A fractured, fluid filled natural reservoir normally consists of three main phases: 
(1) host-rock matrix, (2) fractures, and (3) fluid. The host-rock matrix may itself 
be a complex material, containing contacts, pores and other inhomogeneities and 
inclusions. Similarly, the fluid phase may consist of gas, liquid, or both. The 
mechanical behaviour of a natural fluid-filled fractured reservoir during drilling 
of wells and subsequent extraction of fluids depends much on its overall properties 
and the associated local stresses. The rock properties, in turn, depend strongly on 
the nature and distribution of inhomogeneities and inclusions in the rock and, for 
fractured reservoirs, particularly on the geometry and arrangement of the fractures 
themselves. Young’s modulus, for example, may vary widely within a reservoir 
depending on its fractures and layering.

The heat stored in fractured, crustal rocks can be used to produce geothermal water 
for direct heating, electricity production, or both. There are two basic ways by 
which the heat can be used. One is direct use of natural geothermal fields, such as 
in Iceland; the other is through man-made reservoirs in hot rocks, such as are 
currently being made in Germany. In both cases the fundamental parameter to be 
understood and maintained for economic use of the heat stored is the rock 
permeability. In fractured natural geothermal reservoirs the permeability is largely 
maintained through active (often seismogenic) faulting and hydrofractures. In man-
made geothermal reservoirs, the permeability is primarily initiated and maintained 
through stimulation methods such as hydraulic fracturing, as has been used for 
increasing permeability in petroleum reservoirs for nearly 60 years. 
	
In this talk we first describe natural fractured reservoirs in various rock types, 
using examples from Iceland, Italy, UK, and Germany. Field examples and numerical 
models are used to explain the effects of mechanical layering in reservoirs on 
fracture propagation, fracture arrest, and the development of interconnected 
fracture networks. Then we show, using theoretical examples, how the mechanical 
properties of reservoirs, such as Young’s modulus, vary in relation to fracture and 
cavity frequencies and distributions. Finally, we present results on hydraulic 
fracture propagation, and associated permeability effects, in fractured reservoirs. 
These include (1) the effects of the mechanical contrast between the target unit 
(the potential reservoir) and the adjacent rock units in confining the hydraulic 
fracture to the target layer, and (2) the effects of existing faults on hydraulic 
fracture paths and associated permeability in man-made geothermal reservoirs.

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