Borehole Geophysics and the Interpretation of Subsurface Conditions Contributing to the Design of a Sustainable Geothermal Well

Borehole geophysical techniques were used to characterize the subsurface conditions and contribute to the design of a geothermal well in crystalline bedrock in a southern New Hampshire location. Multiple wireline probes were run to help determine lithology, porosity, vertical fluid flow rate, water quality, engineering rock properties, borehole deviation, as well as the orientation of fractures. To achieve these objectives, standard logs and borehole images were recorded using natural gamma, resistivity, density, caliper, full waveform sonic, water quality, heat pulse flowmeter, acoustic televiewer and optical televiewer probes. The results were used to characterize the subsurface and contribute to the design of a geothermal well. The results indicate that the occurrence and direction of ground water flow are controlled by open fractures. The depth, orientation, and aperture of fractures were measured using optical and acoustic televiewers. Deflections in fluid temperature and water quality curves interpreted in conjunction with heat pulse flow measurements indicate which fractures are producing and which are receiving water. Relative fracture transmissivity was also estimated using heat pulse flowmeter measurements made during static and pumping conditions.