Energy Extraction From a Laboratory Model Fractured Geothermal Reservoir

Abstract

Energy recovery by sweep and steam-drive processes is demonstrated in a laboratory model of a fractured-rock geothermal reservoir that can be heated to a reservoir temperature of 500 deg. F at a pressure of 800 psig. Results show that energy up to 75 Btu/lbm of rock was extracted by the sweep process and about 9 Btu/lbm was extracted by the steam-drive process. Introduction Economic development of geothermal resources on a large scale will require advanced technologies to enhance energy recovery from hydrothermal and hot igneous-rock systems that presently are unproductive. Several fracture-stimulation techniques and energy-extraction methods have been proposed to improve total productivity and energy recovery from these geothermal resources. productivity and energy recovery from these geothermal resources. Stimulation techniques include hydraulic fracturing, explosive fracturing, and thermal stress cracking. These techniques are intended to increase the vertical and horizontal formation permeabilities, providing adequate fluid circulation and exposing new rock-surface area to allow heat transfer from hot rock to fluid. Nonisothermal production of the reservoir and energy recovery from hot rock may be achieved by such methods aspressure reduction and in-place boiling,artificial circulation of water, andfluid production using steam-drive. Nonisothermal processes may be achieved in the first method by lowering the system pressure until in-place boiling of the natural or recharged water present in the reservoir occurs. Steam is produced from the top of the reservoir at pressures determined by the saturation curve. The nonisothermal process in the second method may be achieved by injecting cold water at a low point in the hot rock formation under high pressure. The water is heated as it flows upward in the artificially fracture rock media to a higher point, where it is produced as hot water We call this the sweep process. The third method is similar to the first, except that hot water is produced from a point below the water/steam interface. Steam formed by evaporation from hot rock surfaces and noncondensable gases accumulates in a steam cap in the top portions of the reservoir and provides the drive for production. We refer to this as the steam-drive process. Energy extraction from hot rock by the in-place boiling process was studied in a laboratory model using two rock process was studied in a laboratory model using two rock systems with several characteristics resembling those of explosively fracture-stimulated, hydrothermal reservoirs. Results of these experiments reported in Refs. 7 through 9 show that up to 47 Btu/lbm (109 kJ/kg) of rock was extracted by the in-place boiling process for a broad range of experimental conditions. Heat transfer from the rock resulted in an increase in total energy extraction from the hydrothermal (liquid and rock) system ranging from 1.25 to 2.57 times the energy obtained by flashing the liquid. Fluid production and rock heat-transfer analyses were developed that closely predicted the behavior of the physical model as long as the axial liquid-temperature physical model as long as the axial liquid-temperature gradients were small. The rock heat-transfer analysis was applied to hypothetical, large-scale systems having a 30-year production time and similar fluid circulation characteristics and temperature conditions. Results showed that a mean rock size of about 200 ft (60 m) would result in about the same amount of specific energy extraction from the rock as was obtained experimentally. The physical model of the second rock system used for the in place boiling experiments subsequently was used for sweep experiments. JPT P. 712