PROCEEDINGS, Thirty-Ninth Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, February 24-26, 2014

SGP-TR-202

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Change in Permeability of Porous Medium in Silica Scaling Experiment under Different pH

and Suspended Particle Concentration

Loren Tusara, Ryuichi Itoi, Daisuke Fukuda and Yoshitaka Kawahara

Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, JAPAN

ltusara@kyudai.jp

Keywords: silica scaling, suspended particle, codeposition, permeability, pH

ABSTRACT

Geothermal fluid that is being reinjected back to the formation may contain traces of suspended particles in the fluidas it flows from

production wells through the pipes to reinjection wells. Buildup of solid deposits on the pore spaces of the formation reduces the

ease of flow of the geothermal fluid in the porous medium. In this study, bentonite, a common mud drilling material, was used as a

suspended particle in the silica scaling experiment. Different concentrations of bentonite (0, 0.1 and 1ppm) in synthetic silica

solution of 700ppm at 80oC under basic (pH8) and acidic (pH5.5) conditions were investigated to know how these factors affect the

permeability distribution of the porous medium. Result shows that in basic solution, the effect of bentonite concentration, however

in small concentrations, is significant. Experiment run with 1ppm bentonite concentration only lasted for less than a day before the

experiment was terminated due to high pressure in the porous column. Lowering the pH of the solution with the same 1ppm

bentonite concentration greatly improved the permeability distribution of the porous medium. It took five more days before the

permeability of that with low pH dropped to almost the same amount with that of higher pH value. The suspended particles may

have formed an aggregate with the monomeric silica producing larger particle or these suspended particles may have codeposited

with silica through time causing the pore spaces to be blocked by these formed solid particles.

1. INTRODUCTION

Silica deposition has been one of the major problems in reservoir engineering. Silica in geothermal brine that is saturated with

amorphous silica has a tendency to deposit as it is transported through the geothermal pipeline to reinjection wells and into the

formation. Analyses of the solid deposit formed in the pipeline and reservoir suggested that the deposit were not only composed of

silica deposit but also of other particles as components of the geothermal brine and/or reservoir or impurities from surface facilities.

Kuhn et al. (1998), for example, have done an onsite experiment of reinjecting brine into a clastic reservoir during the geothermal

exploitation and observed that the resulting decline in permeability of the reservoir was due to the deposition of small particles such

as fragments of feldspar and quartz in the porous medium. McLin et al. (2006) investigated the loss of reinjectivity of geothermal

fields in Coso and Salton Lake and found that layers of silica scales with minerals deposited in the soil structure. Tusara et al.

(2012) have shown that solid deposits formed in geothermal pipeline contained other particles which have contributed to the

increase in the amount of solid deposit on the surface. These particles may have also affected the silica deposition process in one

way or another.

Henceforth, this study investigated the effect of suspended particle in geothermal fluid to the silica deposition in porous media. In

this experiment, bentonite, a common mud drilling material and a common clay species in the Earth’s crust, is used as a

representative suspended material in the solution.

2. SILICA SCALING EXPERIMENT

2.1 Experimental Setup

Figure 1 shows the experimental setup for the silica scaling experiment. It consisted of two stock solutions containing the silica and

suspended particles in the form of bentonite powder, and the HCl acid to control the pH of the mixed solution. These stock

solutions were pre-prepared in the laboratory and were pumped separately at specific controlled flow rates through the porous

column. Control of the pumps and the log of the pressure readings were interfaced in a computer.

Table 1 shows the operation condition of the experiments. Different concentrations of the suspended material (0, 0.1 and 1ppm of

bentonite) at two varying pH values (pH8 and pH5.5) were investigated in this study.

The silica stock solution was prepared by dissolving sodium metasilicate with distilled water. The initial silica concentrations were

1400ppm and 2330ppm for pH8 and pH5.5, respectively. Different amount of silica and HCl solution were mixed for different case

due to the difficulty in controlling pH5.5. A 1:1 ratio of SiO2:HCl was implemented for pH8 and 3:7 for pH5.5. The HCl solutions

were then prepared at 0.0488N and 0.0359N, respectively. The resulting mixed solution has initial silica concentration of 700ppm

and has a saturation ratio of 1.5 with respect to the solubility of amorphous silica at 80oC (Weres et al., 1981). A commercial

bentonite was added in the silica stock solution at corresponding concentration values taking note of the proportion of silica and

HCl stock solution in the mixed solution.

The two stock solutions were being pumped through 1.5m tubes, which are submerged in the water bath at 80oC, before they get

mixed at the stirrer container. The stirrer ensured that the two solutions mixed well and that suspended particle remained in the

solution before it was transported through the porous column.

Tusara et al.

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The porous column was made up of five 5-cm glass tubes with 0.4cm diameter. Each glass tube was filled with quartz sand with

grain size of 30-50 mesh. The resulting porous column has an estimated porosity of 0.44. The porous column was also placed in the

water bath for the whole duration of the experiment to maintain the temperature of the solution at 80oC.

The experiment was made to run for 6 days or until the pressure difference in the porous column exceeds 3bar. An electronic

pressure sensor of pressure transmitter type XML P (OsiSense XM, Telemecanique) was used in this study. Control of the pumps

and the log of the pressure readings were interfaced in a computer using NI USB-6009 DAQ device (National Instruments).

Figure 1: Schematic diagram of the silica scaling

experiment setup

Table 1: Operation condition for the silica scaling

experiment

RUN No. Bentonite concentration (ppm) pH

RUN1 0.0 8.0

RUN2 0.1 8.0

RUN3 1.0 8.0

RUN4 0.0 5.5

RUN5 1.0 5.5

Figure 2 shows the experimental setup for another experiment conducted to examine how silica polymerizes in a solution pH8 at

80oC for two different suspended material concentrations. 200ml each of silica and HCl solutions were heated separately at 80oC

and were mixed at the start of the experiment. The temperature of the mixed solution is maintained at 80oC by submerging the

container in the same water bath for the duration of the experiment. Aliquots of the solution were collected at 0, 0.5, 1, 2, 5 and 10

minutes. The samples were analyzed of monomeric and total concentration using yellow-molybdate complex method. This same

experiment was carried out for the silica solution with 1ppm bentonite. Table 2 shows the operation condition for the batch

experiment.

Figure 2: Schematic diagram of the batch experiment

setup

Table 2: Operation condition for the batch experiment

Case No. Bentonite concentration (ppm)

Case 1 0.0

Case 2 1.0

2.2 Measurement

Flow rate of discharged solution at the outlet was measured regularly to make sure that constant flow rate is maintained in the

experiment. The pH of the solution was also measured at time interval to monitor any changes in the acidity of the solution.

The pressure was continuously logged throughout the duration of the experiment to monitor the change in permeability in the

porous column as solidsdeposit on the surface. Permeability, k,(m2) is calculated using Darcy’s Law.

)( p

x

A

Qk

(1)

whereQ is the volumetric flowrate (m3/s), A is the cross-sectional area of the porous column (m2), is the viscosity of the fluid (Pa-

s), and ∆x (m) is the length at which the pressure drop, ∆P (Pa), takes place.

At the end of each experiment, a 5-cm tube segment was dismantled and its permeability was measured by flowing distilled water

under a fixed pressure difference across the tube segment of porous medium. The measured discharged flow rate was noted for each

tube and the corresponding permeability was calculated using Eq.(1).

Discharged solution at the outlet was also collected at time interval for monomeric and total silica concentration analysis. Silica

concentrations were analyzed using the yellow-molybdatecomplex method using the Hitachi U-1800 Spectrophotometer.

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3. RESULTS AND DISCUSSION

3.1 Effect of Suspended Material Concentration

Figure 3: Permeability change with time

Figure 4: Permeability change with time

Figure 3 and Figure 4 show the result of permeability with time for solutions with pH8 and pH5.5, respectively. For both cases,

varying the suspended material concentration in the solution shows a significant difference in the average permeability of the

porous column for the whole duration of the experiment. At pH8, the permeability gradient increases as the concentration of

suspended material increases. Even for very small amount of suspended material, as in RUN2, a significant decrease in

permeability is already observed in the porous column compared to the result of RUN1. This change in permeability further

increased in RUN3. There is a sudden drop in the permeability of the porous column within 21 hours. It is worth to note that these

suspended material concentrations come in small amount of bentonite only. Yet, its effect in the permeability of the porous column

is drastic especially for solutions with higher concentration values. Similar result is observed in the permeability of the porous

column for pH5.5 under two different suspended material concentrations. The permeability in RUN5 dropped significantly

compared to that in RUN4 even under acidic condition.

Figure 5 shows the permeability of each tube segment of the porous column at the end of the experiment for RUNs 2, 3 and 4; all of

which contained different concentrations of suspended material in solutions with different pH. Measurement of permeability

distribution for RUNs 1 and 4 are not successful and thus, are not included here. The average permeability of the tube segments for

these RUNs did not match the average permeability of its porous column at the end of the experiment. At pH8, a small amount of

suspended material concentration gives a low permeability ratio along the porous column as in RUN2. Result also shows a slightly

increasing permeability from the inlet to the outlet of the porous column. Increasing the suspended material concentration in the

solution at the same solution pH as in RUN3 shows a total different result in the distribution of the permeability in the porous

column. It shows that the permeability drop is highest at the inlet. The following tube segments in the experiment have almost the

same permeability as in initial value. Result of RUN5, on the other hand, shows a similar result as in RUN2 but at lower

permeability values for all tube segments. A semi-homogeneous permeability distribution is observed for the same suspended

material concentration as in RUN3 but at pH5.5. This clearly suggests that the deposition mechanisms for pH8 and pH5.5 under the

presence of suspended materials are different.

Figure 5: Permeability distribution along the porous

column

Figure 6: Comparison of non-dimensional permeability

between pH8 and pH5.5. The lines indicate the

permeability gradient for each RUN

3.2 Effect of Solution pH

Figure 6 shows the comparison of permeability with time between RUNs 1 and 3, and RUNs 4 and 5. The difference in pH shows a

difference in the permeability gradient in the porous column. Comparing experiments with the same suspended material

concentration, it shows that the permeability of the porous column changes slowly for solution pH5.5. This shows that lowering the

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pH of the solution delays the decrease in permeability of the porous column either with or without suspended particles in the

solution.

3.3 Silica Concentration

Figure 7 shows silica concentrations in the effluent with time for 6 runs of experiment. Except for RUN3, the silica concentrations

of the solution after it flowed out of the porous column had little change in concentration compared to its initial value. It is thought

that only a portion of the saturated monomeric silica is reactive and hence, deposited in the porous column. The change in silica

concentration for RUN3 both for total (t-SiO2) and monomeric (m-SiO2) silica decreased significantly. This is consistent with its

result in permeability of the porous column with time as shown in Figure 3. The amount of silica that deposited at a short period of

time has easily blocked the porous medium causing a sudden drop in its permeability.

Figure 7: Silica concentration with time. m-SiO2 is the monomeric silica concentration; t-SiO2 is the total silica

concentration

3.4 Conceptual Model

Figure 8 shows the silica concentration with time for both cases. The solubility of silica at 80oC is indicated in the figure.

Figure 8: Silica concentration with time (Batch experiment). Case 1 does not contain bentonite; Case 2 has 1ppm bentonite

Estimating the time the silica and HCl solutions mix at the stirrer container and flow out of the porous column in Figure 1, it would

take around 5 minutes. This is the time at which the monomeric silica for Case 1 has started to decrease. The degree of saturation

with respect to monomeric silica for Case 2 has significantly dropped by this time. It shows that the induction time for the solution

at pH8 without suspended material is longer compared to that solution with suspended material at the same solution pH. At pH5.5,

the silica may only have longer induction time for both cases as polymerization rate of silica in acidic solution becomes slower

(Rothbaum et al., 1979).

The behavior of silica with and without the suspended material in the solution as it is transported through the porous column is

deduced according to the result of the batch experiment. This then suggests that monomeric silica and probably a portion of

polymerized silica deposit in the porous column of RUN1. RUN4, on the other hand, may have only monomeric silica to deposit in

the porous column. At low pH, the silica polymerizes slowly in the solution. On the other hand, several deposition mechanisms are

suggested to favor the results of RUNs 2, 3 and 5, where there is a presence of suspended material in the solution. Figure 9 shows a

schematic diagram of these proposed deposition mechanisms.

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Figure 9: Proposed deposition mechanisms

One of the deposition mechanisms that may happen in the silica scaling experiment is the independent deposition of monomeric

silica and suspended material. These are the silica monomers that do not attach to the suspended material while in the solution but

rather deposit independently on the quartz sand surface. Another deposition mechanism is the codepistion of monomeric silica onto

the suspended material forming a larger particle before depositing in the porous column. It could also be that silica polymers in the

solution attach to the suspended material prior to deposition of the formed larger particle on the surface depending if the condition

is favorable. The suspended material then acts as nuclei for silica deposition. Bohlmann et al. (1977) stressed that nucleation is an

important factor on scale formation. They said that the difficulty in reproducing spontaneous nucleation from brines which

produced heavy scaling when seeded is that these seeds often come naturally as component of natural brines; hence, spontaneous

nucleation from clear brines is unlikely. In this study, these seeds could be thought of as the suspended material added in the silica

solution where it caused a significant change in the permeability drop of the porous column in the experiments.

Figure 10: Proposed behavior of silica and suspended material under different pH values

Figure 10 shows a visual interpretation of how the silica and suspended material behaved under pH8 and pH5.5. Due to the less

reactive monomeric silica in pH5.5, the suspended material still becomes available even at the outlet of the porous column.

Deposition of these particles is then expected to occur throughout the porous column. This is consistent with the result of RUN5

where a semi-homogeneous and a higher permeability drop distribution were observed at the end of the experiment. For pH8, a

larger particle formed from monomeric silica or polymerized silica attaching to suspended material may be present in the solution

before it flows through the porous column. Depending on the concentration of the suspended material and the number of reactive

monomeric and polymerized silica, the particle size and distribution of the formed particle may vary. This difference in particle size

and distribution is manifested in the result of RUNs 2 and 3. Due to the abundance of suspended material in the solution at pH8 in

RUN3, the formed particles have immediately blocked the pore spaces near the inlet of the porous column. This is a phenomenon

similar to cake filtration wherein particles larger than the pore grain size easily get filtered near the inlet forming a cake inhibiting

further transport of particles in the porous medium (McDowell-Boyer et al., 1986). In RUN2, on the other hand, the formed

particles were still able to deposit along the porous column giving a larger permeability drop for all tube segments. This drop in

permeability, however, is lesser than that of RUN5 most likely due to lesser suspended material concentration in RUN2. Figure 11

illustrates how silica and suspended materials may have been distributed in the porous column for each experiment.

Figure 11: Conceptual deposition of silica and suspended material in the porous column for each experiment

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4. CONCLUSION

This study investigated the effect of suspended particle in the form of bentonite in the silica scaling experiment. Result showed that

increasing the concentration of the suspended particle lowers the permeability value of the porous column in the duration of the

experiment for both solution pH8 and pH5.5. Looking closely to the permeability along the porous column at the end of each

experiment run, a semi-homogeneous distribution of permeability has been observed for all experiments except for RUN3 where

the decrease in permeability was significant near the inlet of the porous column. Different behavior of silica and suspended material

has also been observed for different solution pH due to the less reactive monomeric silica in acidic solution. Comparing between

solutions with different pH values but the same suspended material concentration, it was observed that the permeability drop is

lesser for solutions with pH5.5. This suggests that acidifying the solution to pH5.5, however difficult to control, may still be an

alternative way to mitigate excess solid deposition in the porous medium even in the presence of suspended material.

Furthermore, two deposition mechanisms have been proposed in this study: (1) independent depostion of silica and suspended

particle and (2) codeposition of silica with suspended particle forming a larger particle prior to deposition onto the surface.

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