Evaluation of Sensitivity and Robustness of Geothermal Resource Parameters Using Detailed and Approximate Stratigraphy

Abstract:

Accurate assessment of the spatial variation of geothermal heat is key to distinguishing among locations for geothermal project development. Resource assessment over large areas can be accelerated by using existing subsurface data collected for other purposes, such as petroleum industry bottom-hole temperature (BHT) datasets. BHT data are notoriously noisy but in many sedimentary basins their abundance offsets the potential low quality of an individual BHT measurement. Analysis requires description of conductivity stratigraphy, which for thousands of wells with BHT values is daunting. For regional assessment, a streamlined method is to approximate the thickness and conductivity of each formation using a set of standard columns rescaled to the sediment thickness at a location. Surface heat flow and related geothermal resource metrics are estimated from these and additional parameters. This study uses Monte Carlo techniques to compare the accuracy and precision of thermal predictions at single locations by the streamlined approach to well-specific conductivity stratigraphy. For 77 wells distributed across the Appalachian Basin of NY, PA, and WV, local geological experts made available detailed information on unit thicknesses . For the streamlined method we used the Correlation of Stratigraphic Units of North America (COSUNA) columns. For both data sets, we described thermal conductivity of the strata using generic values or values from the geologically similar Anadarko Basin. The well-specific surface heat flow and temperature-at-depth were evaluated using a one-dimensional conductive heat flow model. This research addresses the sensitivity of the estimated geothermal output to the model inputs (BHT, thermal conductivity) and the robustness of the approximate stratigraphic column assumptions when estimating the geothermal output. This research was conducted as part of the Dept. of Energy Geothermal Play Fairway Analysis program.