate and would like to leave the figure as it is.
  Some discontinuities appear in black but in the caption the darkest color seems to be violet, maybe choose another color like red.
  
  We would like to abstain from using colors like red and green which could mean ‘bad’ and ‘good’, as the uncertainty of the input parameters for computation of slip and dilation is high and we would not like to ‘scare’ the audience. The values presented in our paper should be treated more in a relative sense and proper probabilistic assessment of fault reactivation potential, including wide uncertainty of all input parameters, in the area should be carried out in the future using our data set. Also, the color scheme we selected is more readable for people with disabilities. Thus, we would like to leave the figure as it is.
  
  Citation: https://doi.org/10.5194/essd-2022-196-AC2
RC2: 'Comment on essd-2022-196', P. Montone, 03 Sep 2022

The present work aims at enhancing the database of in situ stress data in the Ruhr region adopting standard and commonly used strategies. The paper is well written and clear, some figures need to be improved.

In the following I describe the main issues to be addressed in the revised version.

- Figure 1: to clarify the spatial location of data for a wider international audience, Germany has to be included in a wider map (e.g. Europe).

- Figure 1b: it could be helpful for the readers to have information on the kinematics of faults and about their age and relevance. I would suggest also adding a first order tectonic scheme.

- Figure 1a: Morawietz et alii is reported as 2021, please check the year.

- Figure 1c refers to a section that is not shown in Figure 1 a) or b). Please, also add some information about the lithology of the geological formation.

- Line 75 to line 85: for a better comprehension this paragraph should benefit from an additional figure.

- Line 108 and 109: I didn’t find within figure 1b the places and names the authors reported along the text, as well as the toponyms described in the test location description. For those unfamiliar with the region, it is difficult to follow the text and almost useless to report so many names if they are not represented in the figure. Surely most readers do not know the studied region.

- Line 206 and 207: This sentence has already been written on lines 191 and 192.

- Line 231: Please provide more information about Muskat pressure plot.

- Caption Figure 6: not very clear the sentence “is shaded in light grey” (there is no shaded areas in the figure)

- Line 320: please provide the location of Natrap 1 well

- The authors use a standard density of 2.5 g/cm3 in their calculations. I am wondering if this value is not too low for this kind of rocks. Please give more explanations for this choice.

- I am wondering what is the dipping of the induced fractures. Are all verticals?

- The average Shmax orientation is affected by a large error (+-43°): although the statement starting at line 382 is formally correct it must be emphasized the large standard deviation. It may be argued that with such a large uncertainty it is difficult to find something in strong disagreement. In essence, I’m asking the authors to reshape the statement considering the existing important uncertainty.

- Lines 385 and afterwards. I would suggest authors to reshape the conclusion about the “most critically stressed structures” in a smoother way due to the large uncertainties on the results of this study.

Citation: https://doi.org/10.5194/essd-2022-196-RC2
CC2:
'Comment on essd-2022-196', Joschka Röth, 15 Sep 2022
Dear Michal and co-authors,
I really enjoyed reading your very interesting article.

My scientific background is geosciences with focus on sedimentary basin modelling.

Since a couple of years, one of my favorite topics / hobbies is the understanding of strike-slip tectonics.

The main problem in modelling as well as in our brains is probaly that long horizontal distances are hard to imagine and horizontal displacement is not easy to calculate in 3D (most models are based on subsidence).

In this context, what are 1900 m of depth below surface (your studied formation) compared to dozens of km horizontal movements that occured at the nearby Osning Fault?

What does it mean when your data agrees with the regional stress regime (Shmax = +/-160 degrees)? What is the cause for this orientation? Did this stress orientation change over time? What are the major tectonic features in the surrounding?
What I am missing in your analysis / discussion is the regional tectonic context.

In my opinion, this article could be improved a lot by adding aspects like the stress orientation of the Variscan Orogeny and the tectonic features of this tectonic episode (deformation front, orientation of strike-slip features, ...).

Also it could be very useful to include the Lower Rhine Embayment (striking approx 160 degrees) and the Osning Fault / the Inverted Lower Saxony Basin (striking ca. 130 degrees) as eastern and western boundaries of the block that you call the Ruhr Area, but could also be called the Münsterland Basin.

Last but not least, the Alpine Orogeny definitely contributes to the present-day stress field in your study area, however the Alps and their far-field tectonic effects (i.e., basin inversion in Northern Germany and platform tilting in the south) are not mentioned in the text.
Please check consistency of your data with additional literature and include this in your discussion.
I suggest to quickly study (at least) the following publications and briefly include the above mentioned aspects in your manuscript::
Trautwein-Bruns et al., 2010: In situ stress variations at the Variscan deformation front — Results from the deep Aachen geothermal well. https://www.sciencedirect.com/science/article/abs/pii/S0040195110003288?via%3Dihub

Nelskamp, 2011. Structural evolution, temperature and maturity of sedimentary rocks in the Netherlands: results of combined structural and thermal 2D modeling. http://publications.rwth-aachen.de/record/64527/files/3625.pdf

Ahlers et al., 2021. 3D crustal stress state of Germany according to a data-calibrated geomechanical model. https://www.semanticscholar.org/paper/3D-crustal-stress-state-of-Germany-according-to-a-Ahlers-Henk/ed4aeb3f60a817b1a6e4a3234f8d1cbcd0b06aa9

Bruns et al., 2013. Petroleum system evolution in the inverted Lower Saxony Basin, northwest Germany: A 3D basin modeling study. https://onlinelibrary.wiley.com/doi/epdf/10.1111/gfl.12016

Ghazwani et al., 2018. Petroleum generation and storage in the Pennsylvanian coal-bearing strata of the Münsterland Basin, Western Germany: 3D basin modelling approach. https://www.schweizerbart.de/papers/zdgg/detail/169/90583/Petroleum_generation_and_storage_in_the_Pennsylvan?af=crossref

Voigt et al., 2021. Dawn and dusk of Late Cretaceous basin inversion in central Europe. file:///C:/Users/roeth/Downloads/Dawn_and_dusk_of_Late_Cretaceous_basin_inversion_i.pdf

Bradel & Draxler, 1982. ERWEITERTE COMPUTERAUSWERTUNGEN VON BOHRLOCHMESSUNGEN IN ROTLIEGENDSEDIMENTEN NORDDEUTSCHLANDS (OST-HANNOVER). https://pascal-francis.inist.fr/vibad/index.php?action=getRecordDetail&idt=PASCALGEODEBRGM8220175530 or https://www.osti.gov/etdeweb/biblio/5265273 ... original publication probably archived at DGMK e.V.

I would be also very happy to see additional figures in which the relationships between the regional tectonics and your local data are combined.
Looking forward to see an updated version of your manuscript.
Joschka Röth (Dr. rer. nat.)
Citation: https://doi.org/10.5194/essd-2022-196-CC2

Michal Kruszewski et al.

Data sets

In-Situ Stress Orientation Data from the Greater Ruhr Region (Germany) Derived from Hydrofracturing Tests and Borehole Logs Kruszewski, Michal; Klee, Gerd; Niederhuber, Thomas; Heidbach, Oliver http://dx.doi.org/10.24406/fordatis/200

In-Situ Stress Magnitude Data from the Greater Ruhr Region (Germany) Derived from Hydrofracturing Tests and Borehole Logs Kruszewski, Michal; Klee, Gerd; Niederhuber, Thomas; Heidbach, Oliver http://dx.doi.org/10.24406/fordatis/201

Michal Kruszewski et al.

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Short summary

The authors assemble in-situ stress magnitude and orientation data base based on 429 hydrofracturing tests have been carried out in six coal mines and two coal bed methane boreholes between 1986 and 1995 within the greater Ruhr region (Germany). Our study summarises the results of the extensive in situ stress test campaign and assigns quality to each data record using the established quality ranking schemes of the World Stress Map project.


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