KCI MAY IN SOME CASES PREVENT GOOD CASING CEMENT JOBS

Robert L. Dillenbeck, Bill D. Strickland
Western Co. of North America
Oklahoma City

Billy R. Orr
Western Co. of North America
Yukon, Okla.

While the effects of potassium chloride in cement may seem desirable individually or in aggregate, research has shown that in at least some applications, KCI usage may be detrimental to certain aspects of cement slurry performance.

Muriate of potash, more commonly known as potassium chloride (KCI), is a compound that can influence a wide variety of cement performance parameters. Normally loaded at 2-5% by weight of mix water (BWOW), KCI is generally thought to act as a mild accelerator, a slight dispersant or thinner, and finally as a clay/shale stabilizer or control agent.

Ever since researchers discovered that KCI could effectively control swellable clays, KCI usage in oil field cements has been increasing. The concentrations of KCI in these cement slurries also seems to be on a gradual increase. The early applications used 2% KCI BWOW; more recent slurry formulations may use 3% or even 5%.

As KCI became more popular, other benefits supposedly attributable to its usage began to surface. Many of the benefits, such as acceleration, have been documented in print, while others have not.

When the authors began to survey individuals involved with cementing design and applications, it became apparent that the reasons for using KCI were as varied as the convictions about the need for the material in the first place.

While most used KCI in cements at least occasionally, many could only state vague reasons why the material was used in a given application. Many used KCI only on the recommendation of some other perceived authority.

Because of the wide variety of rationale for using KCI, the authors have tried to verify the majority of the effects through laboratory testing and research. Armed with these results, it is hoped that individuals will be able to identify those applications which will benefit from KCI usage, and those which will not.

TEST PARAMETERS

All tests were conducted using API Class H cement. A sufficient volume of one lot of the cement and additives was set aside to conduct all the tests. Most tests were conducted at more than one temperature to try to establish the effect (if any) of temperature on the parameter being tested. The individual slurry compositions were also kept simple with as few additives as possible, thus eliminating unnecessary variables. The maximum loading of KCI used in any of the tests was 5% KCI BWOW.

Other researchers, as well as our own test results, show that beginning at about 6-8% BWOW, KCI will cause viscosities to increase to the point that adequate mixing is no longer possible.

THICKENING TIME

The first series of thickening time tests was run at 110 F.

The base control slurry was Class H cement with 38% mix water (4.29 gal/sack). The second slurry was identical to the control; only 2% KCI BWOW was added. The third slurry was likewise identical to the control except for the addition of 5% KCI.

The results in Fig. 1 show that KCI is in fact an effective accelerator of cement thickening time. Increased loadings yield decreased thickening times.

The second series of thickening time tests was run at 180 F. As with the 100 F. tests, 16.4 ppg Class H slurries containing 0, 2, and 5% KCI were examined.

Again, the results in Fig. 2 show that increasing concentrations of KCI will produce decreasing thickening times. Note that although the results at 180 F. are consistent with thickening time tests at 110 F., the overall magnitude of acceleration is less at 180 F. than at 110 F.

COMPRESSIVE STRENGTH

To examine the effect of KCI on compressive strength development, a series of tests was run at varying times and temperatures on 0, 2, and 5 KCI slurries.

The first compressive strength tests at 130 F. for 24 hr (Fig. 3) indicate only a very slight (3,475 vs. 3,400 psi) increase in compressive strength between the 2% KCI slurry and the slurry containing no KCI.

The 5% KCI slurry showed no increase in compressive strength (24 hr) over the 2% KCI slurry.

To examine even shorter-term compressive strength development, the second series of strength tests was run at 220 F. for both 12 and 24 hr. Of interest is that while Fig. 4 shows enhanced 12 hr strength development with increased KCI concentration, Fig. 5 demonstrates the exact opposite effect at 24 hr.

VISCOSITY

In discussing KCI usage with various operators, the authors frequently were told that lower slurry viscosity is one justification for KCI in completion slurries. While this would seem to make sense, given the ability of low concentrations of sodium chloride (NACI) to disperse cement slurries, a series of Theological tests was run on an example completion cement containing a fixed amount of fluid-loss control HEC (hydroxyethylcellulose) polymer. The individual slurries contained 0, 2, and 5% KCI. The rheologies were analyzed at 180 F.

Despite the fact that small concentrations of NaCl will generally disperse slurries, Fig. 6 indicates the opposite for KCI. Regardless of the shear rate, increased KCI concentration leads to a marked increase in slurry viscosity.

FLUID LOSS

Because most chlorides (especially sodium and calcium) in cements tend to have an effect on fluid-loss control polymers, the authors decided to check the effect of KCI on an example completion cement. The base slurry was Class H with 1% by weight of cement (BWOC) low-to-medium temperature fluid-loss polymer. The slurries contained 0, 2, and 5% KCI.

The test results (Fig. 7) show that increasing the KCI loading has a profound effect on the fluid-loss control of this slurry. Having 2% KCI almost doubled the fluid loss compared to the 0% slurry. With 5% KCI, fluid loss nearly tripled from that of the 0% slurry.

Because a wide variety of polymers is used (in various formulations) for cement fluid-loss control, the response of any one formulation of fluid-loss control polymer to KCI may differ. However, given the degradation of fluid-loss control seen in these tests, careful scrutiny of a given fluid-loss additive would seem advisable whenever KCI is to be added to a formulation.

It should also be noted that the loading of the fluid-loss polymer in both the 2 and 5% KCI slurries was later increased in an attempt to find the necessary polymer loading to equal the fluid-loss values obtained with 0% KCI. In both cases, the slurries became unmixable (due to excessive viscosity) before the fluid loss was reduced to the same level as the 0% KCI slurry.

CLAY CONTROL

To fully comprehend the interaction of clay stabilizing materials with clays, extensive research into existing literature yielded a wealth of information on the subject.1-5 A brief discussion of clay types and their respective reactive properties is necessary to better understand the application of clay stabilizing materials in cement slurries.

A constant problem associated with the oil field has been clay damage.

Clay crystals can occur naturally in reservoirs or be introduced into the formation through clay-saturated drilling muds.

Clays are found in nearly all types of sandstones. Clay content ranges from 1 to 5% in clean sands and up to 15% in dirty sands. Clays can occur either as particles coating individual sand grains or as particles mixed in with the sand grains.

Even carbonate rocks may have associated clays. These usually are enclosed in the tight rock matrix and are not seriously affected by invading drilling or stimulation fluids.

Problems occur when clays or clay-like materials are subjected to contact with fluids not natural to the reservoir such as drilling filtrate, cement filtrate, or stimulation fluids.

Clays are subject to swelling and/or migrating that will cause subsequent loss of permeability or a loss of formation integrity if clays are part of the natural cementation of the reservoir. This can cause major problems in recovering oil and gas from productive formations.

When clay particles are rearranged or disturbed, it is usually impossible to restore the original permeability and,

therefore, clay damage should be prevented rather than cured.

Several groups of clays are found in petroleum reservoirs. These can be classified into four main categories as follows:

• Montmorillonite

• Kaolin

• Illite

• Mixed-layered.

The montmorillonite group, generally termed as swellable, contains clays such as montmorillonite, saporite, nontronite, hectorite, beidellite, and sauconite.

The kaolin group, generally termed as migrating, contains clays such as kaolinite, nacrite, dickite, endellite, and halloysite.

The illite group, generally termed as migrating, contains clays such as hydrobiotite, glauconite, and illite.

The mixed-layered group is a combination of any of the other three groups and those clays that do not fit into the other categories. This group can be termed as swellable and/or migrating.

Clay materials usually occur as minute, plate-like, tube-like, or fiber-like particles having an extremely large surface area as compared to an equivalent quantity of a granular material such as sand.

All clay minerals are constructed of two basic building blocks: the silica tetrahedron and the octahedral aluminum hydroxide. A silicon tetrahedron consists of a central silicon atom surrounded by four oxygen atoms arranged in a basic four-point triangular shape. These silica tetrahedral groups are arranged to form a hexagonal network that is repeated indefinitely to form a sheet.

The other structural element, the aluminum octahedron, is constructed from a central aluminum atom surrounded above and below by oxygen and hydroxyl ions. These structures likewise can be arranged in a repeating network pattern to form sheets.

The structure of some clays, as for instance montmorillonite, can be pictured as a stack of sheet-like, three-layer lattices that are weakly bonded to each other. The structure is expanded by water or other substances that can penetrate between the sheets and separate them. Bonds between the sheets are very poor because these sheets, being silica tetrahedra, have facing oxygen ions and are free to be neutralized by absorbed cations.

Consequently, water molecules are absorbed between the sheets and, because the molecules happen to be the right size to fit within the structure, the water molecules cause very strong swelling pressures.

The illite group is very closely related to the same stacking as seen in montmorillonite except for the fixation of potassium within the basal plane of the two silica sheets. The potassium ion, because of its unique size, bonds the two silica sheets together much more firmly than is the case for montmorillonite. The result is that the lattice is much less susceptible to cleavage and therefore is more stable in a water environment.

The cation exchange capacity (CEC) is the ability of a clay to absorb ions on its surfaces or edges. CEC is usually defined in terms of the weight (as milliequivalents of hydrogen) absorbed per 100 grams of rock/soil. The higher the CEC, the more likely the clay will disperse or swell in an aqueous fluid.

All clays are negatively charged. The CEC is measured by the density of the negative charges in the clay crystal.

With montmorillonite, the negative charges are predominant on the faces of the clay crystal. The edges of the crystal are positively charged.

Montmorillonite has greater cation exchange capacity than other types of clay. In other words, montmorillonite will react easier with aqueous fluids and cause damage. Montmorillonite has a CEC of 80-150 compared to 10-20 for illite and 3-15 for kaolinite.

Kaolin mineral elements are the most near-perfectly formed of all the clay mineral types. As a result, the CEC for the Kaolin group is the lowest. Therefore, kaolin clay is the most stable.

The low exchange capacity of Kaolin clays is limited to broken bonds from the edges which tend to migrate. Montmorillonite, on the other hand, tends to swell due to broken bonds between the lattice framework.

Although structurally similar to montmorillonites, the illites have a lower CEC due also to broken bonds occurring along the edges but not along the lattice framework.

The CED of illites is still higher than for kaolins because in certain less crystallized illites there may be some substitution within this lattice framework.

Because clays are negatively charged, clays attract cations from aqueous fluids. The clay surface then takes on a positive ion outer shell. When these now positively charged clays come in contact with each other, the clays will repel each other the same as magnets when their like-polarity ends are touched.

The net result in a claywater system is constant movement (dispersion) of millions of clay particles. These moving particles will destroy permeability by blocking pore throats, thus restricting oil or gas production.

With swellable clays, the aqueous fluids are attracted to the surfaces between the lattice framework and absorbed between these layers. As the spaces between these frameworks absorb more water, the spaces expand and swell to take up several times their original size. This expansion creates blockage by filling up pore throats as opposed to clogging the throats.

To combat this destructive nature of clays, some form of clay stabilization is necessary. The stabilization must have the effect of shrinking hydrated clays and preventing the repulsive forces surrounding the clays from causing clay dispersion and migration.

Today, the three most accepted ways to achieve stabilization are ionic neutralization, organic barrier, and structural fusion.

First, in ionic neutralization, the repulsive forces are controlled by preventing or shrinking, to as small as possible, the positive ion shell. If the shell is deactivated to the point that the shells no longer have the energy to repel, clays will form neutral flocs and will be precipitated from the surrounding fluids or will remain in their natural and original orientation without being disturbed.

The general rule for adjusting the size of the positive ion shell is to increase the valence and concentration of the cations. The higher the valance, the smaller the shell and the more stable the clay floc will remain. Valence is the numerical strength given cations. The replacement of cations in clay crystals usually follows the valence rule where:

• Monovalent-C+ is easiest to replace

• Divalent-C++ is the next easiest

• Trivalent-C+++ is hardest.

Note, however, that the hydrogen ion (H+) is the exception to the rule and is harder to displace than the trivalent cation.

It must be remembered that adjusting the size may be a temporary fix to the clay problem because stability follows the general valence, and concentration rules and can be easily changed. Change the environment to a fresher water and the positive ion shell will return and dispersion will result.

Permanent protection from charge neutralization can only be achieved with hydrolyzable metal ions that form polynuclear ions which possess a very high multiple charge that can completely and permanently neutralize the positive ion shell surrounding the clays.

The most popular charge neutralization used today (by the oil industry) is KCI. It provides a monovalent cation which will control clays as long as the total environment for the clays is KCI water.

Potassium controls clay swelling because its size fits the base plane sites better than other cations.

KCI can also be useful in rebuilding and restabilizing degraded illites that have experienced long periods of leaching that extracted the potassium ion. Furthermore, KCI is used to match the salinity of native formation waters and thus satisfy the concentration and valence rules to control clays.

The second method to stabilize clays is the organic barrier method that deposits an organic film across the clay boundary. The film blocks the repulsive forces found when water contacts the clay and forms the positive ion shell.

The organic barrier is usually some type of quaternary ammonium chloride salt with substituted alkyl or allyl groups. Because the barrier's positive charges are attracted to the negative clay charge, the barrier is attached or plates out through ionic attraction.

With the alkyl or allyl groups pointing away from the clays, these groups form a protective barrier of hydrocarbon that controls or stabilizes the clay by eliminating the repulsive effect of the positively charged ion shell.

The third method of stabilizing clays, the structural fusion method, eliminates the repulsive forces between clays by destroying part of the clay mineral itself, principally the alumina octahedron.

By using various anions of fluoride, borate, phosphate and hydroxide, a very thin connecting sheet is formed around the clay when these anions attack the clay and become mixed with the aluminum from the octahedron layer. This sheet is a very powerful binder that holds clay together and doesn't allow them to migrate or swell.

ANALYSIS

The research shows that KCI does accelerate thickening (or set) times, but has only a marginal or even negative effect on short-term compressive strength development.

KCI tends to increase slurry viscosity as loadings increase, and it has a degrading effect on fluid-loss control.

KCI can be an effective clay control material but its effect is temporary, lasting only until the fluid containing KCI is replaced by another fluid.

Other organic barrier materials exist that are as effective at clay control, and also are much longer lasting than KCI. But these materials tend to be more expensive on a per pound basis.

A point that must always be remembered is that one cannot use KCI (in a slurry) to gain a single benefit, such as shorter set times, without also realizing all the other effects. Depending upon a given application, the other effects may or may not be beneficial, but must always be considered.

With respect to using a clay-control agent during cement operations, an operator must first determine if it is necessary or worth the expense.

First, are potentially swellable clays present in the zones to be actually covered by the spacer or cement? It does little good to run a clay control additive in a slurry or spacer that will never be lifted across the subject zone.

Second, if the problem zones are to be covered during cement operations, have the zones been protected from drilling fluid damage during drilling? Untreated drilling fluid filtrate can do irreversible damage long before the well is ever cemented. All the clay control in the world will not cure a problem already created by a poor drilling fluid.

Finally, if the answer to both previous questions is yes, then which type of control (KCI or organic clay control) is best suited to the well? Although this may seem like a rather simple question given the apparent economic advantage enjoyed by KCI, it is not.

If certain slurry performance characteristics are critical, such as thickening time and fluid-loss control, the actual costs of using KCI can be much higher than is initially apparent.

Because KCI tends to degrade fluid-loss control, at best, additional polymer will be necessary to achieve the desired fluid loss. In the worst case, the operator may have to settle for substandard fluid-loss control (due to mixing problems) and risk slurry dehydration accelerated thickening times produced by KCI. The operator may have to use additional retarder to achieve the necessary placement time.

If the slurry is made excessively viscous by either the KCI itself, chloride degradation, a chemical dispersant or thinner may be necessary to overcome mixing problems or to relieve excessive downhole friction pressures.

Finally, if KCI can be included in the cement without undue difficulty, there still remains the spacer that, if in a freshwater base, must be treated with potassium chloride as well.

If freshwater filtrate from the spacer invades the zone containing swellable clays, the KCI in the cement will be of little use.

The alternative to using KCI as a clay control in the cement is to use a permanent, organic barrier-type material in the spacer or preflush. Some specially designed materials will enhance the fluid-loss control of the spacer. Unlike some earlier organic barrier materials, These new additives will leave clay particles water wet instead of oil wet.

When considering all the adjustments necessary to use KCI in completion slurries, for many applications, the economy and simplicity of using an organic barrier material in the spacer/flush may make more sense.

ACKNOWLEDGMENTS

The authors thank Western Co. management for the opportunity to prepare and present this article and Darrel Jones and the Yukon, Okla., engineering staff for providing the laboratory time and personnel to generate the necessary test data.

REFERENCES

1. Harrisberger, W.H., "Special Study-Review of Clay Mineralogy and the Mechanisms of Clay Stabilization," Aug. 10, 1977, pp. 1-14. (No publication mentioned.)

2. Oil and Gas Consultants International Inc. "Well completion Stimulation and Workover Systems," Production Operations Manual, Formation Damage Section, 1969, pp. 9-16.

3. Garbis, Sam, "Clay Minerals and Formation Damage" internal Western Co. publication, May 13, 1982, pp. 1-14.

4. Moore, John E., "Clay Mineralogy Problems in Oil Recovery," Petroleum Engineer, February 1960, pp. 40-47.

5. "Method of Stabilizing Clay Formations," U.S. Patent 4,158,521.

   Copyright 1991 Oil & Gas Journal. All Rights Reserved.