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BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to downhole percussion drills in oil, gas, geothermal, and hot spring drilling, etc.  
           [0003]    2. Description of the Related Art  
           [0004]    The conventional rotary drilling has been widely used for the drilling of oil, gas, geothermal, and hot spring wells, etc. In this method, rock formations are crushed or cut by both of the rotation of a drill bit and the thrust on it.  
           [0005]    It has been well known that rates of penetration and wellbore deviation problems can be greatly improved by giving impact blows to the drill bit. However, downhole percussion drills, which generate impact blows, have seldom been applied to deep well drilling, since they have problems as described below.  
           [0006]    Air percussion drills for downhole use have been put to practical use in the fields for long time. They use compressed air to reciprocate the hammer to strike the bit and to remove cuttings from the bottomhole to the surface. However, they are not suitable when large influxes of water are encountered, since water invades into the tool and it causes insufficient bottomhole cleaning. Thus, the application of them to the fields has been limited to dry formations.  
           [0007]    In order to solve these issues, downhole percussion drills operated by drilling fluids such as mud and water (called mud-driven downhole hammers, simply mud hammers) have been developed and tested worldwide (refer to the Japanese Utility Model Laid-Open No. 55-21352).  
           [0008]    Mud hammers, in which the drilling fluid (mud or water) reciprocates the hammer to strike the bit, do not have the limitations of air percussion drills. However, they have several problems; for example, the sticking and cavitation of sliding parts, rapid wear of parts, and the clogging of fluid passages, since the drilling fluid itself has low lubricating ability and it contains abrasive fine rock particles. Although it is well recognized that percussion drilling has several advantages over conventional rotary drilling, we cannot find practical percussion drills that could be applied to the fields under various conditions at present.  
         SUMMARY OF THE INVENTION  
         [0009]    The object of this invention is to offer downhole percussion drills with high reliability and durability, which could be used at various field conditions.  
           [0010]    To solve issues mentioned above, a new type of downhole percussion drill was invented, which consists of a hammering mechanism driven by a hydraulic fluid (oil) with high lubricating ability, a hydraulic pump that pressurizes the hydraulic fluid, and a drive unit to operate the hydraulic pump. As the pure hydraulic fluid with high lubricating ability drives the hammering mechanism of this tool instead of drilling mud or water, the sticking and cavitation of sliding parts, rapid wear of parts, and the clogging of fluid passages are minimized. Therefore, this downhole percussion drill provides greatly improved reliability and durability.  
           [0011]    Because drilling fluids such as mud and water can be used for the removal of cuttings in the same manner of the mud hammers, the tools also do not have limitations of air percussion drills. If the drilling fluids, used to remove cuttings, were also utilized as a power source of the drive unit, no extra means for supplying power to the drive unit would be needed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 illustrates a well drilling system (called a drill rig) using the downhole percussion drill invented;  
         [0013]    [0013]FIG. 2 is a diagram showing the concept of the downhole percussion drills to illustrate an embodiment of the invention;  
         [0014]    [0014]FIG. 3 is an illustration showing the composition of a downhole motor;  
         [0015]    [0015]FIG. 4 shows the construction of a hydraulic hammering mechanism; and  
         [0016]    [0016]FIG. 5 exhibits how a hammering piston reciprocates to strike the bit.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    The drill rig shown in FIG. 1 consists of conventional equipments, except for the percussion drill  1 .  
         [0018]    This drill rig is comprised of the drillstring  2  and the ancillary facilities  3  which are installed on the surface.  
         [0019]    The drillstring  2  consists of the drill pipes  4 , drill collars  5 , percussion drill  1 , and drill bit  6 .  
         [0020]    The percussion drill  1  includes the hydraulic hammering mechanism  7  operated by pure oil with high lubricating ability, the hydraulic (oil) pump  8  that pressurizes the oil, and the downhole motor  9  that is used to operate the hydraulic pump  8 .  
         [0021]    The main ancillary facilities  3  installed on the surface are comprised of the mast-derrick  11  used for tripping the drillstring  2 , the rotary table  12  that rotates the drillstring  2 , the drawworks  13  that provides a power source for the drill rig, the mud pump  14  for supplying the drilling fluid W to the bottomhole, the shale shaker for removing cuttings from the drilling fluid W, and the pit for the drilling fluid W storage (the shaker and pit are omitted in the drawing).  
         [0022]    Adding percussion, rotary and weight to the drill bit  6  excavates rock formations in the well.  
         [0023]    A part of the weight of the drill collars  5  is loaded on the bit  6 . This weight is maintained within an appropriate range for drilling, controlling the tension of the wire rope  16  using the drawworks  13 .  
         [0024]    The rotation is transmitted to the drill bit  6  through the rotary table  12 , drill pipes  4 , drill collars  5 , and percussion drill  1 . In addition, the percussion drill  1  gives impact blows to the drill bit  6 .  
         [0025]    During drilling, the drilling fluid W stored in the pit is pressurized by the mud pump  14  and supplied to the percussion drill  1  through the swivel  15 , drill pipes  4  and drill collars  5 , and thereby operates the downhole motor  9 .  
         [0026]    The type of the downhole motor  9  shown in FIG. 3 is a positive displacement motor. The rotor  21  built within the stator  20  is connected to the shaft  23  supported by the bearing  22  via the universal joint  24 .  
         [0027]    In the present invention, however, the type of a downhole motor is not limited to the foregoing.  
         [0028]    When the drilling fluid W passes through the downhole motor  9 , the rotor  21  rotates in the stator  20 . Its rotation, which is transmitted to the hydraulic pump  8  via the shaft  23 , operates the hydraulic pump  8 . The drilling fluid W discharged from the front of the downhole motor  9  passes through the drilling fluid passage  25 . It flows into the water hole  26  of the drill bit  6 , and then is exhausted to the bottomhole through the nozzles in the drill bit  6 .  
         [0029]    The circulation of the drilling fluid W transports rock cuttings from the bottomhole to the surface through the annulus between a well wall and the drillstring  2 .  
         [0030]    The cuttings is removed by the shale shaker from the drilling fluid W discharged to the surface, and the drilling fluid W is stored in the pit and circulated again.  
         [0031]    The oil is filled into the space of the hydraulic pump  8  and the hydraulic hammering mechanism  7 , to avoid mixing gases such as air in them. Furthermore, the flow passages etc. for oil and drilling fluid W are isolated by seals to prevent mixing, or the loss of oil into the drilling fluid W from the hydraulic hammering mechanism  7 .  
         [0032]    The pressure compensator  27  consists of the drilling fluid portion  29 , the oil portion  30 , and the seal  28  that isolates two portions. Apart of the drilling fluid W discharged from the downhole motor  9  is guided to the drilling fluid portion  29  in the pressure compensator  27 . The oil portion  30  communicates with the low-pressure portion passage  31  of the hydraulic hammering mechanism  7 ; therefore, the pressure of the drilling fluid W is transmitted to the oil via the seal  28 . Thus, the mixing of drilling fluid into the oil in the hydraulic hammering mechanism  7  is minimized, since the oil pressure in the low-pressure portion passage  31  is maintained at the same pressure of the drilling fluid W by the pressure compensator  27 , independent of the well depth and small changes of the oil volume.  
         [0033]    In addition, changes of the oil volume, which are caused by changes of the oil pressure, can be minimized by filling the space with the oil so that gasses such as air do not mix in. It is desirable that the oil filled in the space is deaerated beforehand.  
         [0034]    The hydraulic pump  8 , which is driven by the rotation of the rotor  21  in the downhole motor  9 , absorbs and pressurizes the oil in the low-pressure portion passage  31  and exhausts the high-pressure oil to the high-pressure portion passage  32 .  
         [0035]    The hammering piston  33 , included in the hydraulic hammering mechanism  7 , is reciprocated by high-pressure oil supplied from the high-pressure portion passage  32  and repeatedly strikes the drill bit  6 . The oil used for reciprocating motion of the hammering piston  33  returns to the hydraulic pump  8 , through the low-pressure portion passage  31 .  
         [0036]    To reduce oil pressure fluctuations associated with the reciprocating motion of the hammering piston  33 , the high-pressure accumulator  34  and the low-pressure accumulator  35  are included in the high-pressure portion passage  32  and the low-pressure portion passage  31 , respectively.  
         [0037]    An increase of the oil pressure due to increases of the drilling depth decreases the volume of a filled gas in the high-pressure accumulator  34  and the low-pressure accumulator  35 ; therefore, the volume of spaces of hydraulic pump  8  and the hydraulic hammering mechanism  7 , where the oil flows, increases by the same volume reduced. This increment of the space volume is compensated by a change in the volumes of the drilling fluid portion  29  and the oil portion  30  in the pressure compensator  27 .  
         [0038]    In the drilling fluid passage  25  linked to the drill bit  6 , the seal  36  is included to prevent an invasion of the drilling fluid W into the oil in the hydraulic hammering mechanism  7 .  
         [0039]    This hydraulic hammering mechanism  7  employs the method in which the front liquid chamber  38  is always pressurized and the pressure of the rear liquid chamber  39  is changed, as a method to reciprocate the hammering piston  33 . However, in this invention, the operation method of the hammering piston  33  is not limited to this method.  
         [0040]    In the hydraulic hammering mechanism  7 , sliding parts of the hammering piston  33  and the valve  37  are fitted so that they can move forward and backward. In the hydraulic hammering mechanism  7 , the hammering piston  33 , valve  37 , high-pressure accumulator  34 , low-pressure accumulator  35 , and pressure compensator  27  are arranged in a line in the order from the bottomhole, so that they can be set within an outside diameter of the drill collar  5 . The drill bit  6  is connected beneath the hammering piston  33 .  
         [0041]    The hammering piston  33  has the large-diameter portion  33 A in its middle portion, and the front liquid chamber  38  is made beneath the large-diameter portion  33 A. The rear liquid chamber  39  is formed above the hammering piston  33 . In the hammering piston  33 , the area pressurized on the rear liquid chamber  39  is larger than that on the front liquid chamber  38 .  
         [0042]    The high-pressure portion passage  32  communicates with the front liquid chamber  38  and therefore, the oil pressurized by the hydraulic pump  8  is constantly supplied to the front liquid chamber  38 .  
         [0043]    In the front liquid chamber  38 , the valve control port  40  and the liquid discharge port  41  are included so that they are opened and shut by the large-diameter portion  33 A, during the reciprocating motion of the hammering piston  33 . In behind the liquid discharge port  41 , the low-pressure port  42  is provided so that it communicates with the liquid discharge port  41  at an advance position of the hammering piston  33 .  
         [0044]    The valve control port  40  and the liquid discharge port  41  always communicate with the control passage  43 , and the low-pressure port  42  always communicates with the low-pressure portion passage  31 .  
         [0045]    The valve  37  is disposed at behind the hammering piston  33 , in order to communicate the rear liquid chamber  39  of the hammering piston  33  with either of the high-pressure portion passage  32  or the low-pressure portion passage  31 .  
         [0046]    The regulatory liquid chamber  44  and the control liquid chamber  45  are formed in the valve  37 . In the valve  37 , the area pressurized on the control liquid chamber  45  is larger than that on regulatory liquid chamber  44 . The regulatory liquid chamber  44  communicates with the high-pressure portion passage  32 , and therefore, the oil pressurized by the hydraulic pump  8  is always supplied to the liquid chamber  44 . The control liquid chamber  45  always communicates with the control passage  43 .  
         [0047]    The low-pressure port  46  is provided between the regulatory liquid chamber  44  and the control liquid chamber  45 , and always communicates with the low-pressure portion passage  31 .  
         [0048]    When the high-pressure oil enters the regulatory liquid chamber  44  from the high-pressure portion passage  32 , the valve  37  move forward and the rear liquid chamber  39  communicates with the low-pressure portion passage  31 , though the passage  47  and the low-pressure port  46 .  
         [0049]    On the other hand, when the high-pressure oil enters the control liquid chamber  45  from the control passage  43 , the valve  37  moves backward, thereby causing the communication between the rear liquid chamber  39  and the high-pressure portion passage  32 , via the passage  47  and the regulatory liquid chamber  44 . Because, the area pressurized on the control liquid chamber  45  is larger than that on regulatory liquid chamber  44 , as described above.  
         [0050]    The operation of the hydraulic hammering mechanism  7  will be described below by referring to FIGS.  5 ( a ) to  5 ( d ).  
         [0051]    In FIG. 5( a ), the hammering piston  33  locates in a back position. In this condition, the control passage  43  communicates with the front liquid chamber  38  via the valve control port  40 , and the liquid discharge port  41  is shut off from the low-pressure port  42  by the large-diameter portion  33 A. Therefore, the high-pressure oil flows into the control liquid chamber  45  from the control passage  43 , and the valve  37  is kept in the back position.  
         [0052]    The high-pressure oil then enters the rear liquid chamber  39  through the passage  47  and regulatory liquid chamber  44 . Because the area pressurized on the rear liquid chamber  39  is larger than that on the front liquid chamber  38 ; therefore, the hammering piston  33  moves forward.  
         [0053]    As shown in FIG. 5( b ), when the hammering piston  33  has moved forward to a position where just before it impacts the drill bit  6 , the communication between the front liquid chamber  38  and the valve control port  40  is closed by the large-diameter portion  33 A of the hammering piston  33 , providing the communication between the liquid discharge port  41  and the low-pressure port  42 . Therefore, the oil pressure in the control passage  43  and the control liquid chamber  45  becomes low.  
         [0054]    Because the regulatory liquid chamber  44  always communicates with the high-pressure portion passage  32 , the valves  37  moves forward to a position where the rear liquid chamber  33  communicates with the low-pressure portion passage  31 , via the passage  47  and the low-pressure port  46 .  
         [0055]    As can be seen in FIG. 5 ( c ), after the hammering piston  33  gives an impact blow to the drill bit  6 , the oil pressure in the rear liquid chamber  39  of the piston  33  becomes low and the oil pressure in the front liquid chamber  38  is constantly high, with the result that the hammering piston  33  starts to move backward.  
         [0056]    As shown in FIG. 5( d ), the large-diameter portion  33 A shuts off the communication between the liquid discharge port  41  and the low-pressure port  42 , and the control passage  43  communicates with the front chamber  38  through the valve control port  40 , during the backward movement of the hammering piston  33 . Therefore, the oil pressure in the control liquid chamber  45  becomes high again, and the valve  37  begins to move the back position.  
         [0057]    When the valve  37  moves, the communication between the rear liquid chamber  39  of the hammering piston  33  and the low-pressure portion passage  31  is shut off via the low-pressure port  46 , and the rear liquid chamber  39  communicates with the high-pressure portion passage  32  through the passage  47  and the regulatory liquid chamber  44 . Therefore, the hammering piston  33  that has moved backward decelerates and stops by braking, and then moves forward again.  
         [0058]    The same cycles as described above are repeated.  
         [0059]    As can be understood from the above descriptions, in the hydraulic hammering mechanism  7 , sliding parts of the hammering piston  33  and the valve  37  are required to provide the small clearance between the sliding parts and the tool body, in order to improve the hammering efficiency as high as possible. These sliding parts are subjected to severe lubricating conditions due to their high-speed reciprocating motion with the small clearance.  
         [0060]    For this reason, in the prior art we could not often avoid the stop of the hammering mechanism, due to the sticking of the sliding parts caused by abrasive fine rock particles included in the drilling fluids.  
         [0061]    Moreover, in the prior art the impact surfaces both of the hammering piston and the drill bit were covered by the drilling fluid that has low lubricating ability and contains abrasive fine rock particles; therefore, it was impossible to avoid the cavitation and erosion caused by shocks during hammering, and the wear caused by hammering surrounded by abrasive fine rock particles.  
         [0062]    In the downhole percussion drills invented, all these parts are immersed in the pure hydraulic fluid with high lubricating ability. Thus, these issues mentioned above can be avoided.  
         [0063]    As described above, the downhole percussion drills invented have high durability and reliability of the hammering mechanism even in an environment in which ground water is encountered, and can be used in various field conditions.

Summary:
Provided is a downhole percussion drill, which is installed at an end portion of a drillstring and performs drilling by giving impact blows to a drill bit at the bottomhole, which includes a hydraulic hammering mechanism  7  which uses oil having high lubricating ability as a driving medium, a hydraulic pump  8  which pressurizes the oil, and a downhole motor  9  which drives the hydraulic pump  8.