Patent Publication Number: US-2007107940-A1

Title: Drill bit lubricant utilizing a sulfur-phosphorous EP agent

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates generally to a lubricant for lubricating journal bearings in a rock bit for drilling earth formation.  
      2. Background Art  
      Rock bits are employed for drilling wells in subterranean formations. Such bits have a body connected to a drill string and a single roller cone or a plurality (typically two or three) of roller cones mounted on the body for drilling rock formations. The roller cones are mounted on journals or pins integral with the bit body at its lower end. In use, the drill string and bit body are rotated in the bore hole, and each cone rotates on its respective journal as the cone contacts the bottom of the bore hole being drilled.  
      Drill bits are used in hard, often tough formations and, therefore, high pressures and temperatures are encountered. The total useful life of a drill bit is typically on the order of 20 to 200 hours for bits in sizes of about 6 to 28 inch diameter at depths of about 5,000 to 20,000 feet. Useful lifetimes of about 65 to 150 hours are typical. When a drill bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string to replace the bit which is a very expensive and time consuming process. Prolonging the lives of drill bits minimizes the lost time in “round tripping” the drill string for replacing bits.  
      Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. One reason for replacing the rock bits includes failure or wear of the journal bearings on which the roller cones are mounted. The journal bearings are subjected to very high drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures due to drilling, as well as elevated temperatures in the formation being drilled. The operating temperature of the grease in the drill bit can exceed 300° F. Considerable work has been conducted over the years to produce bearing structures and employ materials that minimize wear and failure of such bearings.  
      A variety of grease compositions have been previously employed. U.S. Pat. No. 4,358,384 discloses one prior art grease composition that consists of a petroleum derived mineral oil lubricant basestock and a metal soap or metal complex soap including aluminum, barium, calcium, lithium, sodium or strontium metals. A lighter, lower-viscosity basestock is generally employed to obtain low temperature greases, and a heavier, higher-viscosity basestock is used to obtain high temperature greases.  
      In order to enhance the lubricating capacity of typical petroleum basestock greases, solid additives such as molybdenum disulfide, copper, lead or graphite have been previously added. Synthetic polymer extreme pressure (EP) agents and high viscosity synthetic polymers may also be used. These materials serve to enhance the ability of the lubricant base stock to form a friction-reducing film between the moving metal surfaces under conditions of extreme pressure and to increase the load carrying capacity of the lubricants. The function of the lubricant is to minimize wear and to prevent scuffing and welding between contacting surfaces. U.S. Pat. Nos. 4,358,384, 3,062,741, 3,107,878, 3,281,355, and 3,384,582 disclose the use of molybdenum disulfide, and other solid additives such as copper, lead and graphite, which have been employed to attempt to enhance the lubrication properties of oils and greases.  
      Without being restricted to any method, in drilling applications, the mechanism of lubrication is by way of hydrodynamic lubrication. When at rest, the journal and the journal bearings of a drill bit squeeze out the lubricant and make direct contact. As the journal begins to rotate, the lubricant is drawn into the space between contacting surfaces to form a fluid wedge there between. As the journal rotation increases speed, this fluid wedge pushes the journal off the bearings and forms a lubricating film between the contacting surfaces. The film thickness is determined by both the rotation speed and load capacity of the lubricant. If a film is too thin, the asperities may make contact with a greater force, resulting in shearing action between the surfaces instead of a sliding action, which in turn generates heat and wears down the contacting surfaces. When this happens, EP additives in the lubricant are activated by the high temperature resulting from the extreme pressure to react with the exposed metal surfaces and form a protective coating thereon.  
      However, the use of solid EP agents, which improve the load carrying capacity of a lubricant, has been shown to contribute to excessive seal and gland wear and drill bit seal failure. For example, drill bit lubricant compounds comprising a copper EP agent have displayed seal failure due to copper deposits and loading near the seal area. The copper accumulates near the seal area until the seal is abraded by the constant and progressive erosive contact with the copper deposit. The abraded seal eventually loses its capacity to retain the grease composition in the journal area, permitting metal to metal contact between the roller cone and the journal, causing drill bit failure. Conversely, lubricants that reduce seal and gland wear typically lack sufficient film strength, that is, load carrying capacity, to be used as a drill bit lubricant.  
      Additionally, the use of solid EP agents comprising heavy metal complexes is not desirable due to their general toxicity and environmental impact for risks of leaking or spilling of heavy metal-containing lubricant during use, storage or disposal of the lubricant.  
      Accordingly, there exists a need for a lubricant that exhibits both a tight seal and good load carrying capacity with reduced seal and gland wear.  
     SUMMARY OF THE INVENTION  
      In one aspect, embodiments of the present invention relate to a lubricant for a drill bit, that includes from about 1 to about 15 weight percent of a sulfur-phosphorus EP agent, wherein the sulfur-phosphorus EP agent comprises at least one selected from a substituted 1,3,4-thiadiazole, a sulfunized oxymolybdenum organo phosphorodithioate, sulfur-phosphorus ester amine salts, a zinc dithiophosphate, and combinations thereof, from about 5 to about 40 weight percent of a thickener, and a basestock.  
      In another aspect, embodiments of the present invention relate to a roller cone drill bit including a bit body, at least one leg extending downward from the bit body, wherein each leg has a journal and each journal has a bearing surface, a roller cone mounted on each journal, wherein each roller cone has a bearing surface, a grease reservoir in communication with the bearing surfaces; and a lubricating composition in the grease reservoir and adjacent the bearing surfaces, where the lubricating composition includes from about 1 to about 15 weight percent of a sulfur-phosphorus EP agent, wherein the sulfur-phosphorus EP agent includes at least one selected from a substituted 1,3,4-thiadiazole, a sulfunized oxymolybdenum organo phosphorodithioate, sulfur-phosphorus ester amine salts, a zinc dithiophosphate, and combinations thereof, from about 5 to about 40 percent lubricant weight of a thickener, and a basestock.  
      In another embodiment, the present invention relates to a method for lubricating a roller cone drill bit. The method includes the steps of providing a roller cone drill bit having a bit body, a grease reservoir, and at least one roller cone mounted on the bit body with at least one rotatable journal bearing; and filling the grease reservoir with a lubricant, where the lubricant includes from about 1 to about 15 weight percent of a sulfur-phosphorus EP agent, wherein the sulfur-phosphorus EP agent comprises at least one selected from a substituted 1,3,4-thiadiazole, a sulfunized oxymolybdenum organo phosphorodithioate, sulfur-phosphorus ester amine salts, a zinc dithiophosphate, and combinations thereof; from about 5 to about 40 percent lubricant weight of a thickener, and a basestock.  
      In yet another embodiment, the present invention relates to a method for drilling through an earth formation. The method includes the steps of providing a roller cone drill bit having a bit body, a grease reservoir, and at least one roller cone mounted on the bit body with at least one rotatable journal bearing, wherein the grease reservoir contains a lubricant that includes from about 1 to about 15 weight percent of a sulfur-phosphorus EP agent, wherein the sulfur-phosphorus EP agent comprises at least one selected from a substituted 1,3,4-thiadiazole, a sulfunized oxymolybdenum organo phosphorodithioate, sulfur-phosphorus ester amine salts, a zinc dithiophosphate, and combinations thereof, from about 5 to about 40 percent lubricant weight of a thickener, and a basestock; securing the drill bit to the end of a drill string; and rotating the drill bit under an applied load on the earth formation.  
      Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a semi-schematic perspective of a rock bit lubricated with a lubricant according to the present invention;  
       FIG. 2  is a partial cross-section of the drill bit in  FIG. 1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In one aspect, embodiments of the present invention relate to lubricants having a sulfur-phosphorus EP agent. In particular, embodiments of the invention relate to lubricants for drill bits, methods for lubricating, drill bits comprising the lubricants, and methods for drilling.  
      A lubricant according to the present invention comprises a sulfur-phosphorus EP agent, a basestock, and one or more thickeners. Additionally, additives such as anti-wear agents, corrosion inhibitors, wetting agent, and antioxidants may also be added. Details of each component are further explained below:  
      Sulfur-Phosphorus EP Agents:  
      Applications under extreme pressure conditions often rely on additives. Lubricants containing additives that protect against extreme pressure are called EP lubricants. Common EP compounds include compounds of boron, phosphorus, sulfur, and chlorine. The compounds are typically activated by the high temperature that results from the extreme pressure. It has been shown that the activated compounds react with exposed metal surfaces and form a protected coating which covers exposed asperities.  
      In one embodiment, a sulfur-phosphorous EP agent according to the present invention includes at least one selected from a substituted 1,3,4-thiadiazole, a sulfunized oxymolybdenum organo phosphorodithioate, phosphorous-sulfur ester amine salts, a zinc dithiophosphate, or combinations thereof. In another embodiment, a sulfur-phosphorus EP agent according to the present invention includes a blend of a substituted 1,3,4-thiadiazole and a sulfunized oxymolybdenum organo phosphorodithioate. Specifically, when making a lubricant of the present invention, the lubricant may include from about 1 to about 5 weight percent of the substituted 1,3,4-thiadiazole and from about 1 to about 10 weight percent of the sulfunized oxymolybdenum organo phosphorodithioate. Preferably, about 3 weight percent of the substituted 1,3,4-thiadiazole and about 4 weight percent of the sulfunized oxymolybdenum organo phosphorodithioate are added to the lubricant. One example of 1,3,4-thiadiazole is 1,3,4-thiadiazole-2( 3 H)-thione,5,5-dithiobis. One example of sulfunized oxymolybdenum organo phosphorodithioates is molybdenum di(2-ethylhexyl) phosphorodithioate. One example of a phosphorous-sulfur ester amine salt includes Lubrizol™ 5080A, commercially available from The Lubrizol Corporation (Wickliffe, Ohio). Examples of zinc dithiophosphates include Lubrizol™ 677A, Lubrizol™ 139, and Lubrizol™ 5002, all commercially available from The Lubrizol Corporation (Wickliffe, Ohio).  
      In other embodiments, a sulfur-phosphorus EP agent may be added as an additive to a commercially available grease composition. Advantageously, sulfur-phosphorus EP agents, in accordance with embodiments of the present invention, may be used as an additive in a wide range of basestocks and greases, such as Molykote™ G-0101, available from Dow Corning (Midland, Mich.), Mobilith™ AW-2, available from ExxonMobil Corporation (Fairfax, Va.), and Lucant 600 and Lucant 2000, both available from Mitsui Petrochemical (New York, N.Y.).  
      Basestocks:  
      The basestock, or base oil, form the main lubricating component. Oils are generally classified as refined and synthetic. Refined oils are also referred to as mineral oils or petroleum oils. For example, paraphinic and naphthenic are refined from crude oil while synthetic oils are manufactured by chemical synthesis. The basestock may be selected from any of the basestocks known in the art, including a synthetic base oil, a petroleum or mineral oil, or combinations thereof. In some embodiments, a synthetic lubricant basestock may be preferred over a petroleum derived basestock to increase viscosity. In other embodiments, a high viscosity petroleum derived mineral oil basestock may be used.  
      Suitable synthetic oils for use in a basestock may include synthetic polyalphaolefins, other hydrocarbon fluids and oils, synthetic polyethers, poly-esters, alkylene oxide polymers, and interpolymers, esters of phosphorus containing acids, silicon based oils and mixtures thereof. In one embodiment, the basestock may include a high viscosity index polyalphaolefin based fluid. Suitable polyalphaolefins include those discussed in U.S. Pat. Nos. 5,589,443, 5,668,092, and 4,827,064, which are incorporated herein by reference in their entirety. Other suitable synthetic oils include alkylated naphthalenes, such as Synesstic™ AN, which is available from ExxonMobil Corporation (Fairfax, Va.), polybutenes, such as Indopol™ polybutenes which are available from BP P.L.C. (Warrenville, Ill.), and hydrogenated polybutenes, such as Panalane™ hydrogenated polybutenes, which are available from BP P.L.C. (Warrenville, Ill.).  
      Suitable mineral or petroleum oils may include naphthenic or paraffinic oil. Other suitable mineral oils may include high viscosity index hydroprocessed basestock and bio-based esters.  
      Thickeners:  
      Thickeners give a lubricant its characteristic consistency and are sometimes thought of as a “three-dimensional fibrous network” or “sponge” that holds the oil in place.  
      In one embodiment, the base oil may be thickened with a soap, such as soaps of calcium, aluminum, titanium, barium, lithium, and their complexes. Metal complex soaps may include alkali metals, alkaline earth metals, Group IVB metals, and aluminum. Simple soaps may be formed by combining a fatty acid or ester with a metal and reacting through a saponification process, with the application of heat, pressure, or agitation. While simple soaps are formed by reacting one single organic acid with a metal hydroxide, complex soaps may be formed by reacting two or more organic compounds with the metal hydroxide.  
      In another embodiment, the base oil may be thickened with a non-soap, such as urea, fine silica, fine clay, and/or silica gel. In yet another embodiment, the basestock may be thickened with both soap and non-soap thickening agents. While the above description lists several specific thickening agents, no limitation is intended on the scope of the invention by such a description. It is specifically within the scope of the present invention that other soap and non-soap thickening agents may be used.  
      Other Additives:  
      Additives that are commonly added to lubricants to improve their performances may also be added to a lubricant of the present invention. Examples of such additives are anti-wear agents, corrosion inhibitors, anti-oxidants, zinc- and chlorine-based EP agents, etc, including copper powder molybdenum disulfide, and bismuth ethylhexanoate. Examples of zinc- and chlorine-based EP agents include Lubrizol™ 885 and Lubrizol™ 2501, which are both commercially available from The Lubrizol Corporation (Wickliffe, Ohio). Bismuth ethylhexanoate is commercially available from OMG Americas, Inc. (Cleveland, Ohio).  
      For an excellent review of common lubricant additives, see  Lubricant Additives: Chemistry and Applications , edited by Leslie R. Rudnick (2003, ISBN 0824708571).  
      Application of the Lubricant in a Drill Bit:  
      Referring now  FIGS. 1 and 2 , a sealed bearing rotary cone rock bit, generally designated as  10 , consists of bit body  12  forming an upper pin end  14  and a cutter end of roller cones  16  that are supported by legs  13  extending from body  12 . The threaded pin end  14  is adapted for assembly onto a drill string (not shown) for drilling oil wells or the like. Each of the legs  13  terminate in a shirttail portion  22 . Each of the roller cones  16  typically have a plurality of cutting elements  17  pressed within holes formed in the surfaces of the cones for bearing on the rock formation to be drilled. Nozzles  20  in the bit body  12  introduce drilling mud into the space around the roller cones  16  for cooling and carrying away formation chips drilled by the drill bit. While reference is made to an insert-type bit, the scope of the present invention should not be limited by any particular cutting structure. Embodiments of the present invention generally apply to any rock bit (whether roller cone, disc, etc.) that requires lubrication by grease.  
      Each roller cone  16  is in the form of a hollow, frustoconical steel body having cutting elements  17  pressed into holes on the external surface. For long life, the cutting elements may be tungsten carbide inserts tipped with a polycrystalline diamond layer. Such tungsten carbide inserts provide the drilling action by engaging a subterranean rock formation as the rock bit is rotated. Some types of bits have hardfaced steel teeth milled on the outside of the cone instead of carbide inserts.  
      Each leg  13  includes a journal  24  extending downwardly and radially inward on the rock bit body. The journal  24  includes a cylindrical bearing surface  25  which may have a flush hardmetal deposit  62  on a lower potion of the journal  24 .  
      The cavity in the cone  16  contains a cylindrical bearing surface  26 . A floating bearing  45  may be disposed between the cone and the journal. Alternatively, the cone may include a bearing deposit in a groove in the cone (not shown separately). The floating bearing  45  engages the hardmetal deposit  62  on the leg and provides the main bearing surface for the cone on the bit body. The end surface  33  of the journal  24  carries the principal thrust loads of the cone  16  on the journal  24 . Other types of bits, particularly for higher rotational speed applications, may have roller bearings instead of the exemplary journal bearings illustrated herein.  
      A plurality of bearing balls  28  are fitted into complementary ball races  29 ,  32  in the cone  16  and on the journal  24 . These balls  28  are inserted through a ball passage  42 , which extends through the journal  24  between the bearing races and the exterior of the drill bit. A cone  16  is first fitted on the journal  24 , and then the bearing balls  28  are inserted through the ball passage  42 . The balls  28  carry any thrust loads tending to remove the cone  16  from the journal  24  and thereby retain the cone  16  on the journal  24 . The balls  28  are retained in the races by a ball retainer  64  inserted through the ball passage  42  after the balls are in place. A plug  44  is then welded into the end of the ball passage  42  to keep the ball retainer  64  in place.  
      Contained within bit body  12  is a grease reservoir system generally designated as  18 . Lubricant passages  21  and  42  are provided from the reservoir to bearing surfaces  25 ,  26  formed between a journal bearing  24  and each of the cones  16 . Drilling fluid is directed within the hollow pin end  14  of the bit  10  to an interior plenum chamber  11  formed by the bit body  12 . The fluid is then directed out of the bit through the one or more nozzles  20 .  
      The bearing surfaces between the journal  24  and cone  16  are lubricated by a lubricant or grease composition. Preferably, the interior of the drill bit is evacuated, and lubricant or grease is introduced through a fill passage  46 . The lubricant or grease thus fills the regions adjacent the bearing surfaces plus various passages and a grease reservoir. The grease reservoir comprises a chamber  19  in the bit body  10 , which is connected to the ball passage  42  by a lubricant passage  21 . Lubricant or grease also fills the portion of the ball passage  42  adjacent the ball retainer. Lubricant or grease is retained in the bearing structure by a resilient seal  50  between the cone  16  and journal  24   
      Lubricant contained within chamber  19  of the reservoir is directed through lube passage  21  formed within leg  13 . A smaller concentric spindle or pilot bearing  31  extends from end  33  of the journal bearing  24  and is retained within a complimentary bearing formed within the cone. A seal generally designated as  50  is positioned within a seal gland formed between the journal  24  and the cone  16 .  
      In one embodiment, a lubricant in accordance with this aspect of the invention may include from about 1 to about 15 weight percent of a sulfur-phosphorus EP agent; about 1 to about 10 weight percent of silica; about 5 to about 40 weight percent of a thickening agent, preferably a metal-complex soap, and a balance of a heavy mineral basestock. In another embodiment, the lubricant may further comprise solid additives. In one embodiment, the solid additives are from about 1 to about 15 weight percent of molybdenum disulfide and from about 1 to about 10 weight percent of copper particles. In yet another embodiment, the basestock may be a blend of mineral oil and synthetic oil. Specifically, in one embodiment, the basestock may be a blend of 0 to 100% mineral oil and 100 to 0% synthetic oil with any percentage therebetween, preferably about 50% of each.  
      In one embodiment, the lubricant or grease in the grease reservoir may include from about 1 to about 15 weight percent of a sulfur-phosphorus EP agent; about 1 to about 10 weight percent of silica; about 5 to about 40 weight percent of a thickening agent, preferably a metal-complex soap; and a balance of a heavy mineral basestock. In another embodiment, the lubricant or grease may further comprise solid additives such as molybdenum disulfide and copper particles. In one embodiment, the solid additives are from about 1 to about 15 weight percent of molybdenum disulfide and from about 1 to about 10 weight percent of copper particles. In yet another embodiment, the basestock may be a blend of 0 to 100% mineral oil and 100 to 0% synthetic oil with any percentage therebetween, preferably about 50% of each.  
      Use of the Lubricant in a Method of Drilling:  
      According to one aspect of the present invention, a method for drilling is provided. In one embodiment, the method for drilling includes the steps of providing a roller cone drill bit having a bit body and a plurality of roller cones mount on the bit body with rotatable journal bearings, introducing a lubricating composition to the journal bearings, where the lubricating composition includes a basestock, a thickener, and a sulfur-phosphorus EP agent. In one embodiment, the lubricant in the grease reservoir may include from about 1 to about 15 weight percent of a sulfur-phosphorus EP agent; about 1 to about 10 weight percent of silica; about 5 to about 40 weight percent of a thickening agent, preferably a metal-complex soap; and a balance of a heavy mineral basestock. In another embodiment, the lubricant may further comprise solid additives such as molybdenum disulfide and copper particles. In one embodiment, the solid additives are from about 1 to about 15 weight percent of molybdenum disulfide and from about 1 to about 10 weight percent of copper particles. In yet another embodiment, the basestock may be a blend of 0 to 100% mineral oil and 100 to 0% synthetic oil with any percentage therebetween, preferably about 50% of each.  
      In the above embodiments, if the composition of the basestock is predominantly synthetic oil, an ester-based swelling agent may also be added to enhance the wetting and suspension of silica. One suitable swelling agent includes Esterex C4461, which is available from ExxonMobil Corporation (Fairfax, Va.).  
      Lubricants in accordance with embodiments of the invention have been found to have superior properties, as compared to prior art lubricants and as evidenced by the following examples.  
     EXAMPLES  
     Example 1  
     A Sulfur-Phosphorus EP Lubricant Containing Cu and MoS 2    
      A lubricant in accordance with one embodiment of the present invention was prepared with about 5 weight percent of silica, about 12 weight percent of molybdenum disulfide, about 32 weight percent of a Li-complex soap, about 5 weight percent of copper particles, and about 10 weight percent of a sulfur-phosphorus EP agent, and a balance of a basestock containing 100% mineral oil. The sulfur-phosphorus EP agent used in this example is a blend of about 3 parts of a substituted 1,3,4-thiadiazole (e.g. 1,3,4-thiadiazole,5,5-dithiobis available from R.T. Vanderbilt Company, Inc. under the trade name Vanlube® 829) and about 4 parts of a sulfunized oxymolybdenum organo phosphorodithioate (e.g. molybdenum di(2-ethylhexyl)phosphorodithioate available from R.T. Vanderbilt Company, Inc. under the trade name Molyvan® L). Such lubricant has a film strength of 1000 kg as determined by ASTM-2596 4 ball load EP test. The benefits of using only mineral oil may include causing seal swell, which allows for a tighter seal and wetting silica gel for easier suspension.  
     Example 2  
     A Sulfur-Phosphorus EP Lubricant Without Solid Additives  
      A lubricant in accordance with another embodiment of the present invention was prepared with about 5% of silica, about 32% of a Li-complex soap, and about 10% of the sulfur-phosphorus EP agent and a balance of a basestock containing 100% mineral oil. The sulfur-phosphorus EP agent used in this example is a blend of about 3 parts of a substituted 1,3,4-thiadiazole (e.g. 1,3,4-thiadiazole,5,5-dithiobis available from R.T. Vanderbilt Company, Inc. under the trade name Vanlube® 829) and about 4 parts of a sulfunized oxymolybdenum organo phosphorodithioate (e.g. molybdenum di(2-ethylhexyl)phosphorodithioate available from R.T. Vanderbilt Company, Inc. under the trade name Molyvan® L). Such lubricant has a film strength of 1000 kg as determined by ASTM-2596 4 ball load EP test. No solid particles such as copper or molybdenum disulfide were added.  
     Example 3  
     A Sulfur-Phosphorus EP Lubricant with Synthetic Basestock  
      A lubricant in accordance with yet another embodiment of the present invention was prepared with about 10% of a sulfur-phosphorus EP agent, about 5% of silica, about 32% of a Li complex-metal soap, and a balance of a one-to-one blend of synthetic oil and mineral oil as the basestock. The sulfur-phosphorus EP agent used in this example is a blend of about 3 parts of a substituted 1,3,4-thiadiazole (e.g. 1,3,4-thiadiazole,5,5-dithiobis available from R.T. Vanderbilt Company, Inc. under the trade name Vanlube® 829) and about 4 parts of a sulfunized oxymolybdenum organo phosphorodithioate (e.g. molybdenum di(2-ethylhexyl)phosphorodithioate available from R.T. Vanderbilt Company, Inc. under the trade name Molyvan® L). No solid particles such as copper or molybdenum disulfide were added. The addition of synthetic oil may afford such a lubricant better temperature characteristics, and the lack of solids may render the lubricant more compatible with mechanical seal systems. To enhance wetting and suspension of the silica gel thickening agent, an ester-based swelling agents was also added.  
     Example 4  
     A Sulfur-Phosphorus EP Lubricant Containing Other Additives  
      A lubricant according to yet another embodiment of the present invention was prepared with about 10% of the sulfur-phosphorus EP, about 32% of a Li-complex soap, about 3% of a corrosion inhibitor (such as, for example, Hi Tech  350  available from Hi Tech Company), and a balance of a basestock. The sulfur-phosphorus EP agent used in this example contains about 3 parts of a substituted 1,3,4-thiadiazole (e.g. 1,3,4-thiadiazole,5,5-dithiobis available from R.T. Vanderbilt Company, Inc. under the trade name Vanlube® 829) and about 4 parts of a sulfunized oxymolybdenum organo phosphorodithioate (e.g. molybdenum di(2-ethylhexyl)phosphorodithioate available from R.T. Vanderbilt Company, Inc. under the trade name Molyvan® L). The basestock is a blend of a mineral oil and ultra-high molecular weight polyalphaolefin (such as Lucant 2000 and Lucant 600 available from Mitsui Petrochemical, New York, N.Y.) in one-to-one ratio. About 6% of molybdenum disulfide and about 1.5% of silica were also added. Such composition may have the advantage of avoiding vulcanization of seals that are made of HSN elastomer material.  
      Advantages of the embodiments of the invention may include one or more of the following: adding a sulfur-phosphorus EP agent to make a lubricant according to the present invention may eliminate the need to add solid EP additives such as lead. A lubricant thus produced may be non-toxic, environmentally friendly and still retain exceptional load carrying ability required in applications such as lubrication of rock bits. In addition, by combining a high to medium viscosity synthetic base oil with an almost equal amount of mineral oil, a lubricant that has efficient, controlled seal swell may be produced to ensure the maintenance of a tight seal during the drilling process without excessive seal wear.  
      While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.