The addition of particular crystallization suppressants to high density clear brine fluids significantly lower the true crystallization temperatures of the fluids allowing for higher salt content in the fluids and for the preparation of, e.g., higher density zinc free brines.

BACKGROUND OF THE INVENTION

Clear brine fluids are solids-free, industrial fluids widely used in operations where control of pressure in a well is needed, such as in the oil and gas industry, and play an important role in oil exploration and development of deep water wells, high-pressure and deep oil wells, oil sands, and the like. Clear brine fluids find use in well completion, work-over, drilling and fracturing operations, and serve a variety of functions such as a displacement fluid to remove drilling muds, as drill-in fluids, as permanent packer fluid. They inhibit undesirable formation reactions such as clay swelling and are used in preparing well equipment for production, e.g., during insertion of liners, screens, packers, and other equipment.

Clear brine fluids can be prepared with a variety of salts, generally halide salts, at various concentrations to provide specific densities for particular applications. For example, brines with densities ranging, e.g., from 8.4 to over 22 lbs/gal (ppg), may be desired. Commonly used salts include chloride and bromide salts of sodium, potassium, calcium and zinc. Ammonium salts, iodine salts, and other metals have also been used. More than one salt may be present in the fluid.

Bromide fluids, e.g., sodium, potassium and calcium bromide brines, are high density clear brine fluids that are suitable for deepwater production and high temperature / high pressure oil and gas formations. For example, bromide fluids are used in deepwater fracturing operations in order to provide the necessary pressure in the well to successfully fracture the geological formation area that supplies oil and gas to the wellbore allowing for higher volume flows to the production piping.

Clear brine fluids are solids free and thus contain no particles that might plug or damage a producing well or equipment and are used over a wide temperature range. The amount of a particular salt in a brine fluid, and thus the density of the brine, is limited by the solubility of that salt in water. Precipitation of the salt during use must be avoided, and many salts cannot be used on their own in higher density solutions, e.g., 12 ppg or 14 ppg to 20 ppg. Historically, the need for high density clear brine fluids, e.g., >14.2 ppg, has been met by using zinc bromide to blend up calcium bromide to higher densities. Zinc bromide and cesium formate brines traditionally have been used to achieve higher density in completion fluids, up to 19 ppg for high pressure applications such as kill-fluid and on-the-shelf gas wells (high temperature / high pressure).

However, zinc based fluids have environmental and economic limitations. Zinc is regulated and not environmentally friendly and requires a zero-discharge system when in use. Zinc contaminated flow-back and well-produced water has to be collected and shipped to shore for disposal or treatment and cannot, e.g., be pumped off-rig into the Gulf of Mexico. These regulations and a growing consciousness regarding the contamination of ground water has increased the interest in new clear brine fluids that are more environmentally friendly and do not require zero-discharge. A clear brine fluid is needed that can provide high densities and low crystallization temperatures without using zinc components, which fluids can be used in applications typically served with the present zinc containing fluids such as zinc / calcium bromide brines.

SUMMARY OF THE INVENTION

It has been found that certain organic compounds, e.g., certain sugars and 1 ,3- dicarbonyl compounds, can be added to clear brine fluids to lower the temperature at which the salt precipitates out of solution and increase the amount of a salt that will remain dissolved in the water. The addition of these compounds to non-zinc brines provides for new, high density, zinc-free, clear brine fluids having low true crystallization temperatures and good environmental compatibility.

One broad embodiment of the invention provides a method for reducing or lowering the true crystallization temperature of a clear brine fluid (CFB) by adding from 2 to 20 wt%, based on the combined weight of the halide salt and water, of a crystallization

suppressant additive comprising an aldose or ketose having at least 4 carbon atoms, an oligosaccharide compound, an alditol having at least 3 carbon atoms, or a 1 ,3

dicarbonyl compound, e.g., a malonamide, having from 3 to 7 carbon atoms. A CFB of the invention typically comprises a halide salt and in most embodiments has a density of over 8.4 ppg, e.g., at least 9 ppg, often at least 10 ppg and frequently at least 12, 14, 14.2 or higher.

"True crystallization temperature" is the temperature at which the salt begins to crystallize out from a clear brine fluid (CBF) under conditions where the crystallization temperature is determined by the salt composition, i.e., the composition and the concentration of the salt, in the fluid. The crystallization temperature of a CBF can be influenced by changes in pressure and other factors, but the true crystallization temperature excludes these factors and is determined only by the composition of the fluid itself. In the present disclosure, true crystallization temperatures are determined using API Protocol 13J 5th Edition, October 2014 "Testing of Heavy Brines".

Another broad embodiment of the invention provides a clear brine fluid having a density at 60°F of at least 9 ppg, typically 10 ppg, 12 ppg or 14 ppg and often 14.2 or higher, comprising water, a salt, generally a halide salt, and from 2 to 20 wt%, based on the combined weight of the water and halide salt, of an aldose or ketose having at least 4 carbon atoms, an alditol having at least 3 carbon atoms, or a 1 ,3 dicarbonyl compound having from 3 to 7 carbon atoms as a crystallization suppressant. The clear brine fluids of the invention have a true crystallization temperature lower than similar CFBs of the same density that do not contain the crystallization suppressant. In certain embodiments, excellent results are obtained when the crystallization suppressant comprises a C4-6 alditol, e.g., a C5 or Ce alditol such as xylitol or sorbitol, or a diamide such as a C3-7 malonamide, e.g., the compound malonamide itself.

In many embodiments, the halide salt of the CBF of the invention comprises a chloride or bromide salt of sodium, potassium or calcium. In the present disclosure, the article "a" or "an", unless otherwise indicated or necessarily inconsistent with the context, means one or more than one, and more than one salt may be present. Likewise, one or more than one crystallization suppressant compound of the invention may be used. Other components common in the art may also be present in the inventive fluid, including, e.g., other crystallization suppressants or additives to improve a particular property of the CBF, but any of these other components need to be stable under the potentially harsh conditions the present fluids may face, and, if present, these other components make up only a small part of the fluid, e.g., less than 10 wt% or less than 5 wt%, or less than 2 wt%.

In particular embodiments the clear brine fluid comprises less than 1 ppm, e.g., less than 0.5 ppm of zinc or cesium.

DESCRIPTION OF THE INVENTION

One embodiment provides a method for lowering the true crystallization temperature of a clear brine fluid, typically comprising a halide salt and water, and generally having a density of at least 9 ppg, at least 1 0 ppg, at least 14 ppg, e.g., at least 14.2 ppg, which method comprises adding to the clear brine fluid from 2 to 20 wt%, e.g., from 5 to 20 wt% or from 8 to 20 wt%, based on the combined weight of water and salt, typically a halide salt, of a crystallization suppressant additive comprising an aldose or ketose having at least 4 carbon atoms, e.g., an aldose or ketose having from 4 to 6 or from 5 to 6 carbon atoms, an alditol having at least 3 carbon atoms, e.g., from 4 to 6 carbon atoms, e.g. 5 or 6 carbon atoms, or a 1 ,3 dicarbonyl compound, e.g., a malonamide, having from 3 to 7 or from 3 to 5 carbon atoms. The salt may be a metal or ammonium salt. Typically the salt comprises a chloride or bromide salt of sodium, potassium or calcium. In particular embodiments the CFB is zinc free and/or cesium free meaning that it contains less than 1 ppm, e.g., less than 0.5 ppm, of zinc and/or cesium.

For example, in some embodiments the method comprises adding to a CBF comprising water and a chloride or bromide salt of sodium, potassium or calcium and having a density of at least 9, 10, 12, 14, 14.2 ppg or higher, from 2 to 20 wt%, based on the combined weight of salt and water, of a crystallization suppressant described above, wherein the CFB contains less than 1 ppm, e.g., less than 0.5 ppm, of zinc and/or cesium.

In particular embodiments, the method comprises adding as a crystallization

suppressant malonamide, or a Cs or C6 alditol, such as xylitol or sorbitol.

Other embodiments provide a clear brine fluid that is free of solids comprising a salt, typically a halide salt, and from 2 to 20 wt%, e.g., from 5 to 20 wt% or from 8 to 20 wt%, based on the combined weight of water and salt, of a crystallization suppressant additive comprising an aldose or ketose having at least 4 carbon atoms, e.g., an aldose or ketose having from 4 to 6 or from 5 to 6 carbon atoms, an alditol having at least 3 carbon atoms, e.g., from 4 to 6 carbon atoms and often 5 or 6 carbon atoms, or a 1 ,3 dicarbonyl compound, e.g., a malonamide, having from 3 to 7 or from 3 to 5 carbon atoms.

The clear brine fluids of the invention have a density at 60°F of at least 9 ppg, typically at least 10 ppg, e.g., at least 12 ppg and in many embodiments at least 14 or 14.2 ppg, and often greater than 14.2 ppg. The halide salt generally comprises a chloride or bromide salt of sodium, potassium or calcium, e.g., a chloride or bromide salt of sodium or calcium, and in certain embodiments the salt comprises a bromide salt, e.g., sodium or calcium bromide, often calcium bromide. Generally, the CFB is zinc and/or cesium free meaning that it contains less than 1 ppm, e.g., less than 0.5 ppm, of zinc and/or cesium.

For example, some embodiments of the invention provide a clear brine fluid having a density of at least 1 0, ppg, 1 2 ppg or 14 ppg, e.g., at least 1 2, 14 or 14.2 ppg or higher, comprising water, a halide salt, less than 1 ppm of zinc or cesium, and from 2 to 20 wt%, based on the combined weight of the water and halide salt, of an aldose or ketose having 4 to 6 carbon atoms, an alditol having 4 to 6 carbon atoms, or a malonamide having 3 to 7 or 3 to 5 carbon atoms, e.g., a Cs or Ce alditol, such as sorbitol or xylitol, or malonamide. In some embodiments, the density of the CFB is greater than 1 5 ppg.

As previously mentioned, one or more than one halide salt may be present and more than one crystallization suppressant may be used. For example, in one embodiment, a method for lowering the true crystallization temperature of a clear brine fluid, and the fluid obtained by a method comprising adding to a clear brine fluid a crystallization suppressant additive comprising a mixture of two or more compounds selected from aldoses or ketoses having at least 4 carbon atoms, alditols having at least 3 carbon atoms, and 1 ,3 dicarbonyl compounds having from 3 to 7 carbon atoms.

The amount of salt in the fluid will vary depending on the chemical formula and solubility of the salt, and the desired density of the fluid. The salt must of course be soluble in high enough concentrations to obtain to densities needed. In many embodiments, the fluid comprises as a salt, calcium chloride, sodium bromide or calcium bromide in concentrations of greater than 35 wt%, in some embodiments the fluid comprises sodium bromide or calcium bromide in concentrations of greater than 40 wt%. In particular embodiments, the fluid comprises over 45 wt% calcium bromide, e.g., 50 wt% calcium bromide or higher. For example, zinc and cesium free fluids of the invention comprising an alditol or a malonamide crystallization suppressant and 56 wt%, 60wt%, or 64wt% calcium bromide were clear and solid free at temperatures below 20°F. In one example, at a concentration of 61 .5%, calcium bromide will begin to precipitate from an aqueous solution at approximately 83°F, however, adding a crystallization inhibitor of the invention at a loading of about 15% lowers the temperature at which calcium bromide begins to precipitate to 4.5°F. Table 1 illustrates the activity of crystallization suppressants of the invention when added to a brine comprising 61 .5 wt % CaBr2. TCT is true crystallization temperature.

Table 1 . TCT of 61 .5 wt % CaBr2 aqueous solution

Obviously, to maintain a clear, solid free fluid, each component present in the brine, including the crystallization suppressant, must also be soluble in the brine at the needed concentrations and temperature of use.

Given that clear brine fluids are used over a wide temperature range, not only do the brines need to stay clear and free of solids at lower temperatures, e.g., below 60 or 50°F, and in some applications below 40°F, e.g., below 30 or 20°F, but because the fluids are often used at high temperatures and pressure, the components in the fluid should be, and often must be, stable at high temperatures, e.g., greater than 250°F and often greater than 400°F or 450°F. While the salts are typically stable at such temperatures, an effective crystallization suppressant must also be thermally stable at the temperatures at which the fluid is used. Thus, preferred crystallization suppressants are shown to be thermally stable, as determined by thermal gradient analysis (TGA), above temperatures well above 250°F, typically, preferred suppressants are shown to be thermally stable at temperatures of 400°F or higher, e.g., 450°F or higher. While clear brine fluids are special fluids meeting specific density and stability requirements, other issues common to the handling of any fluid will also play a role in selecting the proper CBF for a particular use. For example, in many applications where clear brine fluids are used, it is important that the fluids can be pumped in large quantities and/or high rates and a fluid that is too viscous may be problematic. It is possible that a stable CBF with the proper density may be undesirable for use because of overly high viscosity. The viscosity of a fluid may therefore need to be evaluated along with other features formulating a CBF, and the formulation may need to be adjusted to provide the proper handling characteristics.

As is well known in the chemical arts, sugars, such as those useful in the invention, i.e., aldoses, hexoses and alditols, are generally available in two optically active forms, D and L, often one of the forms is more prevalent in nature. Generally, the naturally occurring sugar will more economically attractive and will be the one chosen for use in the present invention, e.g., D-sorbitol, but the opposite, less naturally abundant form of such sugars may be used in some embodiments, but mixtures of a D and L sugar may not perform the same as a composition wherein only, or predominately, one optically active form is present.

Alternately, oligosaccharide compounds may be used as a crystallization suppressant as described in this disclosure, instead of or in addition to the disclosed aldoses, hexoses and alditols. For example, the oligosaccharide compound may be a

cyclodextrin, such as a (alpha)-cyclodextrin (a 6-membered sugar ring molecule), β (beta)-cyclodextrin (a 7-membered sugar ring molecule), or γ (gamma)-cyclodextrin (an 8-membered sugar ring molecule) or a mixture of two or more cyclodextrins.

The present invention provides a process for lowering the TCT of a CFB and in certain embodiments provides zinc free clear brine fluids, comprising e.g., halide salts of sodium or calcium, with densities of greater than 14.2 and TCTs or less than 20°F. The invention allows one to move away from zinc based CBF's when preparing higher density brines. The new, zinc free clear brine fluids of the invention are solids free, high density, environmentally friendly, are a cost-effective alternative to zinc bromide and cesium formate completion fluids, and do not require zero-discharge like zinc based CBF's.

EXAMPLES

Aqueous calcium bromide samples comprising 53 to 65 wt% calcium bromide solution and 2 to 20 wt% were prepared by adding the crystallization suppressant, i.e., D- sorbitol, malonamide or xylitol, to an aqueous solution of calcium bromide. Generally, some heating is required prior to addition of crystallization suppressant to create a clear CaBr2 solution at higher assays.

In the following examples, true crystallization temperature was established according to API Protocol 13J 5th Edition, October 2014 "Testing of Heavy Brines". Clear brine fluid density of test samples was determined at 60° F using an Anton PAAR Density Meter set at 60° F and the results compared to the solution without suppressant. Density of the CaBr2 solution without suppressant is determined at 100°F due to the higher TCT of the suppressant free fluids.

In the examples, the assay and density of the starting CBF is lowered due to the mass amount of the crystallization suppressant added, however, much higher aqueous brine concentrations can be reached before crystallization occurs due the drastic drop in TCT.

Examples 1 -3: D-Sorbitol as crystallization suppressant

Ex. 1 - D-Sorbitol was added in progressively larger amounts to a 61 .5% CaBr2 aqueous solution and the TCT and density at 60°F was determined.

Ex 2. - D-Sorbitol was added in progressively larger amounts to a 62% CaBr2 aqueous solution and the TCT and density at 60°F was determined.

Ex. 3 - D-Sorbitol was added in progressively larger amounts to a 64% CaBr2 aqueous solution and the TCT and density at 60°F was determined. Results for examples 1 -3 are shown in the table below:

Sorbitol as crystallization suppressant

Examples 4-6: Malonamide as crystallization suppressant

Ex. 4 - Malonamide was added in progressively larger amounts to a 61 .5% CaBr2 aqueous solution and the TCT and density at 60°F was determined.

Ex. 5 - Malonamide was added in progressively larger amounts to a 63% CaBr2 aqueous solution and the TCT and density at 60°F was determined.

Ex. 6 - Malonamide was added in progressively larger amounts to a 63.5% CaBr2 aqueous solution and the TCT and density at 60°F was determined.

Results are shown in the following table:

Malonamide as crystallization suppressant

Examples 7-9: Xylitol as crystallization suppressant

Ex. 7 - Xylitol was added in progressively larger amounts to a 61 .5% CaBr2 aqueous solution and the TCT and density at 60°F was determined.

Ex. 8 - Xylitol was added in progressively larger amounts to a 63% CaBr2 aqueous

solution and the TCT and density at 60°F was determined.

Ex. 9 - Xylitol was added in progressively larger amounts to a 63.5% CaBr2 aqueous solution and the TCT and density at 60°F was determined.

Results are shown in the table below:

Xylitol as crystallization suppressant

The suppression of TCT allows increases in concentrations of calcium bromide in water to reach higher densities.