Rock-mechanics experiments, geodetic observations of postloading strain transients, and micro- and macrostructural studies of exhumed ductile shear zones provide complementary views of the style and rheology of deformation deep in Earth's crust and upper mantle. Overall, results obtained in small-scale laboratory experiments provide robust constraints on deformation mechanisms and viscosities at the natural laboratory conditions. Geodetic inferences of the viscous strength of the upper mantle are consistent with flow of mantle rocks at temperatures and water contents determined from surface heat-flow, seismic, and mantle xenolith studies. Laboratory results show that deformation mechanisms and rheology strongly vary as a function of stress, grain size, and fluids. Field studies reveal a strong tendency for deformation in the lower crust and uppermost mantle in and adjacent to fault zones to localize into systems of discrete shear zones with strongly reduced grain size and strength. Deformation mechanisms ...

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Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics, Geodesy, and Field Observations

Geothermal energy is the energy contained as heat in the Earth’s interior. This overview describes the internal structure of the Earth together with the heat transfer mechanisms inside mantle and crust. It also shows the location of geothermal fields on specific areas of the Earth. The Earth’s heat flow and geothermal gradient are defined, as well as the types of geothermal fields, the geologic environment of geothermal energy, and the methods of exploration for geothermal resources including drilling and resource assessment. Geothermal energy, as natural steam and hot water, has been exploited for decades to generate electricity, and both in space heating and industrial processes. The geothermal electrical installed capacity in the world is 7974 MWe (year 2000), and the electrical energy generated is 49.3 billion kWh/year, representing 0.3 % of the world total electrical energy which was 15,342 billion kWh in 2000. In developing countries, where total installed electrical power is still low, geothermal energy can play a significant role: in the Philippines 21% of electricity comes from geothermal steam, 20% in El Salvador, 17% in Nicaragua, 10% in Costa Rica and 8% in Kenya. Electricity is produced with an efficiency of 10–17%. The geothermal kWh is generally cost-competitive with conventional sources of energy, in the range 2–10 UScents/kWh, and the geothermal electrical capacity installed in the world (1998) was 1/5 of that from biomass, but comparable with that from wind sources. The thermal capacity in non-electrical uses (greenhouses, aquaculture, district heating, industrial processes) is 15,14 MWt (year 2000). Financial investments in geothermal electrical and non-electrical uses world-wide in the period 1973–1992 were estimated at about US$22,000 million. Present technology makes it possible to control the environmental impact of geothermal exploitation, and an effective and easily implemented policy to encourage geothermal energy development, and the abatement of carbon dioxide emissions would take advantage from the imposition of a carbon tax. The future use of geothermal energy from advanced technologies such as the exploitation of hot dry rock/hot wet rock systems, magma bodies and geopressured reservoirs, is briefly discussed. While the viability of hot dry rock technology has been proven, research and development are still necessary for the other two sources. A brief discussion on training of specialists, geothermal literature, on-line information, and geothermal associations concludes the review.

Geothermal energy technology and current status: an overview

Deep drilling and induced seismicity experiments at several locations worldwide indicate that, in general, the brittle crust in intraplate regions is critically stressed, pore pressures are close to hydrostatic, and in situ bulk permeability is ∼10 −17 to 10 −16 m 2 . This high permeability, three or four orders of magnitude higher than that measured on core samples, appears to be maintained by critically stressed faults and greatly facilitates fluid movement through the brittle crust. We demonstrate that such high permeabilities can maintain approximately hydrostatic fluid pressures at depths comparable to the thickness of the seismogenic crust. This leads to the counterintuitive result that faulting keeps intraplate crust inherently strong by preventing pore pressures greater than hydrostatic from persisting at depth.

How faulting keeps the crust strong

http://hera.wdcb.ru/tols/tecton/method/biblio/magn/Zoback_03.pdf

Determination of stress orientation and magnitude in deep wells

Reservoir Geomechanics: Frontmatter

For many years, in situ stress in the brittle crust has been measured at relatively shallow depth and related to the mechanical behavior of the crust as inferred from laboratory studies and faulting theory. A continuous profile of the magnitudes and orientations of the three principal stresses has been estimated to depths of 7.7 km and 8.6 km in the German Continental Deep Drilling Program (KTB). This was achieved by hydraulic fracturing tests at relatively shallow depth (1–3 km), estimates of the magnitude of the least horizontal principal stress provided by modified hydraulic fracturing experiments at 6 km and 9 km depths, and analysis of compressional (breakouts) and tensile (drilling-induced tensile wall fractures) failures of the borehole wall over nearly the entire depth of the KTB borehole. The orientation of the maximum horizontal principal stress was found to be uniform with depth with an orientation of N160°±10°E, which is consistent with the average orientation found throughout western Europe. The only significant change in stress orientation was observed directly below a major fault zone crosscutting the borehole. The profile of stress magnitudes we have obtained demonstrates that to a depth of 8 km, the state of stress in the brittle crust in southern Germany is in frictional equilibrium. That is, the ratio of shear to normal stress as resolved on preexisting faults which are well-oriented to the in situ stress field is comparable to their frictional strength based on predictions of Coulomb faulting theory for a coefficient of friction of about 0.7 and near-hydrostatic pore pressure.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JB02942

Estimation of the complete stress tensor to 8 km depth in the KTB scientific drill holes: Implications for crustal strength

The deep EGS (Enhanced Geothermal System) project at Soultz-sous-Forêts (Alsace, France)

IT has been suggested1–6that in many cases the average strength of the continental crust is quite low (tens of megapascals), so that the crust has little effect on the large-scale deformation of the lithosphere. But laboratory friction studies7,8, combined with simple faulting theory9,10 (as well as extrapolation ofin situ stress measurements from the upper 3 km of the crust11), imply that if pore pressure is approximately hydrostatic at mid-crustal depth, crustal strength is appreciable (hundreds of megapascals) and would markedly constrain the nature of lithospheric deformation12–15. Here we report estimates of the magnitude of in situstresses to 6 km depth in the KTB borehole in southern Germany. Our results indicate a high-strength upper crust, in which the state of stress is in equilibrium with its frictional strength. We suggest that plate-driving forces in the continental lithosphere in this part of western Europe are transmitted principally through the upper crust, and that this may also be the case in other continental areas of moderate to elevated heat flow.

/pdf/upper-crustal-strength-inferred-from-stress-measurements-to-3kxchitjt8.pdf

Upper-crustal strength inferred from stress measurements to 6 km depth in the KTB borehole

HDR/HWR reservoirs: concepts, understanding and creation

The effects of confining pressure/normal stress and temperature on shear fracture energy of rocks have been studied with intact samples of sandstones. Both the nominal values of the shear fracture energy G nom , calculated using triaxial data at constant confining pressure and the inferred values G for the loading path satisfying constant normal stress show that they at first increase then decrease with increasing confining pressure/normal stress and they decrease with increasing temperature. For Ruhr-sandstone, there is a maximum value of G nom 6.2 × 10 4 Jm −2 at 50 MPa confining pressure and maximum G 2.7 × 10 4 Jm −2 at 140–180 MPa normal stress respectively. They tend to zero at the confining pressure of 300 MPa and the normal stress of 600 MPa; for Bunt-sandstone, the maximum value of G nom and G are 0.69 × 10 4 Jm −2 at 20–30 MPa confining pressure and 0.45 × 10 4 Jm −2 at 60–70 MPa normal stress respectively. They tend to zero at confining pressure of 20–30 MPa and normal stress of 150 MPa. The fracture energy mainly depends on the stress drop during the breakdown process at both different pressures and temperatures and in different loading paths.

Fritz Rummel

Papers

Estimation of the complete stress tensor to 8 km depth in the KTB scientific drill holes: Implications for crustal strength

The deep EGS (Enhanced Geothermal System) project at Soultz-sous-Forêts (Alsace, France)

Upper-crustal strength inferred from stress measurements to 6 km depth in the KTB borehole

HDR/HWR reservoirs: concepts, understanding and creation

Shear fracture energy of rock at high pressure and high temperature