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Geothermal Resource Assessment For Grenada, West Indies

Author: GRENLEC Blogger/Sunday, August 30, 2015/Categories: General

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

 

SUMMARY

 

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

during exploitation.

 

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

project.

 

RECOMMENDATIONS

 

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

conditions.

 

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.


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