Optimizing Geothermal Well Placement: Advantages of a Phased Approach Including Fracture Trace Analysis and Geophysical Techniques

Abstract

Bedrock drilling represents a substantial part of costs associated with the development of geothermal resources. Drilling costs can be reduced utilizing a series of phases that build upon each other to assess favorable drilling targets. Phase I should consist of review of all relevant geological and well data, fracture trace analysis and site reconnaissance. Site reconnaissance should document key geological features including rock lithologies, and fracture characteristics (orientation, spacing, aperture widths). Phase II should consist of surface geophysical surveys to locate transmissive fractures, including very low frequency, seismic, and resistivity. Re-processing available seismic data should be considered using alternative algorithms and/or alternative geophysical survey techniques to gain confidence that potential productive zones are not overlooked. The results of geophysical surveys should indicate the existence of favorable zones for testwell drilling. Phase III should consist of test well drilling and borehole geophysical logging. During drilling, the drill rig behavior, and changes in drill cuttings, water loss or gain, and drill bit advance rate should be documented as well as the depth of potential fracture zones. After cased open-hole test well(s) are completed and cleaned-out, borehole geophysics should be performed to determine breakouts, the depth of transmissive fractures, fracture orientation and fracture spacing. Discrete interval sampling of transmissive fractures, in conjunction with fracture orientation, and knowledge of regional fracture orientations may provide further insights into fluid characteristics and potential production rates. In this paper, I will summarize the advantages of a phased approach utilizing fracture trace analysis, surface geophysics (seismic, resistivity, very low frequency [VLF]), and borehole geophysics (temperature, caliper, heat pulse flow meter, televiewer). Utilizing a phased approach and selected non-intrusive and in situ techniques can result in meeting investigation objectives in a single investigation thereby minimizing drilling costs. The phased approach utilizing applicable techniques, as described in this paper, can minimize drilling costs associated with commercial ground-source heat pump (GSHP) systems, exploration of hydrothermal resources, and stimulation of enhanced geothermal reservoirs.