The effect of CaCl2H2O brines on the dehydration of Ca-exchanged montmorillonite (SWy-1) at elevated temperatures and pressures

Geochimica

et Cosmochimica

Pergamon

Acta, Vol. 60, No. 12, pp. 2167-2172, 1996 Copyright 0 1996 Elsevier ScienceLtd Printed in the USA. All tights reserved

0016.7037/96 $15.00+ .OO

PI1 SOO16-7037( 96)00088-9

The effect of CaCl,-H,O brines on the dehydration of Ca-exchanged (SWy-1) at elevated temperatures and pressures SHIJIEWANG,'

A.F.KOSTERVAN

GROOS,~,* and STEPHEN

montmorillonite

GUGGENHEIM*

’Institute of Geochemistry, Academia Sinica, China * Department of Geological Sciences, University of Illinois at Chicago, Chicago, IL 60680, USA (Received September 11, 1993; accepted in revised

formMarch

1, 1996)

Abstract-The dehydration temperature of a Ca-exchanged montmorillonite (Clay Minerals Society Clay SWy-1) in montmorillonite-CaCl, mixtures containing 0.0, 0.9, 1.8, 5, 10, 20, and 30 wt% CaCl, was determined at pressures to 700 bars, using high-pressure-differential thermal analysis (HP-DTA). The calculated concentrations of the resulting solutions range from 0.0 to 72.8 wt% (0.0-30.3 mol%) CaClz. A substantial decrease of the dehydration temperature was observed, which is caused by the reduction of fHzo in these solutions. The relationship between the CaCl* concentration of the fluid, temperature, and the fugacity coefficient (r,,,) was determined to 600°C. rH20 ranges from 0.125 at 300°C to 0.34 at 600°C in a 30.3 mol% CaClz fluid and from 0.53 at 300°C to 0.70 at 600°C in a 0.91 mol% CaCl, fluid. In the presence of moderately concentrated brines and at slightly elevated temperatures, Ca-rich smectite in sedimentary basins may be capable of buffering the fluid pressure. 1. INTRODUCTION

kbar, using a high-pressure hydrothermal diamond cell. They note that the second Hz0 layer they observed by X-ray techniques may be identical with the outer hydration shell as defined by Koster van Groos and Guggenheim ( 1984). Thus, the dehydration of interlayer water in montmorillonite (Mt) proceeds following three distinct reactions:

The hydration/dehydration reaction of smectite is important in many geological processes, as well as in the petroleum industry and in chemical and nuclear waste disposal. Conditions affecting this reaction at 1 atm were studied, among others, by Rowland et al. ( 1956), Posner and Quirk ( 1964), Grim (1968), Keren and Shainberg (1975), Kawano and Tomita ( 1989), and Slade et al. ( 1991). A limited number of experiments have addressed the hydration/dehydration behavior of smectite at higher pressures, simulating conditions of burial. Eberl et al. (1978), for example, showed that smectites persist to temperatures of 400 to 500°C and pressures of 0.5 and 2 kbar. Koster van Groos and Guggenheim (1984, 1986, 1987) studied the effect of interlayer cations on the dehydration of a montmorillonite ( SWy- 1) at elevated pressures and temperatures. They found that a modest increase in pressure greatly extends the stability of the interlayer water and that dehydration occurs in two separate reactions at approximately 40-70°C and at 95- 135°C above the boiling temperature of water, depending on pressure. They concluded that the interlayer water in their samples is present in two distinct hydration shells, a bonded outer shell and a more strongly bonded inner shell. They observed also that different interlayer cations affect both the amount of interlayer water and its bonding energy. In a study of the dehydration of a montmorillonite in the presence of a brine, Colten ( 1986) demonstrated that Na-saturated Cheto montmorillonite has a two-water-layer structure at hydraulic pressures below 460 bars and temperatures below 200°C when in contact with a l-5 molal NaCl brine. Montmorillonite ( SWy- 1) has a three-water-layer structure at much higher Hz0 activities (Bird, 1984; Huang et al., 1994). Huang et al. ( 1994) used synchrotron radiation to study the d(OO1) of this smectite to near 600°C and 5

* Author to whom correspondence

Mt( 1) + Mt(0)

+ HZO,

(1)

Mt( 1, 2) + Mt( 1) + HzO,

(2)

Mt( 1, 2, 3) + Mt( 1, 2) + HZO,

(3)

where Mt(O), Mt( l), Mt( 1, 2), and Mt( 1, 2, 3) represent montmorillonite with increasing interlayer-water layers as indicated by the enclosed numbers. DTA experiments do not directly identify interlayer-water layers. Instead, the DTA peaks relate to the expulsion of similarly bonded water. In earlier work, Koster van Groos and Guggenheim (1987) described the interlayer water in montmorillonite as related to an inner, strongly bonded hydration shell and an outer, more weakly bonded hydration shell. A third hydration shell was not present, because the conditions of storage at 55% r.h. precludes its presence. Based on the study of Huang et al. ( 1994), we implicitly relate the interlayer-water layers to these hydration shells. Thus, Mt( 1) contains only the innermost hydration shell, Mt( 1, 2) contains also the more weakly bonded water of the second hydration shell, and Mt( 1, 2, 3) contains also the second nearest-neighbor water, referred to above as the third hydration shell. Note that the first and second dehydration reactions of our previous papers (e.g., Koster van Groos and Guggenheim, 1984, 1986, 1987; Guggenheim and Koster van Groos, 1992) are reversed numerically compared to the reactions listed above. Because free Hz0 is a product of the dehydration reactions, the activity of Hz0 will affect the dehydration temperatures. Thus, in the presence of brines, montmorillonite must

should be addressed. 2167

S. Wang, A. F. Koster van Groos,

2168 dehydrate water.

at lower In this

temperatures

study,

we

than

consider

in the presence the

dehydration

of pure of Ca-

SWy-1 at elevated pressures in the presence of CaC&-H20 solutions. Because the cation in both the montmorillonite and chloride solution is the same, the interlayer cation of the montmorillonite remains Ca throughout the experiments. exchanged

2. EXPERIMENTAL

METHOD

2.1. Apparatus The HP-DTA system is described in detail by Koster van Groos ( 1979). It consists of a Cu sample holder, which can hold three capsules. The capsules are made from Au foil, using an extrusion technique. They weigh - 130 mg, have a length of -7 mm, a diameter of 3.2 mm, and a wall thickness of -0.07 mm. A 1 mm deep re-entry well at the base of each capsule positions a Pt-Ptg,,Rh,,, thermocouple at the center of the sample. The capsules can be used open or welded shut. Two capsules are loaded with -30 mg sample, and the third with -20 mg of a reference consisting of a mixture of 80 wt% aluminum oxide and 20 wt% of pure quartz sand (St. Peter sandstone), ground to pass 200 mesh. After placing the capsules in the DTA cell, the remainder of the sample chamber was filled with silica wool to minimize gas convection and capsule movement. The complete assembly was sealed within a horizontally mounted internally heated pressure vessel (Holloway, I97 I ). An important feature of the HP-DTA system is that the geometry of the capsules and their position in the cell is reproducible. This facilitates comparison of the DTA signals. Experiments were made from 30 to 700°C at a heating rate of 2O”C/min. All temperatures were calibrated against the high-low quartz inversion of the reference (Koster van Groos and ter Heege, 1973); they are believed accurate to within 1°C. The differential temperatures were recorded on the 50 PV range of a Kipp recorder. which provides a temperature resolution to within 0.05”C. The temperatures referred to in this study represent the reference temperature when the dehydration reaction is either complete or nearly complete, as indicated by the rapid return of the DTA signal from the dehydration peak to the baseline. All temperatures are corrected for the differential temperature (for a detailed description, see Koster van Groos and Guggenheim, 1987). Pressure was measured using two bourdon-type Heise gauges, with the ranges O&70 bars and O& 1000 bars. They are accurate to within 0.5%. The pressure medium was Ar gas. 2.2. Starting

Material

Ca-exchanged montmorillonite, hereafter referred to as CaSWy, was obtained by ion exchange of the
Procedure

Both open and closed capsules were used in this study. The use of either type of capsule does not affect the DTA signal, since sealing the capsule does not significantly change the thermal characteristics of the capsules. In our experiments, evolving vapor will affect the results. Therefore. at low pressures the use of open capsules was necessary, because the molar volume of H,O gas released from montmorillonite during heating at these pressures is sufficiently large to cause failure of sealed capsules. At pressures above 100-200 bar. the molar volume of Hz0 gas is sufficiently small so that closed capsules could be used.

and S. Guggenheim

For open capsules at low pressures, the amount of H,O gas released during heating expels the argon pressure medium from the capsule, and P,,,,,,, = P,,:,, At higher pressures, mixing between argon gas and the Hz0 gas occurs and P,,,,,, > PIi.<,. In the sealed capsules, P I,,,.,/= P,,,. In addition to these runs, a few runs were made with pure CaSWy with distilled water added for comparison with earlier work (Koster van Groos and Guggenheim, 1987). The starting material was packed by pressing the material in the capsule with a plunger using a constant force. In the open runs with additional water present. IO PL water was pipetted into the capsule. Clay w’as added immediately and compressed carefully to prevent water loss. In the hydrous runs in sealed capsules, 5 PL water was pipetted into the capsule and weighed, after which the desired amount of montmorillonite was added. In all open runs, the sample was covered with tightly packed silica wool and run immediately after loading. The silica wool is used to prevent mixing of the argon gas pressure medium and the evolving HZ0 gas in an effort to thus maintain P,,,,,,, = P,,+). All sealed runs were checked for leaks and weighed before the experiments. If, after the experiment, a sealed capsule
AND DISCUSSION

To evaluate the effect of water on the DTA signal, nine dry and five hydrous runs were made with pure CaSWy (Table 1 ). Representative runs are shown in Fig. 1. The boiling of water produces a distinct additional peak (L, V ). In the dry runs at low pressures (open capsule runs), the dehydration peaks are less well-defined than when initially water was added, e.g., the runs at 75 bars. In these runs, the dehydration temperatures of the hydrous and the dry sample are very similar, indicating that mixing between evolving Hz0 and Ar gas was limited. In the runs with CaCl? present, the DTA curves arc similar to the DTA curves with pure HI0 present, indicating that no additional reactions occur at these experimental conditions. A total of forty-five successful runs were made with CaCl, (Table I ) Several of these runs are shown in Fig. 1. The results for reaction I are presented in a In P- 1000/T diagram in Fig. 2. Linear regression analysis of the data are shown, except for the data with 0.91 and 1.8 wt% CaCl,. For these series. the range of PT conditions was too limited to yield a regression consistent with the other analyses. Therefore. the linear fit for these series, as shown in Fig. 2,

Dehydration of Ca-montmorillonite at high T and P

2169

Table 1. HP-DTA peak position of reaction (1) CaSWy in the CaSWy-CaCl, system CaCl,(wt%)

N44B J-82B J-88B’ J-l-IA J-1-1B’ J-4-4A J4-4B’ J-S-SA J-S-SB’ J-2-2B J-9AlJ J-1OA; :::;g 18A 23A 19A 24A 18B 20B 23B 19B 24A 22A 28A 26A J-70B J-2SA J-66B J-77B J-9SB* J-63B2 J-91A2 J-9OA* J-65B J-26B J-70A J-76A J-66A J-77A J-9SA* J-63A2 J-64A2 J-9lB* J-89B* J-2A2 1lA 13A 4A2 SA2 6A2 2 :AA*

1lB 13B 21A 4B2

P&r)

T(=‘C)

1

214 216 214 326 326 332 333 387 389 397 520 524 563 574 356 391 406 429 344 353 381 393 416 258 311 306 320 350 365 375 441 461 482 479 256 306 310 316 335 350 422 436 444 448 453 472 258 316 395 410 414 418 422 247 303

: 38 33 ;z 106 270 285 500 540 45 86 109 174 : 1E 173 G 2 :: 148 332 424 514 518 30 47 :z 97 148 330 422 471 520 580 698 38 :: 441 457 460 487 38 102 114 334

1

I

5%!bar

, 10%

T

Run

I

I

5%

A75

10% 97 5%

bar

99 bar

<

AT

dry

0%

v

151

100

200

300

400

500

T, “C FIG. 1. Representative HP-DTA patterns with CaSWy under dry and aqueous conditions.

T 600

500

400

(Or) 300

7 6

\ \

5

h

y‘.\

v

1.8

II

.

5.1

,,

0

(1

A . 30.3 20.2 10.1

,, >,

1000

A

1

100

4 3

a 10

2

z:

’hydrous

runs * closed capsule

P

-z

1 0

1

-1 1.00

125

1 50

1000/T is based on a visual interpolation, subparallel to the other regression lines. The R* values of the regressions are better than 0.990. In several open runs (J-1-lB, J-4-4B, J-5-5B, J-2-2B) with pure CaSWy, reaction 1 occurred at lower

175

2.00

2.25

2.50

(K)

FIG. 2. LnP vs. 1000/T projection of reaction 1, which is the dehydration reaction of the first hydration shell of CaSWy in the presence of CaCIZ-HZ0 brines. The dashed line (L, V) represents the boiling curve of water.

2170

S. Wang, A. F. Koster van Groos, and S. Guggenheim

eters and the enthal y of reaction (1) Table 2. LnP-l/T of CaS Wparam y in theA. resence of Ca2 ,s shown The enthaIPy of the m these. fluids at the temperatures at the dissolution of H sot 1 bar intercept,

ET7

1 bar intercept (T in “C)

slope

:::

214 191

-7.229,

60.1 51.2:

-;.9*

::; 10.1 20.2 30.3

186 159 146 127 117

-6.340 :z:;;;*

z;+ 51:7 48.5 46.7

-9.8 -4.1. -11.4 -15.4 -17.7

??

?:% -5.616

estimated

temperatures than was expected on the basis of the other runs. In these runs, it is likely that PHzO< P,,,, , which would result in a lowering of the dehydration temperature. Therefore, these runs were excluded in the regression analysis. The boiling curve for water (L, V), taken from Keenan et al. ( 1978), is shown for comparison. Both slope and dehydration temperature of reaction 1 decrease monotonically with an increase in the CaC& concentration. The slope of reaction 1 at different CaCl, concentrations and their 1000/T intercept at In P = 0 are given in Table 2. From this slope, the enthalpy of dehydration at 1 atmosphere for reaction ( 1) of CaSWy in the presence of these CaCl,-fluids at one atmosphere was calculated (Table 2), using a modified van’t Hoff equation, AH,, = -R(ln P2 - In P,)l( l/T,! - l/T1) (e.g., Anderson, 1977). The dehydration energy of pure CaSWy (60.1 -t 2 k.I) is within error of earlier results (58.2 ? 2 kJ) of Koster van Groos and Guggenheim ( 1987). The enthalpy of this reaction (AH,,) represents the sum of the enthalpies of the interlayer water bond (AH,,,,), the enthalpy of evaporation (AH,,) of the interlayer water, which produces an HZ0 vapor, and the enthalpy of dissolution of this HZ0 vapor in the CaCl*-HZ0 fluid ( AHso,). The difference in slope of reaction 1 of CaSWy in the presence of fluids with different CaCl, concentration and, therefore, the change in AH,, must represent differences in AH,,, and AH,,, assuming that the change in AHi, over the range of the dehydration temperatures in this study is small. AH,, of pure water at 1 bar was evaluated for the extrapolated temperature of dehydration, using the extrapolated data from (L, V),,, (Keenan et al., 1978). Therefore, AH,,, for these different fluids can be calculated using AH,,, = AHdh - AH,, - AH,w (Table 2). The decrease in the temperature of reaction 1 of CaSWy in the presence of increasingly concentrated CaCl*-H,O fluids (Fig. 2) is a reflection of the reduction of fH20 in these fluids. If no CaCl, enters the interlayer of CaSWy, then the PT conditions of the dehydration reaction must reflect &, in the fluid phase rather than the total fluid pressure. In this discussion the effect of the higher total pressure on the clay is ignored. The relation between P,,,,, which was measured, and j&o in the brine, which is obtained from the assumption that it equals the dehydration pressure of CaSWy in the presence of pure Hz0 at the same temperature, is shown in Fig. 3. This diagram is similar to Fig. 2, but with the addition of a second pressure scale reflecting fHZo. Although the PT

conditions illustrated by the diagram are limited, Fig. 3 can be used to determine fa+, in solutions with different CaCl, contents at various temperatures and pressures. Consider, for example, a 5.1 mol% CaClz fluid at 1000 bar total fluid pressure. In this fluid, CaSWy completely dehydrates at 540°C (Fig. 3 ) . At the same temperature but in the presence of pure HZ0 fluid, however, CaSWy dehydrates at PHzo = 400 bars (Fig. 3, 0.0 mol% CaCl, line), indicating that f& = 400 bars in the 5.1 mol% CaCl, fluid. With the same fluid but at 360°C CaSWy dehydrates at -100 bars. However, at 360°C but with pure HzO, the dehydration pressure is 30 bars, indicating that fHlo = 30 bars. Similarly, in the same fluid at IO bars and 240°C fHzo = 2 bars. This indicates that at lower temperatures and pressures, the fugacity of Hz0 in the fluid decreases with respect to the total pressure. From these data the relation between temperature and the fugacity coefficient IHZO (Ial = fHZol[PtoLBl* XH,oI, with XHzobeing the mole fraction of HZ0 in the fluid, was determined (Fig. 4). In this relation, the effect of the additional pressure on the montmorillonite structure is ignored. This should not result in a serious discrepancy because the effect of temperature is likely to be dominant. The diagram illustrates the large effect of temperature on I”+, especially on more concentrated solutions. For example, in a 10.1 mol% CaClz solution, IHzO = 0.18 at 3OO”C, and increases twofold to 0.35 at 6OO”C, whereas in dilute CaCl, solutions IHZ0 increases less than by a factor of I .5. In addition, the diagram illustrates the nonlinear behavior of I,,+ with respect to CaCl, concentration. At low concentrations, IHZo is very dependent on XcaC12,but at high concentrations additional CaCl, does not affect rH20, acting only as a dilutant. These data indicate that in dilute solutions of CaClz at low temperatures, the activity of H20 (a”+,) is strongly reduced. At these conditions, CaCI, is highly ionized. However, the reduction in aa,o at these low CaClz concentrations is large, which suggests that these ions have extensive hydra-

500 400

3oc

_ 0

ZOO

0

E

FIG. 3. Relation between the total fluid pressure and H20 fugacity of the dehydration layer ( 1) of CaSWy in the presence of caC1, H20 brines. The relation of the 0.9 and 1.8 mol% CaClz fluids is based an estimated slope, see text.

Dehydration of Ca-montmorillonite at high T and P

0.9

y

0.0 ;;“‘~“.“.-“““““““““““““‘.“.,‘.100

150

200

250

300

358

-r

(

400

450

500

550

600

c>

FIG. 4. Relation between the fugacity coefficient of HZ0 and temperature in CaCl* fluids. The relation of the 0.9 and 1.8 mol% CaCl* fluids is based an estimated slope, see text.

tion spheres. With increasing temperature and increasing CaCl* concentrations, the degree of ionization decreases substantially, with a simultaneous reduction in the size of the hydration sphere. The data obtained here are for reaction 1, in which all the interlayer water from Mt ( 1) is removed. In a system with pure H20, Mt ( 1,2) + Mt( 1) at a -4O-80°C lower temperature (Koster van Groos and Guggenheim, 1987). Recently, Huang et al. (1994) showed in a high-pressure synchrotron X-ray study of SWy- 1 that Mt ( 1,2,3 ) , present at high water activities, transforms to Mt( 1, 2) at temperatures - 150200°C below reaction 1. Because in the presence of a CaCl,fluid at constant pressure, f&o decreases significantly with temperature (Fig. 4) and the difference in these dehydration temperatures will be significantly greater. 4. GEOLOGICAL

APPLICATIONS

This paper describes a procedure to derive quantitative estimates of the effect of water fugacity on the dehydration of Ca-exchanged montmorillonite. Although the dehydration of montmorillonite is an important process in sedimentary rocks, these results address only the effect of Ca-brines on the Ca component of montmorillonite. Therefore, they cannot be rigorously applied to reactions involving K-fixation and illitization of montmorillonite. It is noted that the dehydration of clay minerals in Gulf Coast sediments also reflects this progressive transformation of montmorillonite to illite by K-fixation (Powers, 1967; Burst, 1969; Perry and Hower, 1970, 1972; Weaver and Beck, 1971; Magara, 1975; and Hower et al., 1976), or to a mixed-layer smectite-illite compound (e.g., Ahn and Peacor, 1986). However, smectite, especially where Mg-rich, may persist to temperatures as high as 500°C (Roy and Roy, 1955) in the absence of K+. Experiments from Koster van Groos and Guggenheim (1984, 1987) and Huang et al. (1994) showed that at the pressure conditions in sedimentary basins, temperatures within the normal geothermal range are too low for montmo-

2171

rillonite to dehydrate in the presence of water. However, as was suggested by Koster van Groos and Guggenheim ( 1987), lowering the activity of water in the sediments may produce conditions compatible with montmorillonite dehydration. The present study supports this suggestion, although the compositional range of the system CaSWy-CaCl*-Hz0 is limited and directly applicable only to Ca-rich brines in equilibrium with Ca-rich montmorillonite. As an example of the effect of Ca-brines, consider the following : In a sedimentary basin at a depth of -4 km, which is equivalent to a lithostatic pressure of - 1 kbar, PHIOranges between 400 to 1000 bars, depending on permeability. The temperature of the sediments may reach 130- 17O”C, assuming a geothermal gradient of 30-4OYYkm and an average surface temperature of 10°C. In the presence of a 10.1 mol% brine, fnzo varies between 20 and 80 bars, following Fig. 4. Under these conditions, reaction 1 will occur at 340-420°C (Fig. 2)) reaction 2 at 300-38O”C, and reaction 3 at about 140-220°C (see the previous section). Under these conditions, transformation of Mt( 1, 2, 3) -+ Mt ( 1, 2) becomes a distinct possibility and the montmorillonite may buffer the fluid pressure. If fHlo is lowered further because of the presence of other solutes, e.g. hydrocarbons, Mt( 1, 2, 3) + Mt( 1, 2) may occur at even lower temperatures or in the presence of less concentrated brines. In most sedimentary basins, fluids are dominated by Nasalts, although CaCl*-dominant fluids have been found to occur in deeper sections of some basins (e.g., Macpherson, 1992). As the dehydration of CaSWy occurs at higher temperatures than Na-, K-, or Mg-exchanged SWy-1 (Koster van Groos and Guggenheim, 1984, 1986, 1987), it is likely that at the same pressure the reaction Mt ( 1, 2, 3 ) -+ Mt ( 1, 2) may occur at lower temperatures and, thus, buffer the fluid pressure at even lower temperatures. Acknowledgments-We gratefully acknowledge the help and the many discussions with T. B. Bai. Reviews of W-L. Huang and two unknown reviewers are greatly appreciated. The study was supported by grants from National Science Foundation (EAR-8816898) and the Petroleum Research Foundation (20016-AC2). Editorial

handling:

M. A. McKibben REFERENCES

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