Direct-Heat Geothermal Systems: steps to improve understanding about the source of heat Rick Allis Utah Geological Survey March Workshop

Typically, direct-heat developments are opportunistic – there is already some indication of anomalous heat near the ground surface Fact of Life #1

Fact of Life #2 The amount and type of “exploration” work that can be done depends on what is already known about the location, and the scale of the proposed development (i.e. how much can you afford to invest to clarify the resource potential?) Since there is probably already a thermal system (spring or well with warm water) in the area of the proposed development, we (UGS) probably already know of its existence and can show you what we have on file. A lot of data is on our website: If we don’t know about it, please come and tell us, because it may indicate a significant new geothermal resource in Utah! Because geothermal resources are sub-surface, there is always uncertainty about the extent of the resource. Any drilling has a risk that things wont be as expected; the appropriate exploration techniques need to reflect both the uncertainties and the potential benefits of the proposed development

Exploration Techniques (Overview, warm water systems) Reassess existing data on file (well data? spring data?) Evaluate all types of relevant surface information (thermal features, geological mapping (faults, rock alteration), air photo interpretation, vegetation anomalies, early snow-melt areas) (Re)Measure temperature in any existing wells; sample, analyze, and get geochemical assessment of the thermal waters; if feasible, pump test well(s) (hydrological assessment) Use appropriate geophysical survey methods to attempt to delineate subsurface thermal system Drilling to confirm and hopefully prove location of best resource Increasing cost

Geophysical methods for “imaging” warm water systems hot water has high conductivity (salty) compared to potable ground water electrical surveying methods can usually detect the presence of high conductivity (low resistivity) at depth (beware of naturally salty soils) a variety of electrical surveying techniques are available – used in mining industry and sometimes also in environmental clean-up industry use experienced contractor, and beware of “new” techniques make sure the survey method is capable of sensing resistivity variations down to the likely well depth (or at least to 1000 feet), and the area of the survey is much larger than the obvious thermal feature (> 5000 feet x 5000 feet survey area) if salty soil or cultural noise is a problem (i.e. resistivity contrast unlikely), try self potential survey over similar-sized area temperature gradient drilling (holes to at least 100 feet; i.e. into water table)

Example of exploration results from Newcastle thermal system, Iron County (site of 25 acre greenhouse development by Milgro; it was discovered as a result of water well drilling) greenhouses

Selected cross-section data from Newcastle: most hot water is withdrawn from about 300 feet Resistivity low anomaly coincides with negative SP anomaly and highest near-surface temperatures, indicating upflow of hot water along range-front fault, and outflow to NW in alluvium.