Patent Publication Number: US-6905319-B2

Title: Stator for down hole drilling motor

Description:
FIELD OF THE INVENTION 
   This invention is directed generally toward down hole motors, and in particular down hole drilling motors used in oil and gas well drilling applications and the like. 
   BACKGROUND OF THE INVENTION 
   Progressing cavity motors, also known as Moineau-type motors (after the inventor of U.S. Pat. No. 1,892,217), including stator devices used therein, have been used in drilling applications for many years. See, for example, the following U.S. Pat. Nos. which are incorporated herein by reference: U.S. Pat. Nos. 3,840,080; 3,912,426; 4,415,316; 4,636,151; 5,090,497; 5,171,138; 5,417,281; 5,759,109; and 6,183,226. 
   Conventional Moineau pump and motor art has used rubber or elastomer materials bonded to steel for the stator contact surface. Such elastomers include not only natural rubber, but also synthetics, such as G.R.S., Neoprine, Butyl and Nitrile rubbers and other types such as soft PVC. For example, U.S. Pat. No. 5,912,303, incorporated herein by reference, discloses a polyene terpolymer rubber composition that is vulcanized for applications in the automotive industry. EPDM, a terpolymer, is highly resistant to weather, ozone and heat aging but is not oil resistant. The &#39;303 patent teaches blending nitrile rubber (NBR), which is oil resistant, with EPDM to obtain the advantages of both NBR and EPDM. The rubber is vulcanized and then used in tires, hoses, windshield wipers and the like that are subjected to weather and the like. 
   Rubber stators in down hole drilling motors are subjected to a harsh environment involving both higher temperatures, hydrocarbon immersion and dynamic loading. The key here in down hole motors has been to make the elastomer property soft enough for injection molding and soft enough to maintain the sealed cavity, yet be hard enough to be able to withstand the abrasive wear from the working contact between the rotor and the stator. U.S. Pat. No. 5,620,313, entitled “Worm Pump For Flowable Media,” utilizes a stator wall composed of a rubber with a Shore A hardness of 90 to 95 (tested in accordance with ASTM D2240). Such a hard elastomer property is desirable for withstanding the abrasive wear found in conventional down hole drilling motors. However, such a hard material is difficult to injection mold, resulting in expensive manufacturing costs. Thus, the prior art has not been able to achieve a satisfactory balance for use in down hole motors, regarding durability in operation but easier to manufacture. 
   Additionally, drilling applications generally involve high-temperature environments. U.S. Pat. No. 6,183,226 teaches that rubber used as the stator contact surface is not desirable in high-temperature environments because of its low heat conductivity. U.S. Pat. Nos. 6,183,226 and 5,417,281 disclose use of composites formed from fiberglass, resin, and elastomer. Further, as progressive cavity devices increase in diameter or length or both (as in oil and gas drilling applications), flow characteristics to maintain a successful and long-lasting bond of the rubber to steel housing becomes quite difficult. Moreover, where hydrocarbons make up the material to be pumped, such as in oil and diesel-based drilling mud used in some drilling operations, some rubber compounds are known to deteriorate. 
   SUMMARY OF THE INVENTION 
   The present invention addresses shortcomings in the field of down hole motors, particularly shortcomings associated with oil drilling applications. An embodiment of the invention comprises a down hole drilling motor comprising a tubular housing and a stator disposed in the tubular housing. The stator disposed in the tubular housing includes a central cavity. A rotor is operatively positioned in the cavity to cooperate with the lobe. The stator comprises at least one lobe, and preferably a plurality of lobes, that define at least a portion of the cavity. A lobe is formed from a compound that comprises nitrile rubber. The nitrile rubber preferably has about 35 percent by weight acrylonitrile (ACN) by Kjeldahl method and has a Mooney viscosity (tested in accordance with ASTM standard D1646) of about 50 (the nitrile rubber those characteristics is also identified herein as: 35-5 NBR). Preferably a substantial portion of the stator is formed from the compound. In one embodiment, the stator compound comprises about 100 parts by weight of the 35-5 NBR per about 231.5 total parts per weight. Conventional ingredients typically account for the remainder of the 231.5 parts. 
   A compound according to an embodiment of the present invention suitable for a drilling motor has a hardness (Shore A), tested in accordance with ASTM Standard D2240, less than 90, and preferably in a range of about 70-75. The compound preferably has a volume percent change less than 10 percent when subjected to a 72 hour 300 degree Fahrenheit test in accordance with ASTM Standard D471 using Versadrill™ drilling fluid. Similarly, the compound preferably has a volume percent change less than 5 percent when subjected to a test with similar test parameters except using sodium silicate. 
   The present invention provides an improved stator for a dynamic down hole drilling motor wherein the stator has improved thermal degradation characteristics. The invention provides a down hole motor with reduced susceptibility to stator damage from the rotor due to water swell of the stator. The preset invention provides a down hole motor with improved sealing characteristics and sufficient wear characteristics. 
   Additionally, the present invention reduces down hole motor manufacturing costs associated with injection-molding the rubber stator while improving rubber-to-model metal bonding characteristics. The present invention improves the wear and performance characteristics of the down hole drilling motor by providing better rubber-to-metal bonding characteristics. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  illustrates a side view of a down hole drilling motor of the present invention with the portions of the tubular housing cut away for purposes of illustrating internal features; and 
       FIG. 2  is a cross-section view showing a rotor operatively positioned in a cavity defined by a stator, wherein the stator is disposed in a tubular housing. 
   

   DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
     FIG. 1  depicts a down hole motor  10  according to one embodiment of the present invention. A down hole motor generally comprises a tubular housing  12  that is preferably formed of steel. Disposed within the tubular housing  12  is a power unit  14  connected to a bearing section assembly  16  via a transmission unit  18 . The power unit  14  comprises a stator  20  and rotor  22 , a cross-section of which is shown in FIG.  2 . The stator preferably comprises a plurality of lobes ( 24 ,  26 ,  28 ,  30 ,  32 ) defining a cavity  34 . It will be understood by those skilled in the art that there may be fewer or more lobes than the 5 illustrated herein. The rotor  22  is operatively positioned in the cavity  34  to cooperate with the plurality of lobes. Applying fluid pressure to the cavity  34  causes the rotor  22  to rotate in cooperation with the lobes in order to allow pressurized drilling fluid  100  that is introduced at an upper end of the pump to be expelled at the lower end and then subsequently exhausted from the bit  36 . Rotation of rotor  22  causes drill teeth  36  to rotate. 
   In operation, drilling fluid (also known in the art as drilling mud)  100  is pumped down the interior of a drill string  50  (shown broken away) attached to down hole drilling motor  10 . Drilling fluid  100  enters cavity  34  having a pressure that is a combination of pressure imposed on the drilling fluid by pumps at the surface and the hydrostatic pressure of the above column of drilling fluid  100 . The pressurized fluid entering cavity  34 , in cooperation with the lobes of the stator and the geometry of the stator and rotor causes the lobes to the stator to deform and the rotor to turn to allow the drilling fluid  100  to pass through the motor. Drilling fluid  100  subsequently exits through ports (referred to in the art as jets) in drill bit  36  and travels up the annulus  102  between the bit, motor and drill string and is received at the surface where it is captured and pumped down the drill string again. 
   Down hole drilling motors fall into a general category referred to as Moineau-type motors. For a further discussion of down hole drilling motors and their operations, see U.S. Pat. Nos. 3,840,080, 5,090,497, and 6,183,226 and Canadian Patent No. 2,058,080, incorporated by reference. Down hole motors are, however, generally subjected to greater torquing loads than simple worm pumps that also fall generally into that category. This is particularly true with high power density (HPD) down hole motors used in oil and gas well drilling. Detailed description of Moineau-type motors may be found in U.S. Pat. Nos.: 3,840,080; 3,912,426; 4,415,316; 4,636,151; 5,090,497; 5,171,138; 5,417,281; 5,759,019; and 6,183,226 and Canadian Patent No. 2,058,080. The above-identified U.S. patents are incorporated herein by reference for their teachings concerning Moineau-type motors. 
   Conventional Moineau pump and motor art has used rubber or elastomer materials bonded to the steel housing for the stator contact surface. However, in dynamic loading conditions, such as is involved in down hole drilling applications, substantial heat is generated in the rubber parts. Since rubber is not a good heat conductor, thermal energy is accumulated in the rubber part. This thermal energy accumulation may lead to thermal degradation and, therefore, damage of the rubber parts and separation from the housing. Drilling operations using HPD down hole motors put more loads on the rubber than traditional down hole motors. Thus, HPD applications result in more heat generated in the rubber. Also, where hydrocarbons make up the material to be pumped, such as in oil-based or diesel-based drilling fluids, rubber is known to deteriorate, such deterioration is exacerbated by the accumulation of thermal energy. Thus, the prior art has taught using composites for the stator rather than rubbers or elastomers. (See U.S. Pat. No. 6,183,226 and Canadian Patent No. 2,058,080). 
   Even mere water is a problem in drilling applications. For optimum performance of the drilling motor, there is a certain required clearance between the rubber parts of the stator and the rotor. When the rubber swells, not only the efficiency of the motor is comprised but also the rubber is susceptible to damage because of reduced clearance between the rotor and the stator. The reduced clearance induces higher loads on the rubber. 
   When a rotor is loaded, the rubber lobes of the stator will be deformed. Rubber with a higher modulus, i.e., a stiffer rubber, will recover faster from the deformation, thus providing better sealing during the drilling operation. Stiffer rubber, however, has disadvantages during the manufacturing processing stages. Processibility is generally inversely related to the stiffness of the rubber. This is particularly true in injection-mold processes. The stator in down hole motors are generally formed using an injection mold process. Due to the length and volume of the down hole motor, very high power is required to injection-mold the rubber. Typically, a stiffer compound will demand much more processing power and time, thereby increasing manufacturing costs. 
   Down hole drilling motors typically utilize a steel metal housing. Therefore, another requirement is that the stator have a good rubber-to-metal bonding strength. If there is not enough bonding strength between the rubber and housing, the rubber will separate from the housing during the operation of the down hole motor. The loading requirements are even more stringent for HPD down hole motor applications. 
   U.S. Pat. Nos. 6,183,226 and 5,417,281 and Canadian Patent No. 2,058,080 teach utilizing composites rather than rubber to overcome the above-discussed disadvantages of rubber. Despite the teachings of the prior art, an embodiment of the present invention utilizes a compound comprising nitrile rubber having about 35 percent by weight acrylonitrile and a Mooney viscosity of about 50, measured in accordance with ASTM Standard D1646, typically designated 35-5 NBR. In a preferred embodiment the compound comprises about 100 parts by weight of 35-5 NBR per about 231.5 total parts by weight. 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Compound 
                 
               HS-40B 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               Tensile Strength (psi) - ASTM D412, Die C 
                 
               2307 
             
             
               Elongation  @ Break - ASTM D412, Die C 
                 
               353 
             
             
               Tear Strength (lb/in) - ASTM D624, Die C 
                 
               195 
             
             
               25% Tensile Modulus (psi) - ASTM D412, Die C 
                 
               228 
             
             
               50% Tensile Modulus (psi) - ASTM D412, Die C 
                 
               331 
             
             
               100% Tensile Modulus (psi) - ASTM D412, Die C 
                 
               615 
             
             
               5% Compression Modulus (psi) - ASTM D575 
                 
               41 
             
             
               10% Compression Modulus (psi) - ASTM D575 
                 
               92 
             
             
               15% Compression Modulus (psi) - ASTM D575 
                 
               151 
             
             
               Hardness (Shore A) - ASTM D2240 
                 
               73.7 
             
             
               Density (gm/cc) - ASTM D1817 
                 
               1.218 
             
             
               Adhesion Peel Tests - ASTM D429 Method B 
                 
               108 
             
             
               Dynamic Properties 
             
             
               Temperature 
             
             
               60° C. 
               E′ 
               12.3 
             
             
               80° C. 
                 
               10.5 
             
             
               100° C. 
                 
               9.6 
             
             
               60° C. 
               E″ 
               2.5 
             
             
               80° C. 
                 
               1.9 
             
             
               100° C. 
                 
               1.5 
             
             
               60° C. 
               tanδ 
               0.20 
             
             
               80° C. 
                 
               0.18 
             
             
               100° C. 
                 
               0.16 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
           
             
               TABLE 2 
             
             
                 
             
           
          
             
               Water Swell (%) - ASTM D417 
             
          
         
         
             
             
             
          
             
                 
               Two Weeks at Room Temperature 
               3.3 
             
          
         
         
             
          
             
               Volume Change (%) - ASTM D417 
             
          
         
         
             
             
             
          
             
                 
               24 hours at 300° F. 
                 
             
             
                 
               Sodium Silicate 
               1.611 
             
             
                 
               KCL Brine - water based mud 
               −0.076 
             
             
                 
               Versaclean - oil based mud 
               0.527 
             
             
                 
               Versadrill - diesel based mud 
               9.271 
             
             
                 
               46 hours at 300° F. 
             
             
                 
               Sodium Silicate 
               2.418 
             
             
                 
               KCL Brine - water based mud 
               −0.140 
             
             
                 
               Versaclean - oil based mud 
               0.154 
             
             
                 
               Versadrill - diesel based mud 
               10.076 
             
             
                 
               72 hours at 300° F. 
             
             
                 
               Sodium Silicate 
               3.883 
             
             
                 
               KCL Brine - water based mud 
               0.042 
             
             
                 
               Versaclean - oil based mud 
               −0.580 
             
             
                 
               Versadrill - diesel based mud 
               8.951 
             
             
                 
               168 hours at 300° F. 
             
             
                 
               Sodium Silicate 
               4.086 
             
             
                 
               KCL Brine - water based mud 
               0.382 
             
             
                 
               Versaclean - oil based mud 
               −1.003 
             
             
                 
               Versadrill - diesel based mud 
               7.081 
             
             
                 
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
           
             
                 
               TABLE 3 
             
             
                 
                 
             
             
                 
               Formulation 
               HS-40B 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
                 
               Nysyn 35-5 
               100 
             
             
                 
               Ultra N774 
               75 
             
             
                 
               Akrochem P55 
               10 
             
             
                 
               85% ZnO MB 
               5 
             
             
                 
               Stearic Acid 
               1 
             
             
                 
               TP-95 
               10 
             
             
                 
               DIDP 
               10 
             
             
                 
               Cumar R-13 
               10 
             
             
                 
               Naugard 445 
               1.5 
             
             
                 
               Vanox ZMTI 
               1.5 
             
             
                 
               75% Sulfur MB 
               4.5 
             
             
                 
               MB Total 
             
             
                 
               50% PVI MB 
               1.8 
             
             
                 
               PB (OBTS)-75 
               1 
             
             
                 
               PB (TMTM)-75 
               0.15 
             
             
                 
               Total 
               231.45 
             
             
                 
                 
             
          
         
       
     
   
   For convenience a preferred compound suitable for use in an embodiment of the present invention is designated herein as HS-40B. Tables 1 and 2 list characteristic properties of the HS-40B compound. Table 1 lists various mechanical properties and Table 2 lists various structural property. Table 2 lists the percent change in volume based on soaking the compound in various mediums. Table 3 lists one preferred formulation for the HS-40B compound. 
   
     
       
         
             
             
             
           
             
               TABLE 4 
             
           
          
             
                 
             
             
                 
               72 Hrs. @ 
               168 Hrs. @ 
             
             
               Versadrill Drilling Mud 
               300° F. 
               300° F. 
             
          
         
         
             
             
             
             
          
             
               Property 
               Original 
               % Change 
               % Change 
             
             
                 
             
          
         
         
             
             
             
             
          
             
               NBR-1 
                 
                 
                 
             
             
               Tensile Strength (psi) 
               2003 
               −51.4 
               −53.0 
             
             
               Elongation @ Break (%) 
               400 
               −23.3 
               −19.3 
             
             
               Tear (lb/in) 
               241 
               −35.3 
               −53.5 
             
             
               50% Tensile Modulus (psi) 
               285 
               −42.5 
               −40.4 
             
             
               100% Tensile Modulus (psi) 
               466 
               −40.3 
               −37.3 
             
             
               10% Compression Modulus (psi) 
               88 
               −37.5 
               −36.2 
             
             
               Hardness (Shore A) 
               74 
               −20.3 
               −20.0 
             
             
               Density (gm/cc) 
               1.189 
               −3.0 
               −4.5 
             
             
               Volume (cu. in.) 
               0.479 
               14.6 
               17.7 
             
             
               NBR-2 
             
             
               Tensile Strength (psi) 
               2004 
               −42.6 
               −43.1 
             
             
               Elongation @ Break (%) 
               477 
               −8.8 
               −2.7 
             
             
               Tear (lb/in) 
               262 
               −45.4 
               −43.9 
             
             
               50% Tensile Modulus (psi) 
               276 
               −63.4 
               −64.9 
             
             
               100% Tensile Modulus (psi) 
               504 
               −63.9 
               −65.1 
             
             
               10% Compression Modulus (psi) 
               68 
               −45.6 
               −45.5 
             
             
               Hardness (Shore A) 
               73 
               −27.4 
               −27.0 
             
             
               Density (gm/cc) 
               1.240 
               −4.8 
               −4.5 
             
             
               Volume (cu. in.) 
               0.480 
               19.8 
               19.1 
             
             
               HS-40B 
             
             
               Tensile Strength (psi) 
               2307 
               −15.5 
               −18.7 
             
             
               Elongation @ Break (%) 
               353 
               −10.2 
               −17.7 
             
             
               Tear (lb/in) 
               195 
               −29.3 
               −28.8 
             
             
               50% Tensile Modulus (psi) 
               331 
               −19.5 
               −15.6 
             
             
               100% Tensile Modulus (psi) 
               615 
               −17.0 
               −12.0 
             
             
               10% Compression Modulus (psi) 
               87 
               −11.2 
               −8.3 
             
             
               Hardness (Shore A) 
               74 
               −7.4 
               −4.6 
             
             
               Density (gm/cc) 
               1.216 
               −2.5 
               −2.3 
             
             
               Volume (cu. in.) 
               0.480 
               9.0 
               7.1 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
           
             
               TABLE 5 
             
           
          
             
                 
             
             
               Sodium Silicate 
               72 Hrs. @ 
               168 Hrs. @ 
             
             
               Drilling Mud 
               300° F. 
               300° F. 
             
          
         
         
             
             
             
             
          
             
               Property 
               Original 
               % Change 
               % Change 
             
             
                 
             
          
         
         
             
             
             
             
          
             
               NBR-1 
                 
                 
                 
             
             
               Tensile Strength (psi) 
               2003 
               −45.6 
               −44.0 
             
             
               Elongation @ Break (%) 
               400 
               −51.9 
               −48.7 
             
             
               Tear (lb/in)                  241 
               −52.7 
               −56.9 
             
             
               50% Tensile Modulus (psi) 
               285 
               4.4 
               −1.0 
             
             
               100% Tensile Modulus (psi) 
               466 
               16.2 
               15.8 
             
             
               10% Compression Modulus (psi) 
               98 
               4.8 
               −9.2 
             
             
               Hardness (Shore A) 
               73 
               −8.2 
               −12.1 
             
             
               Density (gm/cc) 
               1.193 
               −0.75 
               −0.70 
             
             
               Volume (cu. in.) 
               0.478 
               9.45 
               11.83 
             
             
               NBR-2 
             
             
               Tensile Strength (psi) 
               2004 
               −51.9 
               −51.9 
             
             
               Elongation @ Break (%) 
               477 
               −71.8 
               −74.4 
             
             
               Tear (lb/in) 
               262 
               −56.3 
               −62.9 
             
             
               50% Tensile Modulus (psi) 
               276 
               33.8 
               −44.3 
             
             
               100% Tensile Modulus (psi) 
               504 
               45.1 
               54.8 
             
             
               10% Compression Modulus (psi) 
               67 
               21.5 
               19.5 
             
             
               Hardness (Shore A) 
               74 
               −5.0 
               −11.0 
             
             
               Density (gm/cc) 
               1.239 
               −1.30 
               −1.62 
             
             
               Volume (cu. in.) 
               0.479 
               9.94 
               14.06 
             
             
               HS-40B 
             
             
               Tensile Strength (psi) 
               2307 
               −19.8 
               −19.7 
             
             
               Elongation @ Break (%) 
               353 
               −38.9 
               −37.3 
             
             
               Tear (lb/in) 
               195 
               −32.1 
               −34.2 
             
             
               50% Tensile Modulus (psi) 
               331 
               −36.2 
               38.4 
             
             
               100% Tensile Modulus (psi) 
               615 
               43.9 
               43.4 
             
             
               10% Compression Modulus (psi) 
               92 
               13.4 
               18.1 
             
             
               Hardness (Shore A) 
               74 
               0.5 
               −1.6 
             
             
               Density (gm/cc) 
               1.218 
               −0.02 
               0.36 
             
             
               Volume (cu. in.) 
               0.480 
               3.88 
               4.09 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
           
             
               TABLE 6 
             
             
                 
             
             
               Initial Wt. 
               Swollen Wt. 
               Dry Wt. 
               Swell 
               Abstract 
             
             
               (gm) 
               (gm) 
               (gm) 
               % 
               % 
             
             
                 
             
           
          
             
               NBR-1 
               XYLENE 
                 
                 
                 
             
             
               Nov. 17, 1998 
               Nov. 27, 1998 
               Dec. 16, 1998 
                 
                 
             
             
               0.399 
               0.655 
               0.33  
               98.5 
               17.3 
             
             
               0.406 
               0.67  
               0.336 
               99.4 
               17.2 
             
             
               0.402 
               0.657 
               0.332 
               97.9 
               17.4 
             
             
               0.399 
               0.656 
               0.327 
               100.6 
               18.0 
             
             
                 
                 
                 
               99.1 
               17.5 
             
             
               NBR-2 
               XYLENE 
             
             
               Nov. 17, 1998 
               Nov. 27, 1998 
               Dec. 16, 1998 
             
             
               0.442 
               0.749 
               0.365 
               105.2 
               17.4 
             
             
               0.438 
               0.742 
               0.362 
               105.0 
               17.4 
             
             
               0.438 
               0.739 
               0.36  
               105.3 
               17.8 
             
             
               0.445 
               0.755 
               0.369 
               104.6 
               17.1 
             
             
                 
                 
               AVG. 
               105.0 
               17.4 
             
             
               HS-40B 
               XYLENE 
             
             
               Jan. 8, 1999 
               Jan. 14, 1999 
               Jan. 25, 1999 
             
             
               0.423 
               0.634 
               0.354 
               79.1 
               16.3 
             
             
               0.437 
               0.657 
               0.365 
               80.0 
               16.5 
             
             
               0.445 
               0.668 
               0.373 
               79.1 
               16.2 
             
             
               0.435 
               0.653 
               0.366 
               78.4 
               15.9 
             
             
                 
                 
               AVG. 
               79.1 
               16.2 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
           
             
               TABLE 7 
             
             
                 
             
             
               Initial Wt. 
               Swollen Wt. 
               Dry Wt. 
               Swell 
               Abstract 
             
             
               (gm) 
               (gm) 
               (gm) 
               % 
               % 
             
             
                 
             
           
          
             
               NBR-1 
               WATER 
                 
                 
                 
             
             
               Dec. 3, 1998 
               Dec. 18, 1998 
               Dec. 22, 1998 
             
             
               0.411 
               0.415 
               0.402 
               3.2 
               2.2 
             
             
               0.4  
               0.405 
               0.394 
               2.8 
               1.5 
             
             
               0.399 
               0.403 
               0.39  
               3.3 
               2.3 
             
             
               0.406 
               0.418 
               0.398 
               5.0 
               2.0 
             
             
                 
                 
                 
               3.6 
               2.0 
             
             
               NBR-2 
               WATER 
             
             
               Dec. 3, 1998 
               Dec. 18, 1998 
               Dec. 22, 1998 
             
             
               0.431 
               0.469 
               0.411 
               14.1 
               4.6 
             
             
               0.436 
               0.481 
               0.413 
               16.5 
               5.3 
             
             
               0.429 
               0.472 
               0.407 
               16.0 
               5.1 
             
             
               0.424 
               0.461 
               0.405 
               13.8 
               4.5 
             
             
                 
                 
               AVG. 
               15.1 
               4.9 
             
             
               HS-40B 
               WATER 
             
             
               Jan. 8, 1999 
               Jan. 14, 1999 
               Jan. 25, 1999 
             
             
               0.419 
               0.422 
               0.409 
               3.2 
               2.4 
             
             
               0.434 
               0.438 
               0.422 
               3.8 
               2.8 
             
             
               0.427 
               0.432 
               0.42  
               2.9 
               1.6 
             
             
               0.437 
               0.441 
               0.426 
               3.5 
               2.5 
             
             
                 
                 
               AVG. 
               3.3 
               2.3 
             
             
                 
             
          
         
       
     
   
   Tables 4-7 show comparisons between HS-40B, which comprises NBR, and other NBR motor compounds, generically designated NBR 1 and NBR 2. Table 4 shows a comparison and Versadrill™ drilling mud which is a diesel based mud. Table 5 shows a comparison in sodium silicate mud. Tables 6 and 7 show the result of subjecting the NBR compounds to Xylene and water swell tests per ASTM Standard D471, respectively. The NBR 1 and NBR 2 were chosen for their comparable hardness (Shore A) characteristic per ASTM Standard D2240. Reference to Tables 4 and 5 will show that the HS-40B percent change in volume was less than half that of the NBR compounds with comparable hardness characteristics. 
   While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.