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US 6,668,554 B1 12 GEOTHERMAL ENERGY PRODUCTION WITH SUPERCRITICAL FLUIDS This invention Was made With government support under Contract No. W-7405-ENG-36 awarded by the US. Depart ment of Energy. The government has certain rights in the invention. TECHNICAL FIELD This invention relates to a method and apparatus for geothermal energy production from hot dry rock reservoirs usingsupercritical?uids. BACKGROUND ART Therehavebeendevelopedvariousmethodsofextracting heatfromdrygeothermal(hotdryrock)reservoirs,suchas 15 thatdescribedinUS. Pat.No.3,786,858(Potter,etal,Jan. 22, 1974). These methods rely upon Water to hydraulically fracture formations to form the reservoirs. Once a fractured reservoir has been formed, production Wells are drilled to intersect the hot dry rock reservoir. Then Water, used as the geo?uid, is pumped into the reservoir through the injection Well Which Was previously used for hydraulic fracturing during reservoir formation. The Water ?oWs across the fractured surfaces of the hot dry rock, is heated by contact With the hot rock, and then is used to transfer the geothermal heat to the surface by ?oWing upWard through one or more production Wells in a pressuriZed, closed-loop circulating operation referred to as heatmining. At the surface, the heat contained in the circulating geo?uid is transferred to a second ?uid, referred to as a binary Working ?uid, in a high-pressure heat exchanger of conventionaldesign,andthenthecooledgeo?uidreinjected into the hot dry rock reservoir. This second ?uid, Which can alsobeWater,ismorecommonlyammonia,oroneofaclass of halogenated hydrocarbon refrigerants such as the FreonsTM, or one or a mixture of loW molecular Weight hydrocarbons such as isobutane or isopentane. In common practice, even though the hot pressuriZed geo?uid is relatively benign chemically, it is not ?ashed to steam at the surface because of the release to the environ ment of small amounts of environmentally undesirable dis solved materials such as hydrogen sul?de, boron, arsenic, ?uorides and other trace minerals in the aqueous geo?uid. Moresigni?cantquantitiesofsilica,chlorides,andcarbon ates are also typically dissolved in the aqueous geo?uid, potentially causing corrosion and undesirable deposits on turbine blades and other metallic surfaces in poWer plants and in heat exchangers. It is a further object of this invention to provide a method of producing geothermal energy With supercritical carbon dioxide. It is stil another object of this invention to provide a method of producing geothermal energy With improved control of the geo?uid chemistry. It is yet another object of this invention to provide a method of sequestering carbon dioxide in deep rock forma tions. Additional objects, advantages and novel features of the invention Will be set forth in part in the description Which folloWs, and in part Will become apparent to those skilled in the artupon examination of the folloWing or may be learned 25 bypracticeoftheinvention.Theobjectsandadvantagesof the invention may be realiZed and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. The claims appended hereto are intended to cover all changes and modi?cations Within the spirit and scope thereof. DISCLOSURE OF INVENTION To achieve the foregoing and other objects, and in accor dance With the purposes of the present invention, as embod 35 iedandbroadlydescribedherein,therehasbeeninventeda methodforproducinggeothermalenergyfromdeepregions ofhot,essentiallydryrockusing?uidsotherthanWaterfor creation of underground reservoirs, production of geother mal energy, and, optionally, as Working ?uids in poWer plants. Underground reservoirs are created by pumping a supercritical ?uid such as carbon dioxide into a rock mass to fracture the rock. Once the con?ned geothermal reservoirs are formed, the same supercritical ?uids are circulated into the geothermal reservoirs, are alloWed to heat up and 45 expand,andthenarepumpedoutofthereservoirtotransport the heat to surface poWer generating plants or other direct heatingaplications. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying draWings, Which are incorporated in and form a part of the speci?cation, illustrate embodiments of the present invention and, together With the description, serve to explain the principles of the invention. In the draWings: 55 FIG. 1 is a graph of the relationship of amount of injected ?uid to reservoir volume. FIG. 2 is a graph of solubility of Water in carbon dioxide at 250° C. as a function of pressure. FIG. 3 is a schematic of a set up for production of geothermal energy in accordance With the invention. FIG. 4 is a graph of permeability of granitic rock as a function of pressure. BEST MODES FOR CARRYING OUT THE INVENTION It has been discovered that geothermal energy can be produced from con?ned dry geothermal (hot dry rock) Water-based geothermal systems generally have a geochemically determined temperature limit controlled by the critical point of Water (384° C. and 22 MPa). As the critical point for Water is reached and then surpassed, the enhanced dissolution of silica folloWed by retrograde pre cipitation above 384° C. presents a substantial obstacle to operatingahotdryrockgeothermalreservoirathigherthan thecriticaltemperatureforWater.Forhotdryrockreservoirs createdinthemostcommon igneousandmetamorphicrocks andmixturesofthemostcommon igneousandmetamorphic rocks, Where silica is present as either a primary or second ary (i.e., fracture-?lling) mineral, the silica dissolution and reprecipitation problem occurs as the critical temperature for Water is exceeded. Although drilling systems are capable of reaching rock temperatures in excess of 400° C., concerns aboutenhancedgeochemicalinteractionsariseinWater 65 based hot dry rock geothermal energy systems at these temperatures. Becauseoftheexcellentinorganicsolventpropertiesof Water and because of the sloW diffusional ?oW of Water throughthemicrocrackporosityinundergroundreservoirs, it is often dif?cult to control the chemistry of the Water used as the production geo?uid. Thus, there is stil a need for improved methods of producing hot dry rock geothermal energy. Therefore, it is an object of this invention to provide another method for production of geothermal energy. 10 It is another object of this invention to provide a method of producing geothermal energy With supercritical ?uids.PDF Image | GEOTHERMAL ENERGY PRODUCTION WITH SUPERCRITICAL FLUIDS
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