Geothermal stress

In the discussion of the results given in this chapter, correlations are made with respect to coal rank and geothermal stress which, in the case of the five coals studied here, coincides with geological age. Table I gives the latest ranking of coals (2) and Table II (3) gives the geological time scale. 

Since temperature is more important than time, we now construct a model based on the changing molecular weight envelope of porphyrins relative to depth of burial/temperature (Figure 2). As a general rule, the complexity of the mass spectral envelope increases in direct proportion to the extent of geothermal stress that a particular stratum has undergone. 

Nakamura, K. (1977) Volcanoes as possible indicator of tectonic stress orientation — Principle and proposal. J. Volcano Geotherm. Res., 2, 1-16. 

Falsaperla S, Lanzafame G, Longo V, Spampinato S (1999) Regional stress field in the area of Stromboli (Italy) insights into structural data and crustal tectonic earthquakes. J Volcanol Geotherm Res 88 147-166... 

Stress induced permeability change is of crucial importance in various kinds of applications such as nuclear waste disposal in deep geological formations, geothermal energy utilization and underground excavations. In particular, coupling between the stress and permeability is a key element in understanding the nature of flow in the fractured rock (Rutqvist and Stephansson, 2003). This is because fractures, which are the main pathways of fluid flow in fractured hard rocks, are heavily dependent on the stress conditions for their deformations. 

Ito T, Hayashi K, 2003, Role of stress-controlled flow pathways in HDR geothermal reservoirs. Pure appi geophys I60 1103-1124... 

All fractures generated by internal fluid overpressure are here referred to as hydrofractures. The fracture-generating fluid may be oil, gas, magma, groundwater, or geothermal water. Hydrofractures include dykes, inclined sheets, mineral veins, many joints, and the man-made hydraulic fractures that are used in the petroleum industry to increase the permeability of reservoir rocks. Hydrofractures are primarily extension fractures (Gudmundsson et al. 2001). The difference between the total fluid pressure in a hydrofracture and the normal stress, which for extension fractures is the minimum compressive principal stress, oj, is referred to as the fluid overpressure. 

Induced seismicity is currently the most discussed risk aspect of deep geothermal systems. Drilling through and pumping fluids under high-pressure into an underground formation layer under shear stress conditions can potentially induce seismicity by releasing the stress (e.g., DiPippo, 2012). 

Earthquake swarms occur worldwide at boundaries of the Uthosphetic plates (interplate), within the plates (intraplate), as well as in subductimi ZOTies, and they are very often related to volcanic areas, areas with high activity of crustal fluids (particularly geothermal fields), and ocean ridges. Additionally, earthquake swarms are also observed in relation to human-induced stress and pressure perturbations like fluid-injectirm into deep boreholes for purposes of the geothermal energy exploitation. Quite frequent earthquake swarms occur in the Yellowstone volcanic field, which is one of the most seismically active areas of the western U.S. Eurther examples of volcanic swarms include the Hawaiian Islands, Alaska, Japan, New Zealand, and Canary Islands. 

Fig. 10 Schematic illustratimi of seismogenic tensile cracking by thermal stresses caused by convective cooling of rocks at the heat source of a geothermal system (From Foulger 1988)...

Roman D, De Angelis S, Latchman J, White R (2008) Patterns of volcanotectonic seismicity and stress during the ongoing emption of the Soufriere Hills Volcano Montserrat (1995-2007). J Volcanol Geotherm Res 173 230-244... 

Vargas-Bracamontes D, Neuberg J (2012) Interaction between regional and magma-induced stresses and their impact on volcano-tectonic seismicity. J Volcanol Geotherm Res 243-244 91-96... 

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