The Phase I HDR Reservoir. Development of the world's first HDR...

The Phase I HDR Reservoir. Development of the world's first HDR system was initiated at
Fenton Hill in 1974. The first borehole was drilled in granitic rock to a depth of 2900 m where the
temperature was 197°C . After a series of hydraulic fracturing experiments, a second wellbore was
drilled toward the largest of the near-vertical, stimulated natural joints. A good connection was not
immediately achieved, and sidetracking was necessary to establish contact with the initial well via a
combination of induced and natural fracture pathways.
The Phase I system was evaluated in a series of flow experiments from 1978 to 1980 (Dash et
al. 1980). In the first flow test, water was circulated through the reservoir for 75 days in early
1978. The significant thermal drawdown (from 175°C to 85°C ) indicated that only a small heat
transfer area existed. A second 28-day test in late 1978 assessed the effects of imposing a high
backpressure on the production wellbore. This strategy was found to reduce flow impedance but
not to increase the surface area of heat extraction. The reservoir was then enlarged by further
hydraulic fracturing and two more flow experiments were conducted: First, a flow test lasting 23
days was carried out to quantify the operating performance of the enlarged reservoir. This was
followed by a 286-day heat extraction flow test during which the reservoir temperature declined
from an initial value of 156°C to a final level of 149°C .
At the end of this series of flow tests, a short stress-unlocking experiment was performed. It
entailed applying an elevated pressure to the reservoir in order to facilitate relative movement of
joint surfaces and the resulting redistribution of fluid flow and/or the opening of new fluid
pathways in the cooled reservoir rock. There were abundant indications of seismic activity within
the reservoir during the pressurization experiment, and subsequent flow measurements suggested
that the reservoir impedance had indeed been reduced. However, the system was not operated long
enough following the stress-unlocking experiment to demonstrate that the improved flow
conditions could be maintained for an extended length of time.
The pioneering work with the Phase I HDR reservoir proved that heat could be extracted from
HDR using the techniques conceived and developed at Los Alamos. In addition, it indicated that
issues such as induced seismicity, water consumption, and fluid geochemistry (including its effect
on the system components), would not present insurmountable problems in operating an HDR heat
mine. This initial field work highlighted the dynamic nature of HDR reservoirs, even under steady4
state operating conditions, and laid the groundwork for the development of strategies to increase
the productivity of future HDR reservoirs