The energy contained in rock within the earth’s
0 f& X; B( Z0 p. tcrust represents a nearly unlimited energy source, but $ m0 N6 e! S2 ?, W
until recently commercial retrieval has been limited to , G% y8 s2 o% K
underground hot water and/or steam recovery sys-
h* D+ v+ ~- Mtems. These systems have been developed in areas of
) {" o" b: w+ X1 l, E5 B1 ^recent volcanic activity, where high rates of heat flow : q% n) x3 g9 t
cause visible eruption of water in the form of geysers
8 x9 x0 z. e6 F+ J3 Iand hot springs. In other areas, however, hot rock " ^* E8 t# A: J6 l+ v: {, ?3 ?
also exists near the surface but there is insufficient ! J) w, ~; ^& G- w) X5 F8 `4 k: P
water present to produce eruptive phenomena. Thus a ! F' ]0 Y1 e# |. T
potential hot dry rock (HDR) reservoir exists when- 0 h" J' v9 ?: d$ u3 U4 r8 {
ever the amount of spontaneously produced geother- $ d/ V6 ^' X' h6 d$ k* Z+ g
mal fluid has been judged inadequate for existing
( m, K6 P5 R9 v7 F h8 R7 Ncommercial systems.
9 T' f# c4 M' p. B! P As a result of the recent energy crisis, new con- 3 \0 `5 t3 f* E1 ^( N
cepts for creating HDR recovery systems—which 8 J' q& N# |$ @( r0 o9 ]4 o: V
involve drilling holes and connecting them to artificial 1 `0 H8 H/ \" u0 |2 `0 f
reservoirs placed deep within the crust—are being
" R- L4 `2 i" r: |/ C1 {developed. In all attempts to retrieve energy from
% G5 [. G" |) U: v B# wHDR’s, artificial stimulation will be required to create
/ U" o; @: G( b% m' Ceither sufficient permeability or bounded flow paths
R- ^# z/ X8 m. d. R" f! n- i+ _to facilitate the removal of heat by circulation of a , w7 P$ N& v2 @3 }8 k) w/ L
fluid over the surface of the rock.
& h! g! w8 j& |3 C: W The HDR resource base is generally defined to 2 m/ c1 a4 y+ G
include crustal rock that is hotter than 150℃, is at
T5 p7 q6 }; o' o" t7 b3 Fdepths less than ten kilometers, and can be drilled
' G( z" z+ y, \with presently available equipment. Although wells
2 ~1 {1 ] O1 ]deeper than ten kilometers are technically feasible, / S1 }8 B- }" H1 L: N g
prevailing economic factors will obviously determine
m0 ^; o3 M; L& a% S& p2 Athe commercial feasibility of wells at such depths. : ]/ R( F* I" ?
Rock temperatures as low as 100℃ may be useful for " i! H' [7 q& S! I
space heating; however, for producing electricity, 4 r f; T# l- h) B. E
temperatures greater than 200℃ are desirable. * I0 a H& D4 L% i3 k0 P
The geothermal gradient, which specifically deter- % S" }, m* i) Y0 K
mines the depth of drilling required to reach a desired
' ^. E/ F& f- {; f: r7 rtemperature, is a major factor in the recoverability of
# `+ q; c( F; }* T0 Ugeothermal resources. Temperature gradient maps
2 Z/ G( S# ]/ `' m4 j6 ogenerated from oil and gas well temperature-depth re- 7 F8 ?# T" I6 D& y8 W- _+ Y) y$ o
cords kept by the American Association of Petroleum ! \0 G% s. _4 G1 W
Geologists suggest that tappable high-temperature & o" Y) Z' z" j4 }4 d: u
gradients are distributed all across the United States.
" k& M h# w; @! M; }$ M(There are many areas, however, for which no tem- 5 }0 u }3 J' B; K# R, L
perature gradient records exist.)
$ I3 f- S% h4 }# y9 ~& h$ V4 T Indications are that the HDR resource-base is very
0 n4 u( l% V6 x5 {5 x) plarge. If an average geothermal temperature gradient
7 {; Q* h5 z2 l; L9 _! n3 Uof 22℃ per kilometer of depth is used, a staggering 0 J5 g: h( K9 r9 G- \% ~
13,000,000 quadrillion B. T. U.’s of total energy are $ e1 n6 J: |0 [2 v3 d3 F" [# Z
calculated to be contained in crustal rock to a ten- 2 [, O& m% C. J$ i" W# B
kilometer depth in the United States. If we conserva-
, m1 ?+ R- ~5 O5 ?tively estimate that only about 0.2 percent is recover- , ~' r! ?7 n& L) I; g6 j6 W: G
able, we find a total that is comparable to the esti- . I/ a: ?) B3 Q4 S
mated resource base of all the coal remaining in the 5 C$ q7 k0 G+ {$ Y+ S
united States. The remaining problem is to balance
4 i2 ~- S h6 F8 C. _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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