# Presentation: 161190

## Microseismicity in the European Hot Dry Rock Research Project at Soultz-sous-Forêts, France

**Sophie Michelet****+**

**+ now PhD student at ERL-MIT, formerly geophysicist at the**
**European Economic Interest Grouping Heat Mining**

- GPK2 platform

**Notes:**
My talk will be on the microseismicity that occurred after the 2000 and 2003 stimulations. I will show you the study I did for my master and when I worked as a geophysicist in Soultz.

## The Location

**Notes:**
On that slide, you see once again where Soultz is. In alsace, close to the german border, in the Rhine graben.

## The History

**1995**
**1996**
**2000**
**2003**

_**The History**_

- Upper Reservoir
- 1987 - 1997

- Deep Reservoir
- 1988 - 2003

**Notes:**
On that slide are represented the microseismic events from the different phases that occurred in Soultz. At approximately 3.5 km depth, there is the upper reservoir, created between 1987 and 1997. At around 5km depth is the deep reservoir, created between 1988 and 2003. I will stay focused on the deeper reservoir. 

## Seismic Network

**RETS**

**SCHO**

**MEMM**

**STEI**

**KIRC**

**CARO**

**KUTZ**

**LIEN**

**REIM**

**SURB**

**HOEL**

**SCH1**

**SCH2**

**LAMP**

**LANG**

**HOFF**

**KUHE**

**FOR1**
**FOR2**

**GPK1**

**EPS1**

**GPK2**

**OPS4**

**4601**

**4550**

**4616**

**GPK3**

**CAR3**

**PRE3**

**ST03**

**LOB3**

**BET3**

**HOH3**

- N

- 0

- 1

- 2 km

- 2003 Seismic Network

- Surface stations

**Downhole sensors**

**Darnet 2003**

_**Seismic Network**_

_**injection**_

**Notes:**
This slide is a map of the Soultz area, The seismic network is added to it. In blue, surface seismic array and in red, the downhole array. The data I’ll show you are only from the downhole network.

## Seismic Network

- 3 accelerometers
- +
- 2 geophones
- were recording microseismic events during the stimulation

**Notes:**
Once again, the seismic network, but this time, only the downhole array. During the 2003 stimulation, 3 acc and 2 geophones were recording microseismic activity. As you see, acc were @ 1.5km depth and geophones were at 2.2 and 3.5 km depth. In 2000, fluid was injected in the GPK2 well and in 2003, injection took place in the GPK2 and 3 wells.

## Data Acquisition 

**4 axis accelerometers**

**Deployment**
** ****of the sensor**
**in well 4601**

**Sampling rate **
**= **
**2000 Hz**

**Notes:**
This slide shows you what the accelerometers used in Soultz look like. The picture shows the deployment of one of them in weel 4601.

## Hydraulic Stimulation 2000

**Injection rate**

**Wellhead pressure**

**Seismic rate**

**Notes:**
First, let’s start with the 2000 stimulation. On top are represented the WH pressure and the injection rate during the stimulation. On bottom, the seismic rate during the same period of time. The seismicity is high during the stimulation (sometimes more than 200 evts/h). Microseismic events didn’t stop instantaneously after the end of the injection. It persisted for a while. A M2.6 event occurred a week after the end of the stimulation.

## Hydraulic Stimulation 2003

**Ml 2.7**

**Ml 2.9**

_**Hydraulic Stimulation 2003**_

**Notes:**
_x0014_This slide shows you the hydraulic stimulation that took place in 2003. On top is represented the seismic rate per hour during and after the stimulation. In red, during and in blue after shut-in. The rate follows the injection rate. But when the pumps were shut down, the seismicity staid high. Moreover, some events with a magnitude higher than 2.5 occurred around 10 days after the end of the injection. A production test was done in order to relax the reservoir.

## Largest Events 2000/2003

_**Largest Events 2000/2003**_

**The **_**ML>2.5**_** events from 2000 & 2003**

**Notes:**
The largest events from the 2 stimulation are represented on that slide. Both in 2000 and 2003, these events occurred NW of the well GPK2, in the area.

## 3D image of microseismic intensity

**Identify high seismic activity zone**
**i.e. M>2.5 occurred**

_**Calculation of the Seismic Moment M0**_

**Notes:**
We thought that a mean to get a view of the higher seismic zone was to calculate Mo.

## Calculation of the source parameters from the 2000 & 2003 stimulation with 13,000 & 9,000 microseismic events

## Calculation of Source Parameters

**Characterization of **
**source mechanics **

**Window containing the P pulse**

**FFT**

- 16Hz-263Hz

**Sampling rate = 2 kHz**
**Length of the trace = 5 s**

**Notes:**
We calculated source parameters on the vertical traces of the accelerometers. Example of one seismic trace. We selected a window starting at the P onset. This window was then Fourier transform in order to get the acceleration spectrum.

## Method of calculation

- Spectrum corrected from attenuation

- Spectrum without correction

- Corner frequency

**Notes:**
The blue curve is the spectrum without correction from attenuation. This attenuation was corrected first, then the best lines that fit the corrected spectrum were estimated. This enabled us to get the seismic moment and the corner frequencies. 

## Moment Magnitude 2003

**Notes:**
From the seismic moment, we estimated the moment magnitudes of the events. Between -2 and 2.

## Seismic Moment 2000

**GPK2**

_**Gray:**_** events during stimulation****	**_**Black**_**: after shut-in events**

**Notes:**
As I told you, we calculated the seismic moments. The 2 colors represent 2 periods of the stimulation: during the injection and after the shut-in. during the stimulation, the largest events were around the GPK2 well.  I enlighted an area of events that occurred after shut in. The M2.6 event occurred in this zone.

## Seismic Moment 2003

- GPK1

-  GPK2

- GPK3

_**Seismic Moment 2003**_

*AFTER SHUT-IN MICROSEISMIC EVENTS*

**Notes:**
In 2003, after shut-in seismic moments are shown on that slide. The circle shows the same area as the previous slide. It is in this area again that 2 larger events occurred (M2.7 & 2.9)/ It really looks this area is seismically active.

## Relationship between the seismic moment & the injected volume?

**Notes:**
Concerning the 2000 stimulation, we were interested in the possible relationship between the two parameters.

## Seismic moment & Injection Rate

_**Seismic moment & Injection Rate**_

**McGarr (1976)**

**Total Seismic Moment**

**K ~ 0.5**

**Volume added to region in expansion in direction NW/SE(********2) and NE-SW (********3)**

**Fluid injected**

**Notes:**
The total seismic moment is represented as a function of the injected fluid volume. As it can be seen on that slide, there is a clear linear relation between the 2. We applied McGarr’s formula on our data, it is a relation between the total seismic moment and the product of the modulus of rigidity, the injected volume and a constant K. We found a value of around 1/2 for K. Following the author, it means that volume is added to region in expansion in direction NW and NE-SW.

## Calculation of the Stress Drop

**Stress state evolution with time and space**

**Stress front propagation**

**Notes:**
We also calculated the stress drop for the 2 stimulations, in order to get an estimation of the stress evolution inside the reservoir.

## Stress Drop 2000

**GPK2**

*AFTER SHUT-IN MICROSEISMIC EVENTS*

- Higher stress drops
- =
- Envelope of the
- cloud

**Notes:**
You see on that slide the after shut-in stress drop from the 2000 stimulation. The higher values are on the boundaries of the stimulated zone, as an envelope.

## The four Phases of the Stimulation

- Phase 1

- Phase 2

- Phase 3

- Phase 4

**Stress Drop**

**Notes:**
Hydraulic program from the 2003 stimulation. We divided the whole program in 4 parts, to have a view of the propagation of the stress front.

## Stress Drop 2003 (1)

- GPK1

-  GPK2

- GPK3

**Phase 1**

**Notes:**
As the stimulation goes on, the stress front is moving away from the injection point and surrounds the stimulated zone.

## Phase 2

-  GPK2

- GPK3

- GPK1

-  GPK2

- GPK3

**Phase 2**

_**Stress Drop 2003 (2)**_

## Stress Drop 2003 (3)

-  GPK2

- GPK3

**Phase 3**

- GPK1

_**Stress Drop 2003 (3)**_

## Phase 4

-  GPK2

- GPK3

- GPK1

-  GPK2

- GPK3

**Phase 4**

_**Stress Drop 2003 (4)**_

## Summary & Conclusion

**Large number of microseismic events during the stimulation in 2000 and 2003**** **

**[max ML2.6 (2000)] & [max ML2.9 (2003)]**

** ****Linear Relationship between seismic moment and injected volume**** **

- A way to control the seismicity?

**Notes:**
The 2 stimulation generated a large number of events with each time larger events after the end of the injection.
Found an interesting linear relation between the Mo and the injected volume, could help to control the seismicity

## More information can be obtained from source parameter calculation:

**Same area with large seismic moments in 2000 & 2003**
**	****  ****high seismic activity zone**

**Larger stress drop located all around the stimulated area ****	****	****     **
**                 ****stress front propagation **

## Merci beaucoup!

**Merci beaucoup!**

## Slide 29