Additives for preparing rheology-modified aqueous fluids

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PCT/US2004/012198

ADDITIVES FOR PREPARING

RHEOLOGY-mODIFIED AQUEOUS FLUIDS

This invention relates to the field of rheology modification agents for use in fluids such as drilling, mining and milling fluids, including but not limited to those used in natural resources exploration, utility horizontal directional boring, and environmental remedial wells.

The use of rheology modification agents, frequently thixotropic agents, for aqueous fluids has been common practice in a large number of industries. These fluids include, for example, oil field drilling fluids, metal-working fluids, mining fluids, fire control fluids, agricultural organic formulations, water-based paints and coating fluids, stripping fluids, and the like. For each of these, and other, applications, the rheology modification agents serve very specific purposes tailored to the function for which the fluid is being employed. Because a variety of properties are frequently needed for a given fluid, the rheology modification agent is frequently used in conjunction with other types of agents or additives, in order to produce a final fluid suitable to a given application. However, it is generally desirable to reduce the number of such agents or additives as much as possible, in order to facilitate the ease of production and use and therefore to also minimize cost.

Certain inorganic materials, including mineral clays such as bentonite and attapulgite, are commonly used for rheology modification of aqueous fluids. For example, bentonite, which is one of the most popular of the clay materials for use in drilling muds, is thixotropic in combination with water and is also relatively stable to temperature extremes. Furthermore, it imparts resistance to oxidation and durability when exposed to high shear conditions. These clays may be used with other types of agents or densifiers, such as iron oxide or barium sulfate, which enhance the ability of the fluid to resist pressures such as are encountered in subterranean excavations. **

Aqueous fluids containing only clays for rheology modification suffer problems, however, including compromise in the presence of polyvalent cations, such as calcium and magnesium. These cations are frequently present in drilling formations and can cause the drilling mud to become so thick at higher temperatures that thinners may be required. Some clay systems also suffer from undesirable reactivity and temperature degradation, and may not be adequately consistent in composition from batch to batch.

Combinations of clays and polymeric materials have been employed in attempts to counter deficiencies in simple clay/water systems and also to extend the clay. Typical

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PCT/US2004/012198 extenders useful with bentonite systems include polyacrylamide and polyacrylic acid. However, combining clays with polymeric materials increases the complexity of the composition and, therefore, its cost and/or difficulty of preparation, particularly under field conditions. Also unfortunately, the weaknesses of the extending polymer, such as thermal instability, may then dominate the characteristics of the fluid as a whole.

For some applications it is particularly desirable to use a rheology modification agent which exhibits the ability to “shear-thin”, that is, to show a rapid or nearinstantaneous and predictable viscosity reduction upon application of shear forces, followed by a rapid or near-instantaneous return to original viscosity when the shear forces are removed. Clays alone may effectively thicken a composition, but generally do not offer this shear-thinning capability. While there are newer rheology modification agents that do offer shear-thinning capability, they tend to be much more expensive than clay and may be more difficult to employ under typical field conditions.

It would therefore be highly useful to identify a clay-based rheology modification agent for use with an aqueous fluid which takes advantage of the relatively low-cost thixotropy of clay, yet offers shear-thinning capability; which is not cost-prohibitive for large scale application; and which does not undesirably interfere with other commonlysought properties of the fluid.

Accordingly, the present invention is a shear-thinning, clay-based rheology modified composition comprising clay, water, magnesium oxide, aluminum oxide hydroxide, sodium carbonate or potassium carbonate, and calcium oxide or calcium hydroxide; provided that the magnesium oxide and aluminum oxide hydroxide result from heating of appropriate precursors to a temperature of from 500°C to 900°C such that each exhibits a loss on ignition (LOI) test result at 1000°C for 1 hour of less than 3 percent by weight; wherein the pH of the composition is at least 10. Optionally, sodium hydroxide or sodium oxide can be added for pH adjustment.

In another embodiment the present invention is a substantially dry rheology modification agent, suitable for use in an aqueous fluid, comprising clay, magnesium oxide, aluminum oxide hydroxide, sodium carbonate or potassium carbonate, calcium oxide or calcium hydroxide; provided that the magnesium oxide and aluminum oxide hydroxide result from heating of appropriate precursors to a temperature of from 500°C to 900°C such that each exhibits a loss on ignition (LOI) test result at 1000°C for 1 hour of less than 3 percent by weight; and, optionally, sodium hydroxide or sodium oxide.

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In still another embodiment the present invention is a method of preparing a rheology modified aqueous fluid composition comprising admixing clay, water, magnesium oxide, aluminum oxide hydroxide, sodium carbonate or potassium carbonate, and calcium oxide or calcium hydroxide; provided that the magnesium oxide and aluminum oxide hydroxide result from heating of appropriate precursors to a temperature of from 500°C to 900°G such that each exhibits a loss on ignition (LOI) test result at 1000°C for 1 hour of less than 3 percent by weight; and further provided that the pH of the composition is at least 10.

The present invention provides a novel family of compositions which can be classified generally as combinations of a clay-based rheology modification agent and an aqueous fluid. The “rheology modification agent” as defined comprises clay, but the clay may be included with the other agent components as a package, or may be combined separately and directly with the water; or each of the components (including the clay) may be separately admixed with the aqueous fluid to form the rheology modified composition of the present invention in situ. Because each of the components of the rheology modification agent is relatively inexpensive, the resulting rheology modified compositions can be employed in a wide range of applications. A particular advantage of the present invention is that use of the rheology modification agent in an aqueous fluid results in a rheology modified composition which exhibits shear-thinning, that is, the ability to significantly diminish in rheology upon application of shear forces, but to “gel” rapidly upon removal of such shear forces, which operates to effectively suspend solids. This ability to “gel” rapidly, using the term “gel” colloquially and without reference to the precise nature of the chemical and/or ionic bonding and/or composition of the material, is particularly important for applications such as drilling and mining, where solids suspension is critical in maintaining the integrity of the excavation during work stoppages and where pumpability must be easily reinitiated in order to ensure restarts.

Those skilled in the art will understand that the term “drilling” is used herein in its broadest meaning. As such it includes not only the field of exploitation of geological deposits such as petroleum, natural gas, steam and super-heated water (so-called “geothermal” drilling), but also any technical accessory drilling, including but not limited to tunneling, so-called river crossing”, the sealing of dump sites, water well drilling, construction applications such as horizontal directional drilling in general, and the like.

The first necessary component in the present invention’s rheology modification agent is the clay. Such clay is preferably a smectitic clay of any type. Preferred clays

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PCT/US2004/012198 include bentonite, chlorite, polygorskite, saconite, vermiculite, halloysite, sepiolite, illite, kaolinite, attapulgite, montmorillonite, Fuller’s earth, and mixtures thereof. Such are commercially available from a wide variety of sources and are relatively inexpensive.

The second component (without reference to order of mixing) is a magnesium oxide that is prepared by, that is, results from, thermal treatment of an appropriate precursor. Such precursor can be, for example, magnesium oxide itself, magnesium carbonate, magnesium hydroxide, magnesium nitrate, or a combination thereof. Such precursors are available from a number of commercial sources, but generally the thermal treatment must be carried out as a separate step, either by the precursor producer or by a secondary contractor. “Thermal treatment” is defined herein as heating to a temperature of from 500°C to 900°C, preferably for at least 0.25 hour, and more preferably from 700°C to 850°C for at least 0.5 hour. Thermal treatment is considered to be complete when loss on ignition (LOI), as tested according to ASTM D2773-94, at 1000°C for 1 hour, is determined to be less than 3 percent by weight. Those skilled in the art will be easily able to determine, via routine experimentation, optimum times and temperatures to achieve the required LOI. The skilled artisan will also know that increases in time, temperature, or both can be employed when a given material has been shown to not reach the required LOI under given test conditions. It should also be noted, however, that so-called “dead-burned” magnesium oxide (that is, heated to a temperature and/or time such that it has become inert) is strictly proscribed herein. “Thermal treatment,” as defined, can be done using methods and means known to those skilled in the art, but usually rotary kilns or muffle kilns are effective means, with heating ramped from ambient to peak temperature and back again over a period of at least 0.25 hour and preferably longer to achieve the desired LOI test result.

A third component (without reference to order of mixing) is an aluminum oxide hydroxide which is produced via thermal treatment of an appropriate precursor. Such precursors may include aluminum oxide hydroxide (pseudoboehmite or bayrite) itself, or aluminum trihydroxide (boehmite), or alternatively aluminum carbonate or aluminum nitrate, or a combination thereof. As with the magnesium oxide component, the precursors for the aluminum oxide hydroxide component are commercially available from a variety of sources. Thermal treatment of a selected aluminum oxide hydroxide precursor mirrors the options available for treatment of the magnesium oxide precursors, but time is preferably somewhat longer, from 0.5 to 0.75 hour. Determination that such thermal treatment to prepare the aluminum oxide hydroxide is completed can again be

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PCT/US2004/012198 accomplished by measuring the loss on ignition (LOI), using methodology known to those skilled in the art at 1000°C. Again, it is considered to be complete for purposes of the present invention when it is less than 3 percent by weight. In fact, for the sake of convenience the precursors for both the magnesium oxide and the aluminum oxide hydroxide can be treated together for a period of preferably from 0.3 to 0.6 hour, using a temperature of preferably from 500°C to 900°C, until the specified LOI is accomplished.

The fourth component (without reference to order of mixing) is a carbonate, which may include, as its cation, sodium or potassium. Sodium carbonate itself is preferred for reasons of cost and easy availability, but potassium carbonate is also efficacious.

The fifth component (without reference to order of mixing) is calcium hydroxide. Alternatives include calcium oxide, but calcium hydroxide is preferred because of its greater reactivity when used in the present invention.

Finally, the last, and optional, component (without reference to order of mixing) of the rheology modification agent of the present invention is a base selected from sodium hydroxide and sodium oxide. This component can be included where it is necessary to raise the pH of the final aqueous fluid composition to at least 10, preferably from 10 to 12. In some cases the selected clay and other components may provide sufficient basicity such that this component is not needed; however, in most cases at least a small amount of sodium hydroxide and/or sodium oxide is needed to effect the desired pH level. Attainment of this pH operates to more rapidly initiate the thixotropic and shearthinning effect.

While some adjustment of the proportions of the enumerated components is contemplated hereby, it is generally preferred that the components be incorporated within the following weight percentages, based on the weight ofthe aqueous fluid.

Clay is preferably employed in a range of from 0.2, more preferably from 0.5, and most preferably from 1, to 15, more preferably to 10, and most preferably to 4 percent, based on the weight of the aqueous fluid composition. Thus, the clay is preferably present in the minimum concentration which desirably increases the viscosity of the fluid.

The magnesium oxide resulting from thermal treatment is preferably employed in a range from 0.04, more preferably 0.05, to 5.0, more preferably to 0.08, still more preferably to 0.06 percent, based on the weight of the aqueous fluid. Most preferred is

0.057 weight percent.

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The aluminum oxide hydroxide prepared by thermal treatment is preferably from 0.02 to 5.0, more preferably to 0.04, still more preferably to 0.03 percent, based on the weight of the aqueous fluid. Most preferred is 0.028 weight percent.

The sodium carbonate or potassium carbonate is preferably from 0.100 to 3.0, more preferably to 0.003, still more preferably to 0.200 percent, based on the weight of the aqueous fluid. Most preferred is 0.143 weight percent.

Calcium oxide or calcium hydroxide is preferably employed in an amount from 0.05, more preferably from 0.06, and most preferably from 0.07, to 2.0, more preferably to 0.10, still more preferably to 0.08 percent, based on the weight of the aqueous fluid. Most preferred is 0.072 weight percent.

Sodium hydroxide or sodium oxide, where included for pH adjustment, is preferably employed In an amount from 0.002 to 0.005, more preferably to 0.004 percent, based on the weight of the aqueous fluid. Most preferred is 0.003 weight percent.

As already noted, order of mixing of the components is not considered to be critical. Preferably the clay is pre-hydrated, but such is not required. Where it is desired to use a substantially dry rheology modification agent comprising the selected rheology modification agent components except for the clay, such may be admixed using any well-known dry mixing methods and means. As used herein “substantially dry” means that the formulation is in a comminuted form, such as a powder, granules, or pellets, suitable for shipping in bags, and that any waters of hydration that may be found in the chemistry thereof and/or any small amounts of moisture, which may result from, for example, exposure to humidity, are discounted from consideration. Optimization of mixing via any known mechanical means, including for example use of impeller devices, rotational mixing, or other inducement of turbulence, is desirable to ensure consistency in performance. Order of mixing can also be modified according to selections, sources and needs at the preparation or use locations. It is also possible to premix the clay with some or ail of the components; or simply to mix each component separately with the aqueous fluid to form the final rheology modified composition of the present invention in situ.

In preparing a substantially dry rheology modification agent of the present invention it is preferable that the selected clay be from 75 to 91 percent; the magnesium oxide from 2 to 8 percent; the aluminum oxide hydroxide from 1 to 4 percent; the sodium carbonate or potassium carbonate from 4 to 9 percent; and the calcium oxide or calcium

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PCT/US2004/012198 hydroxide from 2 to 8 percent. These percentages are based upon the weight of the agent as a whole.

Other additives may also be included in either a substantially dry package with the rheology modification agent of the present invention (if available in dry form), or in the final rheology modified aqueous fluid composition. Such could include, for example, materials intended to modify lubricity, corrosion resistance, or resistance to geological pressure. In some cases such additional materials may also have limited effect on rheological performance which may be desirable and/or which should be appropriately taken into account. Additional additives may also include fluid loss additives, such as starch, carboxymethylated starch and/or modified polysaccharides; weighting agents, such as calcium carbonate, barium sulfate, and/or magnetite (Fe₃O₄); gas hydrate modifiers, such as glycol and glycerine; and corrosion inhibition agents, such as quaternary halides, especially bromides.

While the present invention has been shown to be usually effective at temperatures up to about 200°F, it may in some instances tend to perform less well at higher temperatures. Enhanced temperature stability can be obtained by including, as another component of either the substantially dry rheology modification agent or of the rheology modified aqueous fluid composition, a small amount of urea or another nitrogen-containing compound such as ethylene amide, propylene amide, mixtures thereof, and the like. Of these, urea is preferred because of its usually low cost and convenience. When employed, the urea is preferably in an amount of from 0.05, more preferably from 0.075, to 2.0, more preferably to 0.2, still more preferably to 0.15, and most preferably to 0.09 percent, based on the weight of the aqueous fluid composition. The inclusion of such a compound substantially enhances the ability of the final rheology modified aqueous fluid composition to maintain the desired rheological properties despite exposure to temperatures of 275°F or even higher.

Where a dry package, suitable for shipping and storage, is desired, it may be appropriate in some cases to limit time of exposure to air. This limitation is desirable due to the inherently hygroscopic nature of some of the components of the present invention and also of some possible additional additives.

In partial summary, it will be noted that, because of the variety of mixing options represented hereinabove, it is possible to prepare a substantially dry composition, suitable for shipping, storage and/or later hydration; a fluid (liquid) composition, particularly suited to small scale batching; or a fluid (liquid) composition prepared in situ.

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PCT/US2004/012198 such as would be encountered when either the substantially dry composition or a small scale liquid composition is added to a much larger liquid environment, such as that encountered in a drilling rig mud pit. The final result, using any of these compositions, will preferably be a viscosified and shear-thinning fluid composition which can be effectively used at a temperature preferably up to at least about 200°F, or, where a nitrogen-containing compound such as urea is also included, up to at least about 275°F.

It is preferred that initial viscosification occurs within 30 minutes from mixing of all components, including pH adjustment where needed; more preferably within 20 minutes; and most preferably within 15 minutes.

In addition to their predetermined maximum viscosity, which is preferably a “gelled” elastic solid phase, the compositions of the present invention may also preferably exhibit desirable shear-thinning, which is also termed “stress-dependent fluidity”. In general, the reduction in viscosity upon stress application can be graphically predicted, with the relationship between viscosity (defined in centipoise) being substantially linear when plotted against shear rate (defined as sec'¹, which is a log scale). In the case of such “gelled” elastic solid phase fluids, under conditions of actual use the phase transition from elastic solid to true fluid under shear conditions is preferably rapid, more preferably within 2 minutes, most preferably effectively instantaneous. It is also preferred that the return to the viscosified elastic solid, or “gelled” state, occurs preferably within 10 minutes, more preferably within 5 minutes, and most preferably within 0.5 minute. This last quality enables the composition to suspehd drill, mill and mining solids particularly well upon cessation of shear forces such as those exerted by drill bits or during pumping. The resultant composition is furthermpre preferably durable, exhibiting no or reduced reduction in its ability to make such rapid viscosity transitions upon intermittent and repeated applications of shear and in a wide variety of environments.

Determination of the degree of modification of rheology can be carried out experimentally onsite, but preferably such can be accomplished beforehand via use of an analytical method. The mbst commonly used methodology is described in detail in Manual of Drilling Fluids Technology, 1985, NL Baroid/NL Industries Inc., whereby a yield point may be obtained using a Fann 35 rheometer.

It is interesting to note that the components of the present invention can still be each individually identified upon appropriate molecular analysis following formation of the rheology modified aqueous fluid composition. Such analysis can include X-ray

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PCT/US2004/012198 diffraction (if crystalline); titration metal analysis; nuclear magnetic resonance-mass spectroscopy; infrared spectroscopy; and other standard molecular analyses.

These and other properties of the present invention will be further illustrated via the following example, which is intended for illustrative purposes only and is not meant to limit, nor should it be construed as limiting, the scope of the invention in any way. Example 1

Bentonite (GOLD SEAL*, a trademark of Baroid, Inc.), in an amount of 70 g, was added in a pre-hydration step to 3,430 g water and mixed for 16 hours to produce a 2 percent by weight clay slurry. The yield point for this slurry was measured and found to be approximately 2.

An amount, 350 g, of this clay slurry was charged in a Hamilton Beach mixer running at medium speed. To this slurry was added a mixture of 0.25 g magnesium oxide (resulting from thermal treatment according to the present invention of an appropriate precursor); 0.25 g of aluminum oxide hydroxide (resulting from thermal treatment according to the present invention of an appropriate precursor); 0.25 g calcium hydroxide, 0.5 g sodium carbonate; and 0.04 g sodium hydroxide. Testing using a Fann 35 rheometer (direct dial read-out) revealed the following results:

<td> Yield Point</td><td> Time</td>

<td> 21</td><td> 20 minutes</td>

<td> 22</td><td> 1 hour</td>

<td> 20</td><td> 2 hours</td>

<td> 24</td><td> 4 hours</td>

Comparative Example A

The procedure of Example 1 was followed to prepare a 2 percent by weight clay slurry which was then charged in a Hamilton Beach mixer running at medium speed. To 350 g of this slurry was added 1 g of magnesium hydroxide which had been pretreated by heating to a temperature of 105°C for a period of 2 hours. The mixture was allowed to sit at ambient temperature for one hour and found upon visual examination to exhibit no significant thickening. It was observed again after about eight hours and found to exhibit significant thickening.

Comparative Example B

A 2 percent by weight clay slurry was prepared as in Example 1 and then 1 g of calcium hydroxide was added to 350 g of this clay slurry and mixed at ambient temperature. Yield point tests exhibited the following results:

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<td> Yield Point</td><td> Time</td>

<td> 4</td><td> 20 minutes</td>

<td> 11</td><td> 2 hours</td>

<td> 31</td><td> 4 hours</td>

<td> 35</td><td> 16 hours</td>

<td> Comparative Example C</td><td></td>

<td colspan="2"> A 2 percent by weight clay slurry was prepared as in Example 1 and then 1 g of</td>

<td colspan="2"> magnesium hydroxide, which had been thermally treated according to the present invention to form magnesium oxide, was added to 350 g of this clay slurry and mixed at ambient temperature under the same mixing conditions as in the previous example. Yield point tests exhibited the following results:</td>

<td> Yield Point</td><td> Time</td>

<td> 2</td><td> 20 minutes</td>

<td> 4</td><td> 2 hours</td>

<td> 5</td><td> 4 hours</td>

<td> 16</td><td> 16 hours</td>

<td> 27</td><td> 32 hours</td>

<td> Comparative Example D</td><td></td>

<td colspan="2"> The procedure of Comparative Example C was carried out, except that</td>

<td colspan="2"> magnesium oxide which had been heated at 900°C for 30 minutes was substituted for the magnesium oxide used in Comparative Example C. Testing revealed the following</td>

<td> results:</td><td></td>

<td> Yield Point,</td><td> Time</td>

<td> 4</td><td> 20 minutes</td>

<td> 12</td><td> 2 hours</td>

<td> 27</td><td> 4 hours</td>

<td colspan="2"> 26 16 hours</td>

<td> 29</td><td> 32 hours</td>

<td> Comparative Example E</td><td></td>

<td colspan="2"> The procedure of Comparative Example D was carried out_t except that the</td>

<td colspan="2"> magnesium oxide was pretreated by heating at 1500°C for 30 minutes. Tests revealed</td>

<td> the following results: Yield Point</td><td> Time</td>

<td> 2</td><td> 20 minutes</td>

<td> 2</td><td> 2 hours</td>

<td> 3</td><td> 4 hours</td>

<td> 2</td><td> 16 hours</td>

<td> 6</td><td> 32 hours</td>

Comparative Example F

The procedure of Example 1 was carried out except that the materials added to the 350 g clay slurry were 0.5 g of magnesium oxide (pretreated at a temperature of

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PCT/US2004/012198 about 900°C) and 0.5 g of calcium oxide (which had not been pretreated). Results were as follows:

<td> Yield Point</td><td> Time</td>

<td> 5</td><td> 20 minutes</td>

<td> 17</td><td> 2 hours</td>

<td> 28</td><td> 4 hours</td>

<td> 29</td><td> 16 hours</td>

<td colspan="2"> Comparative Example G</td>

<td colspan="2"> The procedure of Comparative Example F was carried out except that an</td>

<td colspan="2"> additional material was added, which wais 1 g of soda ash (sodium carbonate, Na₂CO₃).</td>

<td> Results were as follows:</td><td></td>

<td> Yield Point</td><td> Time</td>

<td> 6</td><td> 20 minutes</td>

<td> 12</td><td> 1 hours</td>

<td> 20</td><td> 2 hours</td>

<td> 30</td><td> 4 hours</td>

<td colspan="2"> Comparative Example H</td>

<td colspan="2"> The procedure of Comparative Example G was carried out except that an</td>

<td colspan="2"> additional material was added, which was 0.5 g of aluminum trihydroxide. Results were</td>

<td> as follows:</td><td></td>

<td> Yield Point</td><td> Time</td>

<td> 4</td><td> 20 minutes</td>

<td> 10</td><td> 1 hours</td>

<td> 15</td><td> 2 hours</td>

<td> 20</td><td> 4 hours</td>

<td> 26</td><td> 32 hours</td>

<td> Comparative Example I</td><td></td>

<td colspan="2"> The procedure of Example 1 was carried out except that 0.25 g of AI₂O₃ that had</td>

<td colspan="2"> been pretreated at 1500°C was used instead of the AIO(OH). All other components were the same as in Example 1. Results were as follows:</td>

<td> Yield Point</td><td> Time</td>

<td> 3</td><td> 20 minutes</td>

<td> 9</td><td> 2 hours</td>

<td> 12</td><td> 4 hours</td>

<td> 20</td><td> 16 hours</td>

Example 2

The procedure of Example 1 was carried out except that 0.5 g of aluminum oxide hydroxide which had prepared by heating aluminum trihydroxide to 700°C for 30 minutes was added. Results were as follows:

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Yield Point

Time

23 minutes hours hours 4 hours

Example 3

The procedure of Example 1 was carried out, except that the following materials added to the clay slurry were 0.25 g of magnesium oxide (resulting from heating magnesium hydroxide at 900°G for 30 minutes); 0.5 g calcium oxide; 0.5 g aluminum oxide hydroxide (resulting from heating aluminum trihydroxide at 700°G for 30 minutes); 0.5 g sodium carbonate; and 0.04 g sodium hydroxide. Rheology was tested after 20 minutes using a Fann 35 rheometer and found to be as follows:

RPM*_Dial reading

600 26

300 24

16

14 ‘revolutions per minute, equals shear rate

The results indicated a calculated yield point of 22 and a plastic viscosity of 2. This fluid was then hot-rolled at 275°F for 16 hours, and its rheology was retested with the following results:

RPM_Dial reading

600 4

300 3

1

The results indicated a calculated yield point of 2 and a plastic viscosity of 1. Example 4

The aqueous fluid composition of Example 2 was subjected to rheological testing with the following results, 20 minutes after completing the mixture:

RPM_Dial reading

600 27

300 25

11

11

The calculated yield point was 23 and the plastic viscosity was 2. This fluid was then hot-rolled at 275°F for 16 hours and its rheology retested with the following results:

RPM_Dial reading

600 3

300 1

1

1

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Example 5

The procedure of Example 3 was repeated, except that 0.33 g of urea was also added and mixed for an additional 10 minutes. Rheological testing revealed the following results:

<td> RPM</td><td> Dial readinq</td>

<td> 600</td><td> 27</td>

<td> 300</td><td> 24</td>

<td> 6</td><td> 14</td>

<td> 3</td><td> 12</td>

<td></td><td> Calculated yield point was 21 and plastic viscosity was 3. The aqueous fluid</td>

<td colspan="2"> composition was then hot-rolled for 16 hours at 275°F and the rheology was retested as</td>

<td colspan="2"> follows:</td>

<td> RPM</td><td> Dial readinq</td>

<td> 600</td><td> 26</td>

<td> 300</td><td> 22</td>

<td> 6</td><td> 12</td>

<td> 3</td><td> 11</td>

<td></td><td> Calculated yield point was 18 and plastic viscosity was 4.</td>

<td colspan="2"> Example 6</td>

<td></td><td> The procedure of Example 4 was repeated, except that 0.33 g of urea was also</td>

<td colspan="2"> added and mixing continued an additional 10 minutes. Rheological testing gave the</td>

<td colspan="2"> following results:</td>

<td> RPM</td><td> Dial readinq</td>

<td> 600</td><td> 28</td>

<td> 300</td><td> 24</td>

<td> 6</td><td> 13</td>

<td> 3</td><td> 12</td>

<td></td><td> Calculated yield point was 20 and plastic viscosity was 4. The material was then</td>

<td colspan="2"> hot-rolled for 16 hours at 275°F and the rheology retested as follows:</td>

<td> RPM</td><td> Dial readinq</td>

<td> 600</td><td> 25</td>

<td> 300</td><td> 22</td>

<td> 6</td><td> 14</td>

<td> 3</td><td> 12</td>

Calculated yield point was 19 and plastic viscosity was 3.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof, and has been demonstrated as particularly effective in preparing additives for use in drilling, milling and mining fluids. However, it will be evident that various modifications and changes can be made to the steps and

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PCT/ÜS2004/012198 components used in the method without departing from the broader spirit or scope of the invention as set forth in the appended claims. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense. Thus, specific combinations of selected precursors and of starting materials to form the substantially dry rheology modification agent or the rheology modified aqueous fluid compositions, and specific mixing orders, protocols, and means, falling within the claimed parameters but not specifically identified or tried, are anticipated and expected to be within the scope of this invention.