Page 1 This power point presentation is intended to address the issues of ion exchange transformation of brines, and what it means regarding brine selection. The proper selection of brines for matrix and stimulation work extends back for at least 30 years. Along the way there have been shifts in testing and shifts in viewpoints. This presentation intends to put that history in perspective by demonstrating the link between formation mineralogy, ion exchange transformation of brines, clay swelling, and fluid selection.
Transcript
Page 1
This power point presentation is intended to address the issues of ion exchange transformation of brines, and what it means regarding brine selection. The proper selection of brines for matrix and stimulation work extends back for at least 30 years. Along the way there have been shifts in testing and shifts in viewpoints. This presentation intends to put that history in perspective by demonstrating the link between formation mineralogy, ion exchange transformation of brines, clay swelling, and fluid selection.
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At the fundamental center of the brine selection issue is formation mineralogy. We are interested in two processes that occur that are mineralogy based. The first process is ion exchange. In the absence of ion exchange, brine selection can be based on old style “beaker” tests. However, ion exchange almost always occurs.
Clays are alumino-silicates that have silica layers and alumina layers. In some of the alumina layers there can be substitutions of Aluminum by other ions such as Iron, Calcium, Magnesium, etc. Because Al has a 3+ charge, and the substituting ions have a 2+ charge, the alumino-silicate is not charge balanced and so is left with a residual negative charge. And something MUST neutralize that charge on the surface of the clays. The selection of what cation neutralizes the charge is brine dependant. Sodium brines will have sodium ions on the surface. The ion exchange process occurs because of how readily these surface ions can exchange with the ions that are present in the resident brine.
As new brines are flowed into the matrix, the surface ions exchange with the solutions ions one-for-one. This process exchanges any injected brine into it’s corresponding concentration of salt (NaCl) brine. As a result, 2% KCl transforms to about 1.5% NaCl.
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An X-ray analysis (or a spectral gamma ray log) can give the formation mineralogy. From this we can estimate the ion exchange properties of the formation. Alternatively, core tests can be conducted to determine the ion exchange capacity directly. (if anyone really wants to know. . . The CEC rating is in units of mmol/100 g. . .. Only a geologist would likely want to confirm that with a question.)
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Ion exchange, by itself, doesn’t cause any problems what so ever! Ion exchange can only a problem if there are swelling clays in the matrix. The swelling clays are smectite (also known as montmorillonite or bentonite) and the mixed layer clay illite-smectite. If an X-ray analysis shows a clay as ML(35), this tells us the clay is a Mixed-Layer clay with 35% swellable layers. The amount of swelling is based on the type of brine that the clay is exposed to and the concentration of that brine.
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In 1974, Wayne Hower published experimental work on clay swelling of specially prepared clays in various brines (SPE 4785) and is reproduced in the Table. The experiments were “beaker” type experiments that utilized x-ray diffraction for determination of absorbed water layers. From this work it was apparent that sodium smectite clay (smectite that has been ion exchanged to the sodium state by treatment with >10% sodium chloride brine) required at least 6% sodium chloride solution to prevent the clays from absorbing water and swelling. The transition from non-swelling to swelling brine was most pronounced between 0.4 and 2% NaCl. For KCl, the transition was most pronounced between 0 and 0.4% KCl, using potassium smectite clay. It is the work of Hower that gave us the knowledge that KCl brines, and later NH4Cl brines, are better than NaCl brines. . . . at low concentrations.
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The experiments of Hower explicitly examined the clay swelling properties of possible injection fluids on ion exchanged clays. The magnitude and impact of the ion exchange process, however, remained unnoticed until recent laboratory work using a 3-ft Hassler sleeve and fractional pore volume methodology (SPE 30100). The experimental procedures were designed for observing and measuring transient chemical effects, such as mixing, chemical reactivity, and ion exchange (SPE P&F, Nov. 1998, p. 272). As the consequences of ion exchange induced swelling became apparent, it became important that a clear experimental demonstration of the effect be demonstrated.
The test columns for our demonstration were 3-ft long and were packed with a mixture consisting of 90% sand and 10% smectite clay. The column was flowed with 6% NaCl (1 Molar) for at least 24 hours and equilibrated at 150F. The pore volume (PV) of the column was 345 mL. An injection brine of KCl was then flowed through the column. The maximum volume injected was 2 PV. The KCl concentrations tested ranged from 2% (0.27 M) to 7% (0.94 M). In addition, a common type “KCl substitute” was tested at the normal usage concentration of 0.2% (v/v) in fresh water. The results of the 6 separate flow tests (conducted on 6 freshly prepared columns) are shown in the Figure.
The figure shows that 2% KCl caused a 90% loss of permeability of the column within 0.3 PV of injected brine! In fact, a detailed analysis of the various sections showed that a 6-inch section near the first part of the column had lost over 98% of its original permeability. The figure also shows that it required at least 4.5% KCl just to prevent swelling enough that a complete test could be performed and still the column lost 87% of its original permeability. Notice that even the 5.2% KCl (0.7 M) permanently lost about 40% of its original permeability. The test with 7% KCl (0.94 M), however, was successfully completed with about a 5% improvement in permeability. The “KCl substitute” was the worst of all the fluids tested and was essentially no different from fresh water. In fact, typical substitutes would need to be used at about 20 to 40% (v/v) concentration to prevent clay swelling after ion exchange transformation to NaCl brine occurred.
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The issue of fluid pH is an important one. Some people have concerns that high-pH fluids may cause clay swelling, while others believe that high-pH should reduce swelling. The confusion may arise from the fact that raising the pH causes a bentonite drilling fluid to yield and become viscous. Of course, high KOH concentrations are known to be effective at fixing clays so that they will be immune to swelling in fresh water. So, an experiment was performed to determine if a higher pH would cause significant additional swelling and therefore permeability loss. The most common fluid used in fracturing has historically been 2% KCl. However, the previous figure showed that the permeability loss with 2% KCl was so severe that any additional effect from high-pH would not be detectable. Therefore, 4.5% KCl (0.6 M) was chosen for this experiment. The pH of the brine was raised to about pH 11 by the addition of 15 lb/Mgal of K-35 (0.017 M Na2CO3). It was important that the total cation concentration be held constant, so the KCl concentration was reduced by 0.034 M. Failure to make this adjustment could result in a slightly improved flow performance due to the higher overall salt concentration after ion exchange.
The result of the experiment is shown in this Figure along with the original 3%, 4.5% and 5.2% KCl flow tests. The figure shows that the permeability loss was only a little worse at pH 11 than at the neutral pH of unbuffered 4.5% KCl. There was a far greater impact from increasing or decreasing the KCl concentration by 1% even in the absence of a pH buffer. It can be concluded that raising the pH of 2% KCl probably does cause greater permeability loss. However, the effect of pH is minor compared to the overall problem of having insufficient KCl concentration to be compatible with the clay after ion exchange has transformed the KCl into NaCl water.
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Additional experiments have been performed with high-pH NaCl brines. The focus of the experiments was on total clay acidity, TCA, (SPE 64983) and the development of a clay instability rating, CIR, for clays in high-pH fluids (SPE 73730). Two experiments of interest here are 6% NaCl adjusted to pH 11 with 45 lb/Mgal of Na2CO3 and 6% NaCl adjust to pH 13 with KOH (0.1 M). The results of the two flow tests are shown in this Figure. Remember that a fresh column was used for each experiment and that the original fluid saturation was neutral-pH 6% NaCl. Notice that there was only a 2% loss in permeability as the high-pH brine flowed through the column in both cases. In fact, the slight apparent decrease in permeability is probably due to slightly higher viscosity resulting from the addition of either KOH or Na2CO3 .
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Sandstone 2000 teaches that the only requirement for choosing a brine that will be compatible both before and after ion exchange is that it be able to transform into at least 1 Molar NaCl (6%). For example, it is not necessary to add 7% KCl to seawater to have a compatible brine. Rather, it is only necessary to add enough KCl to give a final chloride concentration of 1 Molar or 35,000 mpl chlorides. Flow tests were conducted to demonstrate this principle. The brines included KCl, NH4Cl, and CaCl2. The results of the three flow tests are shown in this Figure. Notice that all three brines provided slightly improved permeability. The 1 M brines transformed into 1 M NaCl at the leading edge of the injected fluid. The transformed brine was completely displaced with un-modified brine, once the clays had been fully ion exchanged, at about 1.3 PV of injected fluid. Completion of the displacement was observed by the stabilized permeability. In practice, the volume of fluid to completely displace the ion-exchanged brine will vary with clay content, ion exchange capacity of the clay, and injected brine concentration. The 0.6 M CaCl2 brine transformed into 1.2 M NaCl and also gave a slight improvement in permeability.
