GB2066876A - Drilling fluid made from abrasive weighting material - Google Patents

Abstract

Weighting agents having a hardness of at least 4.3, preferably at least 4.5, on the Mhos scale can be used in water-based drilling fluids, without encountering problems of abrasion on the drill string, bit etc., if a water-dispersible lubricant, (e.g. a long chain fatty acid or vegetable oil containing such an acid, with a surfactant) is included in the drilling fluid. Suitable weighting agents are ilmenite and non-micaceous hematite.

Description

SPECIFICATION Drilling fluid made from abrasive weighting material The present invention relates to improved compositions and methods for drilling boreholes such as oil wells, gas wells or the like. More particularly, the invention relates to high density drilling fluids made from 'abrasive weighting agents.

Drilling fluids used in the drilling of oil wells, gas wells and similar earth boreholes are commonly water-based mixtures containing clays or other colloidal materials as well as certain additives which are employed depending on borehole conditions. The drilling fluid or mud primarily functions as a medium to carry cuttings produced by the drill bit to the surface. Additionally, the drilling fluid serves as a lubricant for the bit and drill stem and prevents the ingress of formation fluids such as oil, gas and salt water into the borehole while the drilling proceeds. The drilling fluid serves other purposes and possesses other characteristics which need not be discussed in detail here since they are well known and described in numerous publications such as, for example, Walter F.Rogers' Composition and Properties of Oil Well Drilling Fluids, 3rd Ed., Houston, Texas, 1963; Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Ed., New York, 1965, Vol. 7, pgs. 287-307.

As noted, one of the functions of a drilling fluid is to maintain a sufficient hydrostatic pressure in the fluid column in the borehole to prevent ingress of formation fluids into the borehole, and, under extreme circumstances, to prevent blow out of the well. Depending upon the drilling conditions encountered, sufficient hydrostatic pressure may be achieved almost with plain water. However, under normal circumstances, and usually as a safety precaution, it is necessary, for achieving the desired hydrostatic pressure to load the mud with a suitable weighting agent to increase its density. These weighting agents are generally in the form of finely divided solids of a material which has high intrinsic density.The most common weighting agent is finely ground barite, although iron oxides, celestite, galena and other materials have been used at various times. Baste, because of its relatively high density, insolubility, chemical inertness and heretofore ready availability has been the weighting agent of choice. However, with the decreasing availability of barite, other weighting agents are being sought.

Aside from the fact that barite sources are dwindling, and in some areas of the world where drilling activity is high, practically non-existent, barite is relatively soft, being from 2.5 to 3.5 on the Mhos scale of hardness.

This relatively soft characteristic of barite effects the rheological properties of the drilling mud after the barite has been circulated in the borehole during the drilling operation. In the drilling operation, the entire circulating system behaves somewhat in the manner of a fluid mill such that the softer mineral particles are subject to attrition and become reduced in particle size. This reduction in particle size results in a direct increase in the viscosity of the drilling mud arising from a much smaller particle size and an increase in the total number of particles.

It has been suggested to use other weighting materials than barite to overcome the difficulties described above in connection with relatively soft weighting materials such as barite which is subject to progressive comminution. British Patent 1,495,874 discloses the use of ilmenite and/or hematite as a weighting material for drilling fluids.

Ilmenite is generally widely distributed throughout the world. Ilmenite has an adequate specific gravity, i.e. greater than 4.4, and exhibits low water solubility and high chemical inertness. Additionally, ilmenite occurs in forms which are relatively hard, that is greater than 5 on the Mhos scale of hardness. Ilmenite (ferrous titanate) is a commonly occurring mineral. It is described, for example, in Vol. 5 of the text Rock Forming Minerals by W.A. Deer, et al, London, 1962, pages 28 - 33. Typical analyses show ilmenite contains 45 - 48 wt. % of titanium dioxide and 33 - 37 wt. % of iron, although there is considerable variation, and other elements such as magnesium, manganese, calcium, and vanadium are often present in amounts of less than 1 wt. %.

Additionally, Colombian Patent 9396 discloses the use of micaceous specular hematite for use as a weighting agent in drilling fluids.

Specular hematite is a naturally occuring mineral widely distributed throughout the world in different forms and different purities. For example, the micaceous specular hematite disclosed as a drilling fluid weighting agent in Colombian Patent 9396 is a relatively soft, flaky, micaceous mineral, having a distinct laminar structure. This form of micaceous specular hematite upon grinding tends to form very thin flat platelets and upon use in a drilling fluid these platelets easily comminute into very small particles which tend to increase the viscosity of the drilling fluid. This micaceous specular hematite is substantially non-abrasive under borehole conditions.

This invention provides a drilling fluid which comprises (a) water, (b) a weighting agent selected from non-hydrated minerals having a hardness of at least 4.3 on the Mhos scale, and a specific gravity of at least 4.3, said weighting agent have a particle size such that at least 85% by weight passes through a 325 mesh screen, and at least 98% by weight passes a 200 mesh screen, said weighting agent being present in an amount sufficient substantially to increase the density of the drilling fluid, and (c) an amount of a water dispersible lubricant sufficient substantially to reduce the abrasiveness of the drilling fluid.

In another embodiment, the present invention provides an improved earth borehole drilling method characterized in that the novel drilling fluid above described is circulated in the borehole during the drilling operation.

In a preferred embodiment, the weighting agent is ilmenite, non-micaceous specular hematite, or a mixture thereof.

The specular hematite to which the present invention is addressed is a non-hydrated mineral which although of general laminar structure, is hard and upon grinding forms particles having irregular, abrasive edges. Such specular hematite has a relatively high intrinsic density, i.e. SpG. greater than 4.3, and has a hardness on the Mhos scale of 5 to 6. Upon use as a weighting agent in the drilling fluid, this specular hematite does not tend to comminute into smaller particles which would substantially change the rheological properties of the drilling fluid. However, because of its intrinsic hardness, it is quite abrasive and can result in excessive wear on the drilling equipment particularly the drill string and the drill bit.As the search for oil and gas results in the drilling of deeper and deeper wells, excessive wearing or eroding of the drill string and the drill bit becomes an increasingly more severe problem.

Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a graph showing the abrasive weight loss caused by various drilling fluid compositions.

Figure 2 is a graph showing the abrasive wear characteristics of various drilling fluid compositions.

The weighting agents useful in the drilling fluids of the present invention are hard, non-hydrated minerals having a specific gravity of 4.3 or greater, as exemplified by ilmenite and specular hematite. Ilmenite has been described above. Specular hematite is a commonly occuring mineral which chemically is ferric oxide (Fe2O3). It is described, for example, in Ivan Kostov Mineralogy, (Oliver & Boyd, Edinburgh and London) 1 sot Eng. Ed. (1968), incorporated herein by reference. Typically, specular hematite contains 69.94% Fe and 30.06% 0. Some Ti may be present. Specular hematite is often found mixed with hydrated iron oxides, iron carbonate, and magnitite as well as iron silicates. However, deposits of non-hydrated specular hematite are available, to remove water of hydration is not required.

The weighting agents useful in the present invention have a density of at least 4.3, preferably at least 4.5.

When necessary, the weighting agent can be upgraded by well known metallurgical procedures, such as flotation, to achieve the desired minimum density. The weighting agent will have a hardness, on the Mhos scale of hardness, of at least 4.3. Indeed, many specular hematites have a hardness of 5-6. For use in the present invention, the weighting agents will have a particle size such that at least 85% by weight passes through a 325 mesh screen and at least 98% passes a 200 mesh screen. More preferably, the particle size should be such that 90-92% passes a 325 mesh screen. The desired particle size of the weighting agent can be accomplished by milling procedures well known to those in the art.

The amount of weighting material present in the drilling fluid will vary widely depending upon the desired density of the drilling mud. The latter, of course, is dependent upon the drilling conditions encountered or expected. In any event, the quantity of barite necessary to achieve a desired mud density is well known and tables showing this relation are readily available. From a comparison of the intrinsic density of the weighting agents herein with that of barite, such tables may be used to determine the quantity of the weighting agent necessary to provide drilling muds or fluids of any desired density.

As previously indicated, the weighting agents of the present invention are materials which are relatively hard and hence less susceptible to progressive comminution. Concomitantly, the weighting agents exhibit relatively high abrasion characteristics which can result in excessive erosion of drilling equipment such as pumps, drill pipe and drill bits during the drilling operation. It has now been discovered that if suitable water dispersible lubricants are added to the drilling fluids made with the relatively hard weighting agents of the present invention, the abrasiveness of the drilling fluid can be reduced to the point where virtually no more wear is observed than that which occurs with drilling fluids of similar density employing the much softer barite as the weighting agent.The lubricant used in the drilling fluid of the present invention must be dispersible in water to the extent that it forms a colloid, emulsion, or other such heterogeneous system in which the lubricant is generally uniformly dispersed or distributed in the water phase. The lubricant should be of a type which will permit the desired rheological properties of the drilling fluid to be achieved.

Preferably, the lubricant will be a material which, in sufficient amounts, will produce a useful drilling fluid using the hard weighting agents of the present invention which will exhibit wear and/or abrasion characteristics generally similar or substantially the same as those obtained with a drilling fluid of substantially the same density or weight using barite as a weighting agent. One group of lubricants that has been found to be particularly effective in fresh water based drilling fluids of the present invention are those which contain fatty acids such as oleic acid, stearic acid, linolic acid, paimitic acid, and the like. Such fatty acids are commonly found in vegetable oils such as cottonseed oil, soy bean oil, corn oil, castor oil, linseed oil, etc.The fatty acids, either in pure form or in the form of the vegetable oils mentioned above are admixed with small amounts of surfactants or other agents so as to be rendered water dispersible. Such surfactants can include, for example, long chain alcohols, alkanol amines, sulfonated vegetable oil derivatives such as sulfonated castor oil, etc. A particularly effective lubricant is a mixture of soy bean oil, long chain alcohols, and sulfonated castor oil. This composition and similar materials make ideal anti-wear agents since they need only be added in relatively small amounts to the drilling fluid in order to reduce the abrasiveness to an acceptable level. Additionally, since only relatively small amounts are needed only minor to negligible impairment of rheological properties occurs.

The amount of anti-wear agent in the drilling fluid can vary widely but generally will be in the range of from about 0.1 to about 5% by weight. In general, however, the anti-wear agent need be present only in an amount which will effect substantial reduction in the abrasiveness of the drilling fluid.

Preferably the drilling fluid will include thickening agents. Such thickening agents may consist of clay bearing formations such as shale or argillaceous sands which are encountered in the drilling operation and which are picked up by the drilling fluid or mud and become a part thereof. Alternately or additionally, clays may be added to the mud. Typical non-limiting examples of highly collodial clays include smectites, particularly bentonite, attapulgite, and sepiolite, bentonite being the clay of preference. A wide variety of other water dispersible materials, particularly organic colloids, may serve as thickening agents including starch derivatives, cellulose derivatives, synthetic polymers as sodium polyacrylate, natural gums such as guar gum, gum kiraya, bacterial gums such as that elaborated by certain Xanthomonas species, and the like.

Mixtures of such suitable thickening agents may be employed if desired. The technology of using such thickening agents in drilling muds is well developed and well known to those skilled in the art. The various thickening agents have a wide diversity in weight effectiveness in terms of the number of pounds thereof which must be incorporated in a barrel of drilling fluid or mud of a given type in order to achieve desired consistency.

In addition to the thickening agents, the drilling mud may optionally contain fluid loss control agents, viscosity control agents, anti-foaming additives, freezing point depressants, and the like.

It will be appreciated that in the method of the present invention employing the drilling fluids herein and while the borehole is being drilled during the drilling operation, additional amounts of the weighting agent and/or antiwear agent can be added to maintain or alter the drilling mud density and abrasiveness, as desired.

To more fully illustrate the present invention, the following non-limiting examples are presented.

The specular hematites employed were obtained from U.S. Steel Corporation, were non-hydrated and had a specific gravity range of 4.9 to 5.1.

Example 1 A base drilling fluid or mud was prepared by stirring 25 grams of AQUAGEL (Brand name of a bentonite clay marketed by NL Baroid, Houston, Texas) and 50 grams of Glen Rose shale into 350 ml. of water for 20 minutes using a high speed dispersator. To the mud was then added 5 grams of Q-BROXIN (Brand name of a lignosulfonate marketed by NL Baroid, Houston, Texas), 5 grams of CARBONOX (Brand name of an organic humic acid material marketed by NL Baroid, Houston, Texas), 1.5 grams of caustic soda and 4 grams of AKTAFLO S (Brand name of a drilling fluid surfactant marketed by NL Baroid, Houston, Texas) while stirring for 10 minutes on the dispersator. The mud thus prepared was allowed to set overnight.

Example 2 Using the base mud prepared in Example 1, various weighted drilling muds were prepared using BAROID (Brand name of a barite marketed by NL Baroid, Houston Texas), and specular hematite. Evaluations of the weighted drilling muds thus produced were obtained for a 13 pound per gallon (ppg) mud (Table I), a 16 ppg mud (Table II) and an 18 ppg mud (Table III).

TABLE I Sample Mark A B A B A B A B Bass Mud, bbl. 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 BAROID, lb 270 270 270 270 Specular Hematite, lb 260 260 260 260 Stirred (Multimixer), min. 15 15 15 15 20 20 25 25 Rolled at 150 F, hr. 0 0 16 16 16 16 16 16 Aged at 300 F, hr. 0 0 0 0 0 0 16 16 Plastic Viscosity, cp. 54 47 43 32 65 49 97 72 Yield Point, lb/100 sq.ft. 10 15 7 6 25 22 26 26 10 sec gel, lb/100 sq. ft. 4 9 3 3 5 6 4 5 10 min gel. lb/100 sq. ft. 7 9 5 4 5 6 5 5 pH 10.4 10.5 9.0 9.0 APl Filtrate, ml/30 min. 3.5 3.5 4.5 5.0 3.5 4.0 Filtrate, 300 F, 500 psi, ml 12.4 16.4 Shear Strength, lb/100 sq.ft. 100 100 TABLE 11 Sample Mark A B A B A B A B Base Mud, bbl. 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 BAROID, lb 530 530 530 530 Specular Hematite, lb 512 512 512 512 Stirred (Multimixer), min 15 15 15 15 20 20 25 25 Rolled at 150 F, hr. 0 0 16 16 16 16 16 16 Aged at 300 F, hr 0 0 0 0 0 0 16 16 Plastic Viscosity, cp. 61 51 88 28 73 36 100 38 Yield Point, lb/100 sq. ft. 16 2 34 5 7 8 3 3 10 sec gel, lb/100 sq. ft. 5 4 6 2 5 3 9 2 10 min gel, lb/100 sq.ft. 8 5 8 2 10 4 35 2 pH 8.8 8.7 8.4 8.5 8.6 8.4 APl Filtrate, ml/30 min. 2.5 2.5 2.5 3.0 5.0 8.0 Filtrate, 300 F, 500 psi, ml 40.0 65.0 Shear Strength,lb/100 sq.ft. 400 160 Setting of Weight Material 0 MOD H - High MOD-Moderate TABLE 111 Sample Mark A B A B A B A B Base Mud, bbl 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 BAROID, lb 720 720 720 720 Specular Hematite, lb 665 665 665 665 Stirred (Multimixer), min. 15 15 15 15 20 20 25 25 Rolled at 150 F, hr. 0 0 16 16 16 16 16 16 Aged at 300 F, hr. 0 0 0 0 0 0 16 16 Plastic Viscosity, cp. 77 36 43 18 66 38 H 63 Yield Point, lb/100 sq. ft. 0 4 7 3 3 12 H 19 10 sec. gel. lb/100 sq. ft. 0 3 2 2 3 2 120 6 10 min.gel, lb/100 sq. ft. 1 4 5 4 4 3 228 38 pH 9.9 10.1 9.0 9.5 APl Filtrate, ml/30 min. 5.0 5.0 6.0 5.5 10.0 8.4 Filtrate, 400 F, 500 psi, ml 64.0 118.0 Shear Strength, lb/100 sq. ft. 1000 400 H-High As can be seen comparing the results in Tables l, ll and Ill above, drilling fluids prepared with specular hematite compare favorably with a conventional drilling mud prepared using barite as a weighting agent.

Example 3 To test the comparative attrition rates under high shear of various weighting agents in drilling muds, the 16 ppg drilling muds prepared according to Examples 1 and 2 were used. As seen from the data in Table IV below, the attrition rate of specular hematite is considerably less than that of barite in comparable drilling muds. This resistance to attrition is beneficial in assuring that the rheological properties of the drilling fluid will not be as susceptible to change during drilling operations in which the mud is being constantly circulated in the bore hole.

TABLE IV Wet Screen Analysis % Absolute % Less Than 325 Mesh Increase In Fines Initial Final Mud (15 min) Mud (8 hr) Final Minus initial BAROID 93.2 97.0 3.8 Specular Hematite 92.0 93.0 1.0 Example 4 A water-based drilling mud was prepared using 15 grams of AQUAGEL and 60 grams of Glen Rose shale hydrated into 350 ml. of water followed by the addition of 300 grams of the particular weighting agent to be evaluated. Muds made with specular hematite were thinned with 0.1 ppb BARAFOS (Brand name of a polyphosphate compound marketed by NL Baroid, Houston, Texas). To test the abrasive or wear characteristics of the muds produced, the effect of adding anti-wear agents, two methods were employed.

One method, Method A, took into account the phenomena of wear caused by the movement of particles within a fluid against a metal surface. In this method, the over-all loss of weight from the entire surface was measured.

MethodA A Hamilton Beach mixer, Model 936, operating at 15,000 rpm was used to produce laboratory simulated abrasion effects and particle breakdown. For each drilling mud, a new weighed, mixer blade was used as a reference site for abrasive activity. Periodically, the blade was examined for weight loss after various stirring periods. The muds were immersed in a cooling bath during the mixing to avoid overheating and evaporation. Thickening of the weighted muds during stirring was minimized by the addition of BARAFOS.

The results of the wear rate tests are shown in Table V below.

TABLE V Sample Mark A B C D E F Base Bud 1.0 1.0 1.0 1.0 1.0 0.68 BAROID, lb 555 555 270 Specular Hematite, Ib A-31 0,92.6% 325 mesh 512 270 A-314, 93.1% 325 mesh 512 397 BARAFOS, Ib/bbl. 0.14 0.14 0.52 0.25 Anti-Wear Additive, Ib/bbl. 4.0 Properties: Initial Plastic Visc., cp 18 20 16 12 23 22 YieldPoint,lb/100sq.ft. 3 0 0 14 2 20 10Sec.Gel,lb/100sq.ft. 3 1 0 17 1 9 10Min.Gel,lb/100sq.ft. 12 11 4 51 12 24 pH 8.2 8.2 9.0 9.8 9.6 9.1 Properties: 4 hr.

Plastic Visc., cp 30 29 24 19 28 49 YieldPoint,lb/100sq.ft. 52 36 7 2 153 27 10Sec.Gel,lb/100sq.ft. 57 44 3 2 137 13 10 Min. Gel, lb/100 sq. ft. 130 117 22 12 257 44 pH 8.4 8.3 9.0 9.6 9.5 8.9 Properties: 8 hr.

Plastic Visc., cp 36 31 15 YieldPoint,Ib/100sq.ft. 48 39 81 10 Sec. Gel, lb/100 sq. ft. 55 49 62 10 Min. Gel, Ib/100 sq. ft. 105 109 191 pH 8.2 8.2 9.6 Abrasive Wear, Blade wt.,g 5.17 5.15 5.21 5.18 5.21 5.19 Wt. loss, % Miser Blade after hr.

1 0.24 0.50 3.92 2.85 1.47 2.64 2 0.47 0.98 8.00 5.42 2.50 4.86 4 0.94 1.93 14.10 10.20 4.67 8.56 8 1.52 3.23 14.33 A second wear or abrasion test method, Method B, was employed which took into account the movement of metal surfaces against each other while exposed to a flowing fluid. In Method B, the object was to measure weight loss from the area exposed to loads.

Method B The test block used for measuring extreme pressure (EP) properties was modified by machining so that a constant wear surface would be exposed to the test string during tests. A ring speed of 432 rpm was selected since this is approximately equivalent to drill pipe rotating at 130 rpm and is fairly typical in well drilling methods. Various loads of torque were applied to the ring and block at this speed for intervals of 15 minutes.

The initial weight and final weight of the test block were measured and a weight loss of the block determined in each mud. The wear rate was then determined and calculated for one hour. The results are shown in Tables Vl-VIII below.

TABLE VI Weighting Agent BAROID Load, inch pounds 100 100 200 200 300 300 400 400 Ring Speed, rpm 432 432 432 432 432 432 432 432 Ring Speed, ft/sec. 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 Wear Run, min 15 15 15 15 15 15 15 15 Initial Block Weight, g 9.8453 9.8401 9.8183 9.8100 9.7690 9.7562 9.7143 9.7005 Final Block Weight, g 9.8401 9.8357 9.8100 8.8026 9.7562 9.7423 9.7005 9.6852 Weight Loss, g 0.0052 0.0044 0.0083 0.0074 0.0128 0.0139 0.0138 0.0153 Wear Rate, g/hr. 0.0208 0.0176 0.0332 0.0296 0.0512 0.0556 0.0552 0.0612 Average Wear Rate, g/hr. 0.0192 0.0314 0.0534 0.0582 TABLE VIII Weighting Agent SPECULAR HEMATITE Load, inch pounds 50 50 100 100 200 200 Ring Speed, rpm 432 432 432 432 432 432 Ring Speed, ft/sec. 2.6 2.6 2.6 2.6 2.6 2.6 Wear Run, min. 15 15 15 15 15 15 Initial Block Weight, g 9.4625 9.4523 9.4443 9.4199 9.3950 9.3082 Final Block Weight, g 9.4523 9.4445 9.4199 9.3950 9.3082 9.2116 Weight Loss, g 0.0102 0.0078 0.0246 0.0249 0.0868 0.0966 Wear Rate, g/hr. 0.0408 0.0312 0.0984 0.0996 0.3472 0.3864 Average Wear Rate, g/hr. 0.0360 0.0990 0.3668 TABLE VIII Weighting Agent SPECULAR HEMATITE Anti-Wear Agent, Ib/bbl 4 4 8 8 Load, inch pounds 300 200 300 200 Ring Speed, rpm 432 432 432 432 Ring Speed, ft/sec 2.6 2.6 2.6 2.6 Wear Run, Min 15 15 15 15 Initial Block Weight, g 8.4698 8.4483 8.4318 8.4178 Final Block Weight, g 8.4483 8.4318 8.4178 8.4087 Weight Loss, g 0.0215 0.0165 0.0140 0.0091 Wear Rate, g/hr. 0.0860 0.0660 0.0560 0.0364 As can be seen from the data in Tables V-VIII above, the specular hematite, when compared with barite shows a much higher abrasiveness.With reference to Table V, it can be seen that the addition of an anti-wear agent greatly reduces the abrasive wear of a drilling mud weighted with specular hematite. The results are graphically demonstrated in Figure 1 in which a drilling mud weighted with specular hematite and containing no anti-wear agent (Curve A) is compared with a similarly weighted mud containing about 4 ppb of an anti-wear agent, (Curve B), and a typical barite weighted drilling mud (Curve C).

With reference to Tables Vl-VIII and Figure 2, it can also be seen that the wear rate of muds made with specular hematite is much greater than that of a conventional mud weighted with barite. However, as the data, plotted in Figure 2, shows, the addition of relatively small amounts of anti-wear agent greatly reduces the wear rates. Note for example, that the wear rate of a specular hematite weighted mud (Curve A), which is quite abrasive without the addition of anti-wear agent, exhibits an abrasiveness, when combined with about ppb of anti-wear agent, (Curve D), similar or substantially the same as that of a conventional drilling mud weighted with barite (Curve C). Even at relatively low levels of anti-wear agent, i.e. 4 ppb, (Curve B), the abrasiveness is greatly reduced.

It can thus be seen that vastly improved drilling muds, exhibiting excelient rheological properties, can be made with hard or relatively hard weighting agents without excessive wear and abrasion when the drilling mud contains, in addition, an anti-wear agent such as described above.

Claims (12)

1. A drilling fluid which comprises (a) water, (b) a weighting agent selected from non-hydrated minerals having a hardness of at least 4.3 on the Mhos scale, and a specific gravity of at least 4.3, said weighting agent having a particle size such that at least 85% by weight passes through a 325 mesh screen, and at least 98% by weight passes a 200 mesh screen, said weighting agent being present in an amount sufficient substantially to increase the density of the drilling fluid, and (c) an amount of a water dispersible lubricant sufficient substantially to reduce the abrasiveness of the drilling fluid.

2. A drilling fluid as claimed in Claim 1 wherein the amount of lubricant is from 0.1 to 5% by weight.

3. A drilling fluid as claimed in Claim 1 or 2 wherein the weighting agent has a hardness of at least 4.5 on the Mhos scale.

4. A drilling fluid as claimed in any preceding Claim wherein the weighting agent has a particle size such that 90 to 92% by weight passes through a 325 mesh screen.

5. A drilling fluid as claimed in any preceding Claim wherein the weighting agent is ilmenite, non-micaceous specular hematite, or a mixture thereof.

6. A drilling fluid as claimed in any preceding Claim which comprises barite, celestite, galena, or a mixture thereof, as an additional weighting agent.

7. A drilling fluid as claimed in any preceding Claim which comprises a thickening agent.

8. A drilling fluid as claimed in Claim 7 wherein the thickening agent is a water-dispersible organic colloid, asbestos, or a mixture thereof.

9. A drilling fluid as claimed in any preceding Claim which comprises a fluid loss control agent, and/or a thinning agent.

10. A drilling fluid as claimed in any preceding Claim wherein the lubricant comprises a long chain fatty acid or a vegetable oil containing such a fatty acid, and a surfactant.

11. A drilling fluid as claimed in Claim 1 and substantially as hereinbefore described with reference to any of the Examples.

12. A method for drilling a borehole in the ground which comprises circulating a drilling fluid through the borehole as drilling proceeds, wherein the drilling fluid is a fluid according to any of the preceding Claims.