ADDITIVE FOR REDUCING TORQUE ON A DRILL STRING

BACKGROUND OF THE INVENTION

When oil and gas wells are drilled, fluid formulations with a multitude of properties, including lubricity, are pumped down the well through the drill string and out through nozzles in the drill bit, so that the drilling fluid circulates upward through the annular space between the rotating drill string and the rock formation. The functions of these drilling fluids or "muds" are to cool and lubricate the bit and drill string, to carry the cuttings from the drilling process to the surface, to control and reduce fluid loss into the rock formations, and to support and protect the bore hole until the metal casing can be cemented in place (i.e., create a stable hole).

Mud lubricity (to achieve minimum torque and drag) and mud toxicity (for wells in environmentally sensitive areas, such as offshore drilling) are major concerns when selecting a drilling fluid formulation. Most drilling fluids may be grouped into two major categories: water-based or oleaginous-based. The majority of drilling fluids used today are water-based, i.e., they contain water as the continuous external phase. Although oleaginous-based drilling fluids including the so-called synthetic-based fluids do have performance advantages, drawbacks are higher costs and difficult environmental compliance in specific areas of the world.

The lubricity of a drilling fluid is an important factor in the economics of well drilling and is measured by determining the effect of the fluid upon the coefficient of friction between a moving part, such as the drill string, and a surface in contact with the moving part. The lower the coefficient of friction, the greater the lubricity. The lubricity of a drilling fluid determines the fluid's ability to lower torque and drag forces during the drilling operation.

The prior art is full of reports of various lubricants utilized to reduce the torque of a drill string. For example various types of hydrocarbons, synthetic oils, esters, fatty acids, natural oils, soaps, and other pompounds have been added to drilling fluids to help reduce torque. Organic, oil-based lubricants are often added to water-based drilling fluids to reduce the coefficient of friction. Reduction of friction during drilling is particularly important in drilling operations where the well bore is not vertical. Emulsifiers or surfactants are typically added to drilling fluids to keep these oil-based, water-insoluble lubricant components suspended as droplets in the water-based fluids and to prevent their separation and coalescence. These lubricants can increase the toxicity and irritation level of the fluids.

In addition to liquid lubricants, micrometer-sized solid particles or beads may also be added to water-based drilling fluids to increase their lubricity. Some representative examples of this type of lubricant system are: (1) Abrasion- and fracture-resistant, thermally stable and chemically inert ceramic spheres; (2) Plastic beads, for example, those made from a copolymer of divinyl benzene and styrene; (3) Plastic-coated magnetic particles in bead form, to facilitate the removal and recycling of these bead compositions; (4) Chemically-resistant, lime-silica glass beads; (5) Resilient graphitic carbon particles; (6) Cellulose, peat or bagasse, containing absorbed oil-based liquid lubricants; (7) Mixtures of graphite, silicate and silicone materials. Common difficulties with the above solid lubricants have been the environmental concerns when aqueous based drilling fluids are being used and the loading up of the drilling fluid with solid materials. Further, it should be appreciated that the addition of solid materials that do not contribute to the weighting of the fluid may result in an underweight fluid raising concerns about blow-out or wall collapse. A further problem encountered with solid lubricating agents is small diameter apertures present in the valves and other flow and pressure control equipment used may prevent the use of solid particulate lubricating agents because these material block and plug the narrow restrictions. A more serious concern is that the solids might be difficult to remove from the well bore and thus cause formation damage. Despite the continued efforts in this area, there remains and exists an unmet need for fluids that reduce drill string torque and do not exhibit the problems of solids settling, high viscosity, toxicity and reduced total fluid weight.

SUMMARY OF THE INVENTION

The present invention is generally directed to fluids useful in reducing the torque of drill string, as well as methods for making and methods of using such fluids. The fluids of the present invention include a polymer coated colloidal solid material that has been coated with a polymer added during the cominution (i.e. grinding) process for preparing the polymer coated colloidal solid material.

One illustrative embodiment of the present invention includes a method of reducing the torque in a rotating drill string component. In such an illustrative method, the method includes, injecting into the drilling fluid a composition including a base fluid, and a polymer coated colloidal solid material. The polymer coated colloidal solid material includes: a solid particle coated with a polymeric dispersing agent absorbed to the surface of the solid particle. The polymeric dispersing agent is absorbed to the surface of the solid particle during the

cominution (i.e. grinding) process utilized to make the polymer coated colloidal solid material. The base fluid utilized in the above illustrative embodiment can be an aqueous fluid or an oleaginous fluid and preferably is selected from: water, brine, diesel oil, mineral oil, white oil, n-alkanes, synthetic oils, saturated and unsaturated poly(alpha-olefϊns), esters of fatty acid carboxylic acids and combinations and mixtures of these and similar fluids that should be apparent to one of skill in the art. Suitable and illustrative colloidal solids are selected such that the solid particles are composed of a material of specific gravity of at least 2.68 and preferably are selected from barium sulfate (barite), calcium carbonate, dolomite, ilmenite, hematite, olivine, siderite, strontium sulfate, combinations and mixtures of these and other suitable materials that should be well known to one of skill in the art. In one preferred and illustrative embodiment, the polymer coated colloidal solid material has a weight average particle diameter (d ₅₀ ) less than ten microns. Another preferred and illustrative embodiment is such that at least 50% of the solid particles have a diameter less than 2 microns and more preferably at least 80% of the solid particles have a diameter less than 5 microns. Alternatively, the particle diameter distribution in one illustratvie embodiment is such that greater than 25% of the solid particles have a diameter of less than 2 microns and more preferably greater than 50% of the solid particle have a diameter of less than 2 microns. The polymeric dispersing agent utilized in one illustrative and preferred embodiment is a polymer of molecular weight of at least 2,000 Daltons. In another more preferred and illustrative embodiment, the polymeric dispersing agent is a water soluble polymer is a homopolymer or copolymer of monomers selected from the group comprising: acrylic acid, itaconic acid, maleic acid or anhydride, hydroxypropyl acrylate vinylsulphonic acid, acrylamido 2-propane sulphonic acid, acrylamide, styrene sulphonic acid, acrylic phosphate esters, methyl vinyl ether and vinyl acetate, and wherein the acid monomers may also be neutralized to a salt.

The present invention is also directed to a lubricating composition that includes a base fluid and a polymer coated colloidal solid material . The polymer coated colloidal solid material is formulated so as to include a solid particle coated with a polymeric dispersing agent absorbed to the surface of the colloidal solid particle.

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

BRIEF DESCRIPTION OF THE DRAWINGS

The description is presented with reference to the accompanying drawing which is a graphical representation of the particle diameter distribution of the colloidal barite of the present invention compared to that of API barite.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A new and novel aspect of the present invention is the dual role that the colloidal particles play in the drilling fluid. That is to say, the polymer coated colloidal particles may serve both as a weighting agent and lubricating agent. This dualistic view of the material is novel to the drilling industry because previously the functionality of weighing agent and lubricating agent were distinct.

One of skill in the art should appreciate that the solid lubricating agents noted above generally have a density less than conventionally used weighting agents. For example, mineral derived graphite has a specific gravity of about 2.09 to 2.25. hi contrast conventional weighting agents such as barite has a specific gravity of about 4.50, hematite has a specific gravity of about 5.3. According to one preferred embodiment of the present invention, the lubricating / weighting agent of the present invention is formed of particles that are composed of a material of specific gravity of at least 2.68. In this way the particles can serve as a combination lubricating agent and weighting agent. Materials of specific gravity greater than 2.68 from which colloidal solid particles that embody one aspect of the present invention include one or more materials selected from but not limited to barium sulfate (barite), calcium carbonate, dolomite, ilmenite, hematite or other iron ores, olivine, siderite, strontium sulfate. Normally the lowest well bore fluid viscosity at any particular density is obtained by using the highest density colloidal particles. However other considerations may influence the choice of product such as cost, local availability and the power required for grinding.

One of skill in the art should also understand and appreciate that conventional weighting agents, such as powdered barium sulfate ("barite"), exhibit minimal effects on reducing drill string torque. Physically conventional weighting agents are utilized for their high density and exhibit an average particle diameter (d ₅₀ ) in the range of 10-30 microns. It should be well known to one of skill in the art that properties of conventional weighting agents, and barite in particular are subject to strict quality control parameters established by the American Petroleum Institute (API). To suspend these materials adequately requires the

addition of a gellant or viscosifier such as bentonite for water based fluids, or organically modified bentonite for oil based fluids. Polymeric viscosifiers such as xanthan gum are typically added to slow the rate of the sedimentation of the conventional weighting agent. It is therefore very surprising that the products of this invention, which comprise solid colloidal particles that are coated with a polymeric defloculating agent or dispersing agent, provide fluids that contain high density solids that also reduce the torque in the rotating portions of the drill string without increasing sedimentation or sag.

The additives of this invention comprise dispersed solid colloidal particles that are coated with a polymeric defloculating agent or dispersing agent. The fine particle size will generate suspensions or slurries that will show a reduced tendency to sediment or sag, whilst the polymeric dispersing agent on the surface of the particle control the inter-particle interactions. It is the combination of fine particle size and control of colloidal interactions that reconciles the two objectives of high density and increased lubricity.

According to the present invention, the polymeric dispersant is coated onto the surface of the particulate weighting during the grinding process utilized to form the colloidal particle. It is believed that during the course of the grinding process, newly exposed particle surfaces become polymer coated thus resulting in the properties exhibited by the colloidal solids of the present invention. Experimental data has shown that colloidal solid material created in the absence of the polymeric dispersant results in a concentrated slurry of small particles that is an unpumpable paste or gel. According to the teachings of the present invention, a polymeric dispersant is added during the grinding process. It is believed that this difference provides an advantageous improvement in the state of dispersion of the particles compared to post addition of the polymeric dispersant to fine particles. According to a preferred embodiment, the polymeric dispersant is chosen so as it provides the suitable colloidal inter-particle interaction mechanism to make it tolerant to a range of common well bore contaminants, including salt saturated.

A method of grinding a solid material to obtain the solid colloidal particle so of the present invention is well known for example from British Patent Specification No 1,472,701 or No 1,599,632. The mineral in an aqueous suspension is mixed with a polymeric dispersing agent and then ground within an agitated fluidized bed of a particulate grinding medium for a time sufficient to provide the required particle size distribution. An important preferred embodiment aspect of the present invention is the presence of the dispersing agent

in the step of "wet" grinding the mineral. This prevents new crystal surfaces formed during the grinding step from forming agglomerates which are not so readily broken down if they are subsequently treated with a dispersing agent.

A preferred embodiment of this invention is for the weight average particle diameter (d ₅₀ ) of the colloidal solid particles to be less than ten microns. Another preferred and illustrative embodiment is such that at least 50% of the solid particles have a diameter less than 2 microns and more preferably at least 80% of the solid particles have a diameter less than 2 microns. Alternatively, the particle diameter distribution in one illustrative embodiment is such that greater than 25% of the solid particles have a diameter of less than 2 microns and more preferably greater than 50% of the solid particle have a diameter of less than 2 microns. This will enhance the suspension's characteristics in terms of sedimentation or sag stability without the viscosity of the fluid increasing so as to make it unpumpable.

The polymer coated colloidal particles according the invention may be provided as a concentrated slurry either in an aqueous medium or an oleaginous liquid. In the latter case, the oleaginous liquid should have a kinematic viscosity of less than 10 centistokes (10 mm ² /s) at 40 °C and, for safety reasons, a flash point of greater than 60 ⁰ C. Suitable oleaginous liquids are for example diesel oil, mineral or white oils, n-alkanes or synthetic oils such as alpha-olefm oils, ester oils or poly(alpha-olefins).

Where the polymer coated colloidal particles are provided in an aqueous medium, the dispersing agent may be, for example, a water-soluble polymer of molecular weight of at least 2,000 Daltons. The polymer is a homopolymer or copolymer of any monomers selected from (but not limited to) the class comprising: acrylic acid, itaconic acid, maleic acid or anhydride, hydroxypropyl acrylate vinylsulphonic acid, acrylamido 2-propane sulphonic acid, acrylamide, styrene sulphonic acid, acrylic phosphate esters, methyl vinyl ether and vinyl acetate. The acid monomers may also be neutralized to a salt such as the sodium salt.

It has been found that when the dispersing agent is added during the cominution process (i.e. grinding), intermediate molecular weight polymers (in the range 10,000 to 200,000 for example) may be used effectively. Intermediate molecular weight dispersing agents are advantageously less sensitive to contaminants such as salt, clays, and therefore are well adapted to well bore fluids.

Where the colloidal particles are provided in an oleaginous medium, the dispersing agent may be selected for example among carboxylic acids of molecular weight of at least

150 such as oleic acid and polybasic fatty acids, alkylbenzene sulphonic acids, alkane sulphonic acids, linear alpha-olefin sulphonic acid or the alkaline earth metal salts of any of the above acids, phospholipids such as lecithin, synthetic polymers such as Hypermer OM-I (trademark of ICI).

While not intending to be bound by any specific theory of action, it is believed that the formation of the colloidal solid material by a high energy wet process, in which API Barite of median particle size of 25-30 micron is reduced to a median particle size of less than 2 microns, is more efficient when the milling is done at high density, normally greater than 2.1 sg, preferably at 2.5sg. At these high densities, the volume or mass fraction of barite is very high. For example, at a specific gravity of 2.5, a lOOkgs of the final product contains about 78kgs is barite. However, the resulting slurry still remains fluid. The presence of the surface active polymer during the course of the cominution process is an important factor in achieving the results of the present invention. Further, the surface active polymer is designed to adsorb onto surface sites of the barite particles. In the grinder, where there is a very high mass fraction of barite, the polymer easily finds it way onto the newly formed particle surfaces. Once the polymer 'finds' the barite - and in the environment of the grinder it has every chance to do so — a combination of the extremely high energy environment in the wet grinding mill (which can reach 85 to 90 C inside the mill), effectively ensures that the polymer is 'wrapped' around the colloidal size barite. As a result of this process it is speculated that no polymer 'loops' or 'tails' are hanging off the barite to get attached, snagged, or tangled up with adjacent particles. Thus it is speculated that the high energy and shear of the grinding process ensures the polymer remains on the barite permanently and thus the polymer doesn't desorb, or become detached.

This theory of action is supported by the observation that adding the same polymer to the same mass fraction of colloidal barite at room temperature and mixing with the usual lab equipment results provides very different results. Under such conditions it is believed that polymer doesn't attach itself to the surface properly. This may be due to presence of a sphere of hydration or other molecules occupying the surface binding sites. As a result the polymeric dispersant is not permanently 'annealed' to the surface, and thus, the rheology of the suspension is much higher. It has also been observed that the suspension is not so resistant to other contaminants possibly because the polymer wants to detach itself from the barite and onto these more reactive sites instead.

The following examples are to illustrate the properties and performance of the well bore fluids of the present invention though the invention is not limited to the specific embodiments showing these examples. All testing was conducted as per API RP 13 B where applicable. Mixing was performed on Silverson L2R or Hamilton Beach Mixers. The viscosity at various shear rates (RPM's) and other rheological properties were obtained using a Farm viscometer. Mud weights were checked using a standard mud scale or an analytical balance. Fluid loss was measured with a standard API fluid loss cell

In expressing a metric equivalent, the following U.S. to metric conversion factors are used : 1 gal = 3.785 litres ; 1 Ib. = 0.454 kg ; 1 lb./gal (ppg)=0.1198 g/cm ³ ; 1 bbl=42 gal ; 1 lb./bbl (ppb)=2.835 kg/m ³ ; 1 lb/100ft ² =0,4788 Pa.

These tests have been carried out using different grades of ground barite: a standard grade of API barite, having a weight average particle diameter (D ₅₀ ) of about 20 microns ; a untreated barite (M) having an average size of 3 -5 microns made by milling/grinding barite while in the dry state and in the absence of a dispersant, with and colloidal barite according the present invention with a polymeric dispersant included during a "wet" grinding process. It should be appreciated by one of skill in the art that other solid particulate materials may be used in the practice of the present invention.

A representative sample of particle size distributions are shown figure 1. As shown in figure 1, one of skill in the art should understand and appreciate that the colloidal barite particles of the present invention have a particle size distribution that is very different from that of API barite. Specifically one should be able to determine that greater than about 90% (by volume) of the colloidal barite of the present invention has a particle diameter less than about 5 microns, hi contrast, less than 15 percent by volume of the particles in API specification barite have a particle diameter less than 5 microns.

The polymeric dispersant is IDSPERSE™ XT an anionic acrylic ter-polymer of molecular weight in the range 40,000-120,000 with carboxylate and other functional groups commercially available from M-I LLC. Houston, Texas. This preferred polymer is advantageously stable at temperature up to 200 °C, tolerant to a broad range of contaminant, gives good filtration properties and do not readily desorb off the particle surface.

The following examples illustrate the dual use of the lubricating agent as both weighting agent and as a lubricating agent, (i.e. reducing torque)

Example 1

22 ppg [2.63 g/cm ³ ] fluids based on barium sulfate and water were prepared using standard barite and colloidal barite according to the invention. The 22 ppg slurry of API grade barite and water was made with no gelling agent to control the inter-particle interactions (Fluid #1). Fluid #2 is also based on standard API barite but with a post-addition of two pounds per barrel (5,7 kilograms per cubic meter) IDSPERSE XT. Fluid #3 is 100% new lubricating / weighting agent with 67% w/w of particles below 1 micron in size and at least 90% less than 2 microns. The results are provided in table I. Table I

For Fluid #1 the viscosity is very high and the slurry was observed to filter very rapidly. (If further materials were added to reduce the fluid loss, the viscosity would have increased yet further). This system sags significantly over one hour giving substantial free water (ca. 10% of original volume).

Post addition of two pounds per barrel [5.7 kg/cm ³ ] of IDSPERSE XT to conventional API barite (Fluid #2) reduces the low shear rate viscosity by controlling the inter-particle interactions. However due to the particle concentration and average particle size the fluid exhibits dilatency, which is indicated by the high plastic viscosity and negative yield point. This has considerable consequences on the pressure drops for these fluids while pumping. That is to say the ability to pump this fluid is substantially reduced due to the high viscosity. The fluid #2 sags immediately on standing.

By contrast, Fluid #3 exhibits an excellent, low, plastic viscosity. The presence of the dispersing polymer controls the inter-particle interactions, so making fluid #3 pumpable and not a gel. Also the much lower average particle size has stabilized the flow regime and is now laminar at 1000 s ^"1 demonstrated by the low plastic viscosity and positive yield point.

Example 2 ,

Experiments were conducted to examine the effect of the post addition of the chosen polymer dispersant to a slurry comprising weighting agents of the same colloidal particle size. A milled barite ( D ₅₀ ~ 4um) and a milled calcium carbonate (70% by weight of the particles of less than 2μm) were selected, both of which are of similar particle size to the invention related herein. The slurries were prepared at an equivalent particle volume fraction of 0.282 and compared to the product of the present invention (new barite). See table II.

The rheologies were measured at 120° F (49 ⁰ C) ₅ thereafter an addition of 6ppb (17.2 kg/m ³ ) IDSPERSE XT was made. The rheologies of the subsequent slurries were finally measured at 120 ⁰ F (see table III) with additional API fluid loss test.

Table II

Table III

1 - total fluid loss in 26 minutes; 2 - total fluid loss in 20 minutes

No filtration control is gained from post addition of the polymer as revealed by the total fluid loss in the API test.

One of skill in the art should appreciate and know that the performance parameters of major importance are: low rheology, including plastic viscosity (PV), yield point (YP), gel strengths; minimal rheology variation between initial and heat aged properties; minimal fluid loss and minimal sag or settlement. Sag is quantified in the following examples by separately

measuring the density of the top half and bottom half of an aged fluid sample, and a dimensionless factor calculated using the following equation:

Sag Factor = (density of the top half)/(density of the top half + density of the bottom half)

A factor of 0.50 indicates zero solids separation and a no density variation throughout the fluid sample. A sag factor greater than 0.52 is normally considered unacceptable solids separation. Example 3

In the following example, two 13.0 ppg fluid formulations are compared, one weighted with conventional API barite and the second weighted with polymer coated colloidal barite (PCC barite) made in accordance with the teachings of the present invention, as a 2.2sg liquid slurry. Other additives in the formulation are included to provide additional control of pH, fluid loss, rheology, inhibition to reactive shale and claystones. These additives are available from M-I Drilling Fluids.

PRODUCT Fluid A Fluid B

PCC barite lbs/bbl 320.0

API barite lbs/bbl 238.1

Freshwater lbs/bbl 175.0 264.2

Soda Ash lbs/bbl 0.4 0.4

Celpol ESL lbs/bbl 3.5 4.2

Flotrol lbs/bbl 3.5 0

Defoam NS lbs/bbl 0.4 0

KCl lbs/bbl 32.9 36.1

Glydril; MC lbs/bbl 10.5 10.5

Duotec NS lbs/bbl 0.1 1.4

The fluids were heat aged statically for 48 hrs at 104 ⁰ F with the following exemplary results.

One of skill in the art should appreciate upon review of the above results that Fluid A, formulated with the polymer coated colloidal barite, had no solids separation with a sag factor of zero with a rheological profile much lower than a fluid weighted with conventional API barite. Example 4

In the following example, a 14.0ppg Freshwater fluid was chosen to compare the properties of fluids formulated with a polymer coated colloidal barite; an uncoated colloidal barite and a conventional API barite. Fluid A was formulated with the polymer coated colloidal barite of this invention. Fluid B was formulated with conventional API barite. Fluid C was formulated with a commercial grade of non coated colloidal barite, of median particle size of 1.6 microns available from Highwood Resources Ltd., Canada. Post grinding addition of the coating polymer of the invention are included in the formulation of Fluids B and C to maintain the fluid in a deflocculated condition.

Samples of fluid A, B and C were purposely contaminated with bentonite to simulate the inclusion of naturally drilled solids in the formulation. The samples were heat aged dynamically at 150 ⁰ F for 16 hrs. Exemplary and representative results after aging are shown below.

Upon review of the above data, one of skill in the art should appreciate that the properties of Fluid A remain essentially unchanged, while the Fluid B became very viscous, whereas, the rheology of Fluid C formulated with non coated colloidal barite after aging was too viscous to measure. Example 5

A further comparison between a polymer coated colloidal barite of this invention and conventional API barite was made in a 14ppg fluid, in which the yield point of the fluid has been adjusted such that it is the same between the two fluids before ageing.

PRODUCT Fluid A FMd B

PCC barite (2.4sg) lbs/bbl 265

API barite lbs/bbl 265

Freshwater lbs/bbl 238 293

Soda Ash lbs/bbl 0.5 0.5

KOH lbs/bbl 0.5 0.5

PolyPlus RD lbs/bbl 0.5 0.5

PolyPac UL 2.0 2.0

Duovis lbs/bbl 1.0 0.75

KCl lbs/bbl 8.0 8.0

The fluids were heat aged dynamically for 16 hrs at 15O ⁰ F. The following table presented exemplary results.

FANN 35 Reading Fluid A Fluid B (12O ⁰ F) Initial Aged Initial _Aged_

600 rpm 64 61 80 72 300 rpm 42 39 50 43

Upon review of the above, one of skill in the art should understand that the plastic viscosity for the polymer coated colloidal barite fluids were lower and thus more desirable. The Viscometer Sag Test (VST) is an alternative method for determining 'sag; in drilling fluids and is described in American Society of Mechanical Engineers Magazine (1991) by DJefferson. As indicated above, the VST values for Fluid A, containing the polymer coated colloidal barite of this invention is lower than that of Fluid B formulated with untreated, API barite. Example 6

The long term thermal stability of the colloidal barite fluids of the present invention are shown in the following example at 17.34ppg. ECF-614 additive is an organophilic clay additive available from M-I Drilling Fluids.

PRODUCT Fluid A

PCC barite (2.4sg) lbs/bbl 682

Freshwater lbs/bbl 53.5

ECF-614 lbs/bbl 2.0

The fluid was heat aged statically for 4 days at 35O ⁰ F. The following table provides exemplary results.

lO min gei qbs/lOOsq.ft) 10 11

Sag Factor 0.503

Upon review of the above data one of skill in the art should understand and appreciate the long term thermal stability of the colloidal barite fluids of the present invention Example 7

This test was carried out to show the feasibility of 24 ppg [2.87 g/cm ³ ] slurries (0.577 Volume fraction). Each fluid contained the following components: fresh water 135.4 g, barite 861.Og, IDSPERSE XT 18.0 g. The barite component was varied in composition according to the following table. Table IV

Table V

* os = off-scale

The results provided table V show that API grade barite due to its particle size and the high volume fraction required to achieve high mud weights exhibit dilatancy i.e. high plastic and apparent viscosity and negative yield values.

Introduction of fine grade materials tends to stabilize the flow regime keep it laminar at higher shear rates: plastic viscosity decreases markedly and yield point changes from

negative to positive. No significant increase in low-shear rate viscosity (@ 3 rpm) is caused by the colloidal barite.

These results show that the colloidal material of this invention may advantageously be used in conjunction with conventional API barite. Example 8

An eighteen (18) pound per gallon [2.15 g/cm ³ ] slurry of lubricating / weighting agent according the present invention was formulated and subsequently contaminated with a range of common contaminants and hot rolled at 300 °F (148.9 ⁰ C). The rheological results of before (BHR) and after hot rolling (AHR) are presented below. The system shows excellent resistance to contaminants, low controllable rheology and gives fluid loss control under a standard API mud test as shown in following table VI : An equivalent set of fluids were prepared using API conventional barite without the polymer coating as a direct comparison of the two particle types. (Table VII)

Table VI (New barite)

¹ OCMA = Ocma clay, a fine particle ball clay commonly used to replicate drilled solids contamination acquired from shale sediments during drilling

Table VIK Conventional API baritet

1 - Total fluid loss within 30 seconds

2 - Total fluid loss within 5 minutes.

A comparison of the two sets of data show that the lubricating / weighting agent according the present invention (new barite) has considerable fluid loss control properties when compared to the API barite. The API barite also shows sensitivity to drilled solids contamination whereas the new barite system is more tolerant. Example 9

An experiment was conducted to demonstrate the ability of the new lubricating / weighting agent to formulate drilling muds with densities above 20 pound per gallon [2.39 g/cm ³ ].

Two twenty two pound per gallon [2.63 g/cm ³ ] .mud systems were formulated, the weighting agents comprised a blend of 35% w/w new barite lubricating / weighting agent with 65% w/w API grade barite (Fluid #1) weighting agent and 100% API grade barite (fluid #2), both with 11.5 pound per barrel [32.8 kg/m ³ ] STAPLEX 500 (mark of Schlumberger, shale stabilizer), 2 pound per barrel [5.7 kg/m ³ ] IDCAP (mark of Schlumberger, shale inhibitor), and 3.5 pound per barrel [10 kg/m ³ ] potassium chloride. The other additives provide inhibition to the drilling fluid, but here demonstrate the capacity of the new formulation to cope with any subsequent polymer additions. The fluid was hot rolled to 200 ° F (93.3 ⁰ C). Results are provided in table VIII. Table VIII

os : off-scale

The 100% API grade barite has very high plastic viscosity and is in fact turbulent as demonstrated by the negative yield point. After hot rolling the rheology is so high that it is off scale.

Example 10

This experiment demonstrates the ability of the new lubricating / weighting agent of the present invention to lower the viscosity of fluids. The lubricating / weighting agent is 100% colloidal barite according the present invention. Fluid #15 is based on synthetic oil (Ultidrill, Mark of Schlumberger, a linear alpha-olefm having 14 to 16 carbon atoms). Fluid #16 is a water-based mud and includes a viscosifier (0.5 ppb IDVIS, Mark of Schlumberger, a pure xanthan gum polymer) and a fluid loss control agent (6.6 ppb IDFLO Mark of Schlumberger). Fluid #15 was hot rolled at 200 ⁰ F (93.3 ⁰ C), fluid #16 at 250 °F (121.1 ⁰ C). After hot rolling results are shown table IX. Table IX

¹ A measure of the gelling and suspending characteristics of the fluid, determined at 10 sec/10 min using a Fann viscosimeter.

Even though the formulation was not optimized, this test makes clear that the new lubricating / weighting agent provides a way to formulate brine analogues fluids useful for slimhole applications or coiled tubing drilling fluids. The rheology profile is improved by the addition of colloidal particles. Example 11

An experiment was conducted to demonstrate the ability of the new lubricating / weighting agent to formulate completion fluids, were density control and hence sedimentation stability is a prime factor. The lubricating / weighting agent is composed of the new colloidal barite according to the present invention with 50 pound per barrel [142.65 kg/m ³ ] standard API grade, calcium carbonate, which acts as bridging solids. The 18.6ppg [2.23 g/cm ³ ] fluid was formulated with 2 pound per barrel [5.7 kg/m ³ ] PTS 200 (mark of Schlumberger, pH buffer) The static aging tests were carried out at 400 ⁰ F (204.4 ⁰ C) for 72 hours. The results shown in the table below, before (BSA) and after (ASA) static aging reveal good stability to sedimentation and rheological profile.

*free water is the volume of clear water that appears on top of the fluid. The remainder of the fluid has uniform density.

Example 12

This experiment demonstrates the ability of the new lubricating / weighting agent to formulate low viscosity fluids and show it's tolerance to pH variations. The lubricating / weighting agent is composed of the new colloidal barite according to the present invention. The 16ppg [1.91 g/cm ³ ] fluid was formulated with caustic soda to adjust the pH to the required level, with the subsequent fluid rheology and API filtration tested. The results shown in the table below reveal good stability to pH variation and rheological profile.

Example 13

This experiment demonstrates the ability of the new lubricating / weighting agent to formulate low rheology HTHP water base fluids. The lubricating / weighting agent is composed of the new colloidal barite according to the present invention, with 10 pounds per barrel [28.53 kg/m ³ ] CALOTEMP (mark of Schlumberger, fluid loss additive) and 1 pound per barrel [2.85 kg/m ³ ] PTS 200 (mark of Schlumberger, pH buffer). The 17pρg [2.04 g/m ³ ] and 18pρg [2.16 g/cm ³ ] fluids were static aged for 72 hours at 25O ⁰ F (121 ⁰ C). The results shown in the table below reveal good stability to sedimentation and low rheological profile with the subsequent filtration tested.

Example 14

The following examples illustrate the ability of the fluids formulated utilizing the polymer coated colloidal solid materials of the present invention to reduce the drill string torque and thus act as a lubricating agent.

Field test 1) A 311-mm section of a high temperature, high pressure well was drilled at a 60 degree inclination to 5,121 m using a 1.8 kg/L (15 lb/gal) invert oil (paraffin) based drilling fluid incorporating the polymer coated, colloidal solids of the present invention. The fluid was formulated as an 80:20 oil: water ratio drilling fluid with the following additional components: Emul HT (27.0 lb/bbl); Lime 8.1 lb/bbl; EMI-783 (3.2 lb/bbl); EMI-603 (3.5 lb/bbl); VG Supreme (1.8 lb/bbl). the fluid exhibited the following properties:

The following observations regarding the fluid were made: the fluid system proved stable to maximum downhole temperature of 166° C; in long static periods up to 82 hours duration no evidence of cutting fill or mud weight variation; Plastic Viscosity was 25 cps initially and gradually increased to 41 cps by the end of the section as both mud weight and low gravity solids increased; yield point remained unchanged through the section varying between 3 and 4 1 lbs/100ft ² . Surprisingly, when compared with a conventionally formulated fluid used to drill offset wells, the torque required to rotate the drilling string components was reduced by 22% over the entire interval and up to 25% in the deviated section.

Field test 2) An extended reach 215.9 mm section was drilled offshore in the North Sea in the reservoir using a 1.6 kg/L (13 Ib / gal) oil-based drilling fluid incorporating the polymer coated, colloidal solids of the present invention and having the following formulation:

PRODUCT Fluid

PCC barite lbs/bbl 175.0

Freshwater bbl 0.18

EDC99DW Base bbl 0.5 oil

Lime lbs/bbl 7

Versatrol lbs/bbl 2.8

Bentone 128 lbs/bbl 4.6

Emul HT lbs/bbl 17.5 ,

The fluid exhibited the following properties:

The section was drilled with a mud weight of 13.2 lb/gal and an oil: water ratio of between 72:28 and 84:16. Water activity varied between 0.89 and 0.82 with the electrical stability controlled between 675 and 706 Volts. The observations were: no sag or settlement or change in the mud weight occurred; an aggressive (i.e. finer screen) solids separation program could be used; no differential sticking with 2,321 psi overbalance pressure in the lower part of the reservoir. The fluid system reduced the torque in the open hole by about 28% when compared to the offsets drilled with convention drilling fluids.

One of ordinary skill in the art should understand and appreciate in view of the above data that fluids including the polymer dispersant coated colloidal barite of the present invention reduced the torque required to rotate the drilling string when compared to conventionally formulated fluids.

In view of the above disclosure, one of ordinary skill in the art should understand and appreciate that one illustrative embodiment of the present invention includes a method of

reducing the torque of a drill string utilized to drill subterranean wells. In one such illustrative method, the method includes, injecting into the drilling fluid a composition including a base fluid, and a polymer coated colloidal solid material. The polymer coated colloidal solid material includes: a solid particle having an weight average particle diameter (d ₅ o) of less than ten microns, and a polymeric dispersing agent absorbed to the surface of the solid particle during the course of the cominution process. The base fluid utilized in the above illustrative embodiment can be an aqueous fluid or an oleaginous fluid and preferably is selected from: water, brine, diesel oil, mineral oil, white oil, n-alkanes, synthetic oils, saturated and unsaturated poly(alpha-olefms), esters of fatty acid carboxylic acids and combinations and mixtures of these and similar fluids that should be apparent to one of skill in the art. Suitable and illustrative colloidal solids are selected such that the solid particles are composed of a material of specific gravity of at least 2.68 and preferably are selected from barium sulfate (barite), calcium carbonate, dolomite, ilmenite, hematite, olivine, siderite, strontium sulfate, combinations and mixtures of these and other suitable materials that should be well known to one of skill in the art. In one preferred and illustrative embodiment, the polymer coated colloidal solid material has a weight average particle diameter (dso) less than 2.0 microns. Another illustrative embodiment contains at least 60% of the solid particles have a diameter less than 2 microns or alternatively more than 25% of the solid particles have a diameter less than 2 microns. The polymeric dispersing agent utilized in one illustrative and preferred embodiment is a polymer of molecular weight of at least 2,000 Daltons. In another more preferred and illustrative embodiment, the polymeric dispersing agent is a water soluble polymer is a homopolymer or copolymer of monomers selected from the group comprising: acrylic acid, itaconic acid, maleic acid or anhydride, hydroxypropyl acrylate vinylsulphonic acid, acrylamido 2-propane sulphonic acid, acrylamide, styrene sulphonic acid, acrylic phosphate esters, methyl vinyl ether and vinyl acetate, and wherein the acid monomers may also be neutralized to a salt.

In addition to the above illustrative method, the present invention is also directed to a lubricating composition that includes a base fluid and a polymer coated colloidal solid material . The polymer coated colloidal solid material is formulated so as to include a solid particle having an weight average particle diameter (ds ₀ ) of less than ten microns; and a polymeric dispersing agent coated onto the surface of the solid particle. One illustrative embodiment includes a base fluid that is either an aqueous fluid or an oleaginous fluid and

preferably is selected from, water, brine, diesel oil, mineral oil, white oil, n-alkanes, synthetic oils, saturated and unsaturated poly(alpha-olefins), esters of fatty acid carboxylic acids, combinations and mixtures of these and other similar fluids that should be apparent to one of skill in the art. It is preferred in one illustrative embodiment that the solid particles are composed of a material of specific gravity of at least 2.68 and more preferably that the colloidal solid is selected from barium sulfate (barite), calcium carbonate, dolomite, ilmenite, hematite, olivine, siderite, strontium sulfate and combinations and mixtures of these and other similar solids that should be apparent to one of skill in the art. The polymer coated colloidal solid material utilized in one illustrative and preferred embodiment has a weight average particle diameter (d ₅₀ ) less than 2.0 microns. Another illustrative embodiment contains at least 60% of the solid particles have a diameter less than 2 microns or alternatively more than 25% of the solid particles have a diameter less than 2 microns. A polymeric dispersing agent is utilized in a preferred and illustrative embodiment, and is selected such that the polymer preferably has a molecular weight of at least 2,000 Daltons. Alternatively the illustrative polymeric dispersing agent may be a water soluble polymer is a homopolymer or copolymer of monomers selected from the group comprising: acrylic acid, itaconic acid, maleic acid or anhydride, hydroxypropyl acrylate vinylsulphonic acid, acrylamido 2-propane sulphonic acid, acrylamide, styrene sulphonic acid, acrylic phosphate esters, methyl vinyl ether and vinyl acetate, and wherein the acid monomers may also be neutralized to a salt.

One of skill in the art should understand and appreciate that the present invention further includes a method of making the polymer coated colloidal solid material described above. Such an illustrative method includes grinding a solid particulate material and a polymeric dispersing agent for a sufficient time to achieve an weight average particle diameter (dso) of less than ten microns; and so that the polymeric dispersing agent is absorbed to the surface of the solid particle. Preferably the illustrative grinding process is carried out in the presence of a base fluid. The base fluid utilized in one illustrative embodiment is either an aqueous fluid or an oleaginous fluid and preferably is selected from, water, brine, diesel oil, mineral oil, white oil, n-alkanes, synthetic oils, saturated and unsaturated poly(alpha-olefins), esters of fatty acid carboxylic acids and combinations thereof. In one illustrative embodiment the solid particulate material is selected from materials having of specific gravity of at least 2.68 and preferably the solid particulate material is selected from

barium sulfate (barite), calcium carbonate, dolomite, ilmenite, hematite, olivine, siderite, strontium sulfate, combinations and mixtures of these and other similar solids that should be apparent to one of skill in the art. The method of the present invention involves the grinding the solid in the presence of a polymeric dispersing agent. Preferably this polymeric dispersing agent is a polymer of molecular weight of at least 2,000 Daltons. The polymeric dispersing agent in one preferred and illustrative agent is a water soluble polymer that is a homopolymer or copolymer of monomers selected from the group comprising: acrylic acid, itaconic acid, nialeic acid or anhydride, hydroxypropyl acrylate vinylsulphonic acid, acrylamido 2-propane sulphonic acid, acrylamide, styrene sulphonic acid, acrylic phosphate esters, methyl vinyl ether and vinyl acetate, and wherein the acid monomers may also be neutralised to a salt.

It should also be appreciated by one of skill in the art that the product of the above illustrative process is considered part of the present invention. As such one such preferred embodiment includes the product of the above illustrative process in which the polymer coated colloidal solid material has a weight average particle diameter (d ₅₀ ) less than 2.0 microns. Another illustrative embodiment contains at least 60% of the solid particles have a diameter less than 2 microns or alternatively more than 25% of the solid particles have a diameter less than 2 microns.

While the apparatus, compositions and methods of this invention have been described in terms of preferred or illustrative embodiments, it will be apparent to those of skill in the art that variations may be applied to the process described herein without departing from the concept and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the scope and concept of the invention as it is set out in the following claims.