NO342495B1 - Borehole fluid and process for preparing an additive to increase the density of a fluid - Google Patents

Abstract

A borehole fluid having an oily phase and an additive to increase the density of the borehole fluid. The additive comprises solid colloidal particles coated with a dispersant applied to the colloidal particle during the comminution process to form the particles. Exemplary starting materials for the colloidal particles include commonly known weighting agents including barite, calcium carbonate, dolomite, ilmenite, hematite or other iron ores, olivine, siderite, and strontium sulfate as well as mixtures and combinations thereof and other similar weight materials. The dispersant is in an illustrative embodiment selected from carboxylic acids having a molecular weight of at least 150 Daltons. Alternatively, the dispersant coating may comprise compounds including oleic acid, polybasic fatty acids, alkonobenzene sulfonic acids, alkanesulfonic acid, linear alpha olefin sulfonic acid or alkaline earth metal salts of any of the above acids, and phosphorus lipids as well as mixtures and combinations of these compounds. In a further illustrative embodiment, the dispersant is a polyacrylate ester. The illustrative polymeric dispersant should have an average molecular weight of from about 10,000 Daltons to about 200,000 Daltons.

Description

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Background for the invention

One of the most important functions of a borehole fluid is to contribute to the stability of the borehole and to control the flow of gas, oil or water from the pores in the formation for e.g. to prevent flow or blowout of formation fluids or

5 coincidence of pressurized soil formations. The column of fluid in the hole exerts a hydrostatic pressure proportional to the depth of the hole and the density of the fluid.

High pressure formations may require a fluid with a specific gravity up to 3.0.

A number of different materials are used today to increase the density of borehole fluids. These include dissolved salts such as sodium chloride, calcium chloride

10 and calcium bromide. Alternatively, powdered minerals such as barite, calcite and hematite are added to a fluid to form a suspension with increased density. It is also known to use finely divided metal such as e.g. iron as a weight material. In this connection, the literature deals with a drilling fluid where the weight material includes iron/steel spherical particles with a diameter of less than 250�m and preferably between

15 15 and 75�m. It has also been proposed to use finely powdered calcium or iron carbonate, with the difficulty then being that the plastic viscosity of such fluids rapidly increases as the particle size decreases.

It is a requirement for borehole fluids that the particles form a stable suspension, and are not easily deposited. A second requirement is that the suspension should exhibit a low visco-

20 sited to facilitate pumping and minimize the generation of high pressures. A further requirement is that the borehole fluid slurry should exhibit low filtration losses (fluid losses).

Conventional weights such as e.g. powdered barite exhibits an average particle diameter (d50) in the range of 10 to 30�m. In order to satisfactorily suspend these materials, the addition of a gelling agent such as e.g.

25 bentonite for water-based fluids, or organically modified bentonite for oil-based fluids. A soluble polymeric viscosity-increasing agent such as e.g. xanthan gum can also be added to delay the sedimentation rate of the weighting agent. However, this comes at the expense of the fact that as more gelling agent is added to increase suspension stability, the fluid viscosity (plastic viscosity) increases undesirably and

30 results in reduced pumpability. This is clearly also the case if a viscosity increasing agent is used to maintain a desired level of solids slurry.

The sedimentation (or "seeking") of particulate weights becomes more critical in boreholes drilled at obtuse angles from the vertical direction, as seepage of e.g.

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2.54 cm can result in a continuous column of reduced density fluid along the upper part of the borehole wall. Such butt-angle wells are often drilled over long distances for e.g. to gain access to remote parts of an oil reservoir. In these cases, it is important to minimize the plastic viscosity of a drilling fluid to reduce pressure losses

5 along the borehole length. At the same time, a high density should also be maintained to prevent a blowout. Furthermore, as noted above with particulate weights, issues of seepage become increasingly important to avoid differential wedging or separation of the particulate weights on the lower side of the borehole.

10 Being able to compose a drilling fluid with a high density and a low plastic viscosity is no less important in deep high-pressure wells where high-density borehole fluids are required. High viscosities can result in an increase in bottom hole pressure under pumping conditions. This increase in "equivalent circulating density" can result in the opening of fractures in the formation and

15 severe loss of borehole fluid into the fractured formation. Here too, however, the stability of the suspension is important to maintain the hydrostatic pressure to avoid a blowout. The purpose of high density fluids with low viscosity plus minimal seepage of the weight material can be difficult to reconcile. There is therefore a need for materials to increase the fluid density and which at the same time provide

20 brings improved suspension stability and less viscosity increase.

Patent application WO1998003609 relates to an additive that increases the density of the borehole fluid. The additive described in WO1998003609 comprises colloidal particles with a specific dimension, the particles being deflocculated under the action of a dispersant.

US 5681877 describes the use of AB block copolymers as wetting agents. and dispersants for solid particles suspended in an organic medium comprising a solvent or solvent mixture in which block B is soluble.

US 6204224 relates to a drilling fluid system comprising a formulation in which copolymers of polyalkyl methacrylate, with a selected amount of vinyl pyrrolidone

30 prevents and reduces barite precipitation while providing cuttings transport in the drilling system. In particular, US 6204224 is relevant for modifying the properties of a drilling fluid with regard to rheology and filtration control when using the additives.

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Summary of the invention

In a first aspect, the present invention provides a borehole fluid, which comprises an oily phase and an additive to increase the density of the borehole fluid, where the additive comprises solid colloidal particles coated with a

5 dispersant, wherein the dispersant is a polymeric acrylate ester.

In another aspect, the invention provides a method for producing an additive to increase the density of a fluid, which comprises: comminuting a solid material and a dispersant in a liquid medium so as to produce solid colloidal particles coated with the dispersant, wherein

10 the dispersant is a polymeric acrylate ester.

A drilling fluid additive and a method for producing the additive are described here to increase the density of the fluid while at the same time a usable suspension stability is maintained without a significant increase in viscosity. In an illustrative embodiment, the method includes comminuting a solid

15 material and a dispersant in a liquid medium, so that solid colloidal particles are produced which are coated with dispersants. Preferably, the colloidal particles have a weight average particle diameter (D50) of less than about 10 µm and more preferably less than about 2 µm. The liquid medium is preferably an oily fluid and more preferably an oily liquid which is

20 environmentally acceptable as the continuous phase of an oil-based drilling fluid. In order to achieve an optimal and safe grinding process, the oily fluid has preferred a kinematic viscosity of less than 10 centistokes (10 mm<2>/s) at 40�C and a flash point of more than 60�C. Illustrative examples of such oily fluids include diesel oil, mineral oils or a liquid paraffin, n-alkanes or

25 synthetic oils such as alpha olefin oils, ester oils or polyalpha olefins, as well as combinations and mixtures of these and similar fluids which will be known to one skilled in the art. The dispersant which is coated on the solid particle during the course of the painting is a polymeric acrylate ester. The polymeric acrylate ester should have an average molecular weight of from about 10,000 Daltons to about

200,000 Daltons and more preferably from about 17,000 Daltons to about 30,000 Daltons. The solid material can be selected from a wide variety of known weight materials and in an illustrative embodiment the solid material is selected from the group consisting of barite, calcium carbonate, dolomite, ilmenite, hematite or other iron ores, olivine, siderite and strontium sulfate, mixtures and combinations

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of these and similar weight materials that will be known to a person skilled in the art. In a preferred illustrative embodiment, the fine division of the solid material and the dispersant in the liquid medium is carried out in a stirred fluidized layer of a particulate grinding material.

These and other features of the invention are more fully set forth in the following description of preferred or illustrative embodiments of the invention.

Description of illustrative embodiments

10 It is known in this area that reduced particle sedimentation rates can be achieved by reducing the particle size used. However, the conventional view within the drilling industry is that reducing the particle size causes an undesirable increase in viscosity. The rapid increase in viscosity as the particle size decreases is believed to be due to an increase in the surface area of

15 the particles and which causes adsorption of water on the surface of the particles. As reported in "Drilling and Drilling Fluids" Chilingarian G.V. and Vorabutor P.1981, pages 441-441, "The difference in results (i.e. increase in plastic viscosity) when particle size is varied in a mud slurry is primarily due to magnitude of the surface area, which determines the degree of adsorption (tying up) of water. More water ice

20 adsorbed with increasing area”. Furthermore, it is also stated that "Viscosity considerations often will not permit the addition of any more of the colloidal solids necessary to control filtration, unless the total solids surface area is first reduced by removing a portion of the existing clays". It is thus reported in the literature that colloidal fines, due to their high ratio between surface area and volume, will

25 adsorb significantly more drilling fluid than larger particles. Due to this high adsorption of drilling fluid to the surface of the particles, an increase in the viscosity (ie a decrease in the fluidity) of the mud is observed. For these reasons, a person skilled in the art should understand and realize that in weighted particulate sludge it is necessary to remove the fines in order to reduce the viscosity increase due to such fine particles.

30 This concept is reflected in the API specification for barite as a drilling fluid additive. As stated in the API specification, the %w/w of particles with a diameter below 6�m is limited to a 30%w/w maximum to minimize the increase in viscosity.

In light of the foregoing, a person skilled in the art should immediately realize and understand that

35 it is very surprising that the products according to this invention, as an

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turning particles ground to an average particle size (d50) of less than 2 �m, provides wellbore fluids with reduced plastic viscosity while greatly reducing sedimentation or seepage.

The additives according to this invention comprise solid colloidal

5 particles with a deflocculant or dispersant coated on the surface of the particles. The fine particle size generates high-density suspensions or slurries that show a reduced tendency to sedimentation or seepage, while the dispersant on the surface of the particles controls the interactions between the particles and results in lower rheological profiles. It is

10 thus the combination of high density, fine particle size and control of colloidal interactions by means of surface coating of the particles with a dispersant which combines the purposes of high density, lower viscosity and minimal seepage.

A person skilled in the art will realize and understand that the use of small particles in drilling

15 fluids are well known in the field, but for a totally different purpose. E.g. EP-A-119745 deals with a high-density fluid to prevent blowout and which contains water, a first and possibly a second weighting agent and a gelling agent made of fine particles (average diameter from 0.5 to 10�m). The gelling agent particles are small enough to impart static gel strength to the fluid on the basis of inter-

20 particle attractive forces. A person skilled in the art should also realize that if the concentration of the weighting agent is sufficiently low, no gelling agent is needed in the fluids according to EP-A-119745. The small particle size gives the fluid according to EP-A-119745 viscosity-increasing properties which are evident from the high ratio between surface area and volume for the small particles. The teaching according to EP-A-119745 re-

25 view and other similar references are exactly contrary to the teachings of the present invention. That is to say, the teachings according to the prior art indicate that the material with a small particle size can be added to a drilling fluid, but when this is done, the viscosity of the drilling fluid increases. In contrast to this, the surprising results of the present invention are that one can add

30 very fine particulate material which is coated with a dispersant layer and is not subjected to the rapid increases in viscosity exhibited by the prior art.

According to the present invention, dispersant is coated on the particulate weight additive during the comminution process (grinding process).

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That is to say, coarse weight additive is ground in the presence of a relatively high concentration of dispersant so that the newly formed surfaces of the fine particles are exposed to and thus coated with the dispersant. It is thought that this allows the dispersant to find an acceptable con-

5 formation on the particle surface so that the surface is covered. Alternatively, it is thought that due to a relatively higher concentration of dispersant in the grinding fluid, in contrast to the concentration in a drilling fluid, the dispersant will more likely be absorbed (either physically or chemically) to the particle surface. As this term is used herein, "coating the surface" is intended to indicate that

10 a sufficient number of dispersant molecules are absorbed (physically or chemically) or otherwise closely associated with the surface of the particles so that the fine particles of the material do not cause the rapid increase in viscosity observed in the prior art. Using such a definition, one skilled in the art should understand and realize that the dispersant molecules do not actually come

15 to completely cover the particle surface and that quantification of the number of molecules is very difficult to do. The necessary dependence is therefore made on a result-oriented definition. As a result of the inventive process, it has been discovered that the colloidal interactions of the fine particles can be controlled by coating the particles with a dispersant prior to addition to the drilling fluid. By

20 to do this it is possible to systematically control the rheological properties of fluids containing additives as well as the tolerance to contaminants in the fluid in addition to increasing the fluid loss (filtration) properties of the fluid.

Evidence supporting this result-oriented definition in the foregoing can be found in the embodiment examples that follow as well as in the prior art

25 technique. As is well known to one skilled in the art, in the absence of the coating dispersant, a concentrated slurry of particles with a d50 of less than 2 µm will result in a non-pumpable paste or gel. According to the method and the mixtures according to the invention, a dispersant is coated on the particle surface during the grinding process or the comminution process. This provides

30 a beneficial improvement in the dispersion state of the particles compared to subsequent addition of dispersant to fine particles. The presence of dispersant in the comminution process produces separate particles which can form a more effective compacted filter cake and thus advantageously reduce filtration losses.

According to an illustrative embodiment, the dispersant is selected so that it

35 provides the appropriate colloidal interparticle interaction mechanism to

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making it tolerant of a range of common borehole contaminants, including salt saturation.

According to a preferred embodiment of the invention, the weight agent according to the invention is formed from particles consisting of a material with a specific weight

5 at least 2.68. This allows borehole fluids to be formulated to meet most density requirements but still have a particle volume fraction low enough for the fluid to be pumpable.

A preferred embodiment of this invention is that the weight average particle diameter (d50) of the new weight agent should be less than 1.5 microns

10 meters. This will increase the suspension's characteristics with respect to sedimentation or seepage stability without the viscosity of the fluid increasing so that it becomes unpumpable.

A method for comminuting a solid material to obtain material containing at least 60% by weight of particles smaller than 2�m is known, e.g. from British Patent 1472 701 or 1599 632. As taught therein, a coarse mineral is ground in a

15 aqueous suspension in a stirred fluidized bed of a particulate grinding medium for a time sufficient to provide the required particle size distribution. The same grinding process can be carried out by replacing the water-based fluid with an oily (oil) based fluid. An important preferred embodiment aspect of the invention is the presence of the dispersant in the step of the "wet" painting of

20 the mineral.

The colloidal particles can be provided as a concentrated slurry either in an aqueous medium or more preferably as an organic liquid. In the latter case, the organic should be acceptable as a component and have the necessary environmental characteristics required for additives to oil-based drilling fluids.

25 With this in mind, it is preferred that the oily fluid has a kinematic viscosity of less than 10 centistokes (10 mm<2>/s) at 40�C and, for safety reasons, a flash point of more than 60�C. Suitable oily liquids are e.g. diesel oil, mineral oil or liquid paraffin, n-alkanes or synthetic oils such as alpha olefin oils, ester oils or polyalpha olefins, mixtures of these fluids as well

30 as other similar fluids which will be well known to those skilled in the art of drilling fluid composition.

It is known that polymers with a high molecular weight act as flocculants by bridging between particles, while e.g. low molecular weight polymers

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weight (less than 10,000) act as deflocculating agents by creating overall negative charges.

It has been found that when the dispersant is added during painting, polymers with medium molecular weight (in the range of e.g. 10,000 to

5,200,000). Dispersants with an intermediate molecular weight are advantageously less sensitive to contaminants such as e.g. salt and is therefore well adapted to borehole fluids.

The polymeric acrylate ester used as dispersant should have an average molecular weight of from about 10,000 Daltons to about 200,000

10 Daltons and more preferably from about 17,000 Daltons to about 30,000.

The colloidal particles themselves consist of weighting materials which are well known to a person skilled in the art of weighting drilling fluids. In an illustrative embodiment, the particles are made from one or more materials selected from but not limited to barium sulfate (barite), calcium carbonate, dolomite, ilmenite, hematite

15 or other iron ores, olivine, siderite, strontium sulphate. Normally, the lowest borehole fluid viscosities at any particular density are obtained by using the colloidal particles with the highest density. However, other considerations can influence the choice of product, such as price, local availability and the energy required for painting. Minerals such as calcium carbonate and dolomite have that advantage

20 that residual solids or filter cake can be easily removed from a well by treatment with acids.

The mixtures resulting from the process according to the invention have a surprisingly diverse range of applications in drilling fluids, cement, high density fluids and coiled tubing drilling fluids to name a few. The new particulate weight-

25 agents have the ability to stabilize the laminar flow regime and delay the onset of turbulence. It is possible to combine fluids for several applications including spiral pipe drilling fluids that can be pumped faster before turbulence is encountered, so that mainly lower pressure drops are given at equivalent flow rates. This ability to stabilize the laminar flow regime is true

30 surprisingly but appears adequately in high density muds of 2.39 g/cm<3> (20 pounds per gallon) or more. Such high density slurries using conventional weight agents with a weight average particle diameter of 10 to 30�m would exhibit dilation with the concomitant increase in pressure drops due to the turbulence generated. The ability of the new weight agent to stabilize the flow regime even i

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the presence of a component of larger particles means that high density fluids with acceptable rheology are possible with lower pressure drop.

A further and unexpected application occurs in cement whereby the new weighting agent will generate slurries of more controlled and lower rheology so that the slurry

5 can be pumped more freely into position. One skilled in the art will appreciate that the reduced particle size will have a less abrasive character, while its suspension characteristics will reduce the free water and other suspension issues that occur when the cement sets. The high fraction of fines should also act as an effective filtration loss control agent so that gas migration is prevented

10 and stronger cements are produced.

The fluids according to the present invention can also be used in non-oil field applications such as e.g. separation fluid for compact media (e.g. for the extraction of ore) or as a vessel's ballast fluid.

The following examples illustrate the characteristics and performance of

15 the borehole fluids according to the invention, although the invention is not limited to the specific embodiments according to these examples. All testing was carried out using API RP 13 B where appropriate. Mixing was performed on Silverson L2R or Hamilton Beach mixers. The viscosity at different shear rates revolutions per minute (RPM) and other rheological properties were obtained by

20 use of a Fann viscometer. Sludge weight was checked using a standard sludge scale or an analytical balance. Filtration loss was measured using a standard API filtration loss cell.

When expressing to metric equivalents, the following US to metric conversion factors have been used: 1 gal = 3.785 litres; 1 lb. = 0.454 kg; 1 lb./gal(ppg) =

25 0.120 g/cm3; 1 bbl = 42 gal; 1 lb./bbl (ppb) = 2.835 kg/m<3>; 1 lb/100ft<2>= 0.488 (0.4788) Pa = 48.8 g/m<2>= 4.88 kg/100m<2>.

These tests have been carried out with different grades of barite: a standard grade of API barite, with a weight average particle diameter (D50) of approximately 20�m; a commercial baryte (M) produced by milling/grinding of

30 barite while in this dry state, with an average size of 3 �m to 5 �m and colloidal barite according to the invention (with a D50 from 0.5 �m to 1.5 �m) with a dispersant included under it ” wet” painting process. The corresponding particle size distributions are shown in fig.1. The dispersant is "IDSPERSE" XT (trademark of Schlumberger), an anionic acrylic terpolymer

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with molecular weight in the range 40,000 to 120,000 with carboxylate and other functional groups. This preferred polymer is advantageously stable at temperatures up to 200�C, withstands a wide range of contamination, provides good filtration properties and does not easily desorb from the particle surface.

5 Samples were measured on a Malvern "Microplus" instrument using the display (optical model) RI (particle 1.61; absorption 0.1; RI (dispersant) 1.46). The analysis was carried out using a drop of the ground material in an oil dispersant.

The following examples are included to show preferred embodiments

10 for the invention. It will be realized by those skilled in the art that the methods discussed in the examples that follow represent methods which have been proven by the inventors to work well in the practice of the invention and can thus be regarded as constituting preferred forms of its practice.

15 Example 1

Several 2.63 g/cm<3> (22 ppg) fluids, based on barium sulfate and water, were prepared using standard barite and colloidal barite according to the invention. The 2.63 g/cm<3> (22 ppg) slurry of API grade barite and water was prepared without any gelling agent to control the interparticle interactions (Fluid #1). Fluid #2 is also based

20 on standard baryte but with the subsequent addition of 5.7 kg per cubic meter (two pounds per barrel) "IDSPERSE" XT. Fluid #3 is 100% new weight agent with 67% weight/weight particles with a size below 1 micrometer and at least 90% smaller than 2�m.

The results are shown in Table I.

25 Table I

# Viscosity at different shear rates (rpm Plastic Fluid Agitation): Instrument reading or "Find units" for: viscosity limit 600 rpm 300 rpm 200 rpm 100 rpm 6 rpm 3 rpm mPa.s Pascal (lb/100ft<2>) 1 250 160 124 92 25 16 90 34 (70) 2 265 105 64 26 1 1 160 -26 (55) 3 65 38 27 17 3 2 27 5 (11)

After considering the foregoing data, one skilled in the art should realize that the viscosity of fluid #1 is very high and the slurry was observed to filter very rapidly. It should

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also realize that if additional materials are added to reduce the filtration loss, the viscosity would increase even more. It is also worth noting that this system seeps significantly within 1 hour and provides significant free water (approx. 10% of the original volume).

After the addition of 5.7 kg/cm<3> (two pounds per barrel) "IDSPERSE" XT to

5 this system fluid (#2) is shown to reduce the low shear rate viscosity by controlling the interparticle interactions. However, it should be noted that due to the particle concentration and average particle size, the fluid exhibits dilation, which is indicated by the high plastic viscosity and negative yield strength. It should be realized that this will result in significant pressure drops below

10 the pumping of these fluids. Furthermore, it should be noted that fluid #2 seeps immediately upon respite.

One skilled in the art will note that with regard to fluid #3, the fluid exhibits a substantially lower plastic viscosity when compared to fluids #1 and #2. The presence of the dispersant coated on the particles is found to control the inter-

15 the particle interactions so that fluid #3 becomes pumpable and not gel-like. It will also be appreciated that the much lower average particle size has stabilized the flow regime. That is, a consideration of the data will show that the flow is now laminar at 1000 s<-1> as evidenced by the low plastic viscosity and positive yield strength. After considering the above data one will

20 experts realize that there is an observable and significant effect on the rheological properties of the above-mentioned fluids caused by the coating of the fine particles with the dispersant. That is to say, the properties and results according to the invention are achieved when the particles are first coated with dispersant and then added to the fluid. This is in contrast to the properties and results

25 which is achieved when no dispersant is used or when the dispersant is simply added to the drilling fluid together with the particles. One skilled in the art of drilling fluid composition will realize that it is a widespread practice within the industry to simply combine materials into a base fluid to achieve the desired final composition. As supported by the pre-

30 current data, however, the coating of a dispersant on fine particulate weight materials prior to addition to the base fluid results in a significant and observable difference in rheological properties that is surprising and unexpected.

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Example 2

Experiments were conducted to investigate the effect of post-addition of the selected polymeric dispersant to a slurry composed to include weighting agents of the same colloidal particle size. A ground baryte (D50~ 4�m) and

5 a finely divided calcium carbonate (70% by weight of the particles smaller than 2�m) was chosen, both having similar particle size to the materials discussed herein.

The slurries were prepared at an equivalent particle volume fraction of 0.282. See table II.

The rheologies were measured at 49�C (120�F), then 17.2 kg/m<3>(6 ppb)

10 "IDSPERSE" XT added. The rheologies of the resulting slurries were finally measured at 49°C (120°F) (see Table III) with additional API filtration loss testing.

Table II

# Material Dispersion Density Volume fraction Weight/weight average g/cm<3>(ppg)

4 New baryte Under painting 1.92 g/cm<3>0.282 0.625

(16.0)

5 Ground baryte Nothing 1.92 g/cm<3>0.282 0.625

(16.0)

6 Ground barite Additional addition 1.92 g/cm<3>0.282 0.625

(16.0)

7 Calcium carbonate None 1.48 g/cm<3>0.282 0.518

(12.4)

8 Calcium carbonate Subsequent addition 1.48 g/cm<3>0.282 0.518

(12.4)

15 Table III

# Viscosity at different shear rates Plastic Yield strength API (stirring rpm) : Instrument reading or viscosity Filtration loss "Find units" for : ml 600 300 200 100 6 3 mPa.s g/m<2>

rpm rpm rpm rpm rpm rpm (lb/100ft<2>)

4 12 6 4 2 6 0 11

5 os os os os os

6 12 6 4 2 6 0 total<1>7 os os 260 221 88 78

8 12 6 4 3 1 1 6 0 total<1>1 - total filtering loss 26 minutes

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2 - total filtration loss 20 minutes

os = not measurable (“off-scale”)

rpm = revolutions/min

After considering the above data, an expert will note that it

5 no filtration control is gained by subsequent addition of the polymer as shown by the total filtration loss in the API test.

Example 3

This test was carried out to show that it is possible with 2.87 g/cm<3>(24

10 ppg) slurries (0.577 volume fraction). Each fluid contained the following components, e.g. fresh water 135.4 g, total barite 861.0 g, "IDSPERSE" XT 18.0 g. The barite component was varied in the mixture according to the following table.

Table IV

# API grade barite (%) Colloidal barite (%)

9 100 0

10 90 10

11 80 20

12 75 25

13 60 40

14 0 100

15

Table V

# Viscosity at different shear rates (stirring rpm) Plastic Flow-: Instrument reading or "Find units" for: viscosity limit 600 300 200 117 100 59 30 6 3 mPa.s Pascal (lb/100ft<2>) 9 *os 285 157 66 56 26 10 3 2

10 245 109 67 35 16 13 7 3 2 136 -13 (-27) 11 171 78 50 28 23 10 7 3 2 93 -7 (-15) 12 115 55 36 19 17 8 5 3 2 60 -2 (-5 )

13 98 49 34 21 20 14 10 4 3 49 0

14 165 84 58 37 32 22 18 5 3 81 -1.5 (3) *os = not measurable

rpm = revolutions/min

After considering the results given in Table V, one skilled in the art will realize that API

20 degree baryte, due to its particle size and the high volume fraction needed

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necessary to achieve high mud weights, demonstrated dilatation (ie high plastic and apparent viscosity values and negative yield strength values).

It should also be noted that the introduction of the finely graded materials will tend to stabilize the flow regime by keeping it laminar at higher shear

5 speeds: plastic viscosity decreases markedly and the yield point changes from negative to positive. In addition, it is noted that the colloidal baryte does not cause any significant increase in the low speed shear viscosity (@ 3 rpm).

The above results will show a person skilled in the art that the colloidal weight material coated with dispersant as discussed herein can advantageously

10 is used in conjunction with conventional API barite.

Example 4

A 2.15 g/m<3> (eighteen (18) pound per gallon) slurry of weighting agent according to the invention was composed and then contaminated with a range of common

15 contaminants and hot rolled at 148.9�C (300�F). The rheological results for "before hot rolling" (BHR) and "after hot rolling" (AHR) are listed below.

Table VI (New baryte)

Viscosity (Find units) at PV YP Filter different shear rates -ings-(stirring rpm): loss 600 300 200 100 6 3 mPa.s Pascal ml (lb/100ft<2>) no contamination BHR 21 11 8 4 1 1 10 0 .5 (1) no pollution AHR 18 10 7 4 1 1 8 1 (2) 5.0 227 g/l (+80ppb) NaCl BHR 41 23 16 10 2 1 18 2.5 (5) 227 g/l ( +80ppb) NaCl AHR 26 14 10 6 1 1 12 1 (2) 16 85.1 g/l (+30ppb) OCMA<1>38 22 15 9 2 1 16 3 (6) BHR

85.1 g/l (+30ppb) OCMA 26 14 10 6 1 1 12 1 (2) 6.8 AHR

14.2 g/l (5 ppb) Lime BHR 15 7 5 3 1 1 8 0.5 (1) 14.2 g/l (5 ppb) Lime AHR 10 5 4 2 1 1 5 0 6.4

20<1>OCMA = OCMA clay, a fine-grained ball clay commonly used to simulate contamination with drilled solids is kept from shale sediments during drilling

rpm = revolutions/min

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lime = limestone

BHR: as before

AHR: as before

Upon inspection of the previous results, a specialist will realize that

5 dispersant coated weight material system demonstrated excellent resistance to fouling, low controllable rheology and provides filtration loss control under a standard API sludge test as shown in the following Table VII. An equivalent set of fluids was prepared using API standard barite without the polymer coating as a direct comparison of the two particle types (Table VII).

10

Table VII (Standard API baryte)

Viscosity (Find units) at PV YP Filter different shear rates -ings-(stirring rpm): loss 600 300 200 100 6 3 mPa.s Pascal ml (lb/100ft<2>) no contamination BHR 22 10 6 3 1 1 12 - 2

no pollution AHR 40 24 19 11 5 4 16 8 Total<1>+227 g/l (+80ppb) NaCl BHR 27 13 10 6 2 1 14 -1

227 g/l (+80ppb) NaCl AHR 25 16 9 8 1 1 9 7 Total<1>+85.1 g/l (+30ppb) OCMA 69 55 49 43 31 26 14 31

BHR

85.1 g/l (+30ppb) OCMA 51 36 31 25 18 16 15 21 Total<2>AHR

14.2 g/l (5 ppb) Lime BHR 26 14 10 6 2 1 12 2

14.2 g/l (5 ppb) Lime AHR 26 14 10 6 1 1 12 2 Total<1>rpm = rev./min

lime = limestone

BHR: as before

15 AHR: as before

1 - Total filtering loss 30 seconds

2 - Total filtration loss 5 minutes

After comparing the two sets of data, a person skilled in the art will realize that the weighing agent according to the invention has considerable filtration loss control properties by

20 comparison with API baryte. Furthermore, it is noted that API baryte also shows sensitivity to contamination by drilled solids, while the new baryte system, on the other hand, is more tolerant.

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Example 5

An experiment was conducted to demonstrate the ability of the new weighting agent to compound drilling muds with densities above 2.29 g/cm<3> (20 pounds per gallon).

2.63 g/cm<3>(twenty two pound per gallon) sludge systems were composed,

5 as the weights comprised a mixture of 35% w/w new barite weight agent with 65% w/w API grade barite (fluid #1) weight agent and 100% API grade barite (fluid #2), both with 32.8 kg/m< 3>(11.5 pounds per barrel) “STAPLEX” 500 (trademark of Schlumberger, shale stabilizer), 5.7 kg/m<3>(2 pounds per barrel) “IDCAP” (trademark of Schlumberger, shale inhibitor), and 10 kg/m<3>

10 (3.5 pounds per barrel) KCl. The other additives cause inhibition to the drilling fluid, but show here the capacity of the new composition to handle any subsequent polymer additions. The fluid was hot rolled at 93.3°C (200°F).

The results are listed in Table VIII.

15 Table VIII

Viscosity (Find units) at PV Flow- Filter different shear rates limit -ings-(stirring rpm): loss 600 300 200 100 6 3 mPa.s Pascal ml (lb/100ft<2>) Before hot rolling (#1) 110 58 46 30 9 8 52 2.9 (6) After hot rolling (#1) 123 70 52 30 9 8 53 8.1 (17) 8.0 Before hot rolling (#2) 270 103 55 23 3 2 167 -32 (-64 ) After hot rolling (#2) os 177 110 47 7 5 12.0 os: not measurable

rpm = revolutions/min

After considering the above data, one skilled in the art will realize that 100% API grade barite has very high plastic viscosity and causes turbulence as shown

20 at the negative yield point. Furthermore, it is noted that after hot rolling, the rheology is so high that it is not measurable.

Example 6

This experiment shows the ability of the new weight agent in low viscosity fluids (i.e.

25 compositions with high fluidity). The weighting agent is 100% colloidal barite according to the invention. Fluid #15 is a synthetic base drilling fluid (“Ultidrill”, trademark of Schlumberger, a linear alpha olefin with 14 to 16 carbon atoms). Fluid #16 is a

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water-based sludge and includes a viscosity increasing agent (1.42 g/l (0.5 ppb) “IDVIS”, trademark of Schlumberger, a pure xanthan gum polymer) and a filtration loss control agent (18.7 g/l (6.6 ppb) “IDFLO ”, trademark of Schlumberger). Fluid #15 was hot rolled at 93.3�C (200�F), fluid #16 was

5 hot rolled at 121.1�C (250�F). Results after hot rolling are shown in Table IX.

Table IX

Viscosity (Find units) at PV Gels<1>Flow different shear rates limit (stirring rpm):

600 300 200 100 6 3 mPa.s Pascal Pascal (lb/100ft<2>) (lb/100ft<2>) #15 : 1.63 g/cm<3>(13.6 ppg) 39 27 23 17 6 5 12 11 (7) 7.5 (15) #16 : 1.67 g/cm<3>(14 ppg) 53 36 27 17 6 5 17 - (-5) 10 (19)<1>A measure of the gelling and slurrying characteristics of the fluid, determined at 10 sec/10 min using a Fann viscometer.

10 rpm = revolutions/min

Upon consideration of the above representative test data, one skilled in the art will recognize that the novel weights discussed herein provide a new method of composition of saline analog fluids useful for "slimhole" applications or coiled tubing drilling fluids. It should also be noted that the rheology profile is improved

15 by the addition of colloidal particles.

Example 7

An experiment was carried out to show the ability of the new weight agent to compose completion fluids, where density control and consequently sedimentation

20 stability is a primary factor. The weighting agent consists of the new colloidal barite according to the invention with 142.65 kg/m<3> (50 pounds per barrel) standard API grade calcium carbonate which acts as a self-sealing agent. 2.23 g/cm<3>(18.6 ppg ) fluid was composed with 5.7 kg/m<3> (2 pounds per barrel) "PTS" 200 (trademark of Schlumberger, pH buffer). The static aging tests were carried out at 204.4

25 �C (400�F) for 72 hours. After assessing the exemplary results shown in the following table, an expert will note that "before static aging" (BSA) and "after static aging" (ASA) can be achieved a good stability for sedimentation and rheological profile.

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Viscosity (Find units) at PV YP Freely different shear rates water* (stirring rpm):

600 300 200 100 6 3 mPa.s Pascal ml (lb/100ft<2>) 2.23 g/cm<3>(18.6 ppg) BSA 37 21 15 11 2 1 16 2.5 (5) -2 .23 g/cm<3>(18.6 ppg) ASA 27 14 11 6 1 1 13 0.5 (1) 6 *free volume is the volume of clear water that appears on top of the fluid. The rest of the fluid has uniform density.

rpm = revolutions/min

BSA = before static aging

5 ASA = after static ageing

Example 8

This experiment shows the ability of the new weighting agent to compose low viscosity fluids and shows its tolerance to pH variations. The weight mean

10 consists of the new colloidal barite according to the invention. The 1.91 g/cm<3> (16 ppg) fluid was compounded with caustic soda to adjust the pH to the required level, with subsequent fluid rheology and API filtration testing.

Viscosity (Find units) at PV Flow- Filter different shear rates limit ings-(stirring rpm): loss pH 600 300 200 100 6 3 mPa.s Pascal ml (lb/100ft<2>) 8.01 14 7 5 3 7 0 (0) 8.4 9.03 14 8 5 3 6 1 (2) 8.5 10.04 17 9 6 3 8 0.5 (1) 7.9 10.97 17 9 6 3 8 0.5 ( 1) 7.9 12.04 19 10 7 4 1 1 9 0.5 (1) 8.1 rpm = revolutions/min

15 BSA = before static ageing

ASA = after static aging

By looking at the example results, an expert will conclude that a good stability against pH variation and rheological profile has been established in the fluids composed by the use of coated weight agents referred to herein.

20

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Example 9

This experiment demonstrates the ability of the new weighting agent to compose low-rheology-high-temperature, high-pressure stable water-based fluids. The weighting agent consists of a new colloidal barite according to the invention, with 28.53 kg/m<3> (10 pounds

5 per barrel) "CALOTEMP" (trademark of Schlumberger, filtration loss additive) and 2.85 kg/m<3> (1 pound per barrel) "PTS" 200 (trademark of Schlumberger, pH buffer). 2.04 g/cm <3>(17 ppg) and 2.16 g/cm<3>(18 ppg) fluids were statically aged for 72 hours at 121�C (250�F).

Density pH Viscosity (Find units) at PV Flow Free Filter different shear rates (stirring limit water ing loss rpm):

g/cm<3>600 300 200 100 6 3 mPa.s Pascal ml ml (ppg) (lbs/100ft<2>)

2.04 7.4 28 16 11 6 1 1 12 2 (4) 10 3.1 (17)

2.16 7.5 42 23 16 10 1 1 19 2 (4) 6 3.4 (18)

10 By considering the above illustrative results, a person skilled in the art will realize that the fluids composed in accordance with the present invention exhibit good stability against sedimentation and a low rheological profile with a subsequent filtration test.

15 Example 10

The following example shows that a drilling fluid with an oily base containing the coated solids according to the present invention provides better overall performance, with particular advantages for static seepage, dynamic seepage and filtration loss. Three fluids were composed, the first in accordance with the teachings of the invention,

20 the second with a finely ground "Norway" barite and "EMI" 759 as a dispersant and a third fluid composed of fine "Chinese" precipitated barite and "EMI" 759 dispersant.

The following table indicates the experimental results obtained from particle size analysis using the Malvern "Microplus" instrument. All measurements were

25 taken in oil dispersant.

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Particle size distributions

Baryte D10D50D90 Dispersant 0.31 1.04 3.27 coated baryte

"Chinese" precipitated 0.32 1.30 3.01 baryte

"Norway" finely ground 0.94 7.86 31.25 baryt

An expert will realize that "Norway Fine Grind" baryte will be considered to be on the "fine" end of the API standard grinding grade for baryte. The fluids were combined

5 as indicated in the following table. Each fluid was formulated to a density of 1.56 g/cm<3> (13 ppg) and an oil to water (O/W) ratio of 80/20.

The fluid composition

Product "Norway" baryt Precipitated baryt Dispersant-(g/cm<3>) (g/cm<3>) coated baryt (g/cm<3>) "EDC" 99 As required As required As required "Chinese" pptd baryt - As needed - "Norway" As needed - - fine baryt

”OBWARP” - - As required Polyacrylate ester 0.9 0.9 -dispersant*

Fatty acid amide 1,2 1,2 1,2 emulsifier

Organoclay 0.36 0.36 0.36 thickener

Limestone 0.72 0.72 0.72 CaCl2 salt solution As needed As needed As needed (25% by weight)

Gilsonite-based 0.24 0.24 0.24 filtration loss

control additive

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* Note: The amount added to the drilling fluid is equivalent to the amount of compound coated on the dispersant coated barite.

The fluids were tested before and after aging at 121.1�C (250�F). The rheologies were measured at 49�C (120�F) using a Fann 35 viscometer and filtration loss

5 values were measured at 121.1�C (250�F). The dynamic seepage was measured on a Fann 35 viscometer at 49°C (120°F) after 30 min at 100 rpm. The static seepage was measured after aging the fluid at 121.1°C (250°F) for 40 hours. The following table provides illustrative and exemplary data:

10 Fluid properties - dispersant-coated barite

Fluid

Base 56.8 g/l (20ppb) 10% volume/volume HMP seawater BHR AHR BHR AHR BHR AHR

600 31 37 39 47 40 44

300 17 20 21 25 22 24

200 12 14 15 18 15 17

100 7 8 9 10 9 10

6 1 1 1 1 1 2

3 1 1 1 1 1 1

Gels 2/4 2/5 2/4

PV 14 17 18 22 18 20

YP 3 3 3 3 4 4

ES 731 810 405

HTHP FL 121.1�C (250�F)<2.2 1.2 1.2>

Dynamic seepage factor 0.501 - -Static seepage factor 0.509 - -PV = plastic viscosity

YP = yield strength

HTHP FL = high temp. high pressure rheology

BHR: as before

15 AHR: as before

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Fluid properties with dry powder barytes added

"Norway Fine Grind" baryte "Chinese" precipitated baryte

Base 56.8 g/l 10% Base 56.8 g/l 10% (20ppg) volume/volume (20ppg) volume/volume HMP seawater HMP seawater

BHR AHR BHR AHR BHR AHR BHR AHR BHR AHR BHR AHR

600 36 40 43 46 44 43 36 40 43 60 44 54

300 20 21 23 25 24 23 20 22 24 32 24 30

200 13 14 15 17 16 16 15 16 17 22 17 21

100 7 8 8 10 9 9 8 9 10 12 10 12

6 1 1 1 1 1 1 1 1 1 1 2 2

3 1 1 1 1 1 1 1 1 1 1 1

Gels - 1/3 - 1/5 - 1/5 - 1/4 - 1/6 - 2/4 10'/10''

PV 16 19 20 21 16 20 16 18 19 28 20 24

YP 4 2 3 4 8 3 4 4 5 4 4 6

ES 830 715 355 802 629 371

HTHP FL 2.0 3.6 1.8 8.8 12.0 -121.1�C

(250�F)

Dynamic 0.527 - - 0.525 - -sag factor

Static 0.718 - - 0.664

see factor

PV, YP, ES, HTHP FL: As before

After careful consideration, one skilled in the art will realize that the results show that each base fluid has similar rheological properties after contamination with clay

5 and seawater, but the fluid composed of precipitated barite gives a greater increase in PV than the "Norway" barite and the fluids containing the dispersant-coated weight materials according to the present invention. This increase in plastic viscosity may be due to the "non-coated" fines present in the fluid. Furthermore, when comparing the filtration loss properties of the three fluids, this expert will see

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that the fluids containing the coated weight solids according to the present invention show the best overall performance, even though the "Norway" barite also gives a similar performance. However, the fluid with the precipitated barite has a much worse filtration loss. This is most likely due to the very narrow particle

5 size distribution of the precipitated barite as well as the coating effect of the barite particles are less effective in this process. A person skilled in the field of drilling fluids will note that the most significant difference between the three fluids is in their seepage performance. The fluid containing the coated weight materials discussed herein shows very good seepage properties, both for the dynamic and the static

10 aged saying. The coarser "Norway Fine Grind" baryte gives very poor sieving performance for both the dynamic and static sieving tests. This could possibly be expected for a non-optimized fluid using a coarser formal barite, but it will also be realized that optimization to improve its seepage performance will come at the expense of the low rheological properties of the fluid. The "Chinese" precipitated baryte has a lot

15 finer particle size distribution similar to the solids of the present invention, but its sieving performance was also very poor for both the dynamic and static sieving tests. This may also be due to the ineffective coating of the barite by this process.

After further assessment of the preceding data, an expert con-

20 kludder that when comparing the three different barytes, similar rheologies are obtained despite their different particle sizes. It will further be noted that the three base fluids give very similar rheologies which demonstrate the advantages of a fluid composition using the coated colloidal particles according to the present invention. It should further be noted that fluids containing the solids according to

25 the present invention exhibits excellent results for sieve performance and filtration loss compared to the other fluids. This would logically lead one skilled in the art to conclude that to compound the other fluids to provide similar performance would likely result in a fluid with much higher rheology.

On the basis of the preceding teaching, the ordinary expert will understand and

30 realize that an illustrative embodiment of the invention includes a wellbore fluid with an oily phase and an additive to increase the density of the wellbore fluid. The additive comprises solid colloidal particles coated with a dispersant. The dispersant is coated on the colloidal particle during the comminution process to form the particles. The illustrative particles have a

35 weight average particle diameter (D50) of less than 2�m and preferably a D50 of

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less than 1.5�m diameter. Preferably, the colloidal particles consist of a material with a specific gravity of at least 2.68. Exemplary starting materials for the colloidal particles include many commonly known weighting agents including barite, calcium carbonate, dolomite, ilmenite, hematite or other iron ores, olivine, siderite

5 and strontium sulfate as well as mixtures and combinations of these and other similar weight materials. The dispersant applied to the particles during the grinding process is a polymeric acrylate ester. The polymeric acrylate ester should have an average molecular weight of from about 10,000 Daltons to about 200,000 Daltons and more preferably from about 17,000 Daltons to about 30,000 Daltons.

10 The invention also includes a method for producing an additive to increase the density of a fluid. In an illustrative embodiment, the method includes comminuting a solid material and a dispersant in a liquid medium to produce solid colloidal particles having a weight average particle diameter (D50) of less than 2�m and which are coated with

15 the dispersant. The liquid medium is preferably an oily fluid and more preferably an oily liquid with a kinematic viscosity of less than 10 centistokes (10 mm 2 /s) at 40°C and a flash point of more than 60°C. Illustrative examples of such oily fluids include diesel oil, mineral oils or liquid paraffin oils, n-alkanes or synthetic oils such as e.g. alpha olefin oils,

20 ester oils or polyalphaolefins as well as combinations and mixtures of these and similar fluids. The dispersant that is coated on the particles during the course of the painting is a polymeric acrylate ester. Optimally, the illustrative dispersant is formed from stearyl methacrylate, butyl acrylate and acrylic acid monomers. The polymeric acrylate tester should have an average

25 molecular weight from about 10,000 Daltons to about 200,000 Daltons and more preferably from about 17,000 Daltons to about 30,000 Daltons. The solid material can be selected from a wide variety of known weight materials and in an illustrative embodiment the solid material is selected from the group consisting of barite, calcium carbonate, dolomite, ilmenite, hematite or other iron ores, olivine,

30 side dish and strontium sulphate, mixtures and combinations of these and similar weight materials which will be known to the person skilled in the art. In a preferred illustrative embodiment, the fine division of the solid material and the dispersant in the liquid medium is carried out in a stirred fluidized layer of a particulate grinding material.

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While the apparatus, mixtures and methods according to the invention are described on the basis of preferred or illustrative embodiments, it will be clear to the person skilled in the art that variations can be made to the method described herein without going outside the scope of the invention. All such similar

5 replacements and modifications which will be obvious to the person skilled in the art are considered to be within the scope of the invention as stated in the subsequent patent claims.

Claims (16)

342495 26 PATENT CLAIMS 1. Borehole fluid, c a r a c t e r i s e r t w e d that 5 it comprises an oily phase and an additive to increase the density of the borehole fluid, the additive comprising solid colloidal particles coated with a dispersant, wherein the dispersant is a polymeric acrylate ester. 2. Borehole fluid, according to claim 1, c a r a c t e r i s e r t w e d that 10 the colloidal particles consist of a material with a specific gravity of at least 2.68. 3. Borehole fluid according to claim 1, c a r a c t e r i s e r t w e d that the colloidal particles have a D50 of less than 2.0 m in diameter. 4. Borehole fluid according to claim 1, 15 c a r a c t e r i s e r t w e d that the composition of the colloidal particles is selected from the group consisting of barite, calcium carbonate, dolomite, ilmenite, hematite or other iron ores, olivine, siderite, strontium sulfate and mixtures thereof. 5. Borehole fluid according to claim 1, 20 c a r a c t e r i s e r t w e d that the polymeric acrylate ester is prepared from the monomers stearyl methacrylate, butyl acrylate and acrylic acid. 6. Borehole fluid according to claim 1, c a r a c t e r i s e r t w e d that The polymeric acrylate ester has an average molecular weight between about 10,000 Daltons and 200,000 Daltons. 7. Borehole fluid according to claim 1, c a r a c t e r i s e r t w e d that the polymeric acrylate ester has an average molecular weight between approx 30 17,000 Daltons and 30,000 Daltons. 8. Method for producing an additive to increase the density of a fluid, c a r a c t e r i s e r t w e d that it includes: 342495 27 comminution of a solid material and a dispersing agent in a liquid medium so that solid colloidal particles coated with the dispersing agent are produced, wherein the dispersing agent is a polymeric acrylate ester. 9. Method according to claim 8, 5 c a r a c t e r i s e r t w e d that the liquid medium is an oily fluid. 10. Method according to claim 8, c a r a c t e r i s e r t w e d that the liquid medium is an oily liquid with a kinematic viscosity less than 10 10 centistokes (10 mm<2>/s) at 40�C and a flash point of more than 60�C. 11. Method according to claim 9, c a r a c t e r i s e r t w e d that the oily fluid is selected from the group consisting of diesel oil and mineral or liquid paraffin oils, n-alkanes or synthetic oils such as alpha olefin oils, ester oils 15 or polyalphaolefins. 12. Method according to claim 8, c a r a c t e r i s e r t w e d that the polymeric acrylate ester is prepared from the monomers stearyl methacrylate, butyl acrylate and acrylic acid. 20 13. Method according to claim 8, c a r a c t e r i s e r t w e d that the polymeric acrylate ester has an average molecular weight between 10,000 Dalton and 200,000 Dalton. 14. Method according to claim 8, 25 c a r a c t e r i s e r t w e d that the polymeric acrylate ester has an average molecular weight between about 17,000 Daltons and 30,000 Daltons. 15. Method according to claim 8, c a r a c t e r i s e r t w e d that 30 the fine division of a solid material and a dispersant in a liquid medium is carried out in a stirred fluidized layer of a particulate grinding material. 16. Method according to claim 8, c a r a c t e r i s e r t w e d that the solid material is selected from the group consisting of barite, calcium carbonate, dolomite, ilmenite, hematite or other iron ores, olivine, siderite, strontium sulfate and mixtures thereof.