The energy contained in rock within the earth’s crust represents a nearly unlimited energy source, but until recently commercial retrieval has been limited to underground hot water and/or steam recovery systems. These systems have been developed in areas of recent volcanic activity, where high rates of heat flow cause visible eruption of water in the form of geysers and hot springs. In other areas, however, hot rock also exists near the surface but there is insufficient water present to produce eruptive phenomena. Thus a potential hot dry rock (HDR) reservoir exists whenever the amount of spontaneously produced geothermal fluid has been judged inadequate for existing commercial systems.
　　As a result of recent energy crisis, new concepts for creating HDR recovery systems—which involve drilling holes and connecting them to artificial reservoirs placed deep within the crust—are being developed. In all attempts to retrieve energy from HDR’s, artificial stimulation will be required to create either sufficient permeability or bounded flow paths to facilitate the removal of heat by circulation of a fluid over the surface of the rock.
　　The HDR resource base is generally defined to included crustal rock that is hotter than 150℃, is at depths less than ten kilometers, and can be drilled with presently available equipment. Although wells deeper than ten kilometers are technically feasible, prevailing economic factors will obviously determine the commercial feasibility of wells at such depths. Rock temperatures as low as 100℃ may be useful for space heating; however, for producing electricity, temperatures greater than 200℃ are desirable.
　　The geothermal gradient, which specifically determines the depth of drilling required to reach a desired temperature, is a major factor in the recoverability of geothermal resources. Temperature gradient maps generated from oil and gas well temperature-depth records kept by the American Association of Petroleum Geologists suggest that tappable high-temperature gradients are distributed all across the United States. (There are many areas, however, for which no temperature gradient records exist.)
　　Indications are that the HDR resource base is very large. If an average geothermal temperature gradient of 22℃ per kilometer of depth is used, a staggering 13,000,000 quadrillion B.T.U.’s of total energy are calculated to be contained in crustal rock to a ten-kilometer depth in the United States. If we conservatively estimate that only about 0.2 percent is recoverable, we find a total of all the coal remaining in the United States. The remaining problem is to balance the economics of deeper, hotter, more costly wells and shallower, cooler, less expensive wells against the value of the final product, electricity and/or heat.
　　21. The primary purpose of the passage is to
　　(A) alert readers to the existence of HDR’s as an available energy source
　　(B) document the challengers that have been surmounted in the effort to recover energy from HDR’s
　　(C) warn the users of coal and oil that HDR’s are not an economically feasible alternative
　　(D) encourage the use of new techniques for the recovery of energy from underground hot water and steam
　　(E) urge consumers to demand quicker development of HDR resources for the production of energy
　　22. The passage would be most likely to appear in a
　　(A) petrological research report focused on the history of temperature-depth records in the United States
　　(B) congressional report urging the conservation of oil and natural gas reserves in the United States
　　(C) technical journal article concerned with the recoverability of newly identified energy sources
　　(D) consumer report describing the extent and accessibility of remaining coal resources
　　(E) pamphlet designed to introduce homeowners to the advantages of HDR space-heating systems
　　23. According the passage, an average geothermal gradient of 22℃ per kilometer of depth can be used to
　　(A) balance the economics of HDR energy retrieval against that of underground hot water or steam recovery systems
　　(B) determine the amount of energy that will used for space heating in the United States
　　(C) provide comparisons between hot water and HDR energy sources in United States
　　(D) revise the estimates on the extent of remaining coal resources in the United States
　　(E) estimate the total HDR resource base in the United States
　　24. It can be inferred from the passage that the availability of temperature-depth records for any specific area in the United States depends primarily on the
　　(A) possibility that HDR’s may be found in that area
　　(B) existence of previous attempts to obtain oil or gas in that area
　　(C) history of successful hot water or steam recovery efforts in that area
　　(D) failure of inhabitants to conserve oil gas reserves in that area
　　(E) use of coal as a substitute for oil or gas in that area
　　25. According to the passage, in all HDR recovery systems fluid will be necessary in order to allow
　　(A) sufficient permeability
　　(B) artificial stimulation
　　(C) drilling of holes
　　(D) construction of reservoirs
　　(E) transfer of heat
　　26. According to the passage, if the average geothermal gradient in an area is 22℃ per kilometer of depth, which of the following can be reliably predicted?
　　I. The temperature at the base of a 10-kilometer well will be sufficient for the production of electricity.
　　II. Drilling of wells deeper than 10 kilometers will be economically feasible.
　　III. Insufficient water is present to produce eruptive phenomena.
　　(A) I only
　　(B) II only
　　(C) I and II only
　　(D) II and III only
　　(E) I, II, and III
　　27. Which of the following would be the most appropriate title for the passage?
　　(A) Energy from Water Sources: The Feasibility of Commercial Systems
　　(B) Geothermal Energy Retrieval: Volcanic Activity and Hot Dry Rocks
　　(C) Energy Underground: Geothermal Sources Give Way to Fossil Fuels
　　(D) Tappable Energy for America’s Future: Hot Dry Rocks
　　(E) High Geothermal Gradients in the United States: Myth or Reality?
　　A C E B E A D

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