EGU2020-3567
https://doi.org/10.5194/egusphere-egu2020-3567
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Time-lapse changes within the Groningen gas field caused reservoir by compaction and distant borehole drilling

Hanneke Paulssen and Wen Zhou
Hanneke Paulssen and Wen Zhou
  • Utrecht University, Department of Earth Sciences, Utrecht, Netherlands (h.paulssen@uu.nl)

Between 2013 and 2017, the Groningen gas field was monitored by several deployments of an array of geophones in a deep borehole at reservoir level (3 km). Zhou & Paulssen (2017) showed that the P- and S-velocity structure of the reservoir could be retrieved from noise interferometry by cross-correlation. Here we show that deconvolution interferometry of high-frequency train signals from a nearby railroad not only allows determination of the velocity structure with higher accuracy, but also enables time-lapse measurements. We found that the travel times within the reservoir decrease by a few tens of microseconds for two 5-month periods. The observed travel time decreases are associated to velocity increases caused by compaction of the reservoir. However, the uncertainties are relatively large. 
Striking is the large P-wave travel time anomaly (-0.8 ms) during a distinct period of time (17 Jul - 2 Sep 2015). It is only observed for inter-geophone paths that cross the gas-water contact (GWC) of the reservoir. The anomaly started 4 days after drilling into the reservoir of a new well at 4.5 km distance and ended 4 days after the drilling operations stopped. We did not find an associated S-wave travel time anomaly. This suggests that the anomaly is caused by a temporary elevation of the GWC (water replacing gas) of approximately 20 m. We suggest that the GWC is elevated due to pore-pressure variations during drilling. The 4-day delay corresponds to a pore-pressure diffusivity of ~5m2/s, which is in good agreement with the value found from material parameters and the diffusivity of (induced) seismicity for various regions in the world. 

How to cite: Paulssen, H. and Zhou, W.: Time-lapse changes within the Groningen gas field caused reservoir by compaction and distant borehole drilling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3567, https://doi.org/10.5194/egusphere-egu2020-3567, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 30 Apr 2020
  • CC1: compaction signal, Laura Ermert, 05 May 2020

    Dear Hanneke, thanks for the presentation. I was wondering when you mention that other studies found a larger signal that they attributed to compaction, did they use different methods? I am asking since I am starting to work on basin subsidence and its effects on velocity and curious what I should expect.

    • CC2: Reply to CC1, Wen Zhou, 05 May 2020

      Hi Laura, there she means comparing with 4D seismic survey data from MacBeth et al 2018 Geophysical Prospecting, and Hatchell and Bourne 2005.

      • CC3: Reply to CC2, Laura Ermert, 05 May 2020

        Thanks a lot! So is the difference due to the method or are there different amounts of compaction in the different observation periods?

        • AC2: Reply to CC3, Hanneke Paulssen, 05 May 2020

          The overview by MacBeth et al. (2019) shows large variations for different reservoirs. Reservoir slowdown has even been observed.

    • AC1: Reply to CC1, Hanneke Paulssen, 05 May 2020

      Dear Laura, what I meant is that the pore pressure effect of drilling at a distance of 4.5 km is more than 10 times larger. Our results show that the (potential) effects of compaction that we observe are small, whereas studies of time-lapse 3D seismic surveys typically find travel time changes of 0.1 s per year over 100 m. An overview of those studies is given by  MacBeth et al. (2019). The study by Hatchell & Bourne (2005) assumes a linear relation between compaction/strain and seismic velocity. Hope this helps. 

  • CC4: Comment on EGU2020-3567, Laura Ermert, 05 May 2020

    Yes! Thanks for clarifying, I will have a look at the study you mentioned. I past the previous discussion from the videolog in case people pass by in the following days.

     

    Q: Is it right that you use the direct-wave traveltimes from the train signals in between boreholes, and could you also construct coda wave correlations from the trains?

    A: Indeed we use direct P waves. We would like to model the coda but we would like to model that with synthetics. Something for the future. Note that there are also spurious arrivals from conversions/reverberations above the reservoir.