pH and silica scaling control in geothermal field development

0375 - 6505/83 $3.00 + 0.00 Pergamon Press Ltd. © 1983 CNR.

Geothermics, Vol. 12, No. 4, pp. 307 - 321, 1983. Printed in Great Britain.

pH AND SILICA SCALING CONTROL IN GEOTHERMAL FIELD DEVELOPMENT R. W . H E N L E Y

Chemistry Division, D.S.I.R., Wairakei, Private Bag, Taupo, New Zealand (Received October 1982, accepted for publication April 1983) Abstract--Due to the increase of amorphous silica solubility as significant silicate ion forms in the pH range 7 to 8.5, the potential for the deposition of silica scale during reticulation (pipeline transmission) of waste geothermal waters to disposal sites is dependent on steam separation temperature, silica concentration and the pH of the residual fluid. For low salinity geothermal fluids the latter is related to the composition of the fluid, particularly the concentration of carbon dioxide remaining after the removal of steam for power generation using conventional separators. In general terms the higher the gas content of the initial deep aquifer fluid, the higher will be the pH of the waste waters, so that in many fields, where reservoir temperatures fall in the range 250 to 280°C, careful choice of separation pressures to maximise gas removal and hence elevate the pH of residual waters may obviate the problems of silica deposition, or the need to install costly chemical treatments such as acid or alkali addition. In designing and operating separators or flash plants it is important to avoid carryover of steam (containing COs and H2S) into the water reticulation lines, where, after heat loss, condensation occurs with resultant lower pH and higher scaling potential. Where flash plants are designed to receive a mixture of the two-phase discharges from a number of wells, consideration of the pH of the residual water resulting from steam separation may provide design constraints on the reticulation network such that silica scaling potential is minimised or entirely avoided. The Wairakei and Broadlands (Ohaaki) fields are used as examples of these design considerations in relation to the reservoir chemistry of the field and possible changes during extensive exploitation. INTRODUCTION E n v i r o n m e n t a l c o n s t r a i n t s on the d i s c h a r g e o f waste g e o t h e r m a l waters into i n l a n d w a t e r w a y s a n d a r e q u i r e m e n t to o p t i m i s e l o n g - t e r m p r o d u c t i o n b y m a i n t a i n i n g reservoir pressures, r e q u i r e t h a t r e i n j e c t i o n o f waste w a t e r is an essential f e a t u r e in the design o f m o d e r n g e o t h e r m a l d e v e l o p m e n t schemes. T h e c o n t r o l o f silica scaling in r e i n j e c t i o n pipelines is t h e r e f o r e a n a priori r e q u i r e m e n t o f p r e s e n t a n d f u t u r e g e o t h e r m a l p o w e r d e v e l o p m e n t s . T h e silica c o n t e n t o f the t w o - p h a s e m i x t u r e d i s c h a r g i n g f r o m a g e o t h e r m a l well is a f u n c t i o n o f the d i s c h a r g e e n t h a l p y o f the well, a n d t h r o u g h the s o l u b i l i t y o f q u a r t z in the high t e m p e r a t u r e w a t e r o f the g e o t h e r m a l reservoir, is a sensitive f u n c t i o n o f u n d e r g r o u n d t e m p e r a t u r e ( M a h o n , 1966, F o u r n i e r a n d R o w e , 1966). S e p a r a t i o n o f s t e a m f o l l o w i n g d i s c h a r g e leads to a relative increase in the silica c o n c e n t r a t i o n o f the f l a s h e d r e s i d u a l w a t e r (Fig. 1) a n d eventually, with increasing steam r e m o v a l , to s u p e r s a t u r a t i o n with respect to a m o r p h o u s silica. In the s u p e r s a t u r a t e d region, the rate o f a m o r p h o u s silica d e p o s i t i o n (i.e. scaling) a p p e a r s to be a f u n c t i o n o f several f a c t o r s such as the degree o f s u p e r s a t u r a t i o n , salinity, t e m p e r a t u r e , fluid flow regime a n d the a v a i l a b i l i t y o f n u c l e a t i n g species ( M a k r i d e s et al., 1978, 1980; R i m s t i d t a n d Barnes, 1980; B o h l m a n et al., 1980) while p H is also an i m p o r t a n t r a t e - c o n t r o l l i n g f a c t o r ( M a k r i d e s et al., 1980; R o t h b a u m a n d R h o d e , 1979; W e r e s et al., 1982). T w o m e c h a n i s m s a p p e a r to d o m i n a t e silica d e p o s i t i o n - - m o l e c u l a r d e p o s i t i o n a n d h o m o g e n e o u s n u c l e a t i o n . In the f o r m e r , silicic acid or silicate species b o n d directly o n t o a g r o w t h surface, while in h o m o g e n e o u s n u c l e a t i o n d i s s o l v e d silica p r o c e e d s t h r o u g h a series o f c o n d e n s a t i o n r e a c t i o n s to f o r m high m o l e c u l a r weight p o l y m e r s which t h e n flocculate to f o r m a low d e n s i t y silica scale, this m e c h a n i s m d o m i n a t i n g at high s u p e r s a t u r a t i o n . E x c e p t at high 307

308

t4

i~. Het;/e;

supersaturation where rapid polymer formation and flocculation lead to tligh scahug tal,:,, ~:~,. complexity of these kinetic controls is such as to preclude generalizations about the long-~c~-~J~ scaling properties of flashed water flows in pipelines. A number of studies have focussed ~,x3 possible chemical treatments as means of controlling scale formation (Rothbaum ~,~d Anderton, 1975; Rothbaum and Rhode, 1979; Shannon et al., 1982; Fleming and Crerar, 1982; Weres and Tsao, 1981) and on the use of a fluidised bed to accelerate silica removal (GraniTaylor, 1981). Large power projects involve waste water disposal at an average rate ol t tonne/s/100 MW~ generated, so that such remedial treatments are generally precluded by high costs and operating difficulties unless some useful saleable product can be obtained. The design of large-scale geothermal power developments requires ensured long term troublefree operation of water reticulation systems with simple, low cost means of avoidance or control of scaling. Of all the factors affecting deposition rate, supersaturation (or saturation index i.e. silica concentration/amorphous silica solubility) appears to be the most significam in determining the maximum amount of silica which can deposit from a supersaturated solution as well as affecting the deposition rate and mechanism. In this paper the term 'scaling potential' is used qualitatively to indicate the relative severity of silica scaling which may result during the transmission of a supersaturated silica solution. The potential for silica scale formation from geothermal water exists wherever the silica content of the waste fluid exceeds the solubility of amorphous silica. Conversely, where the solubility is not exceeded, scale formation cannot occur. This paper re-examines the design constraints imposed by the solubility of amorphous silica as a function of the pH* of geothermal waters and outlines some design parameters for geothermal power development which may obviate the problems of silica scaling in pipelines in some fields. Figure 1 shows trajectories of the increase in silica content of geothermal waters resulting from steam separation in a number of developed geothermal fields. In considering silica scaling potential it has in the past become customary to consider only the solubility of amorphous silica in water at around neutral pH; and, on this basis, reservoir fluids derived at all temperatures greater than about 250°C would be supersaturated after flashing to an optimum economic separation of 5.4 b.abs. (155°C) (James, 1967) and silica scaling problems would be anticipated. For example, severe scaling problems occur during the reticulation of waste waters in Cerro Prieto field as a result of the high supersaturations reached by flashing to 7.0 b.abs. (165°C) such that homogeneous nucleation to colloidal silica rapidly occurs (Weres er al., 1981). Even where supersaturation is small, significant troublesome scaling can often occur as a result of the large tonnages of water transmitted through pipelines in the field. Clearly the most effective means of avoiding silica scaling is to choose steam separation temperatures such that the silica content of the waste fluid does pot exceed amorphous silica solubility? but, in most cases, this imposes a design constraint which is unacceptable for economic power generation. Generally steam separation pressures are predetermined by the operation efficiency of the selected turbines and pressure loss during steam transmission. The pH of a geothermal water following steam separation in wellhead separator plants is a function of composition and temperature and is strongly dependent on the efficiency of gas removal during single stage or multistage steam separation. For most dilute geothermal fluids the p H r values of the residual flashed waters are alkaline (1 to 2.5 pH units above neutral):[: and *Subscript Tspecifies the pH of the fluid recalculated from analytical data to the temperature of interest as opposed to that measured directly in the laboratory. ?Chalcedony, cristobalite and tridymite have not been observed to form from flashed New Zealand geothermal waters in an extensive series of tests in the temperature range 100 to 190°C, nor have those polymorphsbeen reported from trials elsewhere. ~For example, the neutral pH, of pure water at 155~'C - ';.8 approx.

pH and Silica Scaling Control in Geothermal Field Development

309

8.5

v o

AMORPHOUS

1000

SILICA

80C ' 27

C~

o_ Q_

d 60C

7,8

....

I

I

100"

I

I

i

~

200*

150 °

T~mperatur¢

i

i

i

i

250"

i

i

i

300 °

°C

Fig. 1. Trajectories for the concentration of silica in a selection of geothermal waters as a function of steam separation temperature. The fluids are assumed to be initially saturated with quartz in the reservoir. Amorphous silica solubility is displayed as a function of p H r (see text) so that the state of saturation may be assessed at any specified separation temperature. For the fluids shown here undersaturation is indicated by the open symbols ana supersaturation by closed symbols at an 'optimum' flash separation temperature of 150°C (5.4 b.abs.) (James, 1967).

in this pH-range the solubility of silica is strongly dependent on pHr as well as on temperature. As a result, the residual flashed waters from many 250 to 290°C reservoirs may not, in fact, reach saturation. Advance knowledge of the pHr of such waters may therefore be an important find economic design factor for future geothermal field developments involving reinjection. This paper deals specifically with the scaling properties of such waste waters during pipeline reticulation, where significant deposition may lead to costly cleaning processes or pipeline and plant replacement. Problems incurred by deposition underground, following reinjection, are not considered here. These relate to the supersaturation of the injected fluid but also to the availability of nucleating surfaces for deposition (Rimstidt and Barnes, 1980) and the rate of dispersal of the injected fluid in the reservoir (Henley and Harper, 1979). Solubility of amorphous silica The total concentration of silicon-containing species in a solution is given by the expression msio2,total = mH4SiO4 + mH3SiO~- + mH2SiO2- + mNaH3SiO~, + mLM P

where m is the molality (moles/kg solution) of the subscripted solution species. (mLMP refers to low molecular weight polymers, see below.) The dissociation of silicic acid to silicate ion may be described by the reaction H4SiO 4 = H ÷ + H3SiO ~-

(1)

for which the equilibrium constant (Kl) is given by K1 -

a H + aH3SiO~aH4SiO4

= aH+

mH3SiO~mH4SiO4

YH3SiO~ "YH4SiO 4

(2)

R. W. Henley

310

where c~and ~' are the activity and activity coefficient of the subscripted species respectively. Values of A~ for silicic acid have been re-determined independently by Seward (1974) and Busey and Mesmer (1977) whose data coincide in the temperature range 140 to 200°C. Values of pK l ( = - log KI) are given in Table 1 for the temperature range 100 to 300°C and these data supersede the earlier high temperature data of Ryzhenko (1967). Fleming and Crerar (1982) have recently reviewed available pK I data in relation to the high temperature solubility of silica. The solubility of amorphous silica at p H < < p K I may be regarded as due to silicic acid alone and may be described by the reaction SiO 2 + 2H20 = H4SiO 4 Amorphous silica

Silicic acid

The equilibrium constant (Ks) for this reaction is given by aH4SiO4

(3)

K S = _ a2H20

Fournier and Rowe (1966, 1977) have experimentally determined values of K s in water and these data may be conveniently expressed by the equation log/~s = 4.52 - - -731 t~bs

(toc = 0 - 250)

(4)

where Ks is written as mg SiO2/kg solution. For the low salinity brines of the majority of geothermal systems (for which chloride concentrations are less than approx. 20,000 mg/kg the effect of salinity on the activities of water and silicic acid are negligible and will be ignored in the discussion below. (For the Salton Sea brines however, where Cl concentrations are approx. 155,000 mg/kg, CtH~o ~ 0.7 and "fSi(OH)4 ~ 1.3 so that the solubility of amorphous silica at 150°C is reduced by approx. 60% compared t6 its solubility in pure water.) Table 1. Dissociation constants of weak acids at elevated temperature at the saturation vapour pressure of water, expressed as pK values Temperature

(of)

H,SiO, H3SiO~

100

150

200

250

300

Ref.

9.10 10.97

8.87 10.95

8.85 11.17

8.96 11.55

9.22 12.06

*~~:

*Seward (1974). tBusey and Mesmer 0977). ~Ryzhenko (1967). In obtaining these solubility data, chemical methods of determining silica concentrations cannot distinguish the presence or absence of low molecular weight polymers (LMP) such as H6Si207, so that the contribution of these species is contained in Ks values determined experimentally. Fournier and Rowe (1977) showed that higher order polymers were absent along the solubility curve. Since the p H r range of flashed geothermal brines is seldom greater than 9.0, the existing data for the second dissociation constant of silicic acid (Ryzhenko, 1967; Volosov, et al., 1972) suggest that H2SiO ~- contributes little to the solubility of silica in these fluids. From the studies of the solubility of quartz in pH-buffered solutions, Seward (1974) has postulated the formation of a sodium silicate ion pair, NaH3SiO4, which at tuna+ = 0.043 and at 150°C would contribute 13, 42 and 75 mg/kg to the solubility of amorphous silica at pHT 7.5, 8.0 and 8.25 respectively (Fig. 4). The existence of this species is equivocal (Busey and Mesmer, 1977) so that its contribution to amorphous silica solubility is not further considered below. Combining equations (1)-(3) the solubility, Ssio2 in mg/kg, of amorphous silica in low salinity waters may be conveniently expressed as a function of pHT, Ki Ssi°2 = J~s[ 1 + l (5) a H + ~'H3SiO4 in the p H r range up to approx. 9.5. Activity coefficients may be calculated using the extended Debye - Htlckel equation (Helgeson, 1969) and the ionic strength of the flashed water under consideration. Substituting equation (4) into equation (5) gives an expression for the solubility of silica directly in mg/kg units. The solubility of amorphous silica is shown in Fig. 2a as a function o f temperature and pHr, and isothermal sections of the solubility surface are shown in projection in Fig. 2b. Figure 1 shows iso-pH solubility curves for silica, the silicate-flee solubility being shown by the pH < 7 curve.

p H and Silica Scaling Control in Geothermal Field Development

311

(b) I

1000

: 0"047

/ / / I

-

J/Ill 60O

C

22:2

o. o" ~o 200-

PHT

6

7 8 9 PHT Fig. 2. (a) Isometric projection of the solubility of amorphous silica as a function of pHr and temperature. (b) Isothermic projections of the silica solubility surface onto concentration-pHr space. The second stage flash composition of the Broadlands BR22 water is shown.

The p H of flashed geothermal waters C o m p a r i s o n of these solubility data to the silica contents of waste separation f r o m geothermal discharges requires data for the pHT affecting p H r during exploitation due to changes in reservoir fluid p H itself is a function of the alkalinity (A) of the geothermal fluid 1975).

water resulting f r o m steam of the waters and factors composition. (Ellis, 1967, 1970; Merino,

A ~. mucof + 2 m c ~ - + mH3SiO~- -~- mBOf q- m i l S - Jr- mNUf --mu+

(6)

where m, is the molarity of the subscripted species. Since the p H of deep aquifer fluids is constrained by a set of m i n e r a l - f l u i d interactions (such as c l a y - f e l d s p a r - f l u i d or m i c a - f e l d s p a r - f l u i d ) to near neutral values (Ellis, 1970;

312

R.

Henley

Giggenbach, 1981) rnnco3 dominates this expression under aquifer conditions. The bicarbonate ion concentration itself is a temperature dependent function of pH and the CO 2 content of the aquifer fluid. This raises the generalisation that the highest alkalinities are attained in high gastype geothermal systems. By the same token, as salinity increases, the mineral-fluid dependent pH of deep aquifer fluids decreases, resulting in relatively low mHco3and low alkalinity. Table 2 gives examples of these relationships for a number of geothermal fields. Table 2. Alkalinity (approximated as mH(.()3 ) of geothermal reservoir fluids related to temperature, salinity ( m c ] ) and CO 2 concentration

Field Wairakei Broadlands Kawerau Ngawha Cerro Prieto

Well No.

T (°C)

WR215 BR22 K 19 NG4 19A

250 - 260 275 260 225 288

mcl 0.05 0.03 0.02 0.03 0.23

n~?o2 × 103

mHco3 × 103

12.5 120.1 91 196 73

0.28 1.82 1.78 4.89 0.20

The pHr of a geothermal water following steam separation is a function of temperature (7) solution composition (total silica, boron, bicarbonate and salinity) and alkalinity. As a result of the carbonic acid-bicarbonate ion dissociation reaction (which dominates the buffer capacities of other weak acid-base pairs in the majority of geothermal waters) pHr is strongly dependent upon the proportion of CO 2 remaining in the liquid phase following steam separation, since this component determines the contribution of HCO~- to the alkalinity of the liquid. As gas removal proceeds the contributions of silicate and borate ion become more significant. The high temperature pHT of a separated water cannot be measured directly but may be obtained from appropriate water samples by laboratory pH measurements (at laboratory temperature), analysis of major dissolved components and an iterative thermodynamic calculation (Truesdell and Singers, 1971). Table 3 shows the high temperature pH and partial analyses of a series of water samples from well BR22 at Broadlands (Ohaaki), New Zealand. The waters sampled were derived by either a single or double stage flash separation from the discharge of the deep well BR22 which supplied water to a number of field experiments relating to silica deposition during hot water reticulation. Complete analyses of these waters are presented and discussed elsewhere (Henley and Singers, 1982). The highest temperature single flash water is clearly undersaturated with respect to silica at 186°C when pHr = 7.42. The additional gas loss which occurs during a second stage flash to 153°C leads to a pHr increase such that although the silica content increases due to steam loss, the solution remains undersaturated with respect to silica. This is a simple but important result because many previous discussions of silica deposition problems have not incorporated the effect of pH on silica solubility and for high alkalinity fluids have resulted in quite erroneous conclusions regarding the state of saturation. In the example discussed here the 153 ° flashed fluid would have been expected to be supersaturated since the silicic acid solubility is 638 mg/kg compared to the 750 ___5 mg/kg silica content of the residual water (Table 3) and the 778 mg/kg solubility of amorphous silica as silicic acid and silicate ion. It is also worthwhile to emphasize that separation temperature is also highly important. For the example discussed above flashing to lower temperatures involves no significant increase in pH but proportionally increases the silica content of the residual fluid. Together with the decrease of solubility with falling temperature, this clearly would lead to a supersaturated state. The pH enhancement of silica solubility discussed in this paper essentially forms a 'window', allowing a rather higher removal of steam for power generation free of silica scaling problems than would be the case considering silicic acid solubility alone.

p H and Silica Scaling Control in Geothermal Field Development

313

Table 3. Silica supersaturation in flashed geothermal waters from BR22 measured data and calculated data (bold)

Sample date July 1980 July 1980 July 1980 June 1980 June 1980 June 1980 June 1980 June 1980 June 1980 June 1980 June 1980 June 1980 June 1980 June 1980 December 1979

Primary flash temperature (°C)

Secondary flash temperature (°C)

186 186 186 184 184 184 200 190 180 170 160 150 197 155 179

-153 153 -155 155 150 150 150 150 150 130 -155 149

pH r

Total SiO 2 (mg/kg)

Silicic acid solubility

Amorphous silica solubility

Saturation index

7.41 8.20 8.08 7.36 8.09 8.01 8.05 8.04 8.03 8.00 7.93 8.08 7.25 7.46 8.15

688 750 742 705 740 753 761 761 761 761 761 795 687 756 744

847 638 638 834 650 650 620 620 620 620 620 509 120 649 614

888 824 778 869 797 772 744 742 738 730 713 598 951 683 769

0.77 0.91 0.95 0.81 0.93 0.98 1.02 1.03 1.03 1.04 1.07 1.33 0.72 1.11 0.97

The saturation index (St) is defined by the ratio total dissolved silica/amorphous silica solubility at pH and T.

Flash plant design and operation The experiments referred to above formed part of the design investigations for the Ohaaki power station to be built at Broadlands, New Zealand (Henley and Singers, 1982). The results obtained were specific to the well discharge and their extension to the full set of producing wells to be used in the power scheme was required. The assessment of the state of silica saturation for any operating geothermal well requires only collection, analysis and a calculation procedure, provided that the well discharge is separated at the required design pressures. This is clearly not possible at the investigation stage of a geothermal field with exploration wells shut and separator plant still to be designed. In order to utilise the pH-based silica solubility to consider advisable separation and reinjection temperatures it is, therefore, necessary to obtain a reliable theoretical method for the calculation of the pH and silica content of flash-separated waters, based on chemical data obtained during output testing of the wells. Since pHr is strongly dependent on the concentration of CO 2 in the flashed water, the calculation of pHr for this purpose requires some practical assumptions concerning distribution of CO 2 (and H2S) between water and steam within the separator plant. Henley and Singers (1982) have shown that conventional cyclone separators of the type installed at Wairakei (Bangma, 1961) achieve a quasi-single stage equilibrium distribution of gases between the separated water and steam phases. For non-volatile components like chloride or silica the molal concentration, mc. , in the separated water phase is related to concentration, mco, of the two phase or single phase mixture entering t~{~Wseparator through

moo = (1 - Yi) mci, w

(7)

where y is the steam fraction (mass) separated in the cyclone. For volatile constituents such as CO 2 or H2S

moo = (1 - yi ) mci, w + Yimci, s

(8)

where Ci. s is the gas concentration in the separated steam phase, e.g. for carbon-containing species, C o = EC o = CO2,aq + H C O 3- + CO~3C o = (1 -- yi)(mco2,aq + mHCO3- + mco~3-)i,w + Yi(mco2)i,s

(9)

The equilibrium distribution of CO 2 between steam and water is given by the temperature-dependent function, B c o 2 (Giggenbach, 1980),

314

R. 14~ Henley {C02),.~

so that equation (9) may be modified for single stage steam separation such that

YC° mc'O2,aq,t, w

I

= (1 - y ) ILl +

mHc°~ m(o2,aq

,

mc'°~ 5:

i ~ .v~B,o~

f/'l(f)2,aq J

"

.~iii

With the appropriate insertion of the first and second dissociation constants for carbonic acid (K~ and K2) and activiL~ coefficients, ~'i, this expression becomes

EC°

Vi) I "

y(,O2,a q

1 YHCO 5

aH + Y c o ~ -

Similar pH-dependent expressions may be written for other volatile constituents such as H2S and NH3 and solved simultaneously by iteration using the normal convergence routine employed in the GEODATA (ENTHALPY) program (Truesdell and Singers, 1971). This FLASH option has been incorporated in the most recent version of this program (Henley and Singers, 1982; Singers, el al., in preparation) to allow the calculation of flashed water compositions, pH, and steam composition for any three sequential separation stages given the total discharge composition of the well. At Ohaaki, the discharges of up to five wells are to be combined for steam separation at a multiple flash plant. To accommodate this design strategy a further refinement of the program has been evolved to allow the calculation of flashed water pHr, etc., for a combination of up to five well discharges by appropriate weighting of the mass flow contribution of each well in the mixed input to the proposed flash plant. T a b l e 3 s u m m a r i s e s the e q u i l i b r i u m flashed water c o m p o s i t i o n s calculated f r o m the analyses o f fluids discharged f r o m well BR22 o n different collection dates a n d at differing separator t e m p e r a t u r e s (after H e n l e y a n d Singers, 1982) a n d c o m p a r e s observed P H r a n d silica c o n t e n t s with those calculated using the F L A S H o p t i o n of the recently m o d i f i e d E N T H A L P Y p r o g r a m described a b o v e (Singers, et al., in p r e p a r a t i o n ) . Since analytical a n d experimental data are involved, e s t i m a t i o n of the precision o f the p H r data is difficult b u t it is p r o b a b l y better t h a n _0.1 p H units ( H e n l e y a n d Singers, 1982). The s a t u r a t i o n index (St) is defined as the ratio, total dissolved silica to a m o r p h o u s silica solubility at the same p H r a n d t e m p e r a t u r e , a n d T a b l e 3 shows a m o r p h o u s silica solubilities a n d s a t u r a t i o n indices calculated using e q u a t i o n (5). T a b l e 3 reveals the interesting result that in dual-flash plants p H r a n d a m o r p h o u s silica solubilities are highest where the s e p a r a t i o n pressure differential is m a i n t a i n e d as high as practicable. The validity of the experimental silica solubility and dissociation constant data discussed above have been confirmed by an extensive series of pilot scale reticulation trials using double-flashed BR22 waters (Henley, unpublished reports 1979- 1981). In trials lasting 1 to 2 months each, double-flashed water has been reticulated through 305 m (1000') of 25 mm OD (1 inch) internal diameter insulated mild steel tube at 149 to 156°C without scale formation. A thin hard black deposit of iron sulphide formed as a surface coating on the inner wall of the pipes. A mixture of silica and iron sulphide was observed during a parallel reticulation trial using water delivered at a relatively higher supersaturation from a small separator at 132°C. This experiment w~s effected by scaling in the separator itself as well as by steam carry-over but, at the effective supersaturation of the reticulated water, scaling of the 305 m pipeline was minor. The a b o v e discussions have focussed o n the solubility of silica in waters derived by steam-gas s e p a r a t i o n in either p r i m a r y or d o u b l e - f l a s h separator systems. As s h o w n by H e n l e y a n d Singers (1982) waterline sample c o m p o s i t i o n s m a y diverge f r o m those calculated by the single stage m o d e l due to: (a) multistep steam s e p a r a t i o n prior to steam r e m o v a l in the well a n d (b) carry-over of separated steam into the separator water line. (a) Multistep steam s e p a r a t i o n in the well leads to a f r a c t i o n a l increase o f p H r over that calculated f r o m the single step model, with a c o n s e q u e n t beneficial increase in silica solubility. I n field o p e r a t i o n s this effect m a y be largely obscured by the d e t r i m e n t a l effect of steam carryover a n d c o n d e n s a t i o n , unless separator design a n d o p e r a t i o n g u a r d against this occurrence. (b) T h e chemical effects of steam carry-over f r o m a second stage separator m a y be calculated f r o m e q u i l i b r i u m flash water c o m p o s i t i o n s by a p p r o p r i a t e a m m e n d m e n t of c o m p o n e n t c o n c e n t r a t i o n s to a c c o m m o d a t e the excess steam ( i n c l u d i n g CO2, H2S, N H 0 . T a b l e 4 shows the v a r i a t i o n of flashed water c o m p o s i t i o n together with p H r a n d s a t u r a t i o n

p H and Silica Scaling Control in Geothermal Field Development

315

Table 4. Effect of steam carry-over on the p H r and silica saturation index of second flash water at 155°C 070

pHr

Y.,C

Y.,S Y.,B (millimoles/kg)

0 1 2 3 4 5 10

8.2 8.1 8.0 7.9 7.8 7.7 7.0

1.139 1.591 2.043 2.495 2.947 3.399 5.658

0.189 0.205 0.220 0.236 0.252 0.268 0.346

4.463 4.418 4.374 4.329 4.285 4.240 4.017

ESi

EN

Saturation index

12.400 12.276 12.155 12.028 11.904 I 1.780 11.160

0.005 0.009 0.012 0.016 0.020 0.230 0.042

0.97 0.99 1.01 1.03 1.05 1.06 1.07

~C, ES, EB, XSi and EN refer to total carbon, sulphur, boron, silica and amonia in the water and condensed steam mixture resulting from carry-over.

index, due to carry-over of second stage steam into the discharge water line. The assumption is made that heat loss from the discharge pipeline is sufficient to condense the steam into the waterflow. A small decrease in total silica occurs due to dilution by the condensed steam but prior to this p H r and supersaturation remain constant. As shown by Table 4, for secondary separator waters, carry-over of steam leads to an increase in the saturation index from 0.97 to 1.067 at approx. 5°70 (mass) steam carry-over and clearly some minor scaling potential is then incurred. Primary-separator steam carry-over is potentially more serious as a result of the high content of CO2 and H2S in the steam fraction. The carry-over and condensation of 2°70 (mass) steam into the primary separator water leads to a p H r decrease relative to that of the equilibrium second flash water and a consequent increase in the saturation index. For the computed first flash data (dated December 1979 in Table 3) single step equilibrium flash gives p H r = 8.1 with saturation = 1.005 at 149°C, but pHT = 7.8 and saturation = 1.12 where 2°70 steam carry-over occurs from the primary separator. In the event of this occurrence in an operating field, subsequent reticulation of water is potentially prone to greater silica deposition than for the case where steam carry-over was avoided (since the excess silica - 4 mg/kg). Operating conditions and separator design for power generation should optimise the effects of maximum gas removal and maximum p H r to avoid silica scaling in water reticulation lines. Dual flash separation is most efficient for this purpose and the difference between separation pressures, as shown above, should be at a maximum. For the data in Table 3, for BR22 as a representative example of the Ohaaki wells, optimum flash pressures would be 14.5 b.gauge and 3.5 b.gauge respectively. Separators should be designed to minimise the effects of vortex formation and steam carry-over in reducing p H r and leading to increased supersaturation. Single stage steam separation is not to be recommended. As shown in Table 3, this results in less effective gas removal and relatively low pH silica supersaturated residual waters. For the B R 2 2 (June 1980) data shown, the p H r and saturation index at 155°C are 8.03 and 0.97; and 7.46 and 1.11 for double and single flash respectively.

Pipeline heat loss The most rapid changes in saturation index occur through heat loss from the transmission pipeline. The relative changes in the dissociation constants of the weak acids, in the temperature range 120 to 200°C, lead with falling temperature to only a small increase in p H r with respect to the p H r of the separator liquid. The change in the saturation index (St) is proportional to the change in Ks, as given by equation (4) such that log (St~St2) = 731

,

T,'

where T ' , < T' 2 and T' is in deg. K.

316

R. 14. Henley

Equation (12) is potentially useful in estimating the 'cooling tolerance' of flashed geothermal brines with respect to the onset of silica deposition. Since silica deposition rates are difficult to reliably predict as functions of S, pH, T and other kinetic variables, cautious field design should be based on S r 2 olax - 1. for example for the BR22 (June 1980) double-flashed residual water, discussed above, supersaturation increases from 0.97 at the separator (155°C) to t.07 after cooling to 145°C and 1.18 at 135°C. These saturation ratio changes represent potential deposition of 50 and 115 g / t o n n e water respectively, whereas no deposition potential occurred at 155°C. Current operating experience from experimental reinjection pipelines suggests thermal gradients along above-ground pipelines are of the order 5 ° C / k m *. Then working backwards from the required spacing between flash plant and reinjection wells, an optimum second stage separation pressure may be selected such that the dangers of silica scaling are obviated. CASE STUDIES

Wairakei geothermal field, New Zealand Since commissioning, the Wairakei field has discharged over 1 tonne/s of atmospheric pressure boiling water via open drains to the Wairakei Stream, a distance of approx. 1 km. The deposition of voluminous silica scale in these drains requires a costly annual clearance p r o g r a m m e , and concern based on this experience has figured large in the costing and design of m a n y other geothermal developments. Further developments of the Wairakei field currently under construction will involve addition of new production wells and, due to environmental constraints, reinjection of waste waters so that design criteria based on chemical data have been required by the field operators. Temperatures in the production zone of the Wairakei geothermal field are in the range of 200 to 260°C. The reservoir fluid is a dilute near-neutral p H chloride water with a relatively low content of dissolved CO2 and H2S. For the generation of electric power, steam is separated f r o m the well discharge at 159°C (6.2 b.abs.) and 124°C (2.3 b.abs.) in some cases with a high pressure stage at about 180°C (10 b.abs.). Figure 3 shows the calculated p H and silica content of some representative waters separated to 140 and 124°C by two-stage steam separation at individual wellheads. The wells shown were selected at random but provide a useful illustration of the following: (a) single-flash separation leads to lower p H and higher scaling potential than does doubleflash, e.g. WR215, (b) calculation of saturation ratios on the basis of amorphous silica solubility in pure water may lead to erroneous conclusions regarding scaling hazards, e.g. wells 72, 108 and 206, and (c) combination of high and low silica discharges in flash plants may ensure scale-free reticulation for the combined residual water mixture. At Wairakei, separation to temperatures of the order 130°C (2.7 b.abs.) in most cases leads to silica undersaturated waters, quite incapable of the deposition of silica scale during reticulation. The practice at Wairakei has been to discharge this residual water to an open drain with further steam loss; the associated decrease in solubility, due to the lower temperature of the drain water, leads to high saturation ratios ( - 2 ) and inevitably to the deposition of voluminous silica scale. Pipeline transmission of waste water at the final separation pressure would have obviated the problem of silica deposition. At this stage of the power project the *James (personal communication) has shown that the heat loss from reinjection pipelines containing boiling water may be calculated from the expression AT = 1270/D (°C/km) where D = pipeline diameter (mm).

pH and Silica Scaling Control in Geothermal Field Development

317

700-

n 28 D215 600"

207E]

E

E]108 /

124°

206D~f_172

0

'r_139

500- /

15

8

pN

Fig. 3. Silica concentration and pHr for a number of Wairakei well discharges double-flashed at 159 and 124°C (6.02 and 2.25 b.abs.). The solid line is the pH dependent solubility of silica and the broken line is the solubility of silica in water at pH < 7.

1000

NaH3Si04 [ ~\] I . I / 251'1 /

I

OHAKI PRODUCTION WELLS Flash 1 14.6ba Flash 2 5.4ba

21

22

2o 800

E

35

v)

Si(OH

600

PHT Fig. 4. Silica concentration and PHr for Ohaaki (Broadlands) production well discharges double-flashed to 196 and 155 °C. Where data are available changes in silica concentration and flashed water pH consequent to an early reservoir discharge and recovery are shown by the vectors. For each of these wells the trend in silica concentration through time during production is toward lower silica concentrations. Solubility curves for amorphous silica at 155 and 140°C are shown, the latter to represent pipeline heat loss during reticulation of around 3 km (see text). (Data for BR22 are shown in Fig. 2b.)

318

i¢. ~ . ilenle~'

replacement of the existing drain b~ a pipehne is ccononncall~ una~lraclivc bul dc~elopnl~nt ,,! the field in the future will probably involve pipeline ~eticttlatioll tinder pressmc ~t!i~ consequent avoidance of silica ,~caling problems

Ohaaki geothermal field, N e w Zealand* The reservoir fluid at Broadlands is similar to that at Walrakei i)ut charactensed by mtlt:h higher gas contents (Mahon and Finlayson, 1972). Reservoir ternperatures in the proposed production field range from approx. 240 to 28(l°C and the discharges of a significant numbeJ of wells contain excess steam relative to a single-phase reservoir fluid. Each of these three factors contribute to the higher pH, and higher silica content of Broadlands flashed waters compared to those at Wairakei. The range of silica contents and pH for the Broadlands wells are shown irt Fig. 4 calculated for double-flash conditions. The data derived from the single well BR22 formed the basis of the above discussion, ghe present discharge of this well at high wellhead pressure contains no significant excess reservoir steam and downhole silica temperatures are close to the average for the Broadlands field, 260°C. The data shown in Table 3 suggest that optimisation of flash pH~ and silica solubility occurs where successive flash temperatures are 200 and 150°C. Independent consideration of the inlet pressures for intermediate pressure turbines gives an optimum value of approx. 3.5 b.gauge which, in turn, requires a steam separation pressure of approx. 4.5 b.gauge (155°C) allowing for pressure loss during steam transmission. Adoption of these separation pressures for the Broadlands - Ohaaki field through these separate criteria will, in this case, allow scalefree waste water reticulation to reinjection wells provided cyclone separator operation and heat loss are controlled as prescribed above. The development of the Ohaaki field includes two-phase transmission of discharges of two~stage flash plants, with subsequent reticulation of the separated steam at 14.3 b.abs. (196°C) and 5.4 b.abs. (155°C) to the power house and reinjection of the 155°C flashed water. Preliminary design of the two-phase network in the West Bank portion of the field has been largely based on the cumulative length and, therefore, cost of pipework involved, leading to the grouping of wells as shown in Fig. 5a. Use of the modified E N T H A L P Y program to provide the silica-pH data for flashed waste water from the individual production wells, with these particular well groupings, showed, using available data, that two of the three groups (flash plants 1 and 3) would produce a waste water with silica saturation <1.0, while the third (flash plant 2) producing from the higher temperature zone at the centre of the field would result in a significant supersaturation in the waste water and therefore a high potential for silica scaling. The design alternatives to minimise or avoid silica scaling are: (a) to separate the discharge from BR21 independently or (b) to reticulate the discharge of BR21 to the proposed flash plant 3. The latter alternative has been adopted as desirable and economic and leads to the flash plant groupings shown in Fig. 6, with the inclusion of well BR31. Recent data for well BR21 suggest a major decrease in underground temperature since the initial discharge 12 years ago, so that the pH-SiO~ parameters for the residual fluid will in fact fall well below the point calculated lot flash plant 3 (Fig. 6). Similarly in flash plant 2 the high silica content is biased toward the initial discharge of BR22 in 1971 and this well, continuously on discharge for the last 6 years, has declined markedly in silica content and discharge enthalpy; flash plant 2 appears therefore to produce a residual fluid of low to zero silica scaling potentialt~

*Prior to the offical change of name, this field has been known as the Broadlands Field in the geothermal literature. tFigure 6 also includes a silica saturation curve for 145°C in order to assess the effect of conductive cooling during reticulation to reinjection wells; flashing to this temperature would lead to higher silica content although similar pH compared to the 155°C second flash.

p H and Silica Scaling Control in Geothermal Field Development

319 [B]

II

17

II

17

9

Q9

/

20

/

'

/

/

,

//

,

'\ .

,

~^^ ~ z,uurn

/

PR000E,0

-

~,, • ~

,

,,'7

~

'L'/

WELL

--2-PHASE PIPELINE ~ " FLASH PLANT - - STEAHLINE TO POWER

,

///'

/0/ /'~/ /i>~--

,

~h,o STATION

~lJ

--- WATER TO REiNJECTION WELLS

,,//'~" /

Fig. 5. Location of West Bank production wells for the Ohaaki geothermal power scheme, showing (A) the initial proposal for two-phase reticulation to flash plants based on pipeline length and (B) the subsequently adopted scheme based on pipeline length and consideration of silica saturation, particularly for well BR21 (see text).

Ftash Ptant 1

900

"7

800

E

Flash Ptanf 2

o

Early discharge sampte

I

Later

•

FLASH PLANT DISCHARGE

Flash Ptanf 3

1.1971

22,1971,/~ I

/,

/ ~ ~1976

t3 SINGLE FLASH

9, 1970 11

bq 700

600

h "W Y 18(~

h

I 7

pH

21,19~811~I 155o/~/~/9

/" %5 ° /

o8

t

l

8

7

pH

8

7

8

pH

Fig. 6. Silica concentration and pH for double-flash separated waters from Ohaaki production wells (West Bank) and from multiple-feed flash plants. The solid symbols and arrows show flashed water compositions for wells following the initial sustained field trails. Notice that the SiOspH values for flash plant discharges fall in a mean position with respect to the individual feed wells and depend on the relative contribution of each well to the flash plant inlet. Since discharge compositions change with time, the trend for flash plant residual water compositions is toward lower silica concentration and slightly higher pH. The total discharge (tonne/h) of the individual wells are: flash plant I--BR2, 152; BRS, 87; BR1 l, 133; BR17, 59; BR18, 21; flash plant 2--BR9, 26; BR20, 80; BR22, 119; and flash plant 3--BRI 2, 70; BRI9, 24.5; BR21, 23; BR23, 48; BR31, 53. (For comparison the solid square indicates the single-flash pH for well BR18.)

320

R. 14'. H e n l e y

O n e o f the most interesting aspects o f the design study for this field has been the i n c o r p o r a t i o n o f aspects o f the O h a a k i g e o t h e r m a l system structure a n d chemistry in assessing silica scaling p o t e n t i a l . E q u a l l y intriguing are the aspects o f time which need to be c o n s i d e r e d in r e c o m m e n d i n g design o p t i o n s . As discussed by H i t c h c o c k and Bixley (1975) enthalpies and mass flows for the B r o a d l a n d s wells are expected to decline d u r i n g e x p l o i t a t i o n o f the field, p e r h a p s with gas c o n c e n t r a t i o n s a n d e n t h a l p i e s passing t h r o u g h an early e x p l o i t a t i o n - p h a s e m a x i m u m ( G r a n t , 1977). M a h o n a n d F i n l a y s o n (1972) s h o w e d that l o n g - t e r m decline o f c h l o r i d e a n d silica c o n t e n t s o c c u r r e d for m o s t B r o a d l a n d s wells d u r i n g sustained e x p l o i t a t i o n due to i n t r u s i o n o f n e a r - s u r f a c e low e n t h a l p y waters into the p r o d u c t i o n zones. A decrease o f s u p p l y w a t e r t e m p e r a t u r e due to mixing with c o o l e r waters leads to a decrease in silica content, as has a l r e a d y been the case for a n u m b e r o f the higher t e m p e r a t u r e wells, e.g. BR11, BR21 and BR22 (Figs 4 a n d 6). CONCLUSIONS A s a result o f p h y s i c a l a n d c h e m i c a l c o n d i t i o n s ( t e m p e r a t u r e , gas c o n c e n t r a t i o n ) in low salinity g e o t h e r m a l reservoirs s t e a m s e p a r a t i o n at surface for p o w e r g e n e r a t i o n m a y resolt in the p H o f the r e s i d u a l waters being sufficiently high that, due to the f o r m a t i o n o f significant silicate ion, silica r e m a i n s u n d e r s a t u r a t e d d u r i n g r e t i c u l a t i o n to d i s p o s a l wells or o t h e r d i s c h a r g e p o i n t s a n d silica scaling p r o b l e m s c a n n o t occur. C a r e f u l design a n d o p e r a t i o n o f flash p l a n t , w i t h i n the design c o n s t r a i n t s i m p o s e d by r e q u i r e d t u r b i n e inlet pressures, is r e c o m m e n d e d for f u t u r e g e o t h e r m a l field d e v e l o p m e n t s such t h a t high gas r e m o v a l efficiency is m a i n t a i n e d a n d the flash p H o f residual fluids is m a x i m i s e d . W h e r e well discharges are to be fed to a c o m m o n flash plant, the flash p H a n d silica content o f the waste water resulting from the c o m b i n e d t w o - p h a s e i n p u t s to the p l a n t m a y be r e a d i l y calculated. This in turn m a y lead to r e c o m m e n d a t i o n s c o n c e r n i n g the c o m b i n a t i o n o f wells to be fed to the p l a n t s so that scaling p r o b l e m s m a y be a v o i d e d by o p t i m i s i n g with respect to flash p H a n d silica c o n t e n t . T h e design o f the O h a a k i field has been stressed as an e x a m p l e o f the c o m b i n a t i o n o f g e o c h e m i c a l a n d engineering principles which is desirable in f u t u r e g e o t h e r m a l d e v e l o p m e n t s . REFERENCES Bangma, P. (1961) The development and performance of a steam-water separator for use on geothermal bores. U.N. Conference on New Sources o f Energy, Rome, 3, pp. 60-78. Bohlman, E. G., Mesmer, R. E. and Berlinski, P. (1980) Kinetics of silica deposition from simulated geothermal brines. Soc. Petrol. Engrs J. (August), pp. 239-248. Busey, R. H. and Mesmer, R. E. (1977) Ionization equilibria of silicic acid and polysilicate formation in aqueous sodium chloride solutions to 300°C. Inorg. Chem. 16, 2444-2450. Ellis, A. J. (1967) Geochemistry of explored geothermal systems. In Geochemistry of Hydrothermal Ore Deposits, 1st edn (Edited by Barnes, H. L.) pp. 465- 514. Holt, Rinehart and Winston. New York. Ellis, A. J. (1970) Quantitative interpretation of chemical characteristics of hydrothermal systems. Geothermics Spec. Issue 2, 2(1), 516-528. Fleming, B. A. and Crerar, D. A. (1982) Silicic acid ionization and calculation of silica solubility at elevated temperatures and pH. Geothermics I I, 15 - 29. Fournier, R. O. and Rowe, J. J. (1966) Estimation of underground temperatures from silica content of water from hot springs and wet steam wells, Am. J. Sci. 264, 685- 697. Fournier, R. O. and Rowe, J. J. (1977) The solubility of amorphous silica in water at high temperatures and pressures. Am. Miner. 62, 1052-56. Giggenbach, W. F. (1980) Geothermal gas equilibria. Geochim. cosmochim Acta 44, 2021 -2032. Giggenbach, W. F. (1981) Geothermal mineral equilibria. Geochim. cosmochim Acta 45, 393 -410. Grant, M. A. (1977) Broadlands--A gas dominated field. Geothermics 6, 9 29. Grant-Taylor, D. (1981) Removal of silica from geothermal waters in a fluidised sand bar. Chemistry Division, D.S.I.R., New Zealand, Technical Note 81/2. Helgeson, H. G. (1969) Thermodynamics of hydrothermal systems at elevated temperatures and pressures. Am. J. Sci. 267, 729- 804. Henley, R. W. and Harper, R. T. (1979) Water-rock interactions during injection of flashed BR2 water at BR34. Chemistry Division, D.S.I.R., New Zealand, Open File Report 30/555/2.

p H a n d S i l i c a S c a l i n g C o n t r o l in G e o t h e r m a l F i e l d D e v e l o p m e n t

321

Henley, R. W. and Singers, W. (1982) Geothermal gas separation in conventional cyclone separators. N.Z. JI. Sci. 25, 37-45. Hitchcock, G. W. and Bixley, P. F. (1975) Observations of the effect of a three year shutdown at Broadlands geothermal field. 2nd U.N. Symposium on the Development and Use o f Geothermal Energy, San Francisco 3, pp. 1657- 1661. James, C. R. (1967) Optimum wellhead pressure for geothermal power. N.Z. Engng 22, 221. Mahon, W. A. J. (1966) Silica on hot water discharged from drillholes at Wairakei, New Zealand. N.Z. JI. Sci. 9, 135 - 144. Mahon, W. A. J. and Finlayson, J. B. (1972) The chemistry of the Broadlands geothermal area, New Zealand Am. J. Sci. 272, 48-68. Makrides, A. C., Turner, M. J. and Slaughter, J. (1980) Condensation of silica from supersaturated silicic acid solutions. J. Colloid Interface Sci. 73, 345 - 367. Makrides, A. C., Turner, M. J., Harvey, W. W., Slaughter, J., Brummer, S. B., Offenhartz, P. O'D and Pearson, G. F. (1978) Study of silica scaling from geothermal brines. U.S. Dept Energy Report EY-76-C-02-2607. U.S. Government Printing Office. Merino, E. (1975) Diagenesis in Tertiary Sandstones from Kettleman North Dome, California--ll. Interstitial solutions: distribution of aqueous species at 100°C and chemical relation to the diagnetic mineralogy. Geochim. cosmochim Acta 39, 1629- 1645. Rimstidt, J. D. and Barnes, H. L. (1980) The kinetics of silica-water reactions. Geochim. cosmochim Acta 44, 1683- 1700. Rothbaum, H. P. and Anderton, B. H. (1975) Removal of silica and arsenic from geothermal discharge waters by precipitation of useful calcium silicates. 2nd U.N. Symposium on the Development and Use o f Geothermal Resources, San Francisco 2, pp. 1417- 25. Rothbaum, H. P. and Rhode, A. G. (1979) Kinetics of silica polymerisation and deposition from dilute solutions between 5 and 180°C. J. Colloid Interface Sci. 7 1 , 5 3 3 - 559. Ryzhenko, B. N. (1967) Determination of the hydrolysis of sodium silicate and the calculation of dissociation constants of orthosilicic acid at elevated temperatures. Geochim. Int. 4, 9 9 - 107 (English translation). Seward, T. M. (1974) Determination of the first ionization constant of silicic acid from quartz solubility in borate buffer solutions to 350°C. Geochim. cosmochim Acta 38, 1651 - 6 4 . Shannon, W. T., Owers, W. R. and Rothbaum, H. P. (1982) Pilot scale solids/liquid separation in hot geothermal discharge waters using dissolved air flotation. Geothermics 11, 4 3 - 58. Truesdell, A. H. and Singers, W. A. (1971) Computer calculation of downhole chemistry in geothermal areas. Report No. C.D. 2136, Dept Scientific and Industrial Research, New Zealand. Volosov, A. G., Khodakovskiy, I. L. and Ryzhenko, B. N. (1972) Equilibria in the system SiO2-H20 at elevated temperatures along the lower three phase curve. Geochim. Int. 9, 362-77 (English translation). Weres, O. and Tsao, L. (1981) Chemistry of silica in Cerro Prieto brines. Geothermics 10, 255- 276. Weres, O., Yee, A. and Tsao, L. (1981) Kinetics of silica polymerisation. J. Colloid and Interface Science 84, 379 - 402. Weres, O., Yee, A. and Tsao, L. (1982) Equations and type curve for predicting the polymerization of amorphous silica in geothermal brines. Soc. Petrol. Engrs J. (February), pp. 9 - 16.