Recovery of Energy From Fracture-Stimulated Geothermal Reservoirs
Authors: Hunsbedt A. , Kruger P. , London A.L. 
Published: December 2007

Energy recovery by in-place boiling was studied with a laboratory to model, of a fracture-stimulated geothermal reservoir that can be heated to 500 deg. F and to a pressure of 800 psig. An analytical mode to evaluate rock-energy extraction was developed and agreement between predicted results and experimental results was satisfactory.

Introduction
Geothermal resources used for electric power generation are hydrothermal systems that exist either as vapor- or liquid-dominated reservoirs. Although the latter type is believed to be more abundant, the extent of hydrothermal resources is considered small compared with hot, igneous rock resources that. do not spontaneously produce steam or hot water because of a lack of permeability, porosity, and fluid storage. Economic development of geothermal resources will require advanced technologies for enhanced recovery of energy from hydrothermal reservoirs and the introduction of artificial circulation systems for extracting energy from hot, igneous rock. Several fracture-stimulation techniques have been proposed, such as explosive fracturing, hydraulic fracturing, and thermal stress cracking. Smith et al. described a large-scale hydraulic fracturing experiment in hot, igneous rock where a closed circulation loop of surface water under high pressure is used to extract the thermal energy by convective heat transfer. A variation of that extraction scheme using multiple fractures was proposed by Grinparten and Witherspoon . Stimulation of geothermal reservoir by nuclear explosive fracturing was proposed by Carlson and Kennedy. Nuclear explosive stimulation of natural gas reservoirs has been demonstrated successfully on an experimental basis . Explosive stimulation of hydrothermal systems was discussed by Ramey et al. They showed that the total energy in a hydrothermal system consists of tie thermal energy stored in the rock and the thermal energy stored in the geothermal fluids. The ratio of the magnitudes of these two components depends principally on the rock porosity. Ramey et (11. also demonstrated that the amount of energy extractable from a hydrothermal system can be enhanced by operating the system nonisothermally (for example, by pressure reduction) so that boiling (flashing) takes place within the rock formation. Nonisothermal heat-transfer processes in fractured rock systems have not been studied experimentally. However, several laboratory experiments on nonisothermal flow in porous media have been conducted.
The studies described in this paper were designed to explore(1) conditions for optimum energy extraction, (2) rock heat-transfer characteristics, (3) moving flash fronts, (4) fluid-withdrawal reservoir pressure behavior. (5) effects of cool and hot fluid recharge, and (6) cyclic production/recharge operation of fracture-stimulated production/recharge operation of fracture-stimulated systems. The experiments were carried out in a laboratory model using rock loadings with characteristics that resemble the highly fractured region of a fracture-stimulated hydrothermal reservoir. Experiments were conducted with two rock loadings of different porosities and mean rock sizes at pressures and temperatures found in geothermal reservoirs. The extent of fluid recharge was limited so that energy addition by fluid recharge was less than the energy extracted from the rock. Preliminary evaluations of the results were reported by Hunsbedt et al. and detailed results were reported by Hunsbedt. This paper highlights these results.
JPT
P. 940