The energy contained in rock within the earth’s / `+ W5 [; y% @
crust represents a nearly unlimited energy source, but ! Q9 z! I8 Q$ Z6 f) o, `7 E. ^
until recently commercial retrieval has been limited to 5 K# e5 E6 _/ L" r* y0 l( P" P
underground hot water and/or steam recovery sys- 0 u9 d, ~9 J+ y7 a2 Y/ s; G! R7 `
tems. These systems have been developed in areas of 9 M+ S/ v; N5 Y" t5 v! b/ B
recent volcanic activity, where high rates of heat flow ' y: ]" H0 w/ H" u$ Y# j6 ~
cause visible eruption of water in the form of geysers
% E" E A8 `7 ^8 o* nand hot springs. In other areas, however, hot rock " l: K1 o" R4 ~/ \0 h# Q
also exists near the surface but there is insufficient % a& y' y! X( `* ?5 c+ p
water present to produce eruptive phenomena. Thus a & z: q1 w! {6 p7 L( a$ A
potential hot dry rock (HDR) reservoir exists when-
$ z& R' Y! d+ ~ever the amount of spontaneously produced geother-
5 A2 a9 b8 a7 y# j" ] `- o, g* `mal fluid has been judged inadequate for existing }% c2 g. c1 N: r6 J: i y$ r
commercial systems. 8 c* Q: T' z2 X# Z+ w
As a result of the recent energy crisis, new con- $ I+ l+ h; @6 A/ I1 M [# w
cepts for creating HDR recovery systems—which & K' v) P. `& d5 |6 ~
involve drilling holes and connecting them to artificial
8 o9 `3 S3 g Q- j+ U# l2 A. Wreservoirs placed deep within the crust—are being
9 G' P! E( @& }: p, |& K- `developed. In all attempts to retrieve energy from 0 Z& u; d$ ` N; ~
HDR’s, artificial stimulation will be required to create 4 m& P6 z6 O# m; C d4 j* T
either sufficient permeability or bounded flow paths
, G9 p$ N, r5 a& i1 d2 V: pto facilitate the removal of heat by circulation of a - ~8 c; `0 _7 E
fluid over the surface of the rock. # V7 V" ?- b) U6 Q! f
The HDR resource base is generally defined to
8 ^4 K* L9 R. n e. Einclude crustal rock that is hotter than 150℃, is at
* ^, K% l. o }) M) rdepths less than ten kilometers, and can be drilled
5 W l5 s: Q I$ _* @with presently available equipment. Although wells
; ^# m/ r$ J' q7 {, rdeeper than ten kilometers are technically feasible,
6 [2 f. P U6 l9 X- sprevailing economic factors will obviously determine 2 [$ Q* l( s6 n/ w
the commercial feasibility of wells at such depths. 2 J% J8 {7 A+ o+ m& `: a2 T/ Y
Rock temperatures as low as 100℃ may be useful for
8 \! l2 u+ W' O) r2 Tspace heating; however, for producing electricity,
5 J5 j% A6 g$ H% T' F2 e l4 l* mtemperatures greater than 200℃ are desirable. $ ~* V& B9 @3 u" i( P- H
The geothermal gradient, which specifically deter- ) U7 Y% j3 S8 n# [! v' D
mines the depth of drilling required to reach a desired ( r" r! q z2 s
temperature, is a major factor in the recoverability of
! U! G/ u/ r8 V$ S6 \geothermal resources. Temperature gradient maps
5 J; M' ^* A8 M3 ?+ a5 J0 [generated from oil and gas well temperature-depth re-
$ f) ]) c7 Y/ d3 Scords kept by the American Association of Petroleum _1 n! A* |9 r
Geologists suggest that tappable high-temperature & v2 U3 v; a0 y/ t+ @: s4 C. B
gradients are distributed all across the United States.
! T; D, Z" D7 A0 q0 b(There are many areas, however, for which no tem-
; y" @* @3 b: h5 Zperature gradient records exist.) 8 f1 _0 d! n: S! H: D7 ~
Indications are that the HDR resource-base is very : N+ d/ G: L3 W- l6 x2 u) f0 ?9 `
large. If an average geothermal temperature gradient
4 p1 `0 x8 M4 @* ?of 22℃ per kilometer of depth is used, a staggering ; n2 ?" k# n# p+ |
13,000,000 quadrillion B. T. U.’s of total energy are
3 f+ l7 s5 }+ E1 Icalculated to be contained in crustal rock to a ten-
+ q) [( x% Y* x1 A$ k. f9 B/ K/ Qkilometer depth in the United States. If we conserva-
9 u: n. n# v/ m$ Q# otively estimate that only about 0.2 percent is recover- . J) b0 w% G% ]# s6 y; N
able, we find a total that is comparable to the esti-
1 \; p, s8 ?6 X* c! ?mated resource base of all the coal remaining in the
- ?; d: g! {2 l5 t- {united States. The remaining problem is to balance # m3 ?; \: M3 E% s: C; X4 P
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. |
发表于 2012-8-15 12:45:20
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