PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, February 11-13, 2013

SGP-TR-198

IMPACTS OF ROCK-BRINE INTERACTIONS ON SANDSTONE PROPERTIES IN LOWER

MIOCENE SEDIMENTS, SOUTHWEST LOUISIANA

Masoud Safari-Zanjania, Christopher D. White

a, Jeffrey S. Hanor

b

Louisiana State University aCraft & Hawkins Department of Petroleum Engineering

bDepartment of Geology and Geophysics

2107 Patrick F. Taylor Hall

Baton Rouge, Louisiana, 70803, United States

E-mail: msafar1@lsu.edu

ABSTRACT

Reinjection of cooled geothermal fluid is an essential

part of geothermal reservoir management, and has

been discussed in many recent reservoir studies.

Geothermal fluid reinjection can improve heat

recovery and maintain pressure. Reinjection may

have unfavorable consequences, such as calcite and

silica scaling in reservoir and injection facilities.

However, the impact of brine-rock interactions on

reservoir properties has not been addressed as fully

for reinjection. In this paper, interactions between

geothermal fluid and reservoir rocks in the West

Hackberry field, Cameron Parish, Louisiana are

examined using geochemical modeling. The

Hackberry Field comprises Miocene sediments on the

flank of a salt dome. These sandstones are variably

cemented with calcite, pyrite, and pyrrhotite. The

field is 1.4-2.1 km deep, with initial temperature and

pressures ranging from 65 to 75 °C and 15 to 27

MPa, respectively. The brine is cooled down to the

estimated output temperature of power plant and re-

equilibrated. Then, reinjected brine-rock interactions

with declining reservoir temperature are simulated,

and the effects on reservoir properties like porosity

and permeability are investigated. Geochemical

reactions between different sampled brine and

reservoir rock compositions have been modeled as

the reservoir is chilled.

Keywords. Geochemical model, geothermal reservoir,

rock-brine interaction, geothermal fluid

reinjection, West Hackberry Field

INTRODUCTION

The transition from fossil fuels to renewable

sources seems mandatory for world’s future

economy. High price and political issues

accompanied by crude oil and also environmental

pollution and climate instability caused by fossil fuels

such as oil, coal and natural gas, have attracted the

considerations of energy industry toward renewable

sources like wind, solar, and geothermal energy.

Almost thirty percent of the world’s current

producing electricity by geothermal power plants is

concentrated in the United States; its 3,187 MW of

installed geothermal capacity is more than any other

country in the world. The majority of geothermal

power plants in the US are located in California and

Nevada. There are also power plants in Hawaii, Utah,

Idaho, Alaska, Oregon and Wyoming. More than 140

projects in 15 states comprise one-third of the land

area of the US. Over 5,000 MW of power potential

was identified under development by April 2012.

(International Market Overview Report, GEA, 2012).

Louisiana is one of the states that recently have

started to develop geothermal source of energy.

Independence of power plant to providing fuel and

production reliability has made the geothermal power

plants as an appealing source of energy to provide

emergency electricity in the time of hurricanes.

Heat flowing form the earth’s core and mantle

and from radioactive isotopes decaying in the earth’s

crust is the energy source for geothermal power

projects. Water adsorbs heat from the rock and

transports it to the earth’s surface, where using

turbines and generators heat energy has been

converted to electrical energy. Moreover, direct

applications of geothermal fluids, which decrease the

need for electricity production and burning of fossil

fuels, have gained importance over the years

Geothermal often cannot compete with fossil

fuels commercially. The cost of drilling enough wells

to supply full plant capacity is almost equivalent to

purchasing most of the fuel required for the next 20

years in a fossil-fired plant (Gallup, 2009). On the

other hand, after operating the geothermal power

plant the costs will be mostly the maintenance

expenses. Therefore, a geothermal power plant has to

work reliable for a long period of time to be

profitable.

The experiences of high pressure drop in the

reservoir and high annual steam decline rates in

2

excess of 25% in some areas of the Geysers steam

field, California, have shown the importance of

reservoir management in the geothermal fields

(Goyal and Conant, 2010). Reinjection of cooled

geothermal fluid not only is an environmental

prerequisite for disposal of the geo fluid, but it also

can maintain reservoir pressure and improves heat

recovery from matrix rocks (Ungemach, 2003).

In the case of high salinity geo fluid, corrosion

and scale forming may cause serious problems in

production and surface facilities and also reinjection

process. The Salton Sea geothermal field in southern

California (USA) is a well-known field for its hyper-

saline, 200,000–300,000 mg/L total dissolved solids,

brines (Gallup, 2009). In this field, corrosion problem

has been successfully controlled by materials

engineers. Also, new technologies created by

production engineering and chemistry efforts like

crystallizer–clarifier and brine acidification, has

effectively solved scale forming problem (Gallup,

2009).

Although, considerable efforts have been done to

solve the scaling problem in production and surface

facilities, the impact of scaling and rock-brine

interactions on reservoir properties has not been

addressed as fully for reinjection. In this paper,

interactions between geothermal fluid and reservoir

rocks in the West Hackberry field, Cameron Parish,

Louisiana are examined using geochemical modeling.

GEOLOGIC SETTING

The West Hackberry salt dome is located in

north-central Cameron Parish, southwest Louisiana

and constitutes the western half of a larger, 16-km-

long salt ridge. The dome is an elongate shallow

piercement dome which intrudes Tertiary and older

sediments (McManus and Hanor, 1988).

Ground elevation in the area is a few meters

above mean sea level (m.s.l). Caprock consisting of

calcite, anhydrite and pyrite (Howe and McQuirt,

1935) is developed on top of the dome and extends to

an elevation of -600 m m.s.l. The top of salt occurs at

an elevation of -700 m m.s.l. (McManus and Hanor,

1988). Mineralogical analyses of the salt stock

indicate that it is composed of 95 percent halite, 4

percent anhydrite, and

3

Table 1: The brine compositions that have been used

in the models, all concentrations are in

mg/kg.

Brine number

1 2 3

Specific gravity (gr/cm3) 1.192 1.161 1.127

pH 4.80 6.35 5.80

CO2 1,258 407 279

HCO3- 49 569 110

Ca2+

6,208 208 10,470

Mg2+

1,023 53 1109

Na+ 84,814 82,394 49,500

K+ 857 832 861

Cl- 145,973 128,488 99,379

SO42-

159 39 1

Fe2+

0 3 0

Table 2: The rock compositions that have been used

in the models by mass percentage.

Plug number

1 2 3

Sampling depth (m) 1,524 1,835 2,134

Quartz 84 35 31

K-feldspar 0 3 11

Plagioclase 13 1 9

Calcite 3 53 12

Pyrite 0 4 0

Siderite 0 0 2

Analcime 0 0 4

Kaolinite 0 0.6 2

Illite 0 0.8 5

Smectite 0 2.6 24

Modeling process

The rock-brine reactions have been modeled

with titration paths. In titration reaction paths, the

program repeatedly adds a small aliquot of reactants

and then recalculates the system’s equilibrium state

as it steps forward in reaction progress (Bethke and

Yeakel, 2012).

The initial and final temperatures of geofluid

have been assumed to be 150 and 25 °C, respectively.

Although, the highest measured temperature in

studied area was 75 °C, for generalization purposes

and compatibility with required brine temperature for

binary geothermal power plants, the final temperature

has been assumed to be 150 °C. First, the brine has

been cooled down to 25 °C. Then, the earliest

geochemical reaction between brine and rock has

been modeled at reservoir temperature; with brine

temperature rising from 25 to 150 °C. Next, reaction

steps were modeled using a constant temperature of

150 °C.

Rock-brine geochemical reactions have been

modeled in two different ways; in the first case, after

the earliest rock-brine interaction, the resulting

minerals are separated and next reaction step happens

between the minerals resulting from last reaction step

and initial brine composition. In the second case,

after the earliest rock-brine interaction, the resulting

fluid is separated and next reaction step happens

between the brine resulting from last reaction step

and the initial rock composition. Figure 2 shows a

schematic drawing for these two cases of modeling.

In all cases, reactions have been modeled for several

steps, until the results for total minerals in the system

show the same trend for two consecutive steps.

Figure 2. Schematic drawing shows two different

cases of modeling.

RESULTS

The authors are aware that some of the mineral

products generated in the Geochemist's Workbench

brine-rock simulations (including muscovite and the

Mg-sheet silicates) are usually associated with high-

temperature hydrothermal or low-grade metamorphic

conditions rather than sedimentary diagenetic

settings. However, these phases can probably be

considered as proxies for more typical diagenetic

products, such as illite and other diagenetic sheet

silicates.

The total amount of minerals in the system

versus reaction progress is considered for brine Brine

1 Plug 3, Figures 3 and 4, for the first and second

reaction steps, respectively. In this Plug, the amount

of smectite is relatively abundant. In the first

reaction, the total amount of quartz, albite,

muscovite, calcite and annite are increased

throughout the reaction. These total amounts include

both the initial concentration of minerals which are

entered to the system and minerals which are

4

produced during the reaction. Reaction progress is a

normalized way to show the reaction development. In

the horizontal axis of Figures 3 and 4, 0 is the start

point of the reaction and 1 shows the completion of

the reaction. The program divides this progress into

100 steps and introduces a small fraction of reactants

to the system in each step.

Figure 3: First brine-rock reaction for Brine 1 Plug 3.

For the next reaction, the fluid part of the system

is separated and reacted with new rock composition.

In the second reaction, quartz, muscovite, albite, and

calcite are the minerals with the highest

concentrations. The concentration of pyrite in both

reactions is increased in the first moments of the

reaction and stays constant for the rest of the reaction

path.

Figure 4: Second brine-rock reaction for Brine 1

Plug 3.

Four consecutive reactions between Brine 3 and

rock composition at depth of 1835 m (Plug 2) were

investigated (Figs. 5-8). At this depth calcite with 53

percent, is aboundant and its amount is more than

quartz with 35 mass percent.

In these models, for each reaction, the resulting

fluid from pervious reaction is separated and reacts

with the initial rock composition. In the first reaction,

Figure 5, calcite, quartz, muscovite, pyrite and

saponite-Ca continuously increase during the reaction

path. Calcite and quartz have the highest amounts in

comparison to other minerals.

Figure 5: First brine-rock reaction for Brine 3 Plug 2.

In the second reaction, Figure 6, in addition to

previous minerals, the system has annite. In the next

two reaction steps, Figures 7 and 8, the amount of

minerals versus reaction progress have the same

trend. Calcite and quartz are the minerals with the

highest concentrations.

Figure 6: Second brine-rock reaction for Brine 3

Plug 2.

For comparision purposes, four consecutive

reactions between Brine 3 and rock composition at

depth of 2134 m (Plug 3) were investigated (Figs. 9-

12). In these models, for each reaction, the resulting

minerals from the previous reaction are separated and

react with the initial fluid composition.

5

Figure 7:Third brine-rock reaction for Brine 3 Plug 2.

Figure 8: Fourth brine-rock reaction for Brine 3

Plug 2.

Figure 9: First brine-rock reaction for Brine 3 Plug 3.

Almost the same trend can be seen in all graphs.

Quartz, muscovite, calcite, saponite, daphnite,

nontronite and dolomite are the dominant products in

the system and the amounts of these minerals

increase during the reaction path. Pyrite also is

produced during the reactions; however, its amount,

around 1 mg/kg, is relatively small.

Figure 10: Second brine-rock reaction for Brine 3

Plug 3.

Figure 11: Third brine-rock reaction for Brine 3

Plug 3.

Figure 12: Fourth brine-rock reaction for Brine 3

Plug 3.

DISCUSSION

Reinjection of cooled geothermal brine is

recognized as an effective way to maintain pressure

and improve heat recovery. Scaling problems caused

by calcite and quartz precipitation is a prevalent

6

disaster which happens in geothermal power plants

and reinjection facilities with high salinity geofluids.

Some research has been done on solving scale

problems in surface facilities, but the impact of

precipitation and scaling on reservoir properties has

not been addressed as fully. In this research the

impact of rock-brine interactions on reservoir

properties has been investigated.

Three brine compositions from West Hackberry

field, Louisiana have been used to model rock-brine

interactions. Cl-, Na

+ and Ca

+ are the species with

highest amount in the brine samples. Brines are

somewhat acidic with pHs equal to 4.80, 6.35 and

5.80, respectively.

Dissolution and precipitation

Three rock samples from the same field have

been used for modeling purposes. Samples were

taken from different depths of 1524, 1835 and 2134

m (McManus, 1991). At the lowest depth, the rock

consists mostly of quartz. Plagioclase and calcite, at

13 and 3 percent respectively, are the other minerals

in the rock. At the depth of 1835 m (Plug 2), the

amount of calcite is considerable. At this depth the

amount of calcite (53 percent) is higher than quartz

(35 percent). Low concentrations of clay minerals are

found at this depth. At the depth of 2134 m (Plug 3),

quartz and calcite, at 31 and 12 percent respectively,

are the main minerals in the rock. Clay minerals at 31

percent make considerable portion of the rock.

Smectite with 24 percent is the main clay mineral.

Almost in all cases that have been investigated in

this paper, quartz and calcite were the two important

minerals which were produced in most rock-brine

interaction model runs.

Quartz is one of the most abundant minerals and

occurs as an essential constituent of many igneous,

sedimentary and metamorphic rocks. The

composition of quartz is normally very close to one

hundred percent SiO2. Quartz is one of the most

stable minerals and it is resistant chemically to most

attacking solutions (Deer et al., 1966).

Calcite, like most carbonates, will dissolve in

most forms of acid. Calcite can be either dissolved by

groundwater or precipitated by groundwater,

depending on several factors, including the water

temperature, pH, and dissolved ion concentrations.

The solubility of calcite in water increases with

increasing partial pressure of CO2 and with

decreasing temperature (Deer et al., 1966). The

dissolution or precipitation kinetics of carbonates is

very fast compared to those of silicates. Therefore,

carbonate phases play an important role in the

evolution of reservoir porosity (Fritz et al., 2010).

In a water injection project the possibility exists

that suspended solids will cause the injection wells to

become impaired (Barkman and Davidson, 1972). In

geothermal reservoirs with high salinity geofluids,

the possibility of forming solid particles in a power

plant’s cooled brine outflow is high. Suspended

particles are the main source of damage to wells and

formations (Ungemach, 2003). With similar

mechanisms, these solid particles can cause clogging

inside the reservoir, especially in vicinity of

reinjection wells.

In addition to solid particles present in reinjected

geofluid, new reactions between brine and reservoir

rocks may produce a considerable amount of solid

particles over the geothermal power plant life time

scale. This is particularly true when dealing with tight

and fine-grained reservoirs and high salinity brines.

On the other hand, dissolution of minerals and

porosity increases are also possible.

In modeling consecutive reactions, the

continuous brine flow into rocks has been simulated.

As it can be seen in Figures 5 through 12, after

second reaction, graphs almost have similar trends.

Apparently, as long as the other conditions of

reaction do not change, considerable changes in

mineral concentrations happen in first reaction steps.

Changes of fluid compositions as a result of brine-

rock reaction in different steps of reaction for Brine 3

Plug 2 and the separating fluid for next reaction (Fig.

13) and Brine 3 Plug 3 and the separating minerals

for next reaction (Fig. 14) show the same result.

Figure 13:Changes of fluid compositions in reactions

of Brine 3 Plug 2.

Figure 14:Changes of fluid compositions in reactions

of Brine 3 Plug 3.

0.001

0.01

0.1

1

10

100

1000

10000

100000

1000000

1 2 3 4 5

Reaction steps

Ele

men

tal

com

po

siti

on

(m

g/k

g)

Aluminum

Calcium

Carbon

Chlorine

Hydrogen

Iron

Magnesium

Oxygen

Potassium

Silicon

Sodium

Sulfur

0.001

0.01

0.1

1

10

100

1000

10000

100000

1000000

1 2 3 4 5

Reaction steps

Ele

men

tal

com

po

siti

on

(m

g/k

g)

Aluminum

Calcium

Carbon

Chlorine

Hydrogen

Iron

Magnesium

Oxygen

Potassium

Silicon

Sodium

Sulfur

http://en.wikipedia.org/wiki/Ion

7

As it can be seen in these Figures, the most

significant changes happen between step 1 (the first

interaction between fluid and rock) and step 2.

Although, some changes can be seen in later steps,

potassium decrease in step 2 of Figure 14, in last two

steps there are almost no changes in the amount of

elemental compositions in fluid.

Principal reaction products

Results from the case study show the production

of quartz and calcite in most rock-brine interactions

(Figs. 15 and 16). The ratios of total volume of quartz

after reaction completion to initial volume of quartz

in both Brine 3 Plug 2 and Brine 3 Plug 3 in all

reaction steps are greater than 1; quartz precipitates.

This result is also correct in case of calcite. For Plug

3 which has lower amount of calcite in comparison to

Plug 2, production of quartz is as high as 37 percent

of the initial amount.

Figure 15: The ratio of total volume to initial volume

in reactions of Brine 3 Plug 2.

Figure 16: The ratio of total volume to initial volume

in reactions of Brine 3 Plug 3.

Effects on porosity and permeability

Some minerals, especially quartz and calcite, are

produced during reaction and increase the ratio of

rock volume to bulk volume. On the other hand,

some minerals, especially albite, anaclime, smectite,

illite, and K-feldspar, are dissolved and increase

porosity (Table 3). In this study, the amount of

dissolution was more than precipitation. Therefore,

an increase in porosity occurs. A volume decrease of

6.85 cm3 has been calculated for Brine 3 Plug 3. In

this simulated experiment, minerals from the

previous reaction products are separated (or “Picked

up”, Fig. 2, top), and reacted anew with the original

brine (Table 3). If one assumes initial porosity of

, the secondary porosity is calculated to be 0.264, a 5.6 percent increase in porosity. Similarly, a

0.53 percent increase in porosity is estimated for

Brine 3 Plug 2. In that simulated experiment, fluid is

separated from previous reaction (refer to Fig. 2,

bottom) and reacted anew with initial mineral

composition (data are not shown here).

Table 3: Change of mineral volume as a result of

brine-rock reaction for Brine 3 Plug 3.

Step 1

Volume

before

reaction

(cm3)

Volume

after

reaction

(cm3)

Volume

difference

(cm3)

Albite 34.406 25.460 -8.946

Analcime 17.643 0.000 -17.643

Calcite 44.268 45.080 0.812

Illite 18.090 0.000 -18.090

K-feldspar 43.025 0.000 -43.025

Kaolinite 7.710 0.000 -7.710

Quartz 117.013 141.100 24.087

Siderite 4.942 0.000 -4.942

Smectite-high-Fe 82.538 0.000 -82.538

Dolomite-ord 0 4.697 4.697

Minnesotaite 0 23.96 23.96

Muscovite 0 82.88 82.88

Nontronite-Ca 0 10.12 10.12

Pyrite 0 5.31E-05 5.31E-05

Saponite-Ca 0 31.31 31.31

Summation 369.63409 364.60705 -5.027

Step 2

Volume

before

reaction

(cm3)

Volume

after

reaction

(cm3)

Volume

difference

(cm3)

Albite 25.460 3.988 -21.472

Calcite 45.080 49.970 4.890

Daphnite-14A 0.000 19.123 19.123

Dolomite-ord 4.697 0.471 -4.226

Muscovite 82.880 83.970 1.090

Nontronite-Ca 10.120 10.150 0.030

Pyrite 0.000 0.000 0.000

Quartz 141.100 159.900 18.800

Saponite-Ca 31.310 36.130 4.820

Minnesotaite 23.96 0 -23.96

Summation 364.607 363.702 -0.905

0.96

0.98

1

1.02

1.04

1.06

1 2 3 4 5

Reaction steps

V t

ota

l/V

in

itia

l

Quartz

Calcite

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1 2 3 4 5

Reaction Steps

V t

ota

l/V

in

itia

l

Quartz

Calcite

8

Table 3 (cont.): Change of mineral’s volume as a

result of brine-rock reaction for

Brine 3 Plug 3.

Step 3

Volume

before

reaction

(cm3)

Volume

after

reaction

(cm3)

Volume

difference

(cm3)

Albite 3.988 0.000 -3.988

Calcite 49.970 48.930 -1.040

Daphnite-14A 19.123 18.544 -0.578

Dolomite-ord 0.471 1.401 0.930

Muscovite 83.970 85.650 1.680

Nontronite-Ca 10.150 10.180 0.030

Paragonite 0.000 0.297 0.297

Pyrite 0.000 0.000 0.000

Quartz 159.900 161.200 1.300

Saponite-Ca 36.13 37.24 1.11

Summation 363.702 363.443 -0.259

Step 4

Volume

before

reaction

(cm3)

Volume

after

reaction

(cm3)

Volume

difference

(cm3)

Calcite 48.930 47.050 -1.880

Daphnite-14A 18.544 17.906 -0.638

Dolomite-ord 1.401 3.064 1.663

Muscovite 85.650 86.180 0.530

Nontronite-Ca 10.180 10.210 0.030

Pyrite 0.000 0.000 0.000

Quartz 161.200 160.900 -0.300

Saponite-Ca 37.240 37.820 0.580

Paragonite 0.297 0.000 -0.297

Summation 363.443 363.131 -0.312

Step 5

Volume

before

reaction

(cm3)

Volume

after

reaction

(cm3)

Volume

difference

(cm3)

Calcite 47.050 45.180 -1.870

Daphnite-14A 17.906 17.260 -0.647

Dolomite-ord 3.064 4.719 1.655

Muscovite 86.180 86.390 0.210

Nontronite-Ca 10.210 10.240 0.030

Pyrite 0.000 0.000 0.000

Quartz 160.900 160.600 -0.300

Saponite-Ca 37.820 38.400 0.580

Summation 363.131 362.789 -0.342

Many studies have been conducted to relate

porosity to permeability. Several different forms that

have been suggested for the porosity function by various authors are shown in Table 4 (Dullien,

1979). Using the equation suggested by Rumpf and

Gupte (1971), the changes of in this specific studied example, for a 5.6 percent porosity increase,

would be equal to 35 percent increase. The calculated

change of using Carman-Kozeny form is equal to a 22 percent increase. In case of 0.53 percent

porosity increase, these quantities for Rumpf and

Gupte and Carman-Kozeny equations would be 2.9

and 1.9 percent, respectively.

Table 4: Different porosity functions for low Reynolds

number flow (See Dullien, 1979 for complete

citations).

Author

Blake (1922), Kozeny (1927), Carmen (1937)

Zunker (1920)

Hulbert and Feben (1933)

Slichter (1898)

Hatch (1934), Mavis and Wilsey (1936)

Rumpf and Gupte (1971)

Assuming that Carman-Kozeny behavior applies

and change in tortuosity and specific surface are

negligible, permeability is proportional to . Percipitation of some minerals like calcite also

may change the elastic module of reservoir rocks as a

result of cementation and therefore the condition of

reservoir’s fractures may change over time as a result

of reinjection.

CONCLUSIONS

In this research the impacts of rock-brine

interactions on sandstone properties have been

investigated. Although the production of quartz and

calcite was observed in the simulations, dissolution

of other minerals results in porosity increase and

therefore a permeability increase. However, porosity

changes caused by rock-brine geochemical reactions

are highly dependent on brine and rock compositions.

Depending on the situation, these reactions could

cause a decrease, an increase, or no change in

reservoir porosity. As a result, geochemical and

geological investigations should be a significant part

of the geothermal resource exploration.

ACKNOWLEDGMENTS

This study was supported by Louisiana State

Board of Regents program enhancement award. The

GDL foundation also is appreciated for a grant which

assisted this research.

9

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