Prospects for Universal Heat Mining: From a Jules Verne Vision to a21st Century Reality

Abstract

The extraction of heat or thermal energy from the Earth -- heat mining -- has the potential to play a major role as an energy supply technology for the 21st century. However, even if reservoir stimulation goals are achieved, the role of heat mining with today's energy prices and development costs is limited to only a small fraction of the earth's surface, specifically to geologically active regions where geothermal gradients are high. This paper examines the prospects for universal heat mining and the types of developments required to make it a reality. A generalized multi-parameter economic model was developed for optimizing the design and performance of hot dry rock (HDR) geothermal systems by linking an SQP nonlinear programming algorithm with a generalized HDR economic model. HDR system design parameters selected for optimization include well depth (or initial rock temperature), geofluid flow rate, number of fractures and injection temperature. The sensitivities of the optimized design parameters, HDR system performance, and levelized electricity price to average geothermal gradient, fractured area/volume, maximum allowable geofluid temperature, reservoir flow impedance, well deviation, and fracture separation have been investigated. Key technical and institutional obstacles to universal heat mining are discussed in a more general context. These include (1) developing methods for stimulating low permeability formations to provide sustained productivity with acceptable flow/pressure losses (2) dealing with barriers to change primary energy supply options when fossil energy resources are abundant and prices are low and (3) lowering the high drilling costs for developing the deep (>5 km) reservoirs required in low gradient areas. Advanced concepts in drilling technology that could lead to a linear as opposed to exponential relationship between cost and depth are discussed in light of their potential impact on heat mining.