Microseismicity and permeability enhancement of hydrogeologic structures
during massive fluid injections into granite at 3 km depth at the
Soultz HDR site
Geophysical Journal International, 160 (1):
389--412 (January 2005)DOI: 10.1111/j.1365-246X.2004.02474.x
Abstract
A high-rate injection of 20 000 m3 of water into granite between 2.8
and 3.4 km depth at the Soultz hot dry rock (HDR) test site in France
in 1993 September led to a 200-fold increase in borehole transmissivity
and produced a subvertical cloud of microseismicity of dimensions
0.5 km wide, 1.2 km long, 1.5 km high and oriented 25 deg NW. The
resulting data set is unusually complete and well suited to studying
permeability creation/enhancement processes in crystalline rock and
the utility of microseismic data for revealing them. Although the
microseismic cloud defined using joint hypocentre determination (JHD)
locations was diffuse and showed little structure, application of
the collapsing method showed it to be composed largely of discrete
tubes and planes that propagated coherently. One prominent structure
that extended 350 m downwards from the vicinity of a flow inlet early
in the injection and that appears to contain a major flow path was
subjected to detailed investigation to establish its hydrogeologic
nature and the mechanisms underpinning its inferred permeability
enhancement. High-resolution microseismic mapping techniques (i.e.
multiplets and clustering) showed it to be a subvertical, NNW-SSE
striking, fracture zone of width 10-20 m. The strike and scale of
the structure identifies it as a member of a family of hydrothermally
altered, cataclastic shear structures that constitute the primary
permeable paths for fluid migration within the rock mass, both under
ambient and forced fluid flow conditions. The microseismicity occurred
on subvertical, small-scale fractures within the cataclastic shear
zone whose azimuths scatter within 22° of parallel to the parent
structure. Although the structure is likely to have been naturally
permeable to some degree, its permeability appears to have been significantly
enhanced as a consequence of the injection. The most likely mechanism
of permeability enhancement, which is in accord with the strong preference
for the microseismicity to grow downwards, involves strike-slip shearing,
which produced the opening of vertical tubes at along-strike jogs
in the fault (the so-called Hill mesh). Seismic moment release averaged
over the structure suggests shear displacements of at least 0.3 mm
occurred, which are sufficient to generate aperture changes that
are hydraulically significant. The preponderance of discrete structures
within the microseismic cloud after collapsing suggests that significant
flow and permeability enhancement (i.e. stimulation) within the rock
mass is largely confined to the interiors of shear zones that appear
to have a spacing of approximately 100 m.