Lubricating grease for rock drill bits and bits so lubricated

Abstract

 @ A lubricating grease suitable for use in rock drill bits comprises:

(1) a major proportion of a mineral or synthetic hydrocarbon base oil,

(2) from 3 to 15% wt by weight of the grease of a lithium complex soap as the sole thickener, and

(3) from 5 to 20% wt by weight of the grease of molybdenum disulphide particles as extreme pressure additive.

Such a grease has the required lubrificating qualities and the required flow properties for use in rock drill bits having a grease reservoir (e.g. a pressure equalisation chamber) and channels connecting the reservoir to the bearing surfaces.

Description

[0001] This invention relates to a lubricating grease for rock drill bits and to rock drill bits lubricated with such a grease.

[0002] Rock drill bits used for drilling to a substantial depth into the earths crust (e.g. in exploration for oil or gas) normally consist of a number of cutter cones rotating on a stem. The bearing surfaces between the cones and stem are lubricated by a grease. The grease is sealed into the bit before use and it should provide lubrication for the lifetime of the bit, which may be several hundreds of hours. Eventually the bit has to be replaced because of wear of the cutting surfaces but, since retrieving and replacing a bit is a time-consuming operation, it is desirable that the bit should last for the full lifetime of its cutting ability and should not fail prematurely through failure of its lubricating system.

[0003] The environment within which a rock drill bit operates can be severe. Heavy loads may be imposed on the rotating cutter cones, particularly when drilling through hard rock formations and high temperatures and pressures may exist in the vicinity of the bit. Further the bore of the hole being drilled may be filled with drilling mud containing abrasive particles.

[0004] The lubrication system of a drill bit includes a pressure equalisation chamber connected to the bearing surfaces by channels. The pressure equalisation chamber has a flexible membrane sealing the chamber from the exterior of the bit and, as its name implies, the object of the chamber is to ensure that the pressure in the lubrication system is the same as the exterior pressure in the bore hole. This helps to minimise the risk of ingress of drilling mud, or water or hydrocarbons, into the lubrication system or loss of lubricating grease out from the system. Such a pressure equalisation chamber is necessary, but the membrane with its necessary seals are points of weakness and there must always be some risk of contamination of the lubricating grease by the drilling mud or other fluids in the bore hole.

[0005] The presence of a pressure equalisation chamber means that the grease has to have good flow properties to pass from the chamber to the bearing surfaces or vice versa. This requirement conflicts with other requirements such as good load bearing qualities, and good high temperature properties.

[0006] It has already been proposed that lubricating greases for rock drill bits should use a metal complex soap as the thickener and should contain molybdenum disulphide as an extreme pressure additive. Thus US Patent 3935114 discloses a grease suitable for lubricating drill bits containing a calcium acetate complex soap, molybdenum disulphide and a selected class of metal oxide powders. US Patent No 4358384 discloses a grease for lubricating rock bits containing a metal soap (the metal being aluminium, barium, calcium, lithium, sodium or strontium), 6 to 14X wt of molybdenum disulphide particles and 3 to 9X wt of copper particles. Preferably a mixture of metal soaps is used and, in the specific examples, the greases have a mixture of an aluminium complex soap and a lithium soap or a mixture of a calcium complex soap and a lithium soap.

[0007] It has now been found that the presence of metal oxide powder or copper particles can be dispensed with provided the metal complex soap is correctly chosen and other characteristics of the grease are also correctly selected.

[0008] According to the present invention, a lubricating grease suitable for use in rock drill bits comprises:

(1) a major proportion of a mineral or synthetic hydrocarbon base oil,

(2) from 3 to 15X wt by weight of the grease of a lithium complex soap as the sole thickener, and

(3) from 5 to 20X wt by weight of the grease of molybdenum disulphide particles as extreme pressure additive.

[0009] The base oil is preferably a mineral hydrocarbon base oil derived from petroleum crude oil by the conventional steps of solvent extraction, dewaxing and purification (e.g. clay treatment or hydrofinishing). It may be paraffinic or naphthenic, preferably the former and may be a distillate or bright stock fraction or a mixture.

[0010] Preferably the base oil has a viscosity of from 100 to 500 cSt at 40°C. As stated earlier, the viscosity chosen is a compromise between good flow properties, high temperature performance and load carrying (lubricant film thickness).

[0011] The lithium complex soap may be formed from a hydroxy and a non-hydroxy mono-carboxylic fatty acid, a dicarboxylic fatty acid and lithium hydroxide. The mole ratio of monocarboxylic fatty acid to dicarboxylic acid may be from 2:1 to 6:1 and the mole ratio of hydroxy to non-hydroxy monocarboxylic fatty acid may be from 2:1 to 15:1. The monocarboxylic fatty acids may have from 12 to 24 carbon atoms and the dibasic acid from 6 to 16 carbon atoms. The molar amount of lithium hydroxide is desirably at least sufficient to neutralise the acids and may give a small amount of free alkalinity in the finished grease (e.g. 0.02 to 0.1X wt of free lithium hydroxide).

[0012] The fatty acids may be prepared synthetically or be derived from vegetable or animal fats and a proportion of them may be present initially as an ester, e.g. hydrogenated castor oil (glyceryl ester of 12-hydroxystearic acid), diisooctylsebacate, or diisooctyldodecanedioate.

[0013] The lithium complex soap may be formed in the base oil by reacting the acids, either separately or together, in part of the oil, with lithium hydroxide, heating to a high temperature, cooling and adding the rest of the oil and additives, and homogenising or milling.

[0014] A particularly suitable lithium complex soap may be formed from 12-hydroxy stearic acid, dodecanedioic acid, distilled tallow fatty acid and lithium hydroxide monohydrate.

[0015] Preferably the molybdenum disulphide content of the grease is from 5 to 12% wt, it having been found that too high a content can adversely affect flow properties without markedly improving wear properties. The molybdenum disulphide may be in the form of particles containing at least 98X of molybdenum disulphide, with about 80% of the particles of a size less than 10 pm and about 99X of the particles of size less than 70 pm. Suitable molybdenum disulphide particles include those sold by Climax Molybdenum (Technical or Technical Fine grades) and by Solids & Dispersions (grades 1404 or 1407).

[0016] The grease may contain minor amounts of conventional grease additives, e.g. oxidation and corrosion inhibitors, anti-wear and extreme pressure additives. The molybdenum disulphide and other additives are incorporated into the grease after cooling and before homogenising or milling.

[0017] A suitable grease may have the following proportions of ingredients:

[0018] The present invention includes a drill bit lubricated with a grease having the characteristics described above. The rock drill bit may be any of those having a grease reservoir e.g. a pressure equalisation chamber and a channel or channels connecting the reservoir to the bearing surfaces.

[0019] The greases of the present invention combine good wear properties with good flow properties. Greases have been prepared with 4 Ball weld loads (at room temperature (unheated) and 200°C) in excess of 700 kg. As regards flow properties, the greases are characterised by a comparatively low rate of change of apparent viscosity with temperature. This is important because, although high temperatures may be encountered at certain stages of drilling, there may be considerable variation in temperature over the whole of a drilling operation.

[0020] The invention is illustrated by the following examples, some of which are comparative.

Example 1

[0021] Three lithium complex greases were prepared as follows:

1. Preparation of LXA3

(a) Base Grease

[0022] 1696 gm of oil were heated in a grease kettle to 60°C. 340 gm 12-hydroxystearic acid, 58 gm dodecanedioic acid, and 32 gm distilled tallow fatty acid were added and heated to 90°C. 74 gm of lithium hydroxide monohydrate dissolved in hot water were added and heating of the mixture continued. When foaming ceased, the mixture was heated to 195°C, held at this temperature for 1 hour, the remaining oil added (1816 gm) and the grease cooled to 90°C. The grease was then passed twice through an APV homogeniser at 200 bar pressure. The worked penetration of the grease was 257, free acidity was nil, free alkalinity was 0.1% wt lithium hydroxide, and the drop point was 220°C.

(b) LXA3 Formulation

[0023] 963 gm of the above base grease were mixed with 120 gm of molybdenum disulphide, 52 gm of zinc dialkyldithiophosphate, 17 gm of zinc naphthenate (8X zinc) and 567 gm of oil. The mixture was homogenised through an APV homogeniser at 35 bar. The worked penetration of the blended grease was 355 and the drop point was 205°C.

2. Preparation of LXB4

(a) Base Grease

[0024] (The preparation was substantially as for LXA3 base except that an oil blend with a higher viscosity was used).

[0025] 1500 gm of oil were heated to 60°C, 320 gm 12-hydroxystearic acid, 55 gm of dodecanedioic acid and 30 gm of distilled tallow fatty acid were added and heated to 90°C. 70 gm of lithium hydroxide monohydrate in hot water were added and heating of the mixture continued. When foaming ceased,- the mixture was heated to 195°C, held at this temperature for 1 hour, the remaining oil added (2036 gm) and the grease cooled to 90°C. The grease was then passed twice through an APV homogeniser at 200 bar pressure.

[0026] The worked penetration of the grease was 278, free acidity nil, free alkalinity 0.1% wt of lithium hydroxide and the drop point was 235°C.

(b) LXB4 Formulation

[0027] 690 gm of the base grease were mixed with 225 gm of molybdenum disulphide, 45 gm of zincdialkyldithiophosphate, 15 gm of zinc naphthenate (8X zinc) and 525 gm of oil. The mixture was homogenised through an APV homogeniser at 35 bar. The worked penetration of the blended grease was 356 and the drop point was 210°C.

3. Preparation of LXB3

[0028] 7000 gm of oil were heated to 50°C and 669 gm of 12rhydroxy- stearic acid added. Heating was continued and at 60°C, 84 gm of diisooctylsebacate were added; at 80°C, 115 gm of dodecanedioic acid were added and at 95°C, 162 gm of lithium hydroxide monohydrate dissolved in hot water were added. Heating was continued to 190°C and 63 gm of distilled tallow fatty acids added. At 195°C, 4401 gm of oil were added, the temperature of the mix dropping to 115°C. Additives were incorporated as follows: 981 gm of molybdenum disulphide, 420 gm of zinc dialkyl-dithiophosphate and 140 gm of zinc naphthenate (8% zinc). The grease was cooled to 100°C and homogenised through an APV homogeniser at 200 bar pressure. The worked penetration of the grease was 352 and the drop point was 227°C.

[0029] The compositions of the three greases and their characteristics were as shown in Table 1 below:

Example 2

[0030] Greases LXA3, LXB3 and LXB4, prepared as in Example 1, were tested for load carrying (extreme pressure), wear properties and flow properties. Several alternative greases were also tested for load carrying, wear and flow properties. These alternative greases were:

A2) Aluminium complex soap greases each containing 7% wt

A3) of molybdenum disulphide

C5) Clay/polymer greases containing 15% wt and 7% wt

C6) molybdenum disulphide respectively

D1) Commercially available greases marketed by competitors

D2) as drill bit greases, and believed to be calcium complex soap greases containing EP particles

[0031] The EP wear properties were measured using the 4 Ball method (IP Test No 239). The flow properties were measured in a suitable rheometer by applying a force (shear stress) to the greases and measuring the rate of flow (shear rate). The apparent viscosities shear stress were then calculated. shear rate

[0032] Determinations were first made at 20°C. Considerable variations were found both in the initial shear stress to produce flow and the increase in flow rate with increasing shear stress. Determinations were then made at 100°C, again showing considerable variations. A shear stress of 400 Pa at 20°C was found to be the minimum shear stress to give some flow in all the greases and this shear stress was used to calculate apparent viscosities at 20°C and 100°C and hence to determine the rate of change of apparent viscosity with temperature.

[0033] The EP and wear test results are shown in Table 2 below:

[0034] The flow property results are shown in Table 3 below and are expressed graphically in the accompanying Figure 1.

[0035] Comparing the wear results in Table 2 it will be seen that the greases of the present invention (LXA3, I.XB3 and LXB4) had relatively high weld loads both at starting temperatures of 20°C and 200°C. The aluminium complex and clay greases were inferior, particularly at 200°C. Competitors grease D1 did have a high weld load at both temperatures, but D2 showed a significant decrease at 200°C. The wear results show that LXA3, LXB2 and LXB4 had good qualities, better than the aluminium complex and clay greases, and comparable to the competitors greases.

[0036] Comparing the flow properties (Table 3 and Figure 1) it will be seen that the greases of the present invention (LXA3, LXB3 and LXB4) had a relatively low rate of change of apparent viscosity. The aluminium complex greases (A₂ and A3) had lower apparent viscosities which changed more rapidly. Competitors grease D2 had a high apparent viscosity at 20°C, but it changed rapidly so that it was below that of LXA3 and LXB4 at 100°C. Due to the difficulties of measuring the apparent viscosities of the clay greases (C5 and C6) and Competitors grease Dl at 100°C, the rate of change could not be determined.

Example 3

[0037] Grease LXB3 was subjected to thermo-mechanical analysis along with a number of commercially available greases.

[0038] Thermo-mechanical analysis is a technique for comparing changes in consistency of a material over a temperature range. A small probe 5 mm in diameter is loaded (0.3gm) against a small sample of grease in a hollow about 1 to 2 mm deep. Starting at room temperature, the assembly is heated at 10°C/minute up to 260°C. As the grease softens, the probe sinks further into the grease, ie the probe height is reduced. If any significant expansion occurs, the probe height increases. If hardening takes place, the probe height changes only slightly with temperature.

[0039] The results are shown in Figure 2 in which probe height is plotted against temperature.

[0040] In Figure 2, the curve for grease LXB3 of the present invention shows that the grease remains unchanged in consistency until 150°C and then softens gradually and relatively evenly. All the other greases show inferior properties.

[0041] Thus a commercial conventional lithium soap grease (C) starts to change at 120°C, expands up to 160°C, and then softens rapidly. The commercial aluminium complex soap grease (A) starts to soften almost immediately and is softer than LXB3 up to 200°C. The commercial calcium complex soap grease (B) shows no change in probe height indicating early and maintained hardening. Commercial drill bit grease D1 shows expansion up to 120°C and then a gradual change consistent with hardening. Commercial drill bit grease D2, on the other hand, starts to soften as soon as 80°C and thereafter increases rapidly in softness.

Claims

1. A lubricating grease suitable for use in rock drill bits comprising:

(1) a major proportion of a mineral or synthetic hydrocarbon base oil,

(2) from 3 to 15X wt by weight of the grease of a lithium complex soap as the sole thickener, and

(3) from 5 to 20X wt by weight of the grease of molybdenum disulphide particles as extreme pressure additive.

2. A lubricating grease is claimed in claim 1 wherein the lithium complex soap is formed from a hydroxy and a non-hydroxy mono-carboxylic fatty acid, a dicarboxylic fatty acid and lithium hydroxide.

3. A lubricating grease as claimed in claim 2 wherein the mole ratio of monocarboxylic acid to dicarboxylic acid is from 2:1 to 6:1, the mole ratio of hydroxy to non-hydroxy monocarboxylic acid is from 2:1 to 15:1 and the molar amount of lithium hydroxide is at least sufficient to neutralise the acid.

4. A lubricating grease as claimed in claim 2 or 3 wherein the monocarboxylic acids are 12-hydroxy stearic and distilled tallow fatty acid, the dicarboxylic acid is dodecanedioic acid and the lithium hydroxide is in the form of the monohydrate.

5. A lubricating grease as claimed in any of claims 1 to 4 having the following proportions of ingredients:

6. A lubricating grease as claimed in claim 5 having the following proportions of ingredients:

7. A rock drill bit lubricated by a grease as claimed in any of claims 1 to 6.

8. A lubricating grease as claimed in claim 1 substantially as described in the specific examples.

Drawing