Abstract:
A method and apparatus prevents rotation of a stabilizer which rotates due to friction-induced forces of bearing seals disposed in a shaft. The apparatus includes a stabilizer having a ridged edge where the stabilizer is mounted to a drilling tool using a deformable member. The deformable member assists in creating friction of the stabilizer edge against a drilled bore hole wall and thereby reduces friction-induced rotation.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention provides a novel method and apparatus for preventing the friction induced rotation of non-rotating stabilizers which are attached to a drilling tool to control the direction and orientation of drilling. The invention can be utilized to prevent rotation at any combination of hole angle, curvature rate and bit load. The present invention provides a more efficient method of operating rotary steerable directional tools.  
       BACKGROUND  
       [0002]     Most rotary steerable systems utilize non-rotating stabilizers to control the trajectory of the hole. The rotating friction between the non-rotating stabilizer and the shaft that turns the bit on conventional systems causes the non-rotating stabilizer to rotate in a clockwise direction. With conventionally surfaced stabilizers, the procession rate is related to the ratio of the rotational friction force between the shaft and the fixed stabilizer to the axial drag force between the fixed stabilizer and the borehole wall. The frictional rotation rate decreases as the hole angle, curvature rate, and/or bit weight increases. However, rotation rates may become excessive at low hole angles, low curvature rates and/or low bit weights. The worst conditions are most likely to occur at the kick off point in a vertical hole. This problem prevents the use of conventional rotary steerable systems on many directional drilling applications.  
         [0003]     For example, Table 1 shows the expected frictional rotation rates for a 12 ft non-rotating stabilizer that includes a conventional smooth surfaced adjustable stabilizer, a fixed stabilizer and which utilizes low friction sealed bearings between the shaft and the non-rotating unit.  
                                                           TABLE 1                           CONVENTIONAL NON-ROTATING STABILIZER            Hole   Curvature   Bit   Lateral/axial   Frictional       Angle   Rate   Weight   Slide friction   Rotation rate       deg.   deg/100 ft   kips   Ratio   deg/axial ft                    30.0   6   25   1   14.5       30.0   3   25   1   17.1       30.0   2   25   1   18.4       0.5   3   25   1   37.4       0.5   2   25   1   48.1       0.5   3   10   1   55.5       0.5   2   10   1   68.6                  
 
         [0004]     As suggested by the last column, the adjustable stabilizer blades must be continuously adjusted to compensate for the friction-induced rotation of the non-rotating unit. Even with a hole angle of 30 degrees the expected frictional rotation rates would be a problem. With a 0.5 degree hole angle, the rotation rates are unacceptable.  
       SUMMARY OF THE INVENTION  
       [0005]     Applicant solves the frictional rotation problem by making the stabilizer surface act like a drag bit and by increasing the contact forces between the stabilizer and the bore wall formation. Rotation is prevented whenever the threshold torque required to rotate the drag bit like contacts exceed the rotational driving torque. The design is so effective that it can prevent frictional rotation by only applying the design concept to the fixed stabilizer. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0006]     Preferred embodiments of the invention will be described below in reference to the appended drawings wherein,  
         [0007]      FIGS. 1A-1D  illustrate the relationship between rotational drive torque, bearing forces and frictional rotation rate as observed by the present inventor;  
         [0008]      FIG. 2  illustrates a fixed stabilizer as mounted on a drilling tool according to a preferred embodiment;  
         [0009]     FIGS.  3 A-B illustrate the fixed stabilizer according to a preferred embodiment;  
         [0010]     FIGS.  4 A-B illustrate the forces on a PDC cutter which models drag behavior of the fixed stabilizer according to the present invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENT  
       [0011]     Referring to the accompanying Figures, a preferred embodiment of the invention is described as follows.  
         [0012]     Referring to  FIGS. 1A-1D , the present inventor determined that the mechanics of frictional rotation are defined generally by the following equations:  
             RDT   =         bd   ·   ff   ·   SBF     2     +     2   ·   st               (   1   )             
 
         [0013]     Where:  
                                                       RDT = Rotational driving torque   in lbs.           bd = Shaft bearing diameter   in.           ff = Bearing friction factor   *           SBF = Sum of all the bearing forces   lbs.           st = Rotating seal torque   in lbs                      
 
         [0014]     The present invention is specifically applicable to drilling tools including two to four bearings. However, the invention may be applied to a drilling tool with a different number of bearings as long as the summation of the lateral forces on the bearings (SBF) is taken into account. The rotational driving torque comes from the frictional torque in the Kalsi seals and the lateral contact forces in the bearings between the shaft and the (non-adjustable stabilizer) (NAS). The resisting forces are generated by the lateral contact forces between the stabilizers and the hole. Whenever the resisting forces are larger than the frictional forces rotation is prevented.  
         [0015]      FIG. 2  illustrates the placement of fixed stabilizer blades  1 ,  1 ! on a non-rotating stabilizer according to the present invention. Reference number  2  corresponds to an adjustable stabilizer to position the drilling tool in the bore hole.  
         [0016]     The present inventor noted that the calculated cases for Table 1 (conventionally surfaced stabilizer) assumed that the axial sliding friction factor and the rotating friction factor were equal. If the sliding surface of the stabilizer blade were modified to increase the rotating friction factor, the rotation rate would be reduced.  
         [0017]     Table 2 shows the effect of utilizing a blade surface that provides a rotational friction factor that is 3 times the axial sliding friction factor. The desired effect is enhanced by aligning the edges of the ridges parallel to the axis of the bore hole and making the ridges sharp.  
                                                           TABLE 2                           NON-ROTATING STABILIZER WITH       DIRECTIONAL FRICTION            Hole   Curvature   Bit   Lateral/axial   Frictional       Angle   Rate   Weight   Slide friction   Rotation rate       deg.   deg/100 ft   Kips   Ratio   deg/axial ft                    30.0   6   25   3   4.8       30.0   3   25   3   5.7       30.0   2   25   3   6.1       0.5   3   25   3   12.5       0.5   2   25   3   16.0       0.5   3   10   3   18.5       0.5   2   10   3   22.9                  
 
         [0018]     This improvement makes the frictional rotational rates acceptable in 30 degree holes but still presents a significant problem in 0.5 degree holes, especially at reduced bit weight and curvature rate.  
         [0019]     In the present invention, the contact surface of the stabilizer blade is modified to inhibit lateral movement. The preferred modification places axial ridges on the surface of fixed stabilizer blades. The lateral forces on the stabilizer push the ridges into the bore wall, thereby preventing lateral rotation of the drilling assembly whenever the resisting shear forces in the formation wall exceed the rotational friction force.  
         [0020]     Referring to  FIG. 3A , each stabilizer fin has 6 sharp drag bit shaped cutters  3   a - 3   f . The cutters are equally spaced from the tool center. The cutters are curved along the axial direction. Under low loads only a single cutter will contact the wall of the hole. Referring to  FIG. 3B , the stabilizer fins are supported on load springs. The allowable radial travel is set to provide an under gauge diameter (relative to the bore hole) when the blades are fully collapsed and an over gauge diameter when fully extended. The trailing edge will be 30° (angle A) below the tangential surface.  
         [0021]     The cutters act like polycrystalline diamond compact (PDC) cutters on a PDC bit. The rotational mechanics of this design can be modeled using technology developed for the drill bit industry.  FIGS. 4A-4B  illustrate known configurations for PDC cutters. An excellent source of useful information was published by Glowka of Sandia Nat&#39;l Labs in the Society of Petroleum Engineers Journal of Petroleum Technology in August 1989 pgs 797-849.  
         [0022]     Glowka used a variety of single PDC cutters to measure the mechanics of drilling in three kinds of rock. The test included flat faced cutters as well as sharp edge cutters. Most of the tests measured the axial cutter loads and the penetration forces as a function of the depth of cut.  
         [0023]     They developed the following empirical relationships for cutting dry rock at the surface. 
        FDB=FA·(0.90+2.2·D)     FDT=FA·(0.65−0.58·D)     FDS=FA·(0.63+0.88·D)        
 
         [0027]     where  
                                                       FDB = Cutter drag force in Berea Sandstone   lbs.           FDT = Cutter drag force in Tennessee Marble   lbs.           FDS = Cutter drag force in Sierra White Granite   lbs.           FA = Downward force on the dull cutter   lbs.           D = Depth of cut   in.                      
 
         [0028]     The tests that used sharp cutters required larger cutter drag forces than observed with the dull cutter tests. They also ran tests with drilling fluid. These tests showed that the drilling fluid acted as a lubricant and reduced the cutter drag forces by 10 percent.  
         [0029]     The inventor notes that the cutter drag forces are greater in softer rocks. Using the performance in granite should underestimate the cutter drag forces in all oilfield formations. Combining all these factors gives the following safe estimate for the rotational resistance of the stabilizer design of the invention:  
             RT   =       FS   ⁡     (   0.63   )       ⁢     (   0.9   )     ⁢     sd   2               (   2   )             
 
         [0030]     Where:  
                                                       RT = Rotation resisting torque   in lbs.           FS = Total lateral loads on all of the stabilizer blades   lbs.           sd = stabilizer diameter   in.                      
 
         [0031]     By using a bit cutter like contact, the mechanics are changed from a two dimensional sliding problem to establishing a threshold resisting load that prevents any rotation whenever it exceeds the driving torque RDT defined above in equation (1).  
         [0032]     Referring back to  FIG. 3B , the fixed stabilizer adjusts from 8 ⅜ in. outer diameter to 8 ⅝ in. outer diameter for drilling 8 ½ in. holes. The fixed stabilizer load springs apply 50 to 60 pound loads across the travel limits. The shaft uses three low friction bearings. Both the cutter like contacts and the spring assisted fixed stabilizer blades are needed to completely eliminate frictional rotation. The rotational torque in this situation is modified from equation (2) above to further include the sum of the spring forces.  
               RT   s     =       sd   2     ⁢     (     FS   +     Σ   ⁢           ⁢   F   ⁢           ⁢   spring       )     ⁢     (   0.63   )     ⁢       (   0.9   )     .               (   3   )             
 
         [0033]     Where:  
                                       RT s  = Rotation resisting torque with spring loaded contacts   in lbs.       FS = Total lateral loads on all of the stabilizer blades   lbs.       sd = stabilizer diameter   in.       ΣFspring = Sum of all of the spring forces   lbs.                  
 
         [0034]     Table 3 shows the expected performance of using cutter type contacts without spring loaded stabilizer blades.  
                                                                   TABLE 3                           NON-ROTATING STABILIZER WITH ROTATION AVOIDANCE       CUTTERS ON THE FIXED STABILIZER                        Rotational   Rotational   Avoidance       Hole   Curvature   Bit   Driving   Resisting   Design       Angle   Rate   Weight   Torque   Torque   Design       deg.   deg/100 ft   kips   In lbs   In lbs   Factor                    30.0   6   25   145   1349   9.3       30.0   3   25   131   902   6.9       30.0   2   25   126   750   5.9       0.5   3   25   130   375   2.9       0.5   2   25   126   227   1.8       0.5   1   25   120   73   0.6       0.5   3   10   128   27   0.2       0.5   2   10   124   23   0.2       0.5   1   10   120   14   0.1       0.5   3   5   129   110   0.9       0.5   2   5   125   73   0.6       0.5   1   5   120   37   0.3                  
 
         [0035]     This design easily prevents rotation at both 30 degree and 0.5 degree hole with bit weights of 25,000 lbs and curvature rates of 2 deg/100 ft or more. However, the last five cases in the table would not stop frictional rotation. As shown in Table 4, adding 50 to 60 lb. springs to each of the stabilizer blades completely eliminates any chance of frictional rotation. Five blades are contemplated for the preferred embodiment. At a minimum, the present invention can minimize rotation to 1-3° of roation per foot drilled, even for a verticle hole.  
                                                                   TABLE 4                           NON-ROTATING STABILIZER WITH ROTATION AVOIDANCE       CUTTERS AND 50/60 POUND SPRINGS ON THE       FIVE FIXED STABILIZER BLADES                        Rotational   Rotational   Avoidance       Hole   Curvature   Bit   Driving   Resisting   Design       Angle   Rate   Weight   Torque   Torque   Design       deg.   deg/100 ft   kips   In lbs   In lbs   Factor                    30.0   6   25   145   2020   14.0       30.0   3   25   131   1567   11.9       30.0   2   25   126   1413   11.2       0.5   3   25   130   1040   8.0       0.5   2   25   126   891   7.1       0.5   1   25   120   736   6.1       0.5   3   10   128   699   5.5       0.5   2   10   124   691   5.6       0.5   1   10   120   679   5.7       0.5   3   5   129   779   6.0       0.5   2   5   125   740   5.9       0.5   1   5   120   700   5.8                  
 
         [0036]     The combination of cutter like contacts and spring loaded blades provides a rotational resistance force that is at least 5 times greater than the frictional driving force under all conditions.  
         [0037]     While a preferred embodiment has been described above, one skilled in the art would recognize that the invention can be modified and still fall within the scope of the appended claims. For instance, the load spring can be replaced by alternative mechanism to exert a lateral force against the wall such as a hydraulic system.