COMPREHENSIVE LOGGING OF DEEP BEDROCK WELLS IN VERMONT FOR GEOTHERMAL PURPOSES

During 2010-2011, the Vermont Geological Survey (VGS) received multi-year grants from the U.S. Dept. of Energy (USDOE) to study the deep geothermal potential of Vermont. This grant consists of three tasks: 1) digitization of existing geologic data, 2) analysis of representative lithologies in Vermont for thermal conductivity, and geochemistry, and 3) temperature logging of deep bedrock wells. In order to complete Task #3, the VGS must obtain bottom-hole temperatures on 25 deep bedrock wells that are distributed ~evenly among the six tectonic belts of Vermont which are: 1) Champlain Valley, 2) Taconic Allochthon, 3) Green Mountain, 4) Rowe-Hawley, 5) Connecticut Valley, and 6) Bronson Hill. Temperature data from these 25 wells must be entered into a USDOE geothermal database by the end of 2013.

A well suitable for logging must be ³ 500’ in depth, undisturbed for 2 weeks after completion, and have no pump. The 2 week waiting time is to allow heat induced by drilling into the bedrock to dissipate. Beyond the concern for logging tools getting stuck on wires and tubes connected to the pump, the VGS cannot assume liability for pulling and returning a pump to a well.

Geology faculty at SUNY at Plattsburgh recently acquired Mt. Sopris logging equipment through a National Science Foundation grant. In 2012, the VGS partnered with them to conduct the logging. This apparatus has separate tools to log temperature and conductivity, natural gamma, and caliper (borehole diameter) on individual runs. An additional Acoustic Televiewer tool can take radar images of the borehole.

Although this project only requires bottom hole temperature logging, we are able to acquire valuable additional borehole data that constrain fluctuations in the temperature data. For example, because logging is continuous from top to bottom, we can determine temperature gradients. Caliper logs allow us to determine where fractures or other planar structures may be contributing groundwater to the well. If zones of groundwater temperature and conductivity vary significantly, we can see if those zones correlate with these planar structures. Gamma logs allow us to assess lithologic heterogeneity in the borehole.

We have completed 3 logs in the Green Mountain Belt and 1 in the Champlain Valley. We are currently working with well drillers to identify other suitable wells.