A key technical problem in the exploitation of hot water geothermal energy resources is down-well pumping to inhibit mineral precipitation, improve thermal efficiency, and enhance flow. A novel approach to this problem involves the use of a small fraction of the thermal energy of the well water to boil and super-heat a clean feedwater flow in a down-hole exchanger adjacent to the pump. This steam powers a high-speed turbine-driven pump. The exhaust steam is brought to the surface through an exhaust pipe, condensed, and recirculated. A small fraction of the high-pressure clean feedwater is diverted to lubricate the turbine pump bearings and prevent leakage of brine into the turbine-pump unit. A project demonstrating the feasibility of this approach by means of both laboratory and down-well tests is discussed

Matthews, H. B.

Mcbee, W. D.

English

NASA Technical Reports Server

N75 20854 
GEOTHERMAL DOWN-WELL PUMPING SYSTEM 
I. 
Hugh B. Matthews and Warren D. McBee 
Sperry Research Center 
Sudbury, Massachusetts 
A key technical problem in the exploitation of hot water 
geothermal energy resources is down-well pumping to inhibit 
mineral  precipitation, improve thermal efficiency, and enhance 
flow. A novel approach to this problem involves the use of a small  
fraction of the thermal energy of the well water to  boil and super- 
heat a clean feedwater flow in a down-hole exchanger adjacent to 
the pump. This steam powers a high-speed turbine-driven pump. 
The exhaust steam is  brought to  the surface through an exhaust 
pipe, condensed, and recirculated. A small fraction of the high- 
pressure clean feedwater is  diverted to lubricate the turbine pump 
bearings and prevent leakage of brine into the turbine-pump unit. 
A project demonstrating the feasibility of this approach by means 
of both laboratory and down-well tes ts  has just started under an 
NSF grant. The status and plan of this project a r e  presented. 
INTRODUCTION 
The Sperry Research Center has been engaged for two years in the 
analysis and design of a novel type of pumping system for geothermal 
liquid-dominated wells. Since July 1974 we have been under a grant from-the 
National Science Foundation through its Research Applied to  National Needs 
(RANN) program to build and test  this system. Field testing in a geothermal 
we l l  is expected to begin in July 1975, possibly in the Imperial Valley of 
California. 
II. WELL DYNAMICS 
Figure 1 illustrates in a simplified geologic situation what is known as a 
hydrostatic well. The water table in the strata surrounding the well is 
essentially at the surface of the ground and the water increases in temperature 
as we go deeper in the earth. With the valve closed to prevent flow, 
water inside the well casing w i l l  assume virtually the same thermal gradient 
as  the water outside and, therefore, being of the same average density, wi l l  
r i se  .in the casing essentially to the level of the outside water table. 
the 
If the valve in our illustration i s  opened (Fig. 2) and the we l l  caused to  
start flowing by some means, the hot water flowing up the casing w i l l  pro- 
gressively lower the average density of the water column and the flow velocity 
will  inc rea  s e a c  co r dingly . 
28 1 
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When 2 1 2 ° F  water approaches the surface, a portion of it w i l l  flash into 
steam, lowering the column density even more and again accelerating flow. 
The level at which this flashing begins w i l l  move down the well as  increasingly 
hot water comes to the surface and wi l l  stabilize at a level as determined by: 
(1)  The bottom hole temperature (temperature losses up to the flash 
level in a flowing well a r e  negligible). 
The average density of the mixed liquid and vapor column above the 
flash point. 
(2)  
(3)  The two phase flow losses from the flash level to the surface. 
The mixture of steam and water will arr ive at the surface at  a temperature of 
2 1 2 ° F  and (in a 400°F  well, for example) as approximately 20% saturated steam. 
The quantity of flow from the well will depend on the very complex 
interrelation between the thermal and saline gradients in the water outside the 
well, flow losses in the s t ra ta  feeding the well and in  the well casing, and the 
actual density gradient in the column above the beginning of flashing. When 
this natural flowing 'method is used to produce a well, a s  it i s  in a few places 
in the world for generating electricity, an additional impedence is inserted in 
ser ies  at the surface (equivalent to closing down our valve somewhat) so that a 
pressure drop i s  introduced between atmosphere and the Rankine cycle 
conversion equipment. Thus, the water steam mixture w i l l  enter the energy 
conversion process at a higher temperature than 2 1 2 ° F  (in a 4 0 0 ° F  well, say, at 
325"F,  97-psia pressure,  and 10% steam). 
There a r e  several  drawbacks to  such a naturally produced well: among 
them are:  
( 1 )  W e l l  flow i s  reduced to below that of open wellhead flow by the 
added impedence. 
(2)  Geothermal waters a r e  usually considerably contaminated with 
dissolved solids. The process of flashing causes the most 
troublesome of these compounds to precipitate, which then coat 
t h e  well casing (sometimes seal it) and, more seriously, foul 
surface equipment, such as  heat exchangers. 
The lowered temperature caused by the flashing reduces the 
efficiency of the surface conversion equipment. 
(3 )  
III. WELL PUMPING 
If a pump is introduced into the well below the flash point and sufficient 
pressure is added to  the water so that it w i l l  not flash even on the surface, the 
above problems a re  largely negated: 
(1) Flow can be increased, even above the well's natural flow rate, with 
a consequent increase in available energy. 
282 
(2) Water is brought to the surface at o r  near bottom hc!c temperature 
with a consequent increase in surface conversion efficiency. 
If the water remains ur.der pressure, the dissohed cxrbon dioxide 
remains in solution and prevents the most troublescme constituents 
(mainly carbonates) from precipitating. Thus, exGept for silica, 
which wi l l  precipitate when the water is. cooled, 'the most serious 
problems are eliminated. The effect of silica remains to be 
deiermided, but experiments to date are somewhat encouraging. 
(3)  - 
. 
. . .  - . _  _ -  
. .  . .  %V. . PUMPWORK . ._  : - 
The following factors determine the amount of work to be Gone b y  the 
(1) Requirements. The water must arr ive at the surface somewhat 
above the saturation pressure. In a 400°F well, this is 
approximately 247 psia, and with a 10% pad, 272 psia, If there 
. -  is a pump  ti the surface circulating the hot well water, then the--. . 
net positive suction head fNPSH) requirement of this pump must 
pump: 
~ . -  be added. to t h e  pressure. . .. . . -  
( 2 )  Thermal Liit. As mentioned before, the column of water inside the 
well, when flowing. will be essentially at bottom hole temperature, 
while t h e  water in the strata oxtside will have some thermal 
gradient from the low surface temperature to the high temperature 
at well bottom. This effect, of course, reduces the amount of pump 
work and can be of considerable m.agnitude. A 400°F hydrostatic 
well of relatively pure water, 5000 feet deep and with a linear 
temperature gradient i n  the surrounding strzta, will gain 
approximately 150-psi pressure due to  its thermal lift. 
W d l  Productivity. Bottom hole pressure will, of course, drop as 
flow is increased because of friction in the surrounding strata,  a 
property described in the oil industry by the te rm "productivity 
index, " The pump must make up this pressure loss, and the 
practical matter of energy expenditure would seem tc; limit 
consideration to wells where this pressure loss is no more than 
a few hundred psia at acceptable flow rates. 
- 
(3) 
(4) Casing Flow Loss. The flaw in the casing will always be turbulent, 
and gressure loss will be proportional to flow squared. A 1000-gpm 
flow-in a 8-5/8-in. casing-wiil generate a loss of perhaps 50 to i60 
psi  in a 5000-foot-deep well, depending on pipe roughness. A 
practical casing design for such flows might be 8-518 in. up to just 
below the pump and 10-318 in. or 13-3/8 in, above. 
283 
V. CONVENTIONAL PUMPING SYSTEM 
Figure 3 shows the 
well is pumped only at th 
in the well a sufficient depth below the natural flash level to 
NPSH for  the pump, which is the required pressure 
of the fluid at its pzirticular temperature to prevent 
If a grea ter  quantity than the natural rate-is pumped, then the 
an additional depth as determined by the adde? f!ow los~es  in the strata and in 
the well casing-below the pump. it apeea.rs that this depth could well be 1000 
feet o r  more for optimized well production in the general cask. 
ments of a conventional pumping 
turally flowing rate, then the-pu 
. .  
The pump will be driven by energy sent down from the srirface in some 
sort of conduit, an electric cable or a hydraulic tube, fer example, This 
energy will, in  fact o r  in effect, be obtained from the well water itself and 
before ending downhoie as pressure added to  the well flow be transformed a 
number of times, with each transformation resulting in a loss of efficiency. 
For  example, this can be shown for the hydraulic s'ystcm: 
(1) Heat to 
.. - . -_ ._ . 
(2) Torque through a l t e k i o r  'to -electricity. 
(3) 
(4) 
(5) 
( 6 )  
Electricity through motor to torque. 
Torque through pump to hydraulic power. 
Hydraulic power downhole through hydraulic motor to  torque. 
Torque through pump to pressure and flow in well wa te r .  
VI. THE SPERRY PUMPING SYSTEM 
In o rde r  to avoid this multiple-energy conversion probIc3i-with its 
attendant loss in efficiency, the Sper ry  Research Center is constructing a 
system which w i l l  not bring energy down ft0.m- tbe ?ur_frce tp._ap_rate the pump, 
6,ut will instead &*tract enerky from thG hot.\vcll water downhole at the si te 
of the pump. It will have only two energy conversions--heat to torque and 
torque to .pressure--rather than several. The principle involved is shown , 
used to  drive a turbine and a 
is used to  lubricate 
284 
the turbine and pump bearings. Since this clean water is at a higher pressure 
than the well water, the contaminated well water is prevented from entering 
the turbine. 
The boiler manifold contains a threshold valve, which prevents water from 
entering the boiler unless feedwater pressure is above well water pressure,  
and the A P  valve (mentioned above), which drops feedwater pressure to  permit 
boiling and also to permit changing boiler pressure by varying feedwater 
pressure at the surface. Since mass flow to the turbine is a function of boiler 
pressure,  we can effectively control turbine output f rom the surface. 
An additional energy saving accrues f rom this system, since the energy 
for  pumping ultimately comes from the well water anyway. It is better to remove 
this energy downhole. The temperature is thus lowered somewhat and ar r ives  
at the surface with a correspondingly lower saturation pressure,  requiring less  
pressure added. In a 400°F well, this typically could amount to  an  additional 
20% energy saving, At  first glance, this process would seem to place an 
undue penalty on the efficiency of the surface conversion system as demon- 
strated by the Carnot principle. Nevertheless, it is easy to show that any pump, 
however it operates, w i l l  remove just its required portion of the available 
energy from the water. It does not matter whether it takes this energy from the 
top of the cycle (as we do), f rom the bottom of the cycle, o r  from a n  infinite 
number of points in the cycle (as  in the case of a pump which takes its energy 
electrically f rom the main conversipn system). 
A simplified schematic of the turbine-pump unit for our first tes t  is 
shown in Fig. 5. It is designed to fit in a n  8-5/8-in. casing, although for 
the field tes t  it will be adapted into a larger  casing, 
The bearings a r e  hydrodynamic and a r e  of tilting pad design. The thrust 
bearing has operated on hot water in the lab at three t imes our design load. 
The ball bearing operates only during start-up, when the high-pressure well 
water causes an upward thrust  on the shaft until pump thrust  builds up and 
thrusts the shaft downward through a 0.015-in. end play. 
The turbine is a two-stage pressure,  compounded single-wheel design 
with its exhaust being redirected upward through its spoked hub. 
In the expected tes t  situation, the pump and tes t  parameters will be 
approximately as follows: 
Well temperature 400" F 
Turbine output, nominal 155 HP 
Turbine output, max 220 HP 
Pump output 1000 gpm at 
200 psiAP 
Depth in well 850 f t  
285 
NPSH 100 psi 
W e l l  w a t e r  AT, nominal 15 to 2 0 ° F  
r Pm 
Specific speed 
Suction specific speed 
17,000 
5,400 
8,000 
This first unit has been designed with the objective of a successful tes t  
rather than efficient performance or  maximum output. The system may remove 
f rom the well water as much as 10°F per 100 psia added. (Note that this 
parameter is independent of the quantity pumped. ) With an optimized system 
and some amount of development, however, we feel that this w i l l  drop to 
5" F/ 100 psi  added, which is an amount considerably below that of any other 
approach w e  a r e  aware of. Efficiency in pumping geothermal fluids is, of 
course, of prime importance since they have such a small  amount of available 
energy to begin with. It is  also evident that i f  a well has  an inherently poor 
productivity index, a pump or  any other expedient will not turn it into a 
worthwhile asset .  
VII. PROGRAM 
Lab tes ts  of the impeller w i l l  start by December 1, 1974, and tes ts  of 
the complete turbine-pump in February, 1975. Field testing is scheduled to 
begin in July, 1975, probably in the Imperial  Valley, and we hope to complete 
two months' total  operation during a four-month period. 
286 
t 
\ GROUND SURFACE 
NON-FLOWING 
WELL 
V A L V E  C L O S E D  
/ 
S E V E R  AL 
f H OU SAN 0 
F E E T  
0, . ...:. 
Fig. 1. Schematic of hydrostatic geothermal well 
287 
I. 
GROUND S U R F A C E  
F L O W I N G  
WELL 
WATER T A B L E  I 
S T A R 1  O F  F L A S H I N G  /- 
I 
Fig. 2. W e l l  of Fig. 1 with low flow rate 
288 
PUMPED 
WELL 
POWER SUPPLY 
_ .  c 
POWER 1 RANSMISSION 
/C O NO UIT 
FLASH L E V E L  FOR 
-FLOWING WELL 
Fig. 3. Schematic of conventional well pumping system 
289 
MAN1 F O L D  
BO1 L E R  P I P E  
B O I L E R  
B E A R I N G  LUBE 
P I P E  1/2" 
TURBlN E- PUMP D E P T H  
U N I T  
system for geotherma .1 wells 
Fig. 5. Schematic of turbine-pump unit for Matthews pumping system 


Geothermal down well pumping system

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