WO1997021633A1

WO1997021633A1 - Inhibiting deposition of dissolved silica from brine

Info

Publication number: WO1997021633A1
Application number: PCT/NZ1996/000136
Authority: WO
Inventors: Lewis George Bacon; Brian Stewart Carey; Kevin Laurie Brown; Edward Kazimierz Mroczek
Original assignee: Institute Of Geological & Nuclear Sciences Limited; Contact Energy Limited
Priority date: 1995-12-08
Filing date: 1996-12-09
Publication date: 1997-06-19
Also published as: AU1042797A

Abstract

The invention provides a method of inhibiting deposition of silica from a brine, especially a geothermal brine. The brine is cooled from a temperature above to a temperature below its silica saturation temperature at a rate such that no significant polymerization of the silica takes place during this cooling. The brine is then aged at a temperature or at temperatures below its saturation temperature to cause polymerization of the silica. The temperatuer to which the brine is initially cooled and the ageing temperature(s) are chosen so that the polymerization results in a substantially stable suspension of colloidal silica particles in the brine having a substantially stable particle size of less than about 20 nm.

Description

INHIBITING DEPOSITION OF DISSOLVED SILICA FROM BRINE

TECHNICAL FIELD The present invention is particularly concerned with inhibiting deposition of silica from geothermal brine, but is not limited to the treatment of silica-contaiiiing brines from geothermal sources.

BACKGROUND ART Geothermal resources are an attractive alternative energy source from economic and environmental points of view. However, geothermal power production in New Zealand, and in some other parts of the world, is almost entirely from water dominated hydrothermal systems. Therefore, large quantities of "waste" water are produced from the geothermal reservoir along with the steam required for conventional steam turbine power production. As geothermal power production becomes a mature technology, attention is turning to the sustainable management and more efficient use ofthe total resource. Thi_s is a consequence of both the realisation that geothermal reservoirs are finite resources, as well as increasing public pressure for environmentally sensitive development. Consequently, there has been a growing interest in the utilisation and treatment of the hot water fraction that is produced during geothermal reservoir exploitation. This water fraction is commonly called a brine because ofthe presence of various dissolved salts and other minerals.

There are three aspects to the utilisation of geothermal brine. The first is the use ofthe available heat energy and this can take the form of power production through steam turbines or binary cycle plants as well as through direct use of the heat in such applications as district heating, aquaculture, and horticulture, for example. The second aspect concerns the disposal of the geothermal brine in an environmentally acceptable manner, bearing in mind the mineral content of the brine. The third aspect relates to the extraction of minerals, including silica, from the brine. The present invention concerns the first two of these aspects.

Silica is the name by which silicon dioxide, Si0₂, is commonly known. One form of silica, the mineral quartz, is ubiquitous in rocks in the earth's crust. Indeed, silicon is the earth's second most abundant element after oxygen. Hot water in a geothermal reservoir is in contact with quartz and a chemical equilibrium is established where some of the quartz dissolves in the geothermal water where it generally exists as monomeric silicic acid, often simply called monomeric silica. The exact amount that dissolves in the water is almost entirely a function ofthe temperature of he water; the hotter the water, the more quartz that is dissolved in it. The relationship between temperature and the resulting concentration of dissolved quartz, or silica, has been measured in the laboratory and is well known.

When geothermal fluid is brought to the surface for power production, the brine contains an amount of monomeric silica dependent on the temperature of the geothermal reservoir. As the temperature of the brine is lowered during the separation of steam for power production, the concentration of monomeric silica becomes greater than the equilibrium with quartz would allow. However, at temperatures below about 250°C, monomeric silica is in kinetic equiUbrium, not with quartz, but with another type of silica called amorphous silica. Under this new equilibrium, the solubility of silica with respect to amorphous silica is greater than with respect to quartz. When the temperature of the brine is lowered sufficiently it becomes oversarurated with respect to amoφhous silica. Under these circumstances, solid silica would normally be expected to precipitate from the brine. However, silica is unusual, in that the individual molecules of silica, as monomeric silica, join together in a process of polymerization to form nuclei that then grow by direct molecular deposition of silica to form colloidal silica particles and hence a colloidal suspension.

The individual colloidal silica particles are small in size, ranging from less than 10 nm (nanometre) to greater than 5000 nm. These particles can remain suspended in solution for some time. However, under certain circumstances, colloidal silica can deposit very rapidly. Deposition, a process which includes aggregation of individual colloidal particles and their agglomeration to form larger chains and clusters of particles can lead to silica scaling, that being the depositing of a coating of silica on surfaces with which the brine is in contact. Monomeric silica may also deposit directly on such surfaces. The exact conditions required for deposition are not fully understood, but many factors are involved. These include such parameters as particle size, particle density, fluid flow conditions, temperature, pH and the presence of other ions.

Silica scaling is one of the most persistent problems facing geothermal development.

It causes blocking of waste water pipes and can detrimentally affect the injectivity of injection wells whereby waste brine is reinjected back into the ground. Furthermore, it can preclude the use of heat exchangers for low grade heat use, and can hamper the use of binary cycle power plants. One way of dealing with the silica problem is to precipitate the silica from the waste brine into an open accessible drain or pond which can be mechanically cleaned. However, the resulting waste brine still presents a problem in that its discharge into surface waterways, such as rivers, is not now environmentally acceptable because ofthe presence of environmentally sensitive chemicals such as arsenic in the brine. Current environmental philosophy encourages the reinjection ofthe waste geothermal brine back into sub-surface aquifers in the ground.

However, the disposal of waste geothermal brine by reinjection into the ground presents its own problems. In particular, the development and full utilisation of energy from liquid dominated geothermal fields will always be constrained by the need to avoid disposing of waste brine saturated with silica. The major concern is silica fouling in plant, in transmission pipelines, and in injection wells and ground formations. The easiest method for avoiding silica deposition is to maintain the silica concentration below the amoφhous silica solubility. However, that means maintaining the temperature of the waste brine above the temperature at which the brine would be saturated with monomeric silica and this can be very wasteful of the available heat energy. For example, at

Wairakei, reinjection would have to be at a temperature of above 130°C, and at some of the high temperature New Zealand geothermal resources reinjection would have to be at temperatures around 200°C or higher.

If reinjection could take place at lower temperatures then more heat energy would be available for utilisation. Reinjection at lower temperatures, for example, ambient temperatures, can in the absence of treatment ofthe brine suffer from problems caused by silica scaling. Agglomerated silica particles in the brine can and do cause plugging and blockage of underground formations which is made worse by the subsequent deposition of monomer onto those particles. A sufficient fouling of injection wells means that they have to be replaced by new injection wells, that adding to the cost of disposing of the waste brine. The life of injection wells may be extended by back flushing or acid treatment of them to clear some ofthe blockages in the surrounding subterranean structure but that also adds costs and only delays the need for new injection wells.

Chemical treatment of waste geothermal brine can reduce silica scaling problems but introduces other kinds of problems. For example, a sufficient acidification of the brine can reduce the silica scaling problem but the greater acidity of the brine causes greater corrosion problems in plant and pipelines unless either more expensive corrosion-resistant materials or corrosion inhibitors are used. Furthermore, acidic brines reinjected into the ground could facilitate the formation of clays which can be just as effective in blocking the underground aquifers. The cost of the chemicals used to treat the brine is another unwanted expense.

DISCLOSURE OF INVENTION

It was with problems such as those mentioned above in mind that the present invention was devised. In particular, it is an object of the present invention to provide a method of irihibiting deposition of silica from geothermal brine at a relatively low temperature where the brine is oversaturated with silica, thereby allowing a greater utilisation of the available heat energy while avoiding or minimising silica scaling problems in plant and injection wells. The method does not preclude the use of chemicals to assist in avoiding or minimising silica deposition but the method is not primarily reliant on the use of such chemicals.

The present invention broadly consists in a method of inhibiting deposition of silica from a brine which has a total silica concentration such that the brine is oversaturated with respect to the amoφhous silica solubility at the temperature of the brine, the method comprising the steps of: cooling the brine from a temperature above its saturation temperature to a temperature below its saturation temperature at a rate of cooling such that no significant polymerization takes place during this cooling step; and ageing the brine at a temperature or at temperatures below its saturation temperature such that the silica polymerizes to form a substantially stable suspension of colloidal silica particles in the brine which particles have a substantially stable particle size of less than about 20 nm.

The brine is preferably a geothermal brine.

The amoφhous silica saturation temperature of the brine is preferably greater than 100°C.

The brine is preferably cooled to a temperature at which the amoφhous silica solubility is less than 25% of the total silica concentration.

The brine is preferably maintained at, and is therefore aged at, about the temperature to which it has been cooled by the cooling step. In any case, the brine is preferably aged until polymerization of the silica has substantially ceased. The substantially stable particle size of the colloidal silica particles suspended in the brine is preferably less than about 15 nm, and is more preferably less than about 10 nm

MODES FOR CARRYING OUT THE INVENTION As has been menftoned, various factors have some influence over the silica deposition rate One of the most important factors which has guided the direction of research mto inhibiting deposition of monomeric silica from geothermal brine has been the general concept that the lower the temperature and therefore the more oversaturated the waste brine, the higher the deposition rate and the greater the silica foulmg and scalmg problems. The applicant has found that for geothermal brine from Wairakei this generalisation is correct down to temperatures of about 70°C but at temperatures below that range the deposition rate decreases For temperatures below about 40°C and particularly below about 30-35°C the deposition rate is negligible The reasons are thought to be firstly, that below a certain temperature the reaction kinetics leading to deposition decrease with decreasing temperature and secondly, that silica forms colloids in solution While the formation of colloidal silica m oversaturated solutions has been known for some time there are two factors which have not been well understood and which have not been apphed previously in lnhibinng deposition of monomeπc silica from geothermal brine

The first of these factors is that the final size of the colloidal particles is of critical importance in that the smaller they are the more likely they are to stay suspended and not contribute to the deposition process

The second factor is that the rate of cooling appears to be a most important factor m determining final particle size and hence has important implications m terms ofthe use and disposition of waste brine at low temperatures by reinjection mto the ground without silica deposition Large agglomerated silica particles formed at very low rates of cooling, including natural cooling, have, as already indicated, the potential to plug rock fissures and pores The formation of many small colloidal silica particles rather than fewer largei agglomerated particles should considerably reduce that problem Furthermore, the formation of colloidal silica reduces the concentration of the monomeπc silica which under certain conditions can be responsible for substantial scaling The more silica colloid that can be formed therefore, the lower will be the monomer concentration and hence the deposition potential.

In comparing a slower cooling ofthe bnne with a more rapid cooling of the brine from above its saturation temperature to a temperature below the saturation temperature, the slower cooling leads to the formation of fewer nuclei which then grow to form larger particles. More rapid cooling ofthe brine leads to the formation of more nuclei which do grow but to form smaller particles. In either case, where the monomeric silica concentration drops the growth of the particles slows. The applicant has found that a sufficiently rapid cooling of the brine does not give sufficient time for a significant number of nuclei to form during the cooling step. Most ofthe nuclei therefore form at the temperature to which the brine has been initially cooled, and is preferably being aged, the number of nuclei being formed being dependent on that temperature. This temperature to which the brine is initially cooled may be called the nucleation temperature.

Generally, the greater the degree of oversaturation the greater the number of nucleation sites that form, either homogenously or heterogeneously, in the brine. For any particular concentration of monomeric silica in brine, the lower the nucleation temperature, the greater the degree of oversaturation and hence the greater the number of nuclei that form, this leading to the formation of many small colloidal silica particles which, as has been indicated, remain of a substantially stable small particle size when the temperature of the brine is sufficiently low.

According to the present invention, the rapid cooling of the geothermal brine from above its saturation temperature to a nucleation temperature below the saturation temperature induces nucleation of colloidal silica particles, but the cooling rate used is such that no significant polymerization of the monomeric silica takes place during the cooling step. However, it is desirable that colloid formation be completed before disposal ofthe treated brine. For that puφose, the cooled brine is aged at a temperature below its saturation temperature such that the silica polymerizes to form a substantially stable suspension of colloidal silica particles in the brine.

The applicant has found that in order to form a substantially stable suspension of colloidal silica particles in the brine, the particles should have a substantially stable particle size of less than about 20 nm, preferably less than about 15 nm, and more preferably less than about 10 nm. For different compositions of geothermal brine, for example, geothermal brines from different regions, there may be different cooling rates and/or different nucleation temperatures required to achieve those particular colloidal silica particle sizes. However, in any particular case, it is a simple matter of experimentation to determine what is an appropriate cooling rate and what is an appropriate nucleation temperature. The nucleation temperature to which the brine is cooled is preferably such that the amoφhous silica solubility is less than 25% ofthe total silica concentration. The expression "cooling rate" as used in this specification includes "cooling rates" because it is possible for the one brine to be cooled at more than one cooling rate during the course ofthe cooling process.

The ageing of the brine usually takes place at about the nucleation temperature to which the brine has been cooled by the cooling step. Furthermore, this ageing is usually performed for a period of time sufficient for substantially complete colloid formation. However, the brine may be aged at different temperatures, either higher or lower, and usually lower, than the nucleation temperature. For example, the brine may be cooled at a progressively decreasing cooling rate throughout some or all of the ageing step. The temperature or temperatures at which the brine is aged will affect the time required to complete colloid formation. Again, for different compositions of geothermal brine, simple experimentation can determine what are appropriate ageing temperatures and times.

A typical analysis of Wairakei geothermal brine is as follows:

Silica 550 ppm (parts per million)

Sodium 1100 ppm

Potassium 140 ppm

Calcium 30 ppm

Magnesium 0.1 ppm

Rubidium 2 ppm

Cesium 2 ppm

Arsenic 4.5 ppm

Chloride 1900 ppm

Bicarbonate <5

Boron 25 pH 8.4

With a dissolved silica content of about 550 ppm the Wairakei brine is oversaturated at temperatures below about 140°C. Rapid cooling of the brine to temperatures of less than 40°C and then ageing the brine at temperatures of less than 40°C produces colloidal silica particles of less than about 20 nm in size and can produce particles of less than about 15 nm in size depending on the nucleation and the ageing temperatures used. Rapid cooling to ambient temperatures and ageing at ambient temperatures for 2.5-3 hours produces colloidal silica particles of less than about 10 nm in size. These particles, and particularly those less than 10 nm in size, are stable for a considerable period of time, that is, they tend not to agglomerate and their small size means that they are not as prone to deposition, particularly in flowing systems, and are not of a size where the plugging of the porous ground matrix is significant. After such cooling of the Wairakei brine, the monomeric silica concentration reduces to approximately 200 ppm, that considerably reducing the tendency for direct monomeric silica deposition. In contrast, where geothermal brine, initially at high temperatures, is ponded and allowed to cool at a naturally slow rate the silica particles formed are large agglomerated particles which are prone to deposition in plant and pipelines and to plugging of injection wells and ground formations.

In one experiment applicable to systems where the Reynolds number is between 10,000 and 20,000, Wairakei geothermal brine was cooled to ambient temperatures within about 2 minutes by use of a plate heat exchanger provided with cooling water at a temperature of 15°C followed by ageing ofthe cooled brine for about 2.5 hours. This experiment also produced a substantially stable suspension of colloidal silica particles having a substantially stable size of less than 10 nm in diameter. The particle sizes were measured using a Leeds and Northrup Microtrac UPA Particle Size Analyser.

Silica deposition trials carried out at Wairakei at various temperatures gave th following results:

Trial 1 (a comparative example)

BRINE AGE SILICA SILICA PIPE DIAM. REYNOLDS DEPOSITION NATURE OF

TEMP. Cc) (minutes) (monomer, ppm) (total, ppm) (mm) No. (mm/ r) DEPOSIT

99 9 517 532 21 112000 0.65

97 40 512 538 21 112000 2 SOFT

95 84 512 531 21 112000 6 SOFT

93 118 512 533 21 100000 25 HARD

93 118 512 533 15 70000 10 HARDER

91 143 512 532 40 56000 21 VERY SOFT

91 143 512 532 21 106000 15 VERY HARD

Trial 2 (a comparative example)

BRINE AGE SILICA SILICA PIPE DIAM. REYNOLDS DEPOSITION NATURE OF

TEMP. (*c) (minutes) (monomer, ppm) (total, ppm) (mm) No. (mm yr) DEPOSIT

78 8 517 542 21 95000 7.8 VERY HARD

77 35 509 521 21 95000 10.4 VERY HARD

76 71 467 525 21 95000 5.8 HARD

75 96 408 524 21 95000 5.2 SOFT

74 120 385 529 21 95000 4.5 SOFT

Trial 3 (an example according to the invention)

BRINE AGE SILICA SILICA PIPE DIAM. REYNOLDS DEPOSITION NATURE OF

TEMP. Cc) (minutes) (monomer, ppm) (total, ppm) (mm) No. (mm/yr) DEPOSIT

38 18 350 511 15 15000 2 SOFT

37 57 281 509 15 15000 2 SOFT

36 80 249 510 15 15000 2 SOFT

35 121 226 510 15 15000 <1 SOFT

34 147 206 509 15 15000 NIL

In these trials brine was fed into a baffled tank at various initial temperatures an allowed to age at slowly cooling temperatures thereby allowing silica polymerization t proceed if conditions were favourable. Silica deposition rates were measured in pipe leading from various sections of the baffled tank. In this way deposition rates could b determined from brine of various ages and in which silica polymerization had proceede to varying extents.

Trial 1 shows that there was no polymerization at the initial temperature of 99°C an within the 143 minutes ageing time and any deposition which has occurred must b monomer rather than colloidal silica. The small differences shown in the table between the monomer and the total silica are due to differences in analytical methods rather than indicating that a proportion of the monomer has polymerised to form a colloidal silica fraction.

In Trial 2 the initial or nucleation temperature was 78°C. Over the 120 minutes ageing time there was a significant drop in the monomer concentration, the difference shown in the table between the monomer concentration and the total silica concentration representing polymerization of monomers to form a significant colloidal silica fraction.

In Trial 3 the initial or nucleation temperature to which the brine was rapidly cooled was 38°C. Over the 147 minutes ageing time, and even after 121 minutes ageing time, there was an even greater drop in the monomer concentration than in the case of Trial 2, this indicating the formation of an even greater colloidal silica fraction.

The results show generally high deposition rates where nucleation and ageing temperatures are high, that is, in the case of Trials 1 and 2; but not in the case of Trial 3.

Although particle sizes were not measured during this work, it has been determined from work carried out subsequently that silica colloid particle sizes are a function of nucleation temperature and subsequent ageing temperature, ln these subsequent trials it has been determined that particle sizes at 78°C (cf. Trial 2) and 38°C (cf. Trial 3) were approximately 65 and 20 nm respectively. This factor alone is thought to have a substantial effect upon the deposition rates of colloidal silica.

In trials undertaken at Wairakei the effect of other ions in solution has little effect on particle size. In geothermal brines where the concentration of, for example, divalent ions such as Fe, Ca and Mg are higher than those at Wairakei, then the agglomeration of colloidal particles following their formation is likely to have a substantial effect upon silica deposition rates.

The effect of brine pH will also have an effect on silica polymerization rates and deposition rates.

It will therefore be appreciated that different circumstances may apply to different geothermal brines where there will invariably be differences in the temperature of the extracted brine, in the concentration of the silica and hence the saturation temperature of the brine, in the presence of other minerals and in other factors. However, as already indicated, simple testing can be used to determine what cooling rate, what nucleation temperature and what ageing time and temperature(s) are appropriate for any particular brine.

The present invention therefore provides a relatively straightforward method for inhibiting deposition of dissolved silica from geothermal brine. The method does not necessarily require the use of chemical treatment of the brine and does not necessarily require large expenditure on plant in order to carry out the method.

While the invention has particular application to the treatment of geothermal brines, it will be apparent that it can be applied to the treatment of silica-containing brines from other sources.

Claims

1. A method of inhibiting deposition of silica from a brine which has a total silica concentration such that the brine is oversaturated with respect to the amoφhous silica solubility at the temperature of the brine, the method comprising the steps of: cooling the brine from a temperature above its saturation temperature to a temperature below its saturation temperature at a rate of cooling such that no significant polymerization takes place during this cooling step; and ageing the brine at a temperature or at temperatures below its saturation temperature such that the silica polymerizes to form a substantially stable suspension of colloidal silica particles in the brine which particles have a substantially stable particle size of less than about 20 nm.

2. A method according to claim 1 wherein the brine is a geothermal brine.

3. A method according to claim 1 or claim 2 wherein the amoφhous silica saturation temperature ofthe brine is greater than 100°C.

4. A method according to any one of the preceding claims wherein the brine is cooled to a temperature at which the amoφhous silica solubility is less than 25% of the total silica concentration.

5. A method according to any one of the preceding claims wherein the brine is maintained at, and is therefore aged at, about the temperature to which it has been cooled by the cooling step.

6. A method according to any one ofthe preceding claims wherein the brine is aged until polymerization of the silica has substantially ceased.

7. A method according to any one ofthe preceding claims wherein the substantially stable particle size ofthe colloidal silica particles suspended in the brine is less than about 15 nm.

8. A method according to claim 7 wherein the substantially stable particle size of the colloidal silica particles suspended in the brine is less than about 10 nm.

9. A method according to claim 1 and substantially as herein described with reference to any embodiment disclosed.

10. A brine containing a substantially stable suspension of colloidal silica particles having a substantially stable particle size when produced by the method of any one ofthe preceding claims.