# Presentation: 504655

## How far from equilibrium is the Granite at Soultz 

- François Henri Cornet,
- Equipe de Mécanique des Roches
- Institut de Physique du Globe de Paris
- Determination of the regional stress field
- On local stress heterogeneity
- Microseismicity and rupture processes
- Perspectives for controlling the occurrence of large events

## The European Experimental  Hot Dry Rock site at Soultz

## Analysis of drilling induced fracture

- Tensile fractures because of cooling

- Shear induced fractures

## Compression breakouts observed in well GPK1 around 3440 m

- Compression breakouts are indicative of zones of highest tangential compressive stress :
- 	-h +3 H – Pb –f(P0) - E / (1-) = c
- No breakouts seen initially in GPK1, No breakouts in GPK2 just after drilling, some seen sometime after drilling : problem of time dependency for breakout development.
- Recall loading rate effect on rock strength (e.g. Hudson & Brown, 1973)

## Borehole elongation observed in well GK2 between 1600 m and 2900 m

## Analysis of wellbore failure mechanisms :tensile failure and compressive failure

- Tangential stress at the borehole wall
- 	 = (h + H ) – 2 ( H – h   ) cos 2 - Pb –
- 	f(P0) - E / (1-) - 3/8 E/(1-) 
- 	Where  is the mismatch between thermal expansion coefficients (solution for square inclusion in an homogeneous matrix)
- Time dependency of cooling :
- Slow cooling yields borehole elongation (thermal breakouts),
- fast cooling yields macroscopic thermal cracking
- On the problem of time dependency and stress corrosion on “strength” :
- In tension
- In compression

## Principal stress direction determination from shear wave splitting analysis(Gaucher, 1998)

## Stress data from hydraulic reconnaissance GPK1 – 1968-2000 m

- Several straddle packer tests were run for preliminary hydraulic reconnaissance but were hampered by leakages
- Single packer test was run at bottom of well (1968-2000 m) (upper figure). It yields a well defined shut in pressure
- BHTV log shows several vertical fractures striking n170° and two parallel joints striking 150° and dipping 78° NE.
- Same zone was further stimulated in 1991 (lower figure), yields reopening pressure
- Normal stress component derived from both tests yields : 26. 5 MPa (=  h)

## Results from large scale hydraulic reconnaissance test (2850-3400 m)(Sept. 1993)

## Stress magnitudes

- From density data : Sv = 33.8 + 0.0255 (z-1377);       z in m; Sv in MPa
- Vertical gradient of minimum principal stress magnitude :
- At 1980 m, Sh/Sv  = 0.535
- At 2850 m, Sh/Sv  = 0.548
- At 3315 m, Sh/Sv  = 0.541
- Proposition : Sh = 0.54 (33.8 + 0.0255 (z-1377)
- In 2003 , At 4550 m, Sh/Sv  = 0.537
- SH magnitude from focal mechanisms and shear wave splitting:
- 			 0.95 Sv  SH   1.15 Sv

## Analysis of fault plane solutions from induced microseismicity

- 2 nodal planes for each focal mechanism
- Slip vector **S**  in nodal plane is parallel to resolved shear stress ****** **in nodal plane
**S . ******** / ****| ******** ****| = 1**
****** ****= ****Tn – (Tn.n)n**

## Focal mechanisms and stress directions

- Stress perturbation caused by previous events

- Characterization of preexisting stress heterogeneity

## Local Heterogenity in stress direction is biased 

## What failure criterion for the Granite 

- Induced seismicity and rock failure
- Fault geometry and event relocation
- What failure criterion ?
- What controls stress variations with depth ?

## Acoustic emission during triaxial testing

## Fluid flow and induced seismicity

## Multiplets for precise seismic cloud analysis

**Amp.********g**

** ****B, fréquency**

**Amp.********g**

** ****B, fréquency**

**Amplitude**

- 60

- 30

- 00

**Multiplets for precise seismic cloud analysis**

**Time (seconde)**

- Sonogram (St : 4616, Comp : C2, Evt : 05246)

## Multiplets

**|**

- spatially close,
- same source mechanism,
- similar time series.

*Station 4550, Z component,  amplitude in (********g)*

**0.05**
**|**

**0.1**
**|**

**0.15**
**|**

**517**

**479**
**370**

**384**
**314**

**305**
**363**

**349**

**Time (seconds)**

## Results from the search for multiplets

**Results from the search for multiplets**

- Nb of events per multiplet

- Number ofe multiplets

- 600

- 500

- 400

- 300

- 200

- 100

- 0

- 10

- 20

- 30

- 40

- 50

- 60

- 70

|  Levenstein distance: 4e04 |  |  |  |
| --- | --- | --- | --- |
|  597 : Doublets, |  236 : Triplets |  128 : Quadruplets |  222 : Multiplets (  5) |

## Accurate relocation by cross correlation

- Hypothesis : no variation with time of velocity field.
- Events are close, so that rays are parallel, at receptor.
- Accuracy of relative travel times :
- Time correlation :
- 1 sample = ± 0.2 ms
- Spectral cross correlation:
- 1/20 sample = ± 0.01 ms
- Relative relocation :
- ti= t0 – Nx/Vi XS - Ny/Vi YS  - Nz/Vi ZS
-  ti = Difference in arrival times between Master and Slave, I = P or S;
- t0 difference of time occurrence between Master and Slave.
- subtracting tp from ts eliminates t0 and provides means to determine Xs, Ys, Zs

## Identification of fresh fracture zones  orientation

**Identification of fresh fracture zones  orientation**

- 1. For each multiplet
- One multiplet characterizes one single plane (or one line)
- linear regression (Tarantola 1987; Gaucher, 1998).
- 	 x +  y +  z = 1
- 	best plane.
-  Three points method (Fehler et al, 1987)
- 	n(n-1)(n-2)/6
- 	****identifies the direction that has been
- 		 picked the most often.
- 2. Combine multiplets for a given depth interval
- 	and use 3 points methods for all events

## Identifying the fault plane geometry

## Change of orientation with depth

| Depth interval (m)l | Mean azimuth | Mean dip | Number of events |
| --- | --- | --- | --- |
| 2800 - 2900 | N179°E | 87° | 329 |
| 2900 – 3000 | N165°E | 67° | 402 |
| 3000 - 3200 | N146°E | 86° | 416 |
|  |  |  |  |
| 1990 - 2200 | N 147°E | 87° |  |

## What failure criterion ?

- Byerlee’s law or Mohr-Coulomb ?

- 	Evaluation of the rock mass permeability from the rate of growth of  the microseismic cloud (Shapiro et al., 2000) yields pore pressure at time of failure inception

## Conclusions and perspectives

- Determination of regional stress field is necessary before any stability analysis. This requires integration of various techniques.
- Induced seismicity does not prove that the rock mass is close to failure, it only outlines local stress concentrations.
- It takes 4 to 5 MPa pore pressure increments, at Soultz, to structure induced seismicity into a fresh large scale fault that ignores large scale preexisting fractures.
- Stress perturbation caused by these large scale slip events, some part of which is non seismic, may be evaluated by mechanical modelling after identifying its overall extent
- A difficult problem is identifying the failure criterion of large scale peexisting fauls, many of which do have a significant cohesion

## Flow rate measurements during the test