Geothermal energy 4 : The heat of the earth, usually from produced natural steam, heat recovered from circulated water or direct heat-to-energy conversion (on-going research)
George E. King Engineering, 5000 Oilfield Terms: A Glossary of Petroleum Engineering Terms, Abbreviations and Acronyms, US 2010. http://gekengineering.com/Downloads/Free_Downloads /Glossary_of_Petroleum_Engineering_Terms_25_August _2010.pdf, (Oil - Amer) {593}