WO2020242885A1 - Nano-scale weighting agents for use in wellbore fluids, wellbore fluids containing said nano-scale weight agents and methods for precipitating said nano-scale weighting agents - Google Patents

Abstract

Nano-scale weighting agents comprise precipitated weighting agent nanoparticles, and wellbore fluids contain said nano-scale weighting agents. Methods precipitate a nano-scale weighting agent, configured for use in the wellbore fluids, from a solution in the presence of a crystal growth inhibitor to form the precipitated weighting agent nanoparticles. The crystal growth inhibitor comprises at least one carboxylic acid selected from at least one branched carboxylic acid and/or at least one chair-like carboxylic acid.

Description

NANO-SCALE WEIGHTING AGENTS FOR USE IN WELLBORE FLUIDS, WELLBORE FLUIDS CONTAINING SAID NANO-SCALE WEIGHT AGENTS AND METHODS FOR PRECIPITATING SAID NANO-SCALE

WEIGHTING AGENTS

CROSS REFERENCE PARAGRAPH

[001] This application claims benefit of U.S. Provisional Application No.

62/852306 entitled“ NANO-SCALE WEIGHTING AGENTS FOR USE IN

WELLBORE FLUIDS, WELLBORE FLUIDS CONTAINING SAID NANO-SCALE WEIGHT AGENTS AND METHODS FOR PRECIPITATING SAID NANO-SCALE WEIGHTING AGENTS,” filed May 24, 2019, the disclosure of which is

incorporated herein by reference.

FI ELD OF THE DISCLOSURE

[002] The present disclosure is directed to precipitated nano-scale weighting agents, wellbore fluids comprising said nano-scale weighting agents and methods for precipitating said nano-scale weighting agents or weighting agent nanoparticles.

BACKGROUND

[003] When drilling or completing wells in earth formations, various fluids typically may be utilized in the well for a variety of reasons. Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in a petroliferous formation; transportation of drill "cuttings" to the surface; controlling formation fluid pressure to prevent blowouts; maintaining well stability; suspending solids in the well; minimizing fluid loss into and stabilizing the formation through which the well is being drilled; fracturing the formation in the vicinity of the well; displacing the fluid within the well with another fluid; cleaning the well; testing the well; transmitting hydraulic horsepower to the drill bit; fluid used for emplacing a packer; abandoning the well or preparing the well for abandonment; and/or treating the well or the formation.

[004] In general, wellbore fluids, such as, for example, drilling fluids should be pumpable under pressure down through strings of drilling pipe, then through and around the drilling bit head deep in the earth, and then returned back to the earth surface through an annulus between the outside of the drill stem and the hole wall or casing. Beyond providing drilling lubrication and efficiency, and retarding wear, drilling fluids should suspend and transport solid particles to the surface for screening out and disposal. In addition, the fluids should be capable of suspending additive weighting agents (to increase specific gravity of the mud), generally finely ground barites (barium sulfate ore), and transport clay and other substances capable of adhering to and coating the borehole surface.

[005] Barium Sulfate, often in the form of the ore barite, is a traditional weighting agent utilized in wellbore fluids, such as, drilling fluids. Even through weighting agent alternatives have been proposed, barite continues to be one primary weighting agent for wellbore fluids. However, due to the higher barite content required by high-density fluids, often in high-temperature, high-pressure conditions, barite sag and increased plastic viscosity are significant disadvantages associated with the barite weighting agents. Moreover, barite sag and cuttings slip are closely related to particle sizes of the barite weighting agents. For known weighting agent fluids, the settling rate of barite particles relates barite sag and cutting slip.

[006] Accordingly, there is a continuing need for weighting agents, and wellbore fluids including weighting agents, having reduced particle sizes which may reduce settling rates, reduce sag potential, lower plastic viscosity of the wellbore fluid.

BRI EF DESCRI PTION OF THE DRAWI NGS

[007] The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict several embodiments in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings.

[008] FIG. 1 illustrates transmission electron microscopy imaging of dispersed 4- methyinonanoic acid-inhibited BaS0₄ particles showing individual particles in accordance with embodiments disclosed herein.

[009] FIG. 2 is a chemical mechanism for the inhibition of crystal growth and reduction of crystal nucleation potential in accordance with embodiments disclose herein. [0010] FIG. 3 is graphs of particle size distribution (hereinafter“PSD”) of same material calculated by intensity, by volume and by number (particle count) showing the distortion caused by small numbers of large particles to the overall size in accordance with embodiments disclosed herein.

[0011] FIG. 4 is a graph of ZAvg by shear rate of 1 -adamantane carboxylic acid-treated barium sulfate in accordance with embodiments disclosed herein.

[0012] FIG. 5 is a graph ZAvg by shear rate of 4-methylnonanoic acid-treated barium sulfate in accordance with embodiments disclosed herein.

[0013] FIG. 6 is a graph of ZAvg of 1 -adamantane carboxylic acid-treated barium sulfate samples by concentration in accordance with embodiments disclosed herein.

[0014] FIG. 7 is a graph of ZAvg of 1 -adamantane carboxylic acid-treated barium sulfate samples by concentration before and after redispersion in accordance with embodiments disclosed herein.

[0015] FIG. 8. is a graph of ZAvg of 4-methylnonanoic acid-treated barium sulfate samples by concentration in accordance with embodiments disclosed herein.

[0016] FIG. 9 is a graph of ZAvg of 4-methylnonanoic acid-treated barium sulfate samples by concentration before and after redispersion in accordance with embodiments disclosed herein.

[0017] FIG. 10 is a graph of ZAvg of 1 -adamantane carboxylic acid-treated barium sulfate samples by pH in accordance with embodiments disclosed herein.

[0018] FIG. 1 1 is a graph of ZAvg of 4-methylnonanoic acid-treated barium sulfate samples by pH in accordance with embodiments disclosed herein.

[0019] FIG. 12 is a TEM image of dispersed 1 -adamantane carboxylic acid- inhibited BaS04 particles showing individual particles in accordance with embodiments disclosed herein.

[0020] FIG. 13 is a graph of density of the Upper 10 ml of different barium sulfate nanodispersions after static aging in accordance with embodiments disclosed herein. DETAI LED DESCRI PTION

[0021] In the following detailed description, reference is made to the

accompanying drawings, which form a part hereof. The illustrative

embodiments described in the detailed description, examples and drawings are not meant to be limiting and are for explanatory purposes. Other

embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, set forth in the examples and/or illustrated in the drawings, may be arranged, substituted, combined, and designed in a wide variety of different configurations, each of which are explicitly contemplated and made part of this disclosure.

[0022] The present disclosure is generally drawn to precipitated nano-scale weighting agents, wellbore fluids comprising said nano-scale weighting agents and methods for precipitating said nano-scale weighting agents. The nano-scale weighting agents disclosed herein are also referred to as nano-scale weighting agent particles or weighting agent nanoparticles. Additionally, methods of formulating and using wellbore fluids comprising precipitated nano-scale weighting agents or weighting agent nanoparticles are also disclosed herein. Further, the wellbore fluids, disclosed herein, may be at least one selected from drilling fluids, completion fluids, and drill-in fluids. Moreover, the wellbore fluids comprising the nano-scale weight agents or nanoparticles may be referred to as weighted water and/or nanoparticle-weighted fluids.

[0023] Nano-Scale Weighting Agents Formation

[0024] The source of the present nano-scale weighting agents is by precipitation, which allows for attainable nano-scale particle sizes or nanoparticles of the weighting agents. As used herein, the term "precipitated nano-scale weighting agents" refers to nano-scale weighting agents or weighting agent nanoparticles formed synthetically from a solution by chemical precipitation.

[0025] Precipitated nano-scale weighting agents or weight agent nanoparticles used in embodiments disclosed herein include nano-scale barium sulfate particles or nano scale barite particles and/or barium sulfate nanoparticles or barite nanoparticles. Additionally, as the nano-scale weighting agents or nanoparticles of the present disclosure are synthetically produced, one of ordinary skill in the art would appreciate that compounds other than those naturally formed may be formed by precipitation and used as weighting agents in the wellbore fluids of the present disclosure.

[0026] For wellbore fluids comprising traditional barite weight agent having particles sizes larger than nano-scale sizes, the traditional barite weight agent may“sag” out of the wellbore fluid causing density fluctuations. As a result, the density fluctuations may lead to packing-off, increasing Equivalent Circulating Density (hereinafter“ECD”), reducing flow in the wellbore and potential well control issues caused by a lack of density in the wellbore fluid. However, the reduced particle sizes of the present nano scale weighting agents and/or present weighting agent nanoparticles (collectively referred to hereinafter as“the present nanoparticles”) affect particle settling rates of the weighting agent solids of the present nanoparticles within the wellbore fluids. The particles settling rates of the present nanoparticles may decrease with decreasing particle sizes of the present nanoparticles. As a result of the decreased settling rates, the“sag” potential of wellbore fluids, comprising the present nanoparticles, may be reduced.

[0027] For wellbore fluids comprising the nano-scale barium sulfate particles and/or the barium sulfate nanoparticles (collected referred to hereinafter“the barium sulfate nanoparticles”), smaller particle sizes of the barium sulfate nanoparticles may reduce the barium“sag” in the wellbore fluids. Using smaller particle sizes for the barium sulfate nanoparticles may allow the wellbore fluids to be less viscous than wellbore fluids otherwise would have to be to suspend the barite content of the wellbore fluids. Factors, such as,“sag” may be relevant for wellbore fluids circulating in, for example, extended reach (hereinafter“ER”) horizontal wells or directional wells with one or more long sections an inclination between 30° and 60°. Thus, barium sulfate nanoparticles, in dispersion, may having an average particle diameter (hereinafter “dso”) in the range of about 100 nanometers (hereinafter“nm”) or lower which may greatly reduce the settling rates of the barium sulfate nanoparticles.

[0028] For example, a barium sulfate dispersion in non-viscosified water possessing a particle size distribution (hereinafter“PSD”) of 100nm would have a settling rate of 1.903 x 10-8m/sec. Such a PSD would allow fluids either with lower viscosities and thus lower ECDs or eliminating sag on an operational time scale with standard viscosifiers. [0029] One having ordinary skill in the art would recognize that selection of a particular precipitated nano-scale weighting agent material may depend largely on the density of the material because, typically, the lowest wellbore fluid viscosity at any particular density is obtained by using the highest density particles. However, other considerations may influence the choice of product such as cost, local availability, and whether the residual solids or filter cake may be readily removed from the well.

[0030] In some embodiments, the precipitated nano-scale weighting agents may be formed of nanoparticles that are composed of a material of specific gravity of at least about 1.8; at least about 2.2 in other embodiments; at least about 2.26 in other embodiments; no more than about 2.3 in other embodiments; and no more than about 2.27 in yet other embodiments. For example, the nano-scale weighting agents may be formed of nano-particles having a specific gravity of at least 2.20 which may allow wellbore fluids to be formulated to meet most density requirements yet have a particulate volume fraction low enough for the wellbore fluids to be pumpable.

[0031] In some embodiments, the average particle size, dso, of the present

nanoparticles may range from a lower limit of greater than about 1 nm, about 2 nm, about 3 nm, about 5 nm, about 10 nm, about 20 nm, about 50 nm, about 100 nm, about 200 nm, about 250 nm, about 300 nm to an upper limit of less than about 700 nm, about 500 nm, about 350 nm, about 300 nm, about 250 nm, about 100 nm, about 50 nm, about 30 nm, about 20 nm, about 10 nm, about 5 nm, where the present nanoparticles may range from any lower limit to any upper limit. In other

embodiments, the d⁹⁰ (the size at which 90% of the nanoparticles are smaller) of the present nanoparticles may range from a lower limit of greater than about 2 nm, about 5 nm, about 10 nm, about 20 nm, about 50 nm, about 100 nm, micron, about 200 nm, about 300 nm, about 500 nm to an upper limit of less than about 500 nm, about 300 nm, about 200 nm, about 100 nm, about 50 nm, about 20 nm, about 10 nm, about 5 nm, about 3 nm, where the present nanoparticles may range from any lower limit to any upper limit.

[0032] Further, the present nanoparticles may have a PSD other than a monomodal distribution. That is, the present nanoparticles may have a PSD that, in various embodiments, may be monomodal, which may or may not be Gaussian, bimodal, or polymodal. [0033] The present nanoparticles having these average particle sizes may be obtained by chemical precipitation (also referred to herein“precipitation method”), whereby insoluble solid nano-scale weighting agents are produced as a result of a chemical reaction between chemical species in a solution. Precipitation occurs following the mixing of at least two chemical species in solution. One of ordinary skill in the art would appreciate that the chemical identity of those chemicals mixed would depend on the desired resulting compound to be used as nano-scale weighting agent particles.

[0034] In embodiments, a first precursor solution or mixture (hereinafter“the first precursor mixture”) may be mixed with a second precursor solution or mixture

(hereinafter“the second precursor mixture) to provide a precipitation solution. As a result, the present nanoparticles may be precipitated from the precipitation solution which may be or may comprise, for example, at least one precipitate suspension fluid and/or water.

[0035] For example, when the barium sulfate nanoparticles are desired as the nano scale weighting agent, the first precursor mixture may be a barium salt solution and the second precursor mixture may be an alkali sulfate salt solution. In an

embodiment, the barium salt solution may comprise at least one selected from barium hydroxide and barium chloride and the alkali sulfate salt solution may comprise at least one selected from sodium sulfate and sulfuric acid. In another embodiment, the second precursor mixture may comprise ammonium sulfate.

[0036] In embodiments, at least one additive may be present in the precipitation solution prior to or during the precipitation of the present nanoparticles. The at least one additive may be selected from at least one crystal growth inhibitor, at least one surfactant, at least one dispersant or at least one wetting agent. In embodiments, the at least one additive may be at least one carboxylic acid, such as, for example, at least one branched carboxylic acid or at least one chair-like carboxylic acid. In embodiments, the at least one carboxylic acid is at least one selected from

methylnonanoic acid, adamantane carboxylic acid and arachidic acid. In an embodiment, the at lease one carboxylic acid is at least one selected from 1 - adamantane carboxylic acid and 4-methylnonanoic acid.

[0037] A concentration of the at least one additive present in the precipitation solution may range from a lower limit of greater than about .001 mol/l, about 0.03 mol/l, about 0.05 mol/l, about 0.1 mol/l, about 0.2 mol/l, about 0.4 mol/l, about 0.6 mol/l, about 0.8 mol/l, about 1 mol/l to an upper limit of less than about 2 mol/l, about 1.5 mol/l, about 1 mol/l, about 0.8 mol/l, about 0.6 mol/l, about 0.4 mol/l, about 0.2 mol/l, where the present nanoparticles may range from any lower limit to any upper limit.

[0038] In embodiments, the at least one additive may be added to both the first precursor mixture and the second precursor mixture (collectively referred to

hereinafter as“first and second precursor mixtures) prior to precipitation of the present nanoparticles. Each of the first and second precursor mixtures may be treated with the at least additive to inhibit crystal growth in the present nanoparticles precipitated from the precipitation solution. Moreover, the at least one additive may inhibit crystal growth in the present nanoparticles precipitated from the precipitation solution.

[0039] In embodiments, the present methods may utilize mehylnonanoic acid and/or arachidic acid as the at least one additive. The carboxylic acid group of the methylnonanoic acid and/or the arachidic acid may adsorb to a barium sulfate crystal, while the organic tail of the methylnonanoic acid and/or arachidic acid may wrap around the barium sulfate crystal. As a result, growth of the barium sulfate crystal may be reduced, prevented or at least partially prevented by the methylnonanoic acid and/or arachidic acid. By adjusting the pH of the precipitate suspension fluid, a point of zero charge may be reached. As a result, further aggregation may be reduced, prevented or at least partially prevented. The present methods may allow for or achieve a narrow, nano-scale particle dispersion to be created, with a low plastic viscosity.

[0040] In embodiments, the pH of the precipitate suspension fluid may range from a lower limit of greater than about pH 4, about pH 6, about pH 8, about pH 9, about pH 10 to an upper limit of less than about pH 11 , about pH 10, about pH 9, about pH 7, about pH 5, where the present pH may range from any lower limit to any upper limit.

[0041] In other embodiments, the present methods may utilize adamantane carboxylic acid as the at least one additive. The carboxylic acid group of the adamantine carboxylic acid may adsorb to the barium sulfate crystal, but the body of the adamantine group may increase the hydrodynamic diameter of the resulting particle and/or may prevent further nucleation and crystal growth by acting as a 'bumper'.

[0042] The present methods may achieve advantages over known techniques for producing small particles by requiring a smaller/lesser concentration of the at least one additive and/or less energy than grinding larger particles down. The present methods may eliminate the bimodal distributions of known techniques, which generally display dso values in the sub-micron range. The present nanoparticles provided by the methods disclosed herein may have a dgo of about 100 nm or less. As a result, the present nanoparticles may be utilized to produce wellbore fluids, such as, extremely low-sag fluid, a barite-weighted RDF or ^'weighted water'. Thus, wellbore fluids comprising the present nanoparticles may serve or act as a replacement for expensive caesium completions brine.

[0043] In embodiments, present nanoparticles may maintain a nano-scale PSD even when completely dried and re-dispersed, which is an improvement on, and advantage over, known technologies that require and rely on slurries. As a result, the present nanoparticles may be provided, shipped and/or supplied as a dry powder of nanoparticles.

[0044] In embodiments, the concentration of the at least one additive in the

precipitation solution, the first pre-cursor mixture and/or second pre-cursor mixtures may range from a lower limit of greater than about 0.01 mol/l, about 0.03 mol/l, about 0.05 mol/l, about 0.1 mol/l, about 0.2 mol/l, about 0.4 mol/l, about 0.6 mol/l to an upper limit of less than about 1 mol/l, about 0.8 mol/l, about 0.6 mol/l, about 0.4 mol/l, about 0.2 mol/l, about 0.1 mol/l, about 0.05 mol/l, about 0.03 mol/l, about 0.01 mol/l, where the present concentration may range from any lower limit to any upper limit.

[0045] In embodiments, the present nanoparticles, treated with the at least one additive, may have a dgo of less than about 100 nm, less than about 90 nm, less than about 80 nm, less than about 70 nm or less than about 60 nm. In embodiments, the present nanoparticles may have ZAvg particle sizes of less than about 50 nm, less than about 30 nm, less than about 20 nm or less than about 15 nm. In embodiments, dispersed crystals of present nanoparticles, that were treated with the at least one additive, may have crystallite sizes of less than about 20 nm, less than about 15 nm, least than about 10 nm or less than about 5 nm. In embodiments, the present nanoparticles may be suspended in fluid, such as an aqueous-based fluid or water for over a number of days, such as, for example, at least 25 days, at least 100 days, at least 200 days or at least 400 days.

[0046] Fig. 1 shows TEM imaging of dispersed barium sulfate crystals having a crystallite size of about 5 nm, which were produced using methylnonanoic acid, illustrating particle size possibilities achievable by the present methods in

embodiments discussed herein.

[0047] In embodiments, the barium sulfate nanoparticles may be precipitated from the first pre-cursor mixture comprising barium chloride and the second pre-cursor mixture comprising ammonium sulfate, wherein each precursor mixture is treated with the at least one additive selected from arachidic acid, adamantine carboxylic acid or methylnonanoic acid. The additives added to the first and second precursor mixtures may act simultaneously or consecutively to inhibit crystal growth of the barium sulfate crystals. As a result, barium sulfate nanoparticles having a narrow particle size distribution may be formed or provided by the present methods. The additives may also act as dispersants, wetting agents or friction reducers. As a result, the additives may lower a plastic viscosity of a wellbore fluid comprising the barium sulfate nanoparticles. Chemistry of the present methods may inhibit crystal growth of the barium sulfate nanoparticles by steric hindrance and reduction of crystal nucleation potential by increasing the particle's effective hydrodynamic diameter. One chemical mechanism for the inhibition of crystal growth and reduction of crystal nucleation potential of the present methods may be shown in Fig. 2.

[0048] In embodiments, utilizing ammonium sulfate in the second pre-cursor solution may provide greater control of pH conditions such that formation of the barium sulfate nanoparticles may be maintained during the present methods. As a result of controlling pH conditions, a requirement for subsequent pH adjustment during the present methods may be reduce, eliminated or at least partially eliminated. Moreover, particle morphology of the barium sulfate nanoparticles may be altered by pH adjustment during precipitation to pH valves of at least pH 4 and no more than pH 11 .

[0049] Mixing may occur, for example, in stirred tank reactors (batch or continuous), static or rotor-stator mixers. Devices in which the rotor rotates at a high speed (such as at least 120000 rpm) may be suitable for use in forming such precipitated nano scale weighting agents because the shear, transverse, and frictional forces of intermeshing tools (in combination with high speeds) may result in the formation of fine, dispersed nanoparticles. Additional techniques such as the application of impinging jets, micro-channel mixers, or the use of a Taylor-Couette reactor may improve the mixing intensity and result in smaller nanoparticles and better

nanoparticle homogeneity. Alternatively, ultrasonication, which may provide higher shear and stirring energy to induce micromixing and dissipate high power locally, may also provide smaller nanoparticles and better nanoparticle homogeneity by allowing for control of various parameters, such as power input, reactor design, residence time, particle, or reactant concentration independently. After the precipitation solution has passed through the mixer, the resulting precipitated nano-scale weighting agents may be separated out and dried for later use in a wellbore fluid. As a result of drying, the present nanoparticles may be a dry powder or in dry powder form. One mixer that may be used when nano-scale weighting agents are desired is discussed in U.S. Pat. No. 7,238,331 , which is herein incorporated by reference in its entirety.

[0050] During mixing, mechanical shear may be imparted onto the precipitation solution during precipitation at a shear rate to produce nanoparticles having a d 90 of less than about 100 nm, less than about 90 nm, less than about 80 nm, less than about 70 nm or less than about 60 nm. In embodiments, the shear rate may range from a lower limit of greater than about Os-¹ , about 500s ¹, about 1000s-¹ , about 5000s- ¹, about 10000 S ¹ , about 15000s ¹, about 20000s ¹, about 25000s ¹ to an upper limit of less than about 25000s-¹, about 20000s-¹ , about 15000s-¹ , about 10000s-¹, about 5000S ¹ , about 1000s-¹, about 500s ¹, where the present shear rate may range from any lower limit to any upper limit.

[0051] As discussed above, fluids used in embodiments disclosed herein may include precipitated nano-scale weighting agents. In some embodiments, the precipitated nano-scale weighting agents may be uncoated. In other embodiments, the

precipitated nano-scale weighting agents may be coated with the at least one additive, a dispersant or wetting agent. For example, fluids used in some embodiments disclosed herein may include additive and/or dispersant coated precipitated nano scale weighting agents. The coating of the surface of the precipitated nano-scale weighting agents may occur during the precipitation, after the precipitation, or both during and after the precipitation. Inclusion of such coating may be desirable to further prevent agglomeration of the present nanoparticles, and which may also provide desirable rheological effects on the wellbore fluid in which the present nanoparticles may be used. As that term is used in herein, "coating of the surface" is intended to mean that a sufficient number of dispersant molecules are absorbed (physically or chemically) or otherwise closely associated with the surface of the present

nanoparticles so that the fine particles of material do not cause the rapid rise in viscosity observed in the prior art. By using such a definition, one of skill in the art should understand and appreciate that the dispersant molecules may not actually be fully covering the particle surface of the present nanoparticles and that quantification of the number of molecules is very difficult. Therefore, by necessity, reliance is made on a results-oriented definition. As a result of the process, one can control the colloidal interactions of the present nanoparticles by coating the present nanoparticle with the at least one additive and/or at least one dispersant prior to addition to the wellbore fluid(s). By doing so, it is possible to systematically control the rheological properties of fluids containing in the present nanoparticles as well as the tolerance to contaminants in the fluid in addition to enhancing the fluid loss (filtration) properties of the fluid.

[0052] In some embodiments, the precipitated nano-scale weighting agents may include the present nanoparticles having the at least one additive, a deflocculating agent or dispersant coated onto the surface of the nanoparticle. The precipitated particle size may allow for high density suspensions or slurries that show a reduced tendency to sediment or sag, while the dispersant on the surface of the nanoparticle may control the inter-particle interactions resulting in lower rheological profiles. Thus, the combination of high density, fine particle size, and control of colloidal interactions by surface coating the nanoparticles with the at least one additive and/or a dispersant reconciles the objectives of high density, lower viscosity and minimal sag.

[0053] In some embodiments, the nano-scale weighting agents may include nanoparticles with a dso of less than 100 nm or less than 90 nm that are coated, or at least partially coated, with the at least one additive, a polymeric deflocculating agent or dispersing agent. In other embodiments, the nano-scale weighting agents may include nanoparticles with a dgo of less than 100 nm that are coated with the at least one additive, a polymeric deflocculating agent and/or dispersing agent; less than 90 nm in other embodiments; less than 80 nm in other embodiments; less than 70 nm in other embodiments; and less than 60 nm in yet other embodiments. The nanoparticle size(s) may generate suspensions or slurries that will show a reduced tendency to sediment or sag, and the polymeric dispersing agent on the surface of the

nanoparticle may control the inter-particle interactions and thus will produce lower rheological profiles. It is the combination of nanoparticle size(s) and control of interactions that reconciles at least two objectives of lower viscosity and minimal sag. [0054] Coating of the precipitated nano-scale weighting agent with the at least one additive and/or a dispersant may be achieved by adding the at least one additive and/or dispersant to the precipitation solution prior to mixing. Thus, as mixing and precipitation occurs, the nanoparticles are coated. The presence of the dispersant during the mixing and precipitation may also provide for inhibition of crystallite growth of the nanoparticles if ultra-fine and/or nano-scale weighting agents are desired, and also prevention of nanoparticle agglomeration.

[0055] Coating of the precipitated nano-scale weighting agent with the at least one additive and/or a dispersant may also be performed in a dry blending process following precipitation such that the process is substantially free of solvent. The process includes blending the precipitated nano-scale weighting agent and a dispersant at a desired ratio to form a blended material. The blended material may then be fed to a heat exchange system, such as a thermal desorption system. The mixture may be forwarded through the heat exchanger using a mixer, such as a screw conveyor. Upon cooling, the polymer may remain associated with the nano-scale weighting agent. The polymer/weighting agent mixture may then be separated into polymer coated weighting agent, unassociated polymer, and any agglomerates that may have formed. The unassociated polymer may optionally be recycled to the beginning of the process, if desired. In another embodiment, the dry blending process alone may serve to coat the nano-scale weighting agent without heating.

[0056] Alternatively, a precipitated nano-scale weighting agent may be coated by thermal adsorption as described above, in the absence of a dry blending process. In this embodiment, a process for making a coated substrate may include heating a precipitated nano-scale weighting agent to a temperature sufficient to react the at least one additive and/or a monomeric dispersant onto the nano-scale weighting agent to form a polymer coated sized weighting agent and recovering the additive and/or polymer coated weighting agent. In another embodiment, one may use a catalyzed process to form the polymer in the presence of the nano-scale weighting agent. In yet another embodiment, the polymer may be preformed and may be thermally adsorbed onto the nano-scale weighting agent.

[0057] When the at least one additive and/or a dispersant coated, precipitated nano scale weighting agent is to be used in water-based fluids, a water-soluble polymer of molecular weight of at least 2000 Daltons may be used in a particular embodiment. Examples of such water-soluble polymers may include a homopolymer or copolymer of any monomer selected from acrylic acid, itaconic acid, maleic acid or anhydride, hydroxypropyl acrylate vinylsulphonic acid, acrylamido 2-propane sulfonic acid, acrylamide, styrene sulfonic acid, acrylic phosphate esters, methyl vinyl ether and vinyl acetate or salts thereof.

[0058] The polymeric dispersant may have an average molecular weight from about 10,000 Daltons to about 300,000 Daltons in one embodiment, from about 17,000 Daltons to about 40,000 Daltons in another embodiment, and from about 200,000- 300,000 Daltons in yet another embodiment. One of ordinary skill in the art would recognize that when the dispersant is added to the nano-scale weighting agent during a grinding process, intermediate molecular weight polymers (10,000-300,000 Daltons) may be used.

[0059] Further, it is specifically within the scope of the embodiments disclosed herein that the polymeric dispersant be polymerized prior to or simultaneously with the wet or dry blending processes disclosed herein. Such polymerizations may involve, for example, thermal polymerization, catalyzed polymerization, initiated polymerization or combinations thereof.

[0060] Given the particulate nature of the precipitated and the at least one additive and/or dispersant coated precipitated nano-scale weighting agents disclosed herein, one of skill in the art should appreciate that additional components may be mixed with the nano-scale weighting agent to modify various macroscopic properties. For example, anti-caking agents, lubricating agents, and agents used to mitigate moisture build-up may be included. Alternatively, solid materials that enhance lubricity or help control fluid loss may be added to the nano-scale weighting agents and wellbore fluids disclosed herein. In one illustrative example, finely powdered natural graphite, petroleum coke, graphitized carbon, or mixtures of these are added to enhance lubricity, rate of penetration, and fluid loss as well as other properties of the wellbore fluids. Another illustrative embodiment utilizes finely ground polymer materials to impart various characteristics to the wellbore fluids. In instances where such materials are added, it is important to note that the volume of added material should not have a substantial adverse impact on the properties and performance of the wellbore fluids.

In one illustrative embodiment, polymeric fluid loss materials comprising less than 5 percent by weight are added to enhance the properties of the wellbore fluids. Alternatively, less than 5 percent by weight of suitably sized graphite and petroleum coke are added to enhance the lubricity and fluid loss properties of the wellbore fluids. Finally, in another illustrative embodiment, less than 5 percent by weight of a conventional anti-caking agent is added to assist in the bulk storage of the nano-scale weighting agents.

[0061] The particulate materials as described herein (i.e. , the coated and/or uncoated precipitated nano-scale weighting agents) may be added to a wellbore fluid as a weighting agent in a dry form or concentrated as slurry in either an aqueous medium or as an organic liquid. As is known, an organic liquid should have the necessary environmental characteristics required for additives to oil-based wellbore fluids. With this in mind, the oleaginous fluid may have a kinematic viscosity of less than 10 centistokes (10 mm²/s) at 40° C. and, for safety reasons, a flash point of greater than 60° C. Suitable oleaginous liquids are, for example, diesel oil, mineral or white oils, n- alkanes or synthetic oils such as alpha-olefin oils, ester oils, mixtures of these fluids, as well as other similar fluids known to one of skill in the art of drilling or other wellbore fluid formulation.

[0062] In embodiments, the present methods may achieve a low attachment rate of the surfactant or the at least one additive required to achieve a weighting agent nanodispersion, such as, for example, a barium sulfate nanodispersion. The present methods may provide precipitated weighting agent nanoparticles having a dgo of at least less than about 90 nm and/or an average particle size of less than about 20 nm, such as, for example, about 16nm. Further, the present methods may achieve weighting agent nanoparticles have a uniform, or substantially uniform, particle size and/or morphology. In embodiments, the present nanoparticles spherical, or at least mostly spherical, and/or may have near-elimination of bimodal distribution 'peaks' (i.e., no mixture of larger and smaller nanoparticles). As a result of particle size restriction with very low attachment rate of the at least one additive or surfactant, the present methods may require limited or reduced quantities of the at least one additive or surfactant. The nanoparticles provided by the present methods may not require subsequent grinding or high mechanical shear. After drying, the present

nanoparticles may be redispersed to a nano-scale the same as, or very similar to, the initial precipitation size of the nanoparticles. As a result, the present nanoparticles may not be required to be kept in a slurry and/or suspension. In embodiments, wellbore fluids comprising the present nanoparticles may exhibit no or negligable sag in a suspension after long durations of time, such as, for example, in an about 1 9g/cm³ suspension after at least about 400 days. Moreover, wellbore fluids comprising the present nanoparticles may be utilized as 'weighted water' to replace expensive caesium formate brines.

[0063] Wellbore Fluid Formulation

[0064] The precipitated nanoparticles described above may be used in any wellbore fluid such as drilling, cementing, completion, packing, work-over (repairing), stimulation, well killing, spacer fluids, and other uses of high-density fluids, such as in a dense media separating fluid or in a ship's or other vehicle's ballast fluid. Such alternative uses, as well as other uses, of the present nanoparticles and/or fluid should be apparent to one of skill in the art given the present disclosure. In

accordance with one embodiment, the nano-scale weighting agents may be used in a wellbore fluid formulation. The wellbore fluid may be a water-based fluid, a direct emulsion, an invert emulsion, or an oil-based fluid.

[0065] Water-based wellbore fluids may have an aqueous fluid as the base liquid and the precipitated nano-scale weighting agent (coated or uncoated). Water-based wellbore fluids may have an aqueous fluid as the base fluid and the precipitated nano scale weighting agent. The aqueous fluid may include at least one of fresh water, sea water, brine, mixtures of water and water-soluble organic compounds and mixtures thereof. For example, the aqueous fluid may be formulated with mixtures of desired salts in fresh water. Such salts may include, but are not limited to alkali metal chlorides, hydroxides, or carboxylates, for example. In various embodiments of the wellbore fluids disclosed herein, the brine may include seawater, aqueous solutions wherein the salt concentration is less than that of sea water, or aqueous solutions wherein the salt concentration is greater than that of sea water. Salts that may be found in seawater include, but are not limited to, sodium, calcium, sulfur, aluminum, magnesium, potassium, strontium, silicon, lithium, and phosphorus salts of chlorides, bromides, carbonates, iodides, chlorates, bromates, formates, nitrates, oxides, and fluorides. Salts that may be incorporated in a brine include any one or more of those present in natural seawater or any other organic or inorganic dissolved salts.

Additionally, brines that may be used in the wellbore fluids disclosed herein may be natural or synthetic, with synthetic brines tending to be much simpler in constitution. In one embodiment, the density of the wellbore fluid may be controlled by increasing the salt concentration in the brine (up to saturation). In a particular embodiment, a brine may include halide or carboxylate salts of mono- or divalent cations of metals, such as cesium, potassium, calcium, zinc, and/or sodium.

[0066] The oil-based/invert emulsion wellbore fluids may include an oleaginous continuous phase, a non-oleaginous discontinuous phase, and the precipitated nano scale weighting agent. One of ordinary skill in the art would appreciate that the precipitated nano-scale weighting agents described above may be modified in accordance with the desired application. For example, modifications may include the hydrophilic/hydrophobic nature of the dispersant.

[0067] The oleaginous fluid may be a liquid, more preferably a natural or synthetic oil, and more preferably the oleaginous fluid is selected from the group including diesel oil; mineral oil; a synthetic oil, such as hydrogenated and unhydrogenated olefins including polyalpha olefins, linear and branch olefins and the like,

polydiorganosiloxanes, siloxanes, or organosiloxanes, esters of fatty acids, specifically straight chain, branched and cyclical alkyl ethers of fatty acids; similar compounds known to one of skill in the art; and mixtures thereof. The concentration of the oleaginous fluid should be sufficient so that an invert emulsion forms and may be less than about 99% by volume of the invert emulsion. In one embodiment, the amount of oleaginous fluid is from about 30% to about 95% by volume and more preferably about 40% to about 90% by volume of the invert emulsion fluid. The oleaginous fluid, in one embodiment, may include at least 5% by volume of a material selected from the group including esters, ethers, acetals, dialkylcarbonates, hydrocarbons, and combinations thereof.

[0068] The non-oleaginous fluid used in the formulation of the invert emulsion fluid disclosed herein is a liquid and may be an aqueous liquid. In one embodiment, the non-oleaginous liquid may be selected from the group including sea water, a brine containing organic and/or inorganic dissolved salts, liquids containing water-miscible organic compounds, and combinations thereof. The amount of the non-oleaginous fluid is typically less than the theoretical limit needed for forming an invert emulsion. Thus, in one embodiment, the amount of non-oleaginous fluid is less that about 70% by volume, and preferably from about 1 % to about 70% by volume. In another embodiment, the non-oleaginous fluid is preferably from about 5% to about 60% by volume of the invert emulsion fluid. The fluid phase may include either an aqueous fluid or an oleaginous fluid, or mixtures thereof. In a particular embodiment, coated barite or other nano-scale weighting agents may be included in a wellbore fluid having an aqueous fluid that includes at least one of fresh water, sea water, brine, and combinations thereof.

[0069] Conventional methods can be used to prepare the wellbore fluids disclosed herein in a manner analogous to those normally used, to prepare conventional water- and oil-based drilling fluids. In one embodiment, a desired quantity of water-based fluid and a suitable amount of one or more precipitated nano-scale weighting agents, as described above, are mixed together and the remaining components of the wellbore fluid added sequentially with continuous mixing. In another embodiment, a desired quantity of oleaginous fluid such as a base oil, a non-oleaginous fluid, and a suitable amount of one or more precipitated nano-scale weighting agents are mixed together and the remaining components are added sequentially with continuous mixing. An invert emulsion may be formed by vigorously agitating, mixing, or shearing the oleaginous fluid and the non-oleaginous fluid.

[0070] In yet another embodiment, the precipitated nano-scale weighting agents of the present disclosure may be used alone or in combination with conventional

mechanically milled weighting agents. Other additives that may be included in the wellbore fluids disclosed herein include, for example, wetting agents, organophilic clays, viscosifiers, fluid loss control agents, surfactants, dispersants, interfacial tension reducers, pH buffers, mutual solvents, thinners, thinning agents, and cleaning agents. The addition of such other additives should be well known to one of ordinary skill in the art of formulating wellbore fluids and muds.

[0071] Advantageously, embodiments of the present disclosure for wellbore fluids that may possess high density without sacrificing rheology and/or risk of sag. One characteristic of the wellbore fluids used in some embodiments disclosed herein is that the present nanoparticles form a stable suspension, and do not readily settle out. A further desirable characteristic of the wellbore fluids used in some embodiments disclosed herein is that the suspension exhibits a low viscosity under shear, facilitating pumping and minimizing the generation of high pressures and chances of fluid losses or fluid influxes. Further, by using a bottoms up approach as compared to the traditional top down approach, nanoparticles may be achieved without requiring the energy intensive approach of grinding, and in particular precipitated nano-scale weighting agents which were not otherwise realistically attainable may be produced. Additionally, where some mineral ores may be rare, costly, or risking depletion, the present methods may allow for a wellbore fluid to be formulated irrespective of such concerns. Further, it is also noted that as crude mineral ore may contain impurities, which may reduce the specific gravity of the weighting agents, a reduction in impurities (and thus increase in actual specific gravity) may result by synthetically forming the precipitated nano-scale weighting agents in a more controllable

environment.

[0072] In embodiments, the wellbore fluids comprising the present nanoparticles may be a drilling fluid suitable in extended-reach drilling (hereinafter“ERD”) and/or high temperature, high pressure (hereinafter“HTHP”) wells. In other embodiments, the wellbore fluids comprising the present nanoparticles may be utilized in completion operations in place of other wellbore fluids, such as, for example, high density brines.

[0073] In embodiments, the wellbore fluids comprising the present nanoparticles may be completion fluids. The present nanoparticles, which may have low or reduced settling rates, may be utilized as a replacement for, or equivalent to a brine. High densities achievable with the present nanoparticles may enable said wellbore fluids to be a 'weighted water' which may replace some forms of completions brine. The ‘weighted water’ may comprise the barium sulfate nanoparticles suspended therein without the requirement for any viscosifiers. In addition, the barium sulfate

nanoparticles may be utilized in reservoir sections as the nanoparticles may be small enough to flow in and out of formations, without plugging.

[0074] In other embodiments, the present barium sulfate nanodispersions may be suitable for use as a barite-weighted reservoir drill-in fluid. Barite’s insolubility in all but concentrated acid may render it unsuitable for easy removal from reservoir sections, particularly as API grade barite’s PSD easily blocks many oil-producing sands. In view of the PSD of the present barium sulfate nanoparticles, the barite in the present wellbore fluids may flow in and/or out of the formation more easily than API grade barite, which may remove, prevent or substantially prevent possibility of the formation pores being blocked by an insoluble solid. In embodiments, the present barium sulfate nanodispersions may achieve a reduced or lowered plastic viscosity due to the lower solids content which is required to achieve the same density of fluid by barite’s density of about 4.5g/cm³ compared to 2.7g/cm³ of calcium carbonate. [0075] In embodiments, the present barium sulfate nanodispersion may be suitable for use as of a barite-weighted completion fluid with small or reduced particle sizes. In other embodiments, the present barium sulfate nanodispersion may be utilized as a suitable alternative fluid to high-density completion brines or fluids, such as, for example, caesium formate. Formate brines, particularly caesium formate are very expensive and subject to severe supply shortages, the present barium sulfate nanodispersion may allow alternative high density‘brines’ to be used at a

considerably lower cost than current solutions.

[0076] In embodiments, the present methods may provide or produce barium sulfate nanoparticles in dispersion in water by utilizing one or more different crystal growth inhibitors, such as, one or more different branched and/or chair-like carboxylic acids, during precipitation from barium chloride and sodium sulfate. Using the present precipitation method, dispersed barium sulfate nanoparticles may be provided or formed using one or more different branched and/or chair-like carboxylic acids to restrict crystal growth. In embodiments, the precipitated weighting agent nanoparticles may be characterized fully using X-Ray Diffraction (hereinafter“XRD”), with the surface chemistry of each modified nanoparticle tested using solid-state NMR and/or FTIR. As a result, the DPS of the present nanoparticles in a solution or water and/or the implications of the DPS of the present nanoparticles for wellbore fluid chemistry and/or wellbore fluid applications may be determined and/or identified from said characterization of the precipitated weighting agent nanoparticles provided or produced from the present methods.

[0077] Examples

[0078] The present disclosure and proposals are further illustrated by the following specific examples, and these examples are provided to illustrate the disclosure and proposals, and do not unnecessarily limit them.

[0079] Synthesis of Barium Sulfate

[0080] Following the present methods disclosed herein, barium sulfate was

precipitated from 0.1 mol/l solutions of barium chloride and sodium sulfate. To both barium chloride and sodium sulfate mixtures, prior to precipitation, was added a quantity of crystal growth inhibitor, in this case 1 -adamantane carboxylic acid or 4- methylnonanoic acid, dissolved in ethanol. Ethanol was shown to inhibit crystal growth of barium sulfate and for this reason it was deemed a suitable mutual solvent. The ethanol/inhibitor to precursor salt ratio was 50:50 by volume. Mechanical shear was provided to the samples, during precipitation via a Silverson L4-R mixer. After precipitation, samples were centrifuged, washed in a 50:50 ethanol/water mixture and dried in a low-temperature oven. All samples were made to a specific substitution level as‘master’ samples, from which sub-samples could be taken.

[0081] Particle Characterization

[0082] XRD was used to characterize the precipitated samples as pure barium sulfate. The diffractometer used was the Bruker D8 Advance. Copper provided the radiation source, giving a Ka wavelength of 1.54A at 45mA and 45kV. The powder patterns of all samples were collected over the range 2Q = 20-50° and were scanned for 1 hour. This range was chosen, as it was sufficient to detect all major peaks required to identify BaSC The‘EVA’ program was used to identify the peaks present in the pattern and to compare them with those kept on record for specific phases as a Powder Diffraction File (hereinafter“PDF”). The limitation with this technique is that it will only identify phases if they are present in the PDF.

[0083] Crystallite Size Determination

[0084] The program HighScore was used to determine the unit cell dimensions and to calculate the crystallite size. Unit cell refinement was carried out for every barium sulfate sample synthesized. The main purpose of this analysis was to determine if the orthorhombic form of BaS04 had been successfully synthesized.

[0085] Particle Size Measurement

[0086] Due to the extremely small particles being produced in the precipitation process, Dynamic Light Scattering (hereinafter“DLS”) measurements were preferred to laser-diffraction systems. A Malvern Nanosizer S was used to perform DLS measurements, with separate direct measurement of signal intensity and subsequent calculations made for particle size by volume % and a calculated particle count. The testing used a refractive index for barium sulfate of 1.634 and 1.34 for water with all testing being conducted at 21 ° C (sample temperature was adjusted when required). Standard particle size measurements were made on‘raw’ samples i.e. those not dried out/washed first while re-dispersed samples had first been washed & dried before re suspension in water. [0087] FIG. 3 shows the PSD in different forms of a dispersion possessing a ZAvg of 195.9 nm. If the ZAvg was not used and a simple intensity plot was quoted, then clearly a higher-than-actual PSD would be reported. The intensity-based distribution distorts by a factor of around 1 ,000,000 (Rayleigh approximation) causing the dominant particle size range to be 150-500 nm, with no material below 60 nm. When the distribution is reprocessed into a volume-based distribution, the PSD changes dramatically. The large particles ~10 m become a much larger factor due to their huge volume compared to the other particles present. This gives a good indication of the issues encountered when attempting to judge PSD by the dgo value when using laser- diffraction-based techniques, assuming a relatively narrow distribution width (PDi <= 0.5). On a more positive note, the lower distortion factor inherent in a volume-based PSD allows smaller particles to be observed than the intensity distribution. Far more of the distribution is now below 100 nm and a new peak has appeared at 20 nm. It is now clear that a significant proportion of this sample is composed of very small nanoparticles but that a small number of much larger particles are causing a skewing of the overall distribution (particularly in terms of a dgo). When the sample is reprocessed to a number-based or particle count distribution, this effect is enhanced further. The peak at 20 nm is now the dominant size with a vast majority of particles being <100 nm in diameter. The peak at 10 m, which previously made up over 10% of the particle volume has now disappeared, indicating that the number of particles that size is exceedingly small. While a number-based PSD is most representative it is rarely the most accurate value due to the requirement to use Mie theory to convert an intensity distribution to a volume distribution and then to convert that to a particle count. For that reason and the proven relative accuracy of the ZAvg with PDis of the ranges observed in this work, this figure will be quoted for DLS measurements. To give the true dispersed particle size, a combination of DLS measurements, crystallite size and transmission electron microscope (hereinafter“TEM”) imaging is required. An important concept is that of the‘dominant peak’ in a distribution. This is the average size of the largest single peak in a given distribution. In figure A, the dominant peak size is 19.4 nm, indicating that, compared to the overall average particle size that smaller particles are far more common.

[0088] Particle Imaging

[0089] Direct observation of precipitated particles allows a more accurate

representation of the particle size to be obtained The JEOL JEM 201 1 HRTEM has a resolution of 0.18 nm and magnification up to 1 200 OOOx. A lanthanum hexaboride (hereinafter“LaB6”) filament connected to a high voltage is used as the electron source. Samples were finely ground and mixed with acetone; a tiny amount of the acetone-sample mixture is dropped on an amorphous carbon specimen grid which is left to dry. The sample is then inserted into a specimen cartridge which is inserted into the TEM. The electron beam is focused on the specimen and images captured by the CCD camera at column pressures below 1 x 10^-6 mbar.

[0090] Suspension Tests

[0091] 100 ml samples of the nano-dispersions were used, at densities of 2.21 g/cm³ for 1 -adamantane carboxylic acid and 2.27 g/cm³ for 4-methylnonanoic acid, which were sealed and left static at ambient temperature ~ 21⁰ C. The density of the top 10 ml was measured by sealed retort at 21 ° C. Multiple precipitation batches were combined and reduced via heating to produce the large-scale batches used for the suspension tests.

[0092] Controls

[0093] Experimental controls for crystallite size determination were conducted with precipitated barite and standard barite i.e. samples without any inhibitor present in all situations tested - the results for this are not included due to length constraints but are available, if required.

[0094] Results

[0095] Crystallite Size Determination

[0096] Determination of the size of the crystallites formed in each sample is important as it identifies the‘minimum’ particle size that can be achieved from a particular batch. The crystallite sizes are distinct from particle size, which will be discussed in a future section. Particle size, being the size of dispersed crystal aggregates is of most interest in this work, however crystallite size is useful to identify the minimum possible size that can be obtained, assuming ideal dispersion. The crystallite size is calculated using the Debye-Scherrer equation and has been the standard method for assessing ‘particle’ size in barium sulfate research. The limitation of using the Debye-Scherrer equation for this purpose is that it provides no indication of the behavior(s) of the crystallites in dispersion i.e. their useful/effective particle size. For this reason, many papers quote particle sizes, which are in effect crystallite sizes and bear no resemblance to the particle/aggregate size in dispersion. The calculations were conducted using the Highscore Plus software provided with the XRD. All calculations were rechecked‘by hand’ i.e. using the Schemer equation, using a shape factor of 0.9 - assuming spheres, based on TEM results. Each modified sample was split into three sub-samples, with the resulting XRD patterns being used to calculate the crystallite sizes. An average of these three readings was then calculated and presented in Table 1 below.

concentration

[0097] Crystallite diameter decreases with increasing inhibitor concentration, as shown in Table 7 across all tested products. At concentrations of 0.1 mol/l, both inhibitors produce crystallites smaller on average than those produced by simple aqueous precipitation and those produced in a 50:50 water/ethanol mix albeit with a small advantage when using 4-methylnonanoic acid. The results agree broadly with the findings of the dispersed particle size analyses, i.e. that increasing inhibitor concentration decreases the dispersed particle size. The crystallite size provides confidence that the apparent discrepancy between the TEM imaging 5-20 nm and the dispersed ZAvgs is due to larger particles skewing the distributions, as is

demonstrated in FIG. 3.

[0098] Effect of Shear on the particle ZAvq 0.2mol 1-1 inhibitor concentration

Table 2: Effect of shear rate on ZAvg in barium sulfate inhibited with 1 -adamantane carboxylic acid - Average of 3 batches.

[0099] FIG. 4 shows that the ZAvg of the unsheared sample using 1 -adamantane carboxylic acid as an inhibitor was 4339 nm. A shear rate of 5,000s-¹ produces a virtually-unchanged ZAvg of 4392 nm. While the ZAvgs of the individual batches are grouped closer than the unsheared sample, there is no real difference between the two. There was an increase in the ZAvg of the major peak detected from 434.3 nm to 597 nm and the high PDi makes it likely that the sample is very polydisperse, accounting for this discrepancy. Using a shear rate of 10,000s-¹ results in no significant change. The ZAvg increases marginally to 4482 nm with a corresponding increase in major peak to 636 nm, as shown in table 1. The batches are again very close together, but little separates the particles produced at these shear rates. The first sign of a real change is at 15,000s ¹ where a ZAvg of 3072 nm is produced. This is the first change in particle size that has been seen with 1 -adamantane carboxylic acid under shear. The batches are closely grouped, and the major peak size has also dropped to 280 nm. Particle size reduces further when the shear rate is increased to 20,000s ¹ to 2697 nm.

Table 3: Effect of shear rate on ZAvg in barium sulfate inhibited with 4- methylnonanoic acid - Average of 3 batches

[00100] Immediately it can be seen in FIG. 5, that unsheared samples treated with 4- methylnonanoic acid are producing nanoparticles at 0.1 mol/l. Imparting shear at a rate of 5,000s^·1 reduces the ZAvg from 792 nm to 632 nm however the size of the dominant distribution peak has increased from 207 nm to 248 nm, suggesting that shear may have reduced the number of larger particles but has not decreased the size of the lowest particles present. Table 2 indicates that increasing the shear rate to 10,000s-¹ had little effect on the sample with ZAvg effectively unchanged at 61 1 nm and a small decrease in major distribution peak size to 226 nm. A more significant change is noted however at an increased shear rate of 15,000s ¹ with the ZAvg decreasing to 513 nm. The sample sheared at 20,000s-¹ shows very little change from the previous shear rate with a ZAvg of 549 nm. 4-methylnonanoic acid showed little variation between shear rates and while a marginal benefit was observed, it would appear that shear rate is a less important factor in the production of nanoparticles using 4-methylnonanoic acid as an inhibitor than in the case of 1 -adamantane carboxylic acid. The consistency in the size of the major peak size also suggests that 4-methylnonanoic acid is a more effective inhibitor than 1 -adamantane carboxylic acid. [00101] Effect of inhibitor concentration on the particle ZAvg - Unsheared

Table 4: Effect of inhibitor concentration in barium sulfate inhibited with 1 -adamantane carboxylic acid - Average of 3 batches; low/high values in parentheses () after average particle size [00102] The lowest concentration of barium sulfate precipitated using 1 -adamantane carboxylic acid, 0.03 mol/l, produces a ZAvg that is considerably lower than untreated and unsheared barium sulfate. Some sub-micron dominant peaks are detected at 0.03 and 0.05 mol/l, but the relatively high Pdi renders this dubious, in any case the ZAvg is well above nano-scale. At concentrations of 0.2 mol/l and above however a different situation occurs. Table 3 shows an inhibitor concentration of 0.2 mol/l seeing the overall ZAvg drop to 899 nm, which was the first overall nano-scale dispersion observed during testing. Pdi for these samples is low, indicating a narrow particle size distribution and when combined with the dominant peak sizes being close to the ZAvg, at this concentration, the inhibitor is producing a narrow dispersion of nanoparticles. A further concentration increase to 0.4 mol/l brings a dramatic drop in ZAvg, with an overall average particle size of 266 nm. Again, a low Pdi is seen and the dominant peak sizes are close to that of the ZAvg, which indicates that the dispersion is narrow and nano-scale in nature. Furthermore, the major peak size tells us that the most common particle size is lower than the ZAvg itself - from this it is clear that a large proportion of the sample is lower than the ZAvg and that it should be possible to produce an even smaller dispersion. These suspicions were confirmed when the concentration was raised to 0.6 mol/l at which point a ZAvg of 47 nm was recorded, as shown in FIG.5. This ZAvg represents the desired particle size range of <100 nm and was the first sample that to breach this barrier. Due to these ZAvgs being close to the reported crystallite size for this barium sulfate sample, it is unlikely that without shear, pH adjustment or ultrasonic agitation, increased concentration will yield a significantly reduced particle size. We also see that the ZAvg is higher, consistently than the dominant peak size. This is most likely due to skewing of the ZAvg by small numbers of large particles. To test this, the 0.6 mol/l samples were centrifuged for 2 minutes at 2000 rpm. This resulted in a ZAvg of 26 nm, which is very close to the dominant peak size. This centrifuging resulted in a decrease in density from 2.21 g/cm³ to 2.19 g/cm³. This suggests strongly that the distribution can be narrowed significantly without altering the density by a large factor. This reinforces the suggestion that the number of larger particles, relative to the smallest is low.

[00103] 1 -adamantane carboxylic acid as a crystal growth inhibitor also shows good properties as a dispersant. This can be seen from the ZAvgs of the different concentrations, displayed in FIG. 7, before and after dispersion. Between 0.03 and 0.1 mol/l the redispersed ZAvg is no greater than 2.82 % of the original. This difference grows to 28.1 % at 0.2mol/l; while disappointing, is mostly due to a large ZAvg in the redispersed batch 3. The gap increases to 40.82 % at 0.4 mol/l; however, batch 3 again stands out as being noticeably larger than the others. At 0.6 mol/l the redispersed ZAvg is 30.46 % larger than the original dispersion and while the percentage increase after redispersion is initially disappointing, it should be remembered that this is an increase of only 27 nm and that a nano-scale dispersion has still been formed.

Table 5: Effect of inhibitor concentration in barium sulfate inhibited with 4- methylnonanoic acid - average of 3 batches; low/high values in parentheses () after average particle size

[00104] The initial concentration of 4-methylnonanoic acid reduces particle size over untreated barium sulfate considerably, with a ZAvg of 3518 nm instead of 19,971 nm. When compared to previous inhibitors a steadier reduction in particle size is seen in Table 4. The decrease in particle size occurs earlier than in 1 -adamantane carboxylic acid but it is at 0.1 mol/l that the largest change is seen. ZAvg drops to 616 nm, which is considerably lower than any previous inhibitor in water. Interestingly, the dominant peak size is 221 nm, which indicates that most of the dispersion is smaller than the ZAvg and that a small number of larger particles are skewing the data. FIG. 7 shows that the ZAvg decreases with each increase in inhibitor concentration from 307nm to 135 nm and finally to 16 nm at 0.2, 0.4 and 0.6 mol/l. The Pdi shows that relatively narrow distributions are being formed with a good correlation between particle size and major peak size. 0.6 mol/l of inhibitor produces very small particles ca. 50 nm in diameter. Particles of this size are well below the desired 100 nm limit and are likely to provide excellent suspension properties.

[00105] FIG. 9 shows that barium sulfate precipitated in the presence of 4- methylnonanoic acid produced redispersed ZAvgs that were close to that of the original suspension at concentrations of 0.03, 0.05, 0.4 and 0.6mol/l. The outlier is the batch with an inhibitor concentration of 0.1 mol/l where the redispersed ZAvg is over twice as high as the original suspension. Both it and the batches at 0.2 mol/l were considerably larger than those at lower concentration levels (In percentage terms). As particle size decreases then any change in ZAvg will cause a disproportionately large percentage change in diameter. This does not explain the situation here however with the margin of increase i.e. 698 nm is considerably larger than those seen at lower concentrations. The increase of 54 Onm in the case of 0.2 mol/l is considerably higher than those at 0.03 and 0.05 mol/l and correspondingly makes up a much larger percentage of the suspension size at the lower diameters. 4-methylnonanoic acid gives a slightly lower increase after redispersion than 1 -adamantane carboxylic acid at 0.4 mol/l at 39.64 % vs 40.82 %. This lower percentage also equates to a lower increase in absolute terms i.e. 54 nm vs 109 nm. This is also the case at 0.6mol/l where 1 -adamantane produces a higher percentage increase of 34.1 % vs 27.22 % and in absolute terms of 27 nm instead of 13 nm. Centrifuging the 0.6mol 1-1 samples after precipitation, as was done with 1 -adamantane carboxylic acid-inhibited samples, yields a ZAvg of the dispersion of 16 nm, with a small reduction in density, from 2.27 g/cm³ to 2.24 g/cm³ again indicating that a small number of larger particles is skewing the ZAvg away from the dominant peak size.

[00106] Effect of pH on the particle ZAvg - 0.2mol/l Inhibitor concentration, unsheared

[00107] The pH of a colloidal system is very important in determining the particle size of the dispersion, due to the electrochemical dynamics of the particle interaction with the suspension medium. The point of zero charge (hereinafter“PZC”) is the condition where the charge density of a surface is zero. The PZC is usually determined in relation to the pH of an electrolyte, with the PZC being assigned to the colloidal particle. In effect, the PZC is the pH at which the colloidal particle exhibits zero net charge. This point is of interest due to the phenomenon, which occur at the PZC. Particles at their PZC exhibit no zeta potential and as such will display no movement in an electrical field. The PZC is also the point at which the particle will display its maximum solubility, maximum viscosity and, most relevant to this project, minimum stability i.e. the point at which the particles are most prone to

flocculation/agglomeration. PZC is identical to the Isoelectric point (hereinafter ΊER”) if there are no adsorbed molecules on the particle surface. The IEP of barium sulfate has been determined to be 6.92 and as such any adsorbed molecules would be expected to change this and thus alter the particle size of colloidal barium sulfate dispersions. To investigate the effect of pH on particle size, tests were conducted on barium sulfate dispersions with inhibitor concentrations of 0.2mol/|-¹, i.e. the highest concentrations synthesized at that time. The samples had their pH increased from their precipitated‘natural’ pH. The reasoning for this was twofold, firstly studies have shown that alkaline pH conditions provide lower particle sizes in dispersion and secondly that all modern water-based drilling fluids are at an alkaline pH, generally 9.5 - 10.05.

Table 6: Effect of pH in Darium sulfate inhibited with 1 -adamantane carboxylic acid -

Average of 3 batches; low/high values in parentheses () after average particle size

[00108] In the case of 1 -adamantane-carboxylic acid-treated samples, the behavior expected by the crystal growth inhibition mechanism is borne out. We see in Table 5 that as the pH increases, deprotonation of the R-COOH group increases, which in turn increases adsorption of the 1 -adamantane carboxylic acid to the barium sulfate crystals. This pH increase gives an effective increase in inhibitor concentration and reduces ZAvg. The pH effect on particle size is dramatic from the initial pH and this reduction occurs in (mildly) acidic conditions. In addition to this, while the pH at which the steric hindrance is overcome i.e. >10.0 is in line with previous inhibitors, the lowest ZAvg is not reached immediately before the particles agglomerate rapidly (see FIG. 10). This is likely due to the shape of 1 -adamantane carboxylic acid, which, taking the ‘armchair’ configuration gives the particles a lower hydrodynamic diameter, reducing the hydrophobicity of adsorbed inhibitor. The rigidity of the armchair configuration may also play a role in this. The fact that the ZAvg reaches a minimum at a pH of 8 before rising slightly at 10.1 is worth noting, but given the very small differences in size, can effectively be treated as the same result. This, by extension, applies also to the ZAvg at pH 6.15, meaning that past this point, no significant reduction in particle size occurs. It can be said that 1 -adamantane carboxylic acid will produce its lowest ZAvg at a given concentration at the desired pH for a water-based drilling fluid. Clearly the 1-adamantane-treated samples are relatively monodispersed - with the notable exception of pH 12.7 - and this is reflected in the much lower particle size measured. The possibility remains that more monodispersed samples attain their lowest ZAvg at pH levels below 10, but without further testing, this is only an assumption.

Table 7: Effect o pH in barium sulfate inhibited with 4-methylnonanoic acid - Average of 3 batches; low/high values in parentheses () after average particle size.

[00109] The particle size of barium sulfate using 4-methylnonanoic acid as the crystal growth inhibitor behaves in line with expectation from the proposed deprotonation mechanism. The values in Table 6 show that the ZAvg decreases with increasing pH until a point is reached where massive agglomeration occurs. Unlike 1 -adamantane carboxylic acid, the particle size reduction continues until the agglomeration point is reached and while a lower ultimate ZAvg is reached, 4-methylnonanoic acid achieves this at a higher pH, i.e. pH 10.7. The reasons for this are not immediately clear but the very low solubility of 4-methylnonanoic acid may require a higher pH to encourage adsorption to the barium sulfate surface as opposed to forming an emulsion, but as this is the result of one trial, it is difficult to draw conclusions. The results so far, highlighted in FIG. 11 , suggest that crystal growth inhibition is correct and that it is suitable for creating dispersions. The smaller-still‘armchair’ configuration of 1 - adamantane carboxylic acid may also provide a clue as to why it produces successful nano-scale dispersions, but a more likely cause is the rigidity of the 1 -adamantane group, giving a‘bumper’ like effect as opposed to the‘flagellation’ hindrance provided by a long chain.

[001 10] TEM Imaging

[0011 1] At maximum magnification, BaS0₄ inhibited by 1 -adamantane carboxylic acid, as shown in FIG. 12, produces very small particles of ca. 4 nm. The particles imaged are virtually identical to those inhibited by 4-methylnonanoic acid, demonstrating that the two inhibitors are equal in terms of maximum performance. The shape of the particles is also identical between the two inhibitors. The major difference between the two inhibitors is that the very smallest particles are manifestly more common with 4- methylnonanoic acid ~ 3x more sub 10 nm particles.

[00112] FIG. 1 shows that at very high magnification levels, the smallest particles can be observed. Particles of around 3nm have been formed and are relatively dispersed with no clusters or aggregates being formed. It would be inaccurate to claim that these particles were the median size in the precipitated batch, but it is possible that they are the modal particle size. Even if this was the modal particle size TEM images are not a suitable method for any official counts, despite their use as an indicator. Particle morphology is not easy to assess clearly but all particles appear to be round/spherical and do not appear to be elongated. 1 -adamantane carboxylic acid and 4- methylnonanoic acid can be used to produce extremely small particles but this does not indicate the size of particles when dispersed and as such TEM is used only as proof-of-concept. [001 13] Suspension Tests

T able 9: Density of top 10 ml of 100 ml dispersion in water - no viscosifier.

[00114] Suspension tests were conducted using the batches with the lowest ZAvg, i.e. those with an inhibitor concentration of 0.6mol/l, but unsheared and at their respective best pH levels, which were 10.1 for 1 -adamantane carboxylic acid and 10.7 for 4- methylnonanoic acid. The maximum density achieved for 1 -adamantane carboxylic acid-inhibited barium sulfate was 2.210 g/cm³, while for 4-methylnonanoic acid- inhibited samples, a dispersion with a density of 2.27 g/cm³ was used. The results (see Table 8) showed that after standard testing times for static aging, there was no density fluctuation that would not otherwise be explained as being within experimental error. To determine if standard tests were simply too short to detect sagging, a longer testing schedule was planned.

[00115] Extended static sag tests were conducted on the non-viscosified dispersions for a total of 428 days, as shown in FIG. 13, at which point testing was suspended. Both samples showed extreme resistance to sag, despite their densities. 4- methylnonanoic acid-inhibited barium sulfate had no appreciable sag at all by the end of testing, despite its density being close to that of‘spike’ (maximum density - 2.3 g/cm³) caesium formate brine. This was also the case with barium sulfate treated with 1-adamantane carboxylic acid although there does appear to be a slow decline after 250 days. Even if this is a steady decline, after 428 days the sag factor is still < 0.51 , which is a comfortable pass. These results indicate that true colloidal suspension is occurring and that in the presence of a compatible viscosifier, sag will be effectively eliminated.

[00116] Experimental Summary

[00117] Increasing inhibitor concentration progressively may reduce crystallite size for both experimental inhibitors. Alkaline pH may cause a marginal decrease in crystallite size due to the increased deprotonation of the carboxylic acid groups of each (acid) inhibitor, which may, in turn, lead to greater attachment of the RCOO- ion to the Ba²⁺ ion. Inhibitor concentration may be an effect in reducing crystallite size, but that optimizing pH and shear rate may reach the minimum crystallite diameter.

[00118] Mechanical shear may reduce ZAvg with both inhibitors. Additionally, the mechanical shear may increase reproducibility of results, with all batches’ ZAvg aligning more closely after the application of shear compared to when shear is not used. This may be caused by the shear dispersing the inhibitor more evenly and replenishing areas faster after precipitation has occurred. Without inhibitor present, mechanical shear may have slight or substantially slight effect on particle size, at least on a measurable timescale. Shear may impart temperature into the precipitation mixture which may have affected the results. Even if the temperature was a factor, it may be consistent across inhibitors and as such may not have led to any false results. DLS shows that concentration may by an effect on particle size reduction. The presence of any inhibitor may reduce particle size but may increase above 0.03 mol/l to have a significant or relevant effect. The ZAvg of all inhibited samples may decrease steadily with increasing inhibitor concentration. Inhibited BaSC may display a consistent reproducible ZAvg even after redispersion, after drying. Not only are the ZAvgs very close to the original values (in some cases even smaller), the ZAvgs may be self-consistent and follow a same or similar declining trend as inhibitor

concentration may increase. This may demonstrate its suitability to acts as either a suspension or as a dry agent. The redispersion may aided by the mesopores and hydrogen bonding present on the surface of the BaSC None of the particle sizes for each inhibitor had reached an asymptote, suggesting that lower particle sizes may be possible at higher conditions.

[00119] The ZAvg was subject to diminishing returns so while the minimum size may have not yet been reached, it may be likely that the minimum was not significantly smaller, particularly as the crystallite sizes may be approached. Increasing the pH of the suspension above its‘natural’ level may cause a general decrease in ZAvg prior to a large increase. It is likely that pH alteration, in conjunction with higher

concentrations and mechanical shear may represent an optimum scenario for obtaining the lowest possible nanoparticle sizes. [00120] Proof of extremely small, dispersed crystals may be provided by TEM analysis of barium sulfate inhibited by the at least one additive, such as, for example, 1- adamantane carboxylic acid and 4-methylnonanoic acid. Extremely small crystals of ~3 nm may be observed with the TEM images supporting a possibility that the present particles may be the modal particle size.

[00121] Extended suspension tests indicate that even at very high densities, no appreciable sag may be detected. Coupled with the extremely low particle sizes, this may indicate that these nanoparticles may be suitable for use as a heavy brine replacement in drilling fluid, particularly in reservoir drill-in fluids.

[00122] Overview

[00123] Wellbore fluids comprising the present nanoparticles (also referred to hereinafter as“the present nanoparticle-weighted fluids”) may achieve several benefits or advantages over known weighted fluids. For example, the present nanoparticle-weighted fluids may be high-density drilling fluids, which may be utilized as an alternative to expensive heavy brines or as barite-weighted reservoir drill-in fluids. The present nanoparticle-weighted fluids may eliminate, or substantially eliminate, barite sag in wellbore fluids. By using at least one branched and/or chair like carboxylic acid, weighting agent nanodispersions may be achieved that may be stable in water, with no detectable agglomeration. As a result, the weighting agent nanodispersions may be self-dispersing after drying. In embodiments, greater steric hindrance and smaller nanoparticle sizes may be achieved by utilizing the at least one branched and/or chair-like carboxylic acid. In embodiments, the concentration of the at least one branched and/or chair-like carboxylic acid may reduce, prevent or substantially prevent crystal growth and/or may provide particle growth retardation.

[00124] The present methods may produce weighting agent nanoparticles having low or reduced contact areas and/or a dispersed ZAvg of less than about 20 nm, such as, for example, about 16nm. The present methods may produce nanodispersions with a density of less than about 2.30 g/cm³, such as, for example, about 2.27g/cm³. These nanodispersions may display no detectable‘sag’ after a long duration of time in suspension, such as, for example more than about 400 days in suspension. Thus, these nanodispersion may achieve colloidal stabilization. Further, the present methods may achieve decreases in nanoparticle diameter which may be achievable through a combination of mechanical shear during precipitation and pH modification after precipitation has ceased.

[00125] In embodiment, the present methods may achieve an optimum pH post precipitation of less than about pH 11 , such as, pH 10.4, which is close to targeted pH levels of water-based reservoir drill-in fluids. As a result, the present additive- inhibited barium sulfate nanoparticles may be suitable for use as a density agent for wellbore fluids, such as, for example, drilling fluids. Using pH to modify the PSD of the present nanoparticle dispersions strongly may allow these nanodispersions to be tuned to one PSD suitable for an intended operation. In embodiments, the at least one additive or crystalline growth inhibitors utilized during precipitation may be low-cost and non-toxic and/or may enable the dry nanoparticles to disperse to comparable PSDs after drying to their precipitated values. The present nanoparticles incorporated into wellbore fluids may provide high-density brine replacement fluids, which may provide a significant cost saving over the known alternative, such as, caesium formate.

[00126] In embodiments, the present methods provide or produce barium sulfate nanoparticles via the at least one branched or chair-like carboxylic acid which may be more effective and in significantly lower concentrations than known approaches. As a result, equivalent nanoparticle size distributions at ultra-low adsorption levels are achievable by the present methods utilizing the at least one branched and/or chair-like carboxylic acid.

[00127] While various aspects and examples have been disclosed herein, other aspects and examples will be apparent to those skilled in the art. The various aspects and examples disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

What is claimed is:

1. A nano-scale weighting agent configured for use in wellbore fluids, comprising: precipitated weighting agent nanoparticles having a dgo of less than 100 nanometers.

2. The nano-scale weighting agent of claim 1 , wherein the precipitated weighting agent nanoparticles are precipitated barium sulfate nanoparticles having

a dgo of less than 90 nanometers.

3. A method, comprising:

precipitating a nano-scale weighting agent, configured for use in wellbore fluids, from a solution in the presence of a crystal growth inhibitor to form precipitated weighting agent nanoparticles,

wherein the crystal growth inhibitor comprises at least one carboxylic acid selected from at least one branched carboxylic acid and at least one chair-like carboxylic acid.

4. The method of claim 3, wherein the precipitated weighting agent nanoparticles have a dgo of less than 100 nanometers.

5. The method of claim 4, wherein the precipitated weighting agent nanoparticles are precipitated barium sulfate nanoparticles having a dgo of less than 90 nanometers.

6. The method of claim 3, further comprising:

drying the precipitated weighting agent nanoparticles to provide a dried nano scale weighting agent.

7. The method of claim 6, wherein the dried nano-scale weighting agent is dry powder.

8. The method of claim 7, further comprising:

adding the dried nano-scale weight agent to a base fluid to form a wellbore fluid; and

circulating the wellbore fluid into a wellbore.

9. The method of claim 3, further comprising:

adding precipitated weighting agent nanoparticles to a base fluid to form a wellbore fluid; and

circulating the wellbore fluid into a wellbore.

10. The method of claim 3, wherein the at least one carboxylic acid is selected from mehylnonanoic acid, adamantane carboxylic acid and arachidic acid.

11. A method comprising:

treating a first precursor mixture and a second precursor mixture with at least one crystal growth inhibitor; and

precipitating a nano-scale weighting agent, configured for use in wellbore fluids, from a solution to form precipitated nano-scale weighting agent,

wherein the solution comprises at least the treated first precursor mixture and the treated second precursor mixture and the at least one crystal growth inhibitor comprises at least one carboxylic acid selected from at least one branched carboxylic acid and at least one chair-like carboxylic acid.

12. The method of claim 11 , further comprising:

drying the precipitated nano-scale weighting agent to provide a dried nano scale weighting agent.

13. The method of claim 12, wherein the dried nano-scale weighting agent is dry powder.

14. The method of claim 13, further comprising:

adding the dried nano-scale weight agent to a base fluid to form a wellbore fluid; and

circulating the wellbore fluid into a wellbore.

15. The method of claim 1 1 , further comprising:

adding precipitated nano-scale weighting agent to a base fluid to form a wellbore fluid; and

circulating the wellbore fluid into a wellbore.

16. The method of claim 11 , wherein the first precursor mixture comprises a barium salt solution and the second precursor mixture comprise an alkali sulfate salt solution.

17. The method of claim 11 , wherein the at least one carboxylic acid is selected from mehylnonanoic acid, adamantane carboxylic acid and arachidic acid.

18. The method of claim 11 , wherein the precipitated nano-scale weighting agent has a dgo of less than 100 nanometers.

19. The method of claim 18, wherein the precipitated nano-scale weighting agent comprises precipitated barium sulfate nanoparticles having a dgo of less than 90 nanometers.

20. The method of claim 18, further comprising:

changing a particle size of the precipitated nano-scale weighting agent by adjusting a pH level of the solution or a mechanical shear rate imparted onto the solution during precipitation of the nano-scale weighting agent.