1. Field of the Invention
The present invention relates to utilization of geothermal energy and more particularly to an improved system for maximizing the transfer of geothermal energy from a subsurface zone to the surface for utilization.
2. Description of the Prior Art
There exists, essentially untapped, massive quantities of heat available from magma which has migrated to zones close to the surface. Such conditions exist in large regions of the United States and in other locations throughout the world such as in Italy, New Zealand and Japan. Frequently, tectonic activity has produced fault lines which have permitted deep, subsurface waters to come in contact with magmatic rocks and return to the surface along fault lines as heated water, or in a few cases as steam. Similarly, tectonic activity below ancient subsidence areas, more recently overlain, have resulted in migration of heat into brine pools of varying salinity which lodged in these sinks. There are some 1.8 million acres in the United States which are designated as KGRA (Known Geothermal Resource Areas) and most of these KGRA are situated in the Western states.
The only operating geothermal plants in the United States are those operating at The Geysers, Sonoma, California, and others are formulating plants for geothermal power development in the Imperial Valley of California. The plants in operation in the United States and Italy rely on direct thermal fluid mining methods. Such methods have entailed serious problems due to the high salinity of the steam causing cavitation, abrasion, scalling and corrosion of the equipment over short intervals. Moreover, geological prospecting techniques are not very accurate and if a dry well results or a well with insufficient steam pressure the venture is a total loss. Drilling of adjacent thermal direct fluid recovery wells entails the risk of lowering the bottom hole pressure of the whole field. It is estimated that a brine pool exists in the Niland area of the Imperial Valley of California which occupies an area of 25 square miles. About a dozen geothermal wells have been drilled in the area, which produce as much as one million pounds per hour of brine per well for sustained periods. Flashing this brine would produce about 200,000 pounds per hour of steam which in turn could produce about 10,000 kW of electric power per well.
However, because of the high salinity of the brines, all attempts to utilize these brines have been unsuccessful. Since the discovery of these wells, several companies have spent millions of dollars trying to extract chemicals and generate power from the brines. Neither operation has been commerically successful because of the high operating costs and associated material costs necessary to withstand the corrosive and erosive environment and to dispose of the salt and concentrated salt bitterns.
Further to the South in Mexico, another brine pool exists of a size comparable to the Niland pool. The brine is lower in salinity in this pool. Exploratory investigation suggests the existence of seven or eight similar brine pools between Niland and the pool at Cerro Prieto in Mexico. In all, the power generating potential from geothermal energy in the Imperial Valley is estimated to be as high as 30,000 MW. Successful exploitation of this potential by conventional direct thermal mining methods would require either selective use of the lower salinity brine or disposal of vast amounts of salt and concentrated salt bitterns. Geothermal energy represents a clean, pollution free alternative to fossil fuel energey sources and does not entail the hazards or the environmentally unacceptable aspects of nuclear produced power.
The problems inherent in the conventional direct thermal mining approaches are avoided by use of the downhole heat exchanger disclosed in U.S. Pat. No. 3,470,943 since the geothermal brines remain in the pool and heat is extracted by in situ circulation of a clean, stable, secondary heat transfer fluid inside the downhole heat exchanger which is placed at the lower part of the casing within the geothermal zone. Thus, the downhole heat exchanger provides a means for utilizing the heat contained in the brine pools by extracting only the heat energy, leaving the brine recirculating in the underground pool. The advantages are many.
No saline fluids are brought to the surface; hence there are no disposal problems and reinjection wells are not required. The reservoir inventory and pressure is undisturbed. Except for extraction of heat, which is readily replenishable, nothing has been changed in the reservoir. Subsidence therefore will be avoided by this method.
Since the interior of the casing is contacted only by pure fluids, no corrosion or scaling will occur internally. The outside of the casing is in contact with the reservoir aquifer but the brine is at a pressure which does not allow the dissolved salts to precipitate. Hence, there is no abrasive action from the solids as in a flowing well. The convective currents within the aquifer are expected to be of insufficient velocity to result in abrasion. The low velocity should also promote the retention of a thin, passive coating of corrosion products which inhibit further corrosion.
The downhole heat exchanger system represents a truly non-polluting source of energy in that no pollution products are permitted to reach the surface. However, the energy capacity of a single well is not sufficient to justify the installation and operation of a generation plant. Therefore, multiple wells are required to develop sufficient steam to operate the turbine generator. This requires the utilization of a greater area of the surface for drilling the multiple wells and a greater improvement cost to drill the well at each site. Furthermore, the separate location of the multiple wells requires water injection lines running to each well site and steam gathering lines running from each site to the generating plant. All of this involves capital investment and entails heat loss each time the steam or water is moved.