F.H. Cornet; IPG-Strasbourg, francois.cornet@unistra.fr Depth dependent stress field and its consequences for intraplate seismicity F.H. Cornet; IPG-Strasbourg, francois.cornet@unistra.fr What controls the vertical stress variations and their lateral variations in sedimentary formations ? The question of stress decoupling and its consequences for estimating the stress field at depth Some results from focal plane inversions in the Rhine Graben and the problem of mapping plate scale tectonic stress fields Discussion on the origin of the stress field in domains of very slow deformation Wileveau, Cornet, Desroches and Blumling; Physics and chemistry of the earth, 2007; vol. 32, pp 866-868; Cornet and Roeckel, 2011, accepted in Tectonophysics + Julie Maury’s Ph D thesis

Local site conditions at the Underground Research Laboratory Geological conditions Initial borehole reconnaissance

Hydraulic fracturing The stress field close to a cylindrical hole in an elastic field is : If the borehole is parallel to a principal stress direction (Vertical) and a pressure is applied in the hole: σθθ = (σH + σh ) – 2 (σH - σh ) cos 2θ - Pw, and rupture occurs for : If the rock has been cooled down by mud circulation, the hoop stress is : σθθ = -KΔT /E, where K is coefficient of thermal expansion, E is Young’s modulus and ΔT is the difference between far-field and borehole temperature Both hydraulic fracturing and thermal cracking are mode I fractures perpendicular to the minimum principal stress

Results from initial Hydraulic fracturing tests Results from fracture orientations Results for stress magnitudes

Hydraulic fracture in argilite UBI image Hydraulic test

A strategy for getting the complete stress tensor Techniques based on hydraulic testing: Use standard hydraulic fracturing tests in vertical wells for principal stress direction and minimum horizontal principal stress magnitude Test sleeve fracturing and sleeve fracture reopening (there is no fluid penetration into the fracture but a major question remains on the role of pore pressure) Use Hydraulic tests on preexisting fractures technique for measuring vertical component Use real hydraulic fracturing tests in horizontal wells parallel to the principal stress directions ; but if σh = σv < σH , then such tests do not yield the σH magnitude . Run hydraulic tests in inclined wells so as to create en echelon fractures. If five of the six stress components are known, then these tests yield the maximum horizontal principals stress magnitude Interpret borehole breakouts, both in vertical and inclined wells (differences if wells are drilled with water based mud or oil based mud) Interpret convergence measurements in vertical shaft

Is the vertical component equal to the weight of overburden? Sources for possible lateral variation in vertical stress component are : Lateral variation in vertical stiffness, either because of local alteration in the limestone layers or because of lateral variations in compaction of the argilite Fracturing of the stiffest layers (beam effect) Topographic effects HTPF helps reopen bedding planes and solves for the vertical component of the stress tensor At the URL site, the gradient of the vertical stress component is found equal to 0.0254 Mpa/m. In the North East Corner it is found equal to 0.0234 Mpa/m. This has been attributed to topography

Generating en echelon fractures in inclined wells effects on borehole failure + Pw - Pw Zones where maximum stress component reach its maximum value Azimuth where minimum principal stress reaches its lowest value TOH BOH TOH BO IF B0 IF Smin(θech) = ½ ( + zz ) - [ ( - zz )2 / 4 + z2 ] 0.5 On the necessity to know the five stress tensor components within the same structural sedimentary layer Inclination of plane in which shear stress is zero

Results in inclined wells at Bure from electrical imaging (only in water based mud) Tensile fracture Compressive failure Q = 2 – (σv - σh )/ (σH - σh )

σH orientations derived from borehole breakouts

Borehole breakout data are not uniquely related to present-day state of stress. The BO angular width depends on chemical effects, for clay rich materials. They are absent in wells drilled with oil based mud. This helps place bounds on horizontal principal stress magnitudes. The BO amplitude strongly depends on time Yet, Breakouts demonstrate the existence of differential stress in the horizontal plane

Results from the stress evaluation Stress profile obtained by combining all data initial results

Lateral variability of the σh/σv vertical profile URL (South corner) σv measured North west corner σv estimated

Lateral variability of vertical profile of σh/σv North East corner , σv measured

Factors controlling vertical stress variation with depth at Bure Although a linear trend may be fitted to the vertical variation of the minimum principal stress, this linear trend is not consistent with the accuracy of the stress measurements. The factor controlling the vertical stress variation is not friction on supposedly critically oriented preexisting fractures, (that have not been observed) but the rheological properties of the material

How to explain the present day stress field How do we explain a small shear component in the Callovo-oxfordian if it creeps ? How do we explain shear stresses in the overlying oxfordian limestone if the shear stress has been relaxed in the callovo-oxfordian argilite ? We need to introduce present day deformation in the overlying limestone. Are present day stresses in the Paris Basin controlled by tectonics ?

No evidence for present day large scale deformation No seismic activity within the Paris Basin, Grünthal et al., 2007; No detectable displacement field from continuous GPS, Noquet and Calais, 2004

Evidence from Vertical Seismic Profiles Vertical seismic profiles run in various wells have shown the occurrence of shear wave splitting, which characterizes velocity anisotropy. This anisotropy reflects density of microfractures. A lack of anisotropy is noticed in the shaly layer (Lefeuvre et al., Geophysics, 1992).

Need of internal mechanism: Could it be fluid-solid chemical coupling ? Existence of preferential fracture orientation induces flow in preferential direction that may induce dissolution in preferential direction; Effect of pressure solution on asperities; Pulsing effect linked to glaciations (Jost et al., 2007)

Hydraulic head in COX layer (ANDRA reports)

Conclusions from the Paris Basin The vertical variation of the stress field is controlled by material properties rather than by friction along critically oriented faults. The observed small shear stress observed in the argilite layer cannot be caused by tectonic stresses. The circulation of fluid is the likely cause for the observed differential stress in the limestone layers. It leads to a present day local deformation.

Copyright: Piewak & Partner GmbH The Southern Permian Basin Spatial Extent of the Zechstein evaporitic formation dated from the Permo-Triasic transition. Spatial extent of the Zechstein evaporitic formation 26.04.2017 Copyright: Piewak & Partner GmbH 22

Stress direction above Zechstein salt 26.04.2017 Stress direction above Zechstein salt 26.04.2017 Copyright: Piewak & Partner GmbH 23 23

Stress direction above Zechstein salt 26.04.2017 Stress direction above Zechstein salt 26.04.2017 Copyright: Piewak & Partner GmbH 24 24

Schematic Stress Profile in the North German Basin 26.04.2017 Schematic Stress Profile in the North German Basin 26.04.2017 Copyright: Piewak & Partner GmbH 25 25

Conclusions from sedimentary basins Combination of hydraulic testing and borehole imaging in wells with various orientations, but including one direction parallel to a principal stress direction, provides means to determine the complete stress field in soft material. The vertical stress profile is controlled by material rheology and possibly fluid circulation rather than by friction Modeling Stress field in sedimentary layers that include competent (solid type) material may not be simply derived from pure continuum mechanics principles but may requires taking into account chemical coupling. Stress fields in sedimentary formations in domains of very slow deformation may not represent the stress field in the basement. The only information we get on the stress field at seismogenic depths is from focal mechanisms

Focal mecanisms inversion in an increasing volume Sierentz : 33 focal mechanisms 73% data explained S1 : 340/0 ; S2 : 250/79 ; S3 : 70/12 ; r=0.35 misfit : 4.5°

Focal mecanisms inversion in an increasing volume 20 km around Sierentz : 49 focal mechanisms 71% data explained S1 : 160/0 ; S2 : 249/78 ; S3 : 70/13 ; r=0.44 Reduced misfit : 2.5° Global misfit : 6.6° 40 km around Sierentz : 73 focal mechanisms 63% data explained S1 : 138/4 ; S2 : 344/86 ; S3 : 228/2 ; r=0.5 Reduced misfit : 2.9° Global misfit : 12.0° Rhine graben : 99 focal mechanisms 64% data explained S1 : 142/2 ; S2 : 25/96 ; S3 : 232/4 ; r=0.40 Reduced misfit : 4.2° Global misfit : 18.6° Converges toward SH=N142°

What controls the stress field in domains of very slow deformation rates ? The various controlling factors Rheological consideration

Conclusion In sedimentary formations, the stress field does not vary linearly with depth. In crystalline formation, although the stress field does vary linearly with depth in limited depths intervals, some rotations of principal stress directions are observed at the multi-kilometer scale. For depths greater than 6 km, only focal mechanisms may help constrain the stress field. But focal mechanisms solutions alone do not constrain the principal stress directions at better than 20 to 25°. Additional independent constraints must be introduced , whether from an extrapolation of borehole data or from a numerical model if a resolution better than 20 ° is desired.