Geothermal Resource Assessment for Grenada, West Indies
for Grenada Electricity Services Ltd. (Grenlec)
22 February 2009
Rohrs Consulting, Inc.
David T. Rohrs
1 March 2009
GEOTHERMAL RESOURCE ASSESSMENT FOR GRENADA, WEST INDIES
The distribution of thermal features, primarily warm springs, indicates that Mt. St. Catherine hosts a low
to moderate temperature geothermal reservoir. Despite age dates of about 1 million years for the most
recent volcanic products, Mt. St. Catherine still provides a viable heat source for the system.
The geothermal system appears to be centered beneath the upper slopes of Mt. St. Catherine with
outflow north and northeast towards the chloride warm springs at Castle Hill and Chambord.
Temperature estimates from chemical geothermometry suggest a reservoir temperature of 350- 400 :F,
and potentially higher. The majority of thermal features, including the chloride springs, appear to lie on
the periphery of the geothermal reservoir. Despite high salinities, the warm springs at Castle Hill and
Chambord, have low temperatures, generally less than 110 :F, indicating substantial conductive cooling
along their flow paths.
Geologic traverses found ample of evidence of fractured rocks, and, when combined with the large
amount of leakage from the system, indicates that reservoir permeability should be good. The reservoir
may be partially hosted by sedimentary rocks, which could lower overall permeability and possibly
increase the risk of calcite deposition in wellbores. Additionally, the caprock overlying the system may
be relatively permeable, raising the risk of cold water influx should reservoir pressures be drawn down
Reservoir chemistry issues are likely to be low. The warm spring chemistry indicates that the system
hosts a neutral pH fluid. While there is potential for calcite scaling in the wellbores during production,
as suggested by the travertine deposits associated with some of the warm springs, the scaling could be
mitigated by preventing boiling in the wellbore or through downhole chemical mitigation.
The highest resource risks are reservoir temperature and volume. The volume of high temperature
fluids may be quite small, deep, and difficult to locate through exploration drilling. Additionally, the
topography, combined with temperature, may be a significant issue for well productivity. Wells drilled
at high elevation locations may be unable to produce due to a combination of low reservoir temperature
and pressure. This would favor drilling wells from lower elevation locations directionally towards the
peak of Mt. St. Catherine. However, such wells may have difficulty encountering adequate reservoir
temperatures for commercial production. Nevertheless, should the exploration wells encounter
commercial reservoir temperatures, there should be sufficient resource volume to support a 30 MW
To further evaluate the development potential of the geothermal resource at Mt. St. Catherine, the
following recommendations are offered.
Acquire a Contract Area. Mt. St. Catherine hosts a single geothermal system. Therefore, the contract
area should cover the entire prospective area, including all thermal manifestations that can be related
to Mt. St. Catherine.
Wellbore Modeling. Because of the potential for drilling high elevation wells into a moderate
temperature, under pressured resource with a liquid level near 1000 ft. elevation, wellbore modeling is
recommended. This modeling should help with the selection of exploration drilling locations and well
targets. The following issues should be addressed through the wellbore modeling:
· Influence of elevation on well productivity, especially the role of boiling in providing steam lift to the produced fluids.
· The placement of pumps in the wellbore to aid production under low reservoir temperature
Geologic mapping. A good geologic map should be developed for the prospective area. This map should
show major geologic units, volcanic structures, and faults. The mapping should help establish the
potential reservoir rocks and structural targets for the exploration wells. To assist the geologic mapping,
remote sensing images and either LIDAR images or aerial photography of the contract area should be
obtained. The aerial photography, LIDAR, or overlapping remote sensing images should be used to
develop a Digital Elevation Model for the contract area. This model will assist the geologists and
geophysicists by ensuring that they can accurately locate themselves in the field and should also help
project engineers with the selection and design of roads and locations. Adequate regional geologic
maps and air photos may already be available from an agency on Grenada. If not, then the area should
be flown so that new aerial photography or LIDAR can be obtained.
Geophysical Surveys. A very important next step for the exploration of the resource is a geophysical
survey to measure the resistivity of the earth in the prospective area. This survey should involve both
magneto-telluric (MT) and Time-Domain Electromagnetic (TDEM) stations. This survey should help
resolve the distribution of the low resistivity clay caprock overlying the geothermal system. The shape
and distribution of the low resistivity layer should be integrated into alternative conceptual models of
the reservoir in order to identify the highest potential exploration drilling targets. Gravity and magnetic
surveys are not recommended for this prospect.
Exploration Well Planning. Three exploration wells are recommended to be drilled at Mt. St. Catherine.
These wells should incorporate a slimhole well design that will allow the wells to be flow tested. The
wells should be targeted to 6000 ft. total depth and should be drilled directionally towards the upper
slopes of Mt. St. Catherine. Without the geophysical results, siting of drilling locations is difficult. The
current thought is to build a total of four drilling locations, with two locations each on the west and east
slopes of the volcano. The three wells should be drilled sequentially. Once a well achieves drilling
success, it should be followed up by a second exploration well from a nearby location to confirm the
extent of the resource. Two successful slim holes should be adequate to demonstrate the feasibility of a
30 MW project.