241
that everywhere on Earth has a geothermal gradient, where the heat of the rock
increases with depth (USDOE 2016). Some of this is primordial heat left over from
when the Earth accreted from smaller objects some 4.5 Ga, but most of it is generated from the decay of naturally-occurring radioactive elements in the subsurface
rocks. A borehole drilled deep enough anywhere on Earth will eventually encounter
rocks hot enough to provide energy. The problem is that this is much deeper in some
places than others, and the major economic barrier to widespread EGS development
is the cost of drilling.
Heat from the rocks has to be transported to the surface by some kind of fluid to
be useful for energy generation. For natural geothermal energy systems, the fluid is
commonly high temperature groundwater moving upward from below. EGS injects
fluid into the ground and recovers it to extract the heat. This can be accomplished by
drilling two parallel boreholes as horizontal laterals through the hot rocks at depth.
Engineered flowpaths are required for the introduced fluids from one well to collect
heat from the deep rocks and enter the other well to be transported back to the
surface.
Hydraulic fractures are created to connect the two laterals and introduce flowpaths through the hot rocks for fluids to circulate between the two wells. The vertical hydraulic fractures would be expected to intercept horizontal boreholes much
more readily than vertical wells. Although this sounds easy in theory, the practice
has been challenging (Ye et al. 2020). The permeability of most rocks at great depths
is very low, and hydraulic fractures or some other form of permeability enhancement are needed to circulate fluids and extract heat.
One of the other challenges of EGS is the nature of the hydraulic fractures themselves. Fractures tend to have different apertures or widths where some are much
narrower than others. Wider fractures are more permeable and narrow ones less so.
In shale gas or tight oil production, this makes little difference because flow is from
the reservoir rock into the fracture system. In EGS however, the flow is between
horizontal wells via the fracture system, and the fluids will preferentially follow the
wider fractures with higher permeability. This ends up extracting more heat from
the rocks containing larger-aperture fractures, and less heat from rocks with narrower fractures, creating an unbalanced, inefficient system that leaves a lot of heat
behind in the subsurface. Research efforts are focused on understanding the behavior of fluid movement through these induced fracture systems (Ye et al. 2020).
A potential improvement on EGS is a hybrid technology that uses solar heat to
increase the temperature of marginal underground geothermal reservoirs (Zhou
et al. 2013). A version of this idea called Solar-Assisted Geothermal Energy or
SAGE was developed in Oman and patented in 2006. A company in the United
States has licensed the process, and started field tests in 2018 on existing geothermal
production wells in Nevada.
The principle behind the hybrid is to circulate solar-heated water or other fluids
into a warm but not hot deep aquifer and transfer the heat to the rocks. The system
uses an injection and production well like EGS, but with a solar heating component
added at the surface. These are typically mirror-lined parabolic troughs with a black
water pipe running through the center. The solar-heated water is injected into the
12.1 Technological Solutions
that everywhere on Earth has a geothermal gradient, where the heat of the rock
increases with depth (USDOE 2016). Some of this is primordial heat left over from
when the Earth accreted from smaller objects some 4.5 Ga, but most of it is generated from the decay of naturally-occurring radioactive elements in the subsurface
rocks. A borehole drilled deep enough anywhere on Earth will eventually encounter
rocks hot enough to provide energy. The problem is that this is much deeper in some
places than others, and the major economic barrier to widespread EGS development
is the cost of drilling.
Heat from the rocks has to be transported to the surface by some kind of fluid to
be useful for energy generation. For natural geothermal energy systems, the fluid is
commonly high temperature groundwater moving upward from below. EGS injects
fluid into the ground and recovers it to extract the heat. This can be accomplished by
drilling two parallel boreholes as horizontal laterals through the hot rocks at depth.
Engineered flowpaths are required for the introduced fluids from one well to collect
heat from the deep rocks and enter the other well to be transported back to the
surface.
Hydraulic fractures are created to connect the two laterals and introduce flowpaths through the hot rocks for fluids to circulate between the two wells. The vertical hydraulic fractures would be expected to intercept horizontal boreholes much
more readily than vertical wells. Although this sounds easy in theory, the practice
has been challenging (Ye et al. 2020). The permeability of most rocks at great depths
is very low, and hydraulic fractures or some other form of permeability enhancement are needed to circulate fluids and extract heat.
One of the other challenges of EGS is the nature of the hydraulic fractures themselves. Fractures tend to have different apertures or widths where some are much
narrower than others. Wider fractures are more permeable and narrow ones less so.
In shale gas or tight oil production, this makes little difference because flow is from
the reservoir rock into the fracture system. In EGS however, the flow is between
horizontal wells via the fracture system, and the fluids will preferentially follow the
wider fractures with higher permeability. This ends up extracting more heat from
the rocks containing larger-aperture fractures, and less heat from rocks with narrower fractures, creating an unbalanced, inefficient system that leaves a lot of heat
behind in the subsurface. Research efforts are focused on understanding the behavior of fluid movement through these induced fracture systems (Ye et al. 2020).
A potential improvement on EGS is a hybrid technology that uses solar heat to
increase the temperature of marginal underground geothermal reservoirs (Zhou
et al. 2013). A version of this idea called Solar-Assisted Geothermal Energy or
SAGE was developed in Oman and patented in 2006. A company in the United
States has licensed the process, and started field tests in 2018 on existing geothermal
production wells in Nevada.
The principle behind the hybrid is to circulate solar-heated water or other fluids
into a warm but not hot deep aquifer and transfer the heat to the rocks. The system
uses an injection and production well like EGS, but with a solar heating component
added at the surface. These are typically mirror-lined parabolic troughs with a black
water pipe running through the center. The solar-heated water is injected into the
12.1 Technological Solutions