Environmental Engineering Reference
In-Depth Information
tracted by circulating water through the reservoir in a closed loop and can be used for
power generation with binary-cycle plants and for industrial or residential heating.
These experiments have proved the technical viability of the EGS approach, and
confirm that knowledge of temperature at drillable depth is a prerequisite for site se-
lection in any EGS development. The challenge remains to develop methodologies
to simulate and design viable economic systems. In this respect, one relevant project
being carried out at Tohoku University is the numerical simulator FRACSIM 3-D as
one of recently developed analogue and numerical models which provide insights
useful for geothermal exploration and production (Watanabe and Takahashi (
1995
);
Willis-Richards et al. (
1996
); Jing et al. (
2000
); Jing et al. (
2014
)). Taking account
of fracture distributions, the permeability of the rock before and after fracturing can
be simulated. This has been applied to the long-term circulation and heat extraction
experiment at Hijori, Japan.
6.5
EGS Modelling
The HDR/HWR projects shown in Fig.
6.7
have confirmed that one of the crucial
pre-requisites is to form an extensive fracture network sufficient for sustainable
heat extraction. For the formation of EGS, as mentioned previously, it is required to
stimulate and maintain multiple reservoirs with sufficient volumes to sustain long-
term production at acceptable rates: in other words to establish and maintain the
water supply and viable flow paths from the high-temperature rock. A small num-
ber of multiple fractures may only result in an early thermal drawdown due to the
limited area of heat exchanging surface (hydraulically induced fracture surface).
The majority of the rock mass types encountered and/or selected for the sites of the
HDR/HWR projects included a number of naturally occurring fractures. Further-
more, hydraulic stimulations conducted have been shown to induce primarily shear
dilation (aperture increase due to slip deformation along the natural fracture) rather
than formation of new fractures. Thus, based on the experience obtained from the
HDR/HWR projects, it appears to be a viable approach to utilize and stimulate a
natural fracture system in order to create a water circulation loop with sufficiently
high permeability for geothermal energy extraction.
The above-mentioned observation underlines the importance of characterizing
and modelling the distribution of natural fractures and their mechanical response
for the design of engineered geothermal reservoirs. Figure
6.9
shows a schematic
of the fracture network model employed for the development of FRACSIM-3D.
FRACSIM 3-D takes into account the complexity of natural fractures, and utilizes
the phenomenon of fractal geometry (because most fractures exhibit the character-
istics of fractals) to simulate the reservoirs shown in red in Fig.
6.9
. In FRACSIM-
3D, the reservoir is assumed to consist of a number of circular cracks whose length
distribution follows the fractal geometry.
As illustrated in Fig.
6.10
, FRACSIM-3D firstly generates a natural fracture net-
work based on the fractal characteristics, which can be obtained from surface obser-
vations. Observation of cores and well logs may be used to determine the number
