We are working in an area of the world where few deep wells exist. Our drilling contractor stated, “[we are] recommending that a test hole be drilled as soon as possible on this site. We would use modern diamond drilling techniques to economically drill a hole to a depth of 600 meters with core samples recovered through the hole…At this time there is no information available about the ground conditions and what might be encountered at these depths. Therefore it has been very difficult to plan and budget for a large diameter well drilling project.”
Although prepared for a negative result from the test borehole, we consider the risk of failure to be relatively low, based on the following facts:
- In the Bahamas, which have similar hydrogeology to Grand Turk, cooling water for large-scale air conditioning is pumped from wells at high rates. For example, the Holiday Inn on Paradise Island has a 115 m deep well from which natural coolant is abstracted at 188 L/s [Weech, P. S. (1997) Deep-well disposal in the Bahamas. Bahamas J. of Science, 6/97, 6-13]. Our 4-well design needs rates of 64 L/s per well
- The recent (2010) Nassau, Bahamas airport project used wells 120 m deep for its chilled water air conditioning system [thebahamasweekly.com - New Providence, Bank of the Bahamas gets firsthand tour of airport facility, By Diane Phillips and Associates, Aug 30, 2010]
- An existing 15 m deep well at the inland Greenhouse site exhibited water quality parameters for sodium, sulphate, magnesium, potassium, and bicarbonate that are similar to ocean water [Roland Wahlgren interpreting water quality data collected by Hydrogeologist E. Hall for the Pre-Drilling Environmental Impact Assessment in our CIDA project]
- The temperature to depth profile of the existing well, if extrapolated linearly, predicts that cool 15 °C saline water should be encountered at a depth of about 140 m [Roland Wahlgren interpreting water temperature to depth data collected by Hydrogeologist E. Hall for the Pre-Drilling Environmental Impact Assessment in our CIDA project]
- Oceanographic soundings in the waters surrounding Grand Turk show the 15 °C isotherm to be at the 450 m depth [Wahlgren, R. V. (2002) Technical Feasibility Study for our CIDA Project]
Hydrogeologists have noted that islands of the Bahamas platform are cavernous, even at depth. The evidence comes from drilling bit drops and loss of circulation of drilling mud, even to depths of 3000 m [Whitaker F.F and Smart, P.L. (1990) Active circulation of saline ground waters in carbonate platforms: Evidence from the Great Bahama Bank. Geology 18, 200-203]. Pump tests in the Bahamas have shown relatively high hydraulic conductivities [Whitaker F.F and Smart, P.L. (2000) Characterising scale-dependence of hydraulic conductivity in carbonates: evidence from the Bahamas. Journal of Geochemical Exploration 69-70, 133-137].