WorldWide Drilling Resource

47 WorldWide Drilling Resource ® JUNE 2019 Environmental Monitoring by Thomas Kwader, Ph.D., P.G. What is the Geothermal Gradient? The geothermal gradient is the rate at which the temperature in the earth increases with depth. An “average” geothermal gradient in the southern United States is typically about one degree Fahrenheit per 70 feet of depth. The actual gradient depends upon the type of sediment or rock, thickness of the earth’s crust, seismic and any volcanic activity present, etc. In the last year, you may have seen on national television the lava flowing at land surface on the Hawaiian Islands that was approaching 2000ºF (1093ºC). Most of you also know about Yellowstone National Park and the geysers which spew boiling water every hour or so due to the high heat located much closer to land surface in this area. Fortunately, most water well drilling professionals do not have to concern themselves with the “small” temperature rises in the few hundred or so feet to drill water wells. “Hot water” often contains high concentrations of dissolved solids (salts, metals, ions, etc.). Over the last ten years or so, I have had the opportunity to deal with the consequences of geothermal gradients with some deep wastewater disposal wells along the U.S. Gulf Coast. These wells are approximately 10,000 feet deep where the actual temperature is near 250ºF (121ºC). Due to slightly corrosive fluid being injected, steel pipe lined with a fiberglass coating was utilized to protect the pipe. Due to the extremely high temperature in the injection zone, the lining and epoxy resin were impacted as a result of being at its thermal limit. The lining eventually began to “soften” and lose its strength. Steep geothermal gradients can also impact the use of plastic pipe in some situations. If the bottom hole temperature is not known and needs to be determined, you may want to call a borehole geophysical logging company to run a continuous temperature profile of the hole to obtain an actual temperature profile of the borehole. Tom Tom Kwader may be contacted via e-mail to ENV