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  3. The energy contained in rock within the earth’s crust represents a nearly unlimited energy source, but until recently commercial

The energy contained in rock within the earth’s crust represents a nearly unlimited energy source, but until recently commercial

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问题     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 HDRs, 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 include crustal rock that is hotter than 150°C, 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°C may be useful for space heating; however, for producing electricity, temperatures greater than 200 °C 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 °C 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.
The primary purpose of the passage is to______.
选项 A、alert readers to the existence of HDRs as an available energy source
B、document the challenges that have been surmounted in the effort to recover energy from HDRs
C、warn the users of coal and oil that HDRs are not an economically feasible alternative
D、encourage the use of new techniques for the recovery of energy from underground hot water and steam
答案A
解析 本题为主旨题。通读全文可知，本文主要讲述了干热岩(HDR)作为一种可再生能源的前景。A选项“提醒读者HDR作为一种可利用能源的存在”准确概述了全文内容，因此A项正确。
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