PROBLEMS DRILLING AND COMPLETING GEOTHERMAL WELLS IN A PALEOKARST TERRAIN

To use more green technologies, Ohio State University is constructing three geothermal wellfields that will provide heating and cooling for five, large undergraduate dormitories. The plan entails drilling 481 wells to a depth of 550 feet on 20-foot centers. One 550-foot test was drilled at each geothermal field using a direct rotary drill rig and advancing steel casing to the bottom of the well. Construction drilling began in early 2011 using multiple air-rotary rigs and advancing steel casing only through the glacial sediments to the top of bedrock, a depth of about 80 feet. The remainder of the well is open borehole through about 20 feet of shale, followed by several hundred feet of limestones and dolostones. No problems were encountered completing the first well. Drilling the second well, however, caused groundwater to rise more than 30 feet in the air in the first well. By the time the eighth well was being drilled, seven wells were spouting groundwater. One of the pressurized wells was more than 300 feet from the drilled well. Unknown to the drillers or the project engineers were the two paleokarst zones underlying campus. Both zones occur below major disconformities. The shallower zone contains several commercial caves. The deeper (Newburg) zone is locally known for its enhanced permeability and high yielding wells. Borehole videos from older water supply wells on campus and in the area show voids 1 to 8 feet deep along bedding planes in the deeper zone and solution enhanced vertical fractures extending tens of feet up and down the borehole. Mapping karst features in a series of limestone quarries oriented up dip (northwest) toward the crest of the Findlay Arch reveals karst features below both disconformities. The features include speleothems, anastomosing channels, layered sediments in voids, vertical shafts, caves, and breccias. It is these karst features that enable the pressurized air from the drilled well to lift the groundwater in nearby wells. After three months of drilling using air-rotary rigs, the project was halted, in part because of fines to be assessed for sediment pollution of the Olentangy River. The project is proceeding with a field-tested technique using direct rotary drilling, steel casing to the top of rock, and backfilling with gravel after the geothermal loop is placed in the borehole.