ALIGNMENT OF NEAR‐SURFACE INCLUSIONS AND APPROPRIATE CRACK GEOMETRIES FOR GEOTHERMAL HOT‐DRY‐ROCK EXPERIMENTS1

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Abstract
Ubiquitous splitting of seismic shear‐waves indicates that most rocks in the upper half of the crust are pervaded by stress‐aligned fluid‐filled inclusions, called EDA‐cracks. These inclusions are expected to be aligned perpendicular to the minimum compressional stress by stress relationships similar to those aligning industrial hydraulic fractures. At depths where the overburden stress is sufficiently large (typically below a few hundred metres), this minimum stress is usually horizontal, so that the EDA‐cracks and hydraulic fractures are typically aligned vertically, striking parallel, or subparallel, to the direction of maximum compression. This is confirmed by the polarizations of the split shear‐waves along raypaths at depth in the crust. At the free surface, however, the vertical stress is zero (or approximately zero) and cracks (and hydraulic fractures) at shallow depths in intact rock tend to be horizontal. Thus, the directions of minimum stress, and the orientations of hydraulic fractures, are likely to swing through 90° near the surface of the Earth. Since the behaviour of cracks and stress is often crucial to drilling operations, the rotation of the crack‐ and stress‐geometry near‐surface has important implications, particularly for optimizing hydrocarbon production and geothermal reservoir management. Consequently, evidence gained from experiments, for example in hot‐dry‐rock geothermal heat extraction, in inappropriate crack geometries at shallow depths, may not be valid when applied to other crack‐ and stress‐geometries at depth in hot rock.