Abstract:
A drilling system utilizing the drilling fluid in a borehole to drive an apparatus to generate a high-pressure jet of fluid to facilitate the drilling operation. A pressure intensifier disposed between a drilling motor and the drill bit generates high pressure fluid jet. The drilling motor rotates the pressure intensifier. Fluid enters a high pressure chamber in the pressure intensifier at selected location during each rotation. A piston in the pressure intensifier discharges the fluid from the high pressure chamber to the drill bit bottom at a high pressure. An electrically-operated pulse frequency control device generates fluid pulses of at least two frequencies, each such frequency defining a bit of a binary system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application takes priority from U.S. patent application Ser. No. 60/070,753, filed Jan. 8, 1998, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to drilling wellbores and more particularly to a drilling system utilizing a downhole pressure intensifier for jet-assisted drilling. 
     2. Background of the Art 
     To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached to a drill string. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth&#39;s formations. 
     Modern directional drilling systems generally employ a drill string having a drill bit at the bottom that is rotated by a drill motor (commonly referred to as the “mud motor”). A plurality of downhole devices are placed in close proximity to the drill bit to measure certain downhole operating parameters associated with the drill string and to navigate the drill bit along a desired drill path. 
     Positive displacement motors are commonly used as mud motors. U.S. Pat. No. 5,135,059, assigned to the assignee hereof and which is incorporated herein by reference, discloses a downhole drill motor that includes a power section having a housing, a stator having a helically-lobed inner elastomeric surface secured within the housing and a rotor having a helically-lobed exterior metallic surface disposed within the stator. Pressurized drilling fluid (commonly known as the “mud” or “drilling mud”) is pumped into a progressive cavity formed between the rotor and stator. The force of the pressurized fluid pumped into the cavity causes the rotor to turn in a planetary-type motion. A suitable shaft connected to the rotor via a flexible coupling compensates for eccentric movement of the rotor. The shaft is coupled to a bearing assembly having a drive shaft (commonly referred as the “drive sub”) which in turn rotates the drill bit attached thereto. Radial and axial bearings in the bearing assembly provide support to the radial and axial movements of the drill bit. For convenience, the power section and bearing assembly are collectively referred to herein as the “motor assembly.” Other examples of the drill motors are disclosed in U.S. Pat. Nos. 4,729,675, 4,982,801 and 5,074,681, the disclosures of which are incorporated herein by reference. 
     For drilling in rock, the assistance of a jet of high pressure fluid facilitates the drilling operation. Some of the current operations supply the high pressure directly from the surface by either generating the high pressure for the entire fluid flow or operating a smaller amount of high pressure fluid via additional conduits inside the drill pipe. These prior art high pressure systems utilize high pressure pumps or pressure intensifiers at the surface. These systems are relatively expensive and unreliable and thus have not gained commercial acceptance. 
     The present invention addresses the above-described problems with the prior art methods for jet-assisted drilling and provides novel apparatus and methods for generating high pressure fluid flow downhole. 
     SUMMARY OF THE INVENTION 
     The present invention provides apparatus and methods for generating high pressure fluid jet downhole during drilling of the boreholes. The high pressure jet is discharged at the drill bit bottom to aid drilling of the boreholes. A preferred embodiment of the system includes a pressure intensifier disposed between a drilling motor and the drill bit. The drilling motor produces a rotary force as the drilling fluid passes through the drilling motor. The pressure intensifier includes a rotatable sleeve having at least one port for admitting drilling fluid. The rotary force of the drilling motor rotates the rotating sleeve causing the drilling fluid to enter a chamber. A reciprocating differential piston in the rotating sleeve discharges the fluid from the chamber at a high pressure to the drill bit bottom. The preferred embodiment utilizes a dual acting piston that reciprocates between two chambers. During each rotation of the rotating sleeve, the piston discharges at high pressure the fluid from each such chamber. 
     The pressure intensifier generates pulses of a defined frequency, which act as a carrier of signals and data transmitted uphole (to the surface). A pulse frequency control device or valve coupled to the drilling motor acts as the frequency modulator. A controller or processor in the downhole assembly operates the pulse control frequency device at at least two (at two or more) frequencies, each such frequency representing a binary bit of a digital signal. 
     Examples of the more important features of the invention have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, and wherein: 
     FIG. 1 shows a schematic diagram of a drilling system having a drill string containing a drill bit, mud motor and pressure intensifier according to a preferred embodiment of the present invention. 
     FIGS. 2A-2E show a cross-sectional view of a portion of a downhole assembly which includes a pressure intensifier that is driven or controlled by a mud motor and a data transmission apparatus that utilizes the pulses generated by the pressure intensifier to transmit data to the surface. 
     FIG. 2F is a section view taken from FIG. 2B along line  2 F— 2 F showing the flow of low-pressure mud from the inlet channel to the pressure intensifier via the upper port of the pressure intensifier. 
     FIG. 2G is a section view taken from FIG. 2B along line  2 G— 2 G showing the flow of low-pressure mud from the lower port to the outlet channel of the pressure intensifier. 
     FIG. 3 ( 3 A- 3 B) is a partial, cross-sectional view of a second preferred embodiment of a double-acting pressure intensifier with a control valve sub used as the driving mechanism for the pressure intensifier. 
     FIG. 4 ( 4 A- 4 B) is a partial, cross-sectional view of a preferred embodiment of a driving mechanism operating with a single-acting pressure intensifier. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In general, the present invention provides a drilling system that utilizes a downhole pressure intensifier that provides high-pressure fluid jet or pulses which discharges at the telemetry to the drill bit bottom to more efficiently drill the boreholes. The drilling system further incorporates a system that utilizes the pressure pulses to transmit measurement-while-drilling (“MWD”) signals and data uphole (to the surface). FIG. 1 is a schematic diagram showing a drilling system  10  which utilizes a drill string  20  for drilling a borehole  24 . The drill string  20  includes a drill bit  26  at its bottom end carried by a bottom hole assembly or drilling assembly  74 . FIGS. 2A-2G show an embodiment of a rotating pressure intensifier  100  for use in the drilling assembly  74  of the system  10 . FIGS. 3-4 show alternative embodiments  100 A- 100 B of the pressure intensifier  100  for use in a drill string  20 . 
     The drilling system  10  of FIG. 1 is a schematic diagram of a typical drilling system  10  utilizing a mud motor  12  for driving the drill bit  26 . The drilling system  10  includes a conventional derrick  14  erected on a platform  16  that supports a rotary table  18  that is rotated by a prime mover (not shown) such as a motor at a desired rotational speed. It is contemplated that the mud motor  12  of this invention may also be used with the so-called snubbing and coiled-tubing units (not shown). 
     A drill string  20 , that includes a tubing  22 , extends downward from the rotary table  18  into the borehole  24 . The drill bit  26  disintegrates the earth formation  28  at the borehole bottom  50  when the drill bit  26  is rotated to drill the borehole  24 . The drill string  20  is coupled to a drawworks  30  via a kelly joint  32 , a swivel  34  and a line  36  through a pulley  38 . During the drilling operation, the drawworks  30  is operated to control the weight-on-bit (“WOB”) and the rate-of-penetration (“ROP”) of the drill string  20  into the borehole  24 . The operation of the drawworks  30  is well known in the art and is thus not described in detail herein. 
     During drilling operations a suitable drilling fluid (commonly referred to in the art as the “mud”)  40  from a mud pit  42  is circulated under pressure through the drill string  20  by a mud pump  44 . The mud  40  passes from the mud pit  42  into the drill string  20  via a desurger  46 , a fluid line  48  and the kelly joint  32 . The mud  40  flows downward through the tubing  22  and then the bottom hole assembly  74  and is discharged at the bottom of the borehole  24  through one or more openings  52  in the drill bit  26 , such as passages  338   a-   338   b  and  339  shown in FIG.  3 B. The drilling mud  40  carrying the cuttings circulates uphole through the annular  54  between the drill string  20  and the borehole  24  and is discharged into the mud pit  42  via a return line  56 . 
     A surface control unit  60  coupled to a sensor  62  placed in the fluid line  48  is used to control the drilling operation and to display desired drilling parameters and other information on a display/monitor  64 . The surface control unit  60  preferably contains a computer, memory for storing data, recorder for recording data and other peripherals (not shown). The control unit  60  processes data with a central processing unit (not shown) and executes program instructions and responds to user commands entered through a suitable means, such as a keyboard, a graphical pointing device or any other suitable device (not shown). The surface control unit  60  preferably activates alarms  66  when certain unsafe or undesirable operating conditions occur. The surface control unit  60  also operates as the receiver for the mud pulse data transmission. 
     The drilling motor or mud motor  12 , coupled to the drill bit  26  via the drive shaft (not shown) disposed in the bearing assembly  70 , rotates the drill bit  26  when the drilling mud  40  is passed through the mud motor  12  under pressure. The bearing assembly  70  supports the radial and axial forces of the drill bit  26 , the downthrust of the drill motor  12  and the reactive upward loading from the applied weight-on-bit. A stabilizer  72  coupled to the bearing assembly  70  acts as a centralizer for the lowermost portion of the mud motor assembly  74 . 
     The first preferred embodiment of the pressure intensifier system  100  is illustrated in FIGS. 2A-2G. This embodiment also includes a data transmission apparatus or device  110  for transmitting data pulses to the surface in the form of modulated pressure pulses generated by the pressure intensifier. 
     The various devices of the pressure intensifier system  100  are disposed in an outer housing  105  which connects at its upper end to a tubing (not shown). Various electronic circuits and components relating to the system  100  are preferably disposed in a pressure tight housing  106  disposed uphole of the data transmission apparatus  110 . The operation of the mud motor  130  and the pressure intensifier  200  will be described before describing the operation of the data transmission apparatus  110 . 
     The mud motor  130  includes a power section that contains an elastomeric stator  132  having an inner lobed surface  134 . The stator  132  is securely affixed in an outer housing  136 . A rotor  140  having an outer lobed surface  142  is rotatably disposed in the stator  130 . The lobes of the stator  130  and the rotor  140  are such that they create a series of cavities  144  between the rotor and stator lobed surfaces. The rotor  140  has a passage  146  which can be utilized to bypass a certain amount of the drilling fluid to alter the mud motor  130  rotational speed. As the mud  40   a  flows from the pulse frequency controller  110  to the mud motor  130 , it passes through the cavities  144 , thereby turning (rotating) the rotor  140 . The mud  40   a  leaves the mud motor  130  at the lower end of the power section of the drilling motor and enters the pressure intensifier  200  at ports  232   a.  The bypass fluid leaves the rotor at ports  149 . The rotor  130  rotates a flexible shaft  150 , which is coupled to the pressure intensifier  200  via a coupling  152 . 
     The pressure intensifier  100  is preferably integrated into the mud motor assembly which is usually composed of the mud motor  130 , flexible shaft  150  and the bearing assembly  160 . The pressure intensifier  100  is shown disposed between the flexible shaft  150  and the bearing assembly  160  in the configuration of FIGS. 2A-2G. The pressure intensifier  200  includes a rotatable housing  225 , which is coupled at its upper end  225   a  to the flexible shaft  150  at the coupling  154 . The lower end  225   b  of the housing  225  is coupled to the drive shaft  162  in the bearing assembly  160  via a coupling  226 . As the rotor  140  rotates, it rotates the flexible shaft  150 , which rotates the coupling  154  and thus the pressure intensifier housing  225 . The housing  225  in turn rotates the coupling  226 , which rotates the drive shaft  162  and thus the drill bit  170 . In the system  100 , the mud motor  130  drives the pressure intensifier  100  rather than a separate driving mechanism, such as shown in FIGS. 3-4. 
     The rotating housing  225  is disposed in a non-rotating valve sleeve  227 , which is fixed within the outer housing  105 . The non-rotating sleeve  227  creates two channels: an inlet fluid channel  232  (FIG. 2F) between the outer housing  105  and the non-rotating sleeve  227  that receives the low pressure drilling fluid  40   a  from the motor  130  and an outlet channel  231  for discharging the low pressure fluid  40   a  to the bearing assembly  160 . An upper seal  260   a  and a lower seal  260   b  provide seals between the non-rotating sleeve  227  and the outer housing  105 . The non-rotating sleeve  227  has openings  227   a  and  227   b,  which allow fluid  40   a  to flow from the channel  232  to the rotating sleeve  225 . The rotating sleeve  225  has an upper port  225   a  and a lower port  225   b,  each of which comes in fluid communication with fluid  40   a  via the openings  227   a  and  227   b  during each rotation of the rotating sleeve  225 . 
     A double acting piston  235  reciprocates between an upper chamber  236   a  and a lower chamber  236   b  which are formed by the piston and the rotating sleeve  225 . The upper end of the piston  235  has an upper pressure plunger  240   a  that reciprocates in an upper plunger chamber  242   a.  The lower end of the piston  235  has a lower pressure plunger  240   b  that reciprocates in a lower plunger chamber  242   b.  An upper suction check valve  245   a  is disposed in a hydraulic line  244   a  connecting the upper chamber  236   a  and the upper plunger chamber  242   a  to allow the fluid  40   a  to flow from the upper chamber  236   a  to the upper plunger chamber  242   a.  Similarly, a lower suction check valve  245   b  is disposed in a hydraulic line  244   b  that connects the lower chamber  236   b  and the lower plunger chamber  242   b  to allow the fluid  40   a  to flow from the lower chamber  236   b  to the lower plunger chamber  242   b.  An upper outlet check valve  250   a  allows the high pressure fluid  40   b  to discharge from the upper plunger chamber  242   a  into a high pressure channel  248 . Similarly, a lower outlet check valve  250   b  allows the high pressure fluid  406  to discharge from the lower plunger chamber  242   b  into the high pressure channel  249 . 
     The operation of the pressure intensifier  100  will now be described while referring to FIGS. 2A-2G. The low pressure drilling fluid  40   a  causes the mud motor  130  to rotate, which rotates the rotating sleeve  225  causing the upper port  225   a  and the lower port  225   b  to come in fluid communication with the inlet channel  232  depending on the rotational position of the rotating sleeve  225  relative to the non-rotating sleeve  227 . FIG. 2F is the cross-section of the pressure intensifier  200  taken along  2 F— 2 F. It shows the upper port  225   a  in fluid communication with the inlet channel  232 . FIG. 2G is the cross-section of the pressure intensifier taken at  2 G— 2 G when the rotating sleeve is in the same position as shown in FIG.  2 F. It shows the lower port  225   b  in fluid communication with the outlet channel  231  after a rotation of ninety degrees (90°). Here the rotating sleeve  225  is in transition phase i.e., from connecting the upper port  225   a  with the inlet channel  232  and the lower port  235   b  with the outlet channel  231  to connecting the upper port  225   a  with the outlet channel  231  and the lower port  235   b  with the inlet channel  232 . For a certain amount of time during this transition phase, each of the ports  235   a  and  235   b  connects to both the inlet channel  232  and the outlet channel  231 . During this time, the fluid  40   a  bypasses the pressure intensifier  200 , which ensures continuous supply of the fluid  40   a  to the drill bit  170  and a constant rotation of the mud motor  130 . During each revolution of the rotating sleeve  225 , (i) the upper port  225   a  comes in fluid communication with the outer channel  231  for a portion of the rotation, (ii) the lower port with the inlet channel  232  for a portion of the rotation, and (iii) for a portion of the rotation such fluid communications occur simultaneously. This is accomplished by configuring the radial dimensions of the inlet channel  232 , outlet channel  231 , and the upper and lower ports  225   a-   225   b  such that there always is a certain amount of low pressure fluid  40   a  flowing from the inlet channel  232  to the outlet channel  231 , which ensures continuous rotation of the mud motor  130 . 
     When the upper port  225   a  is in fluid communication with the inlet channel  232 , the low pressure fluid  40   a  enters the upper chamber  236   a  as shown by arrow  260  pushing the piston  235  and the lower plunger  240   b  downward. The downward movement of the piston  235  (a) discharges the low pressure fluid  40   a  from the lower chamber  236   b  into the outlet channel  231  and (b) causes the lower plunger  240   b  to discharge the fluid from the lower plunger chamber  242   b  into the high pressure channel  248  via check valve  250   b.  The high pressure fluid  40   b  from the line  248  passes to the drill bit  270  via a connecting high pressure line  249 . Simultaneous with the discharge of the fluid from the lower chamber  236   b,  the low pressure fluid  40   a  enters into the upper chamber  236   a  and into the upper plunger chamber  242   a  via suction check valve  245   a  and line  244   a.  It should be noted that the inlet channel  232 , the outlet channel  231  and the upper and lower ports  225   a-   225   b  are configured such that there always is a certain amount of the low pressure fluid  40   a  flowing from the inlet channel  232  to the outlet channel  231  to ensure continuous rotation of the mud motor  130 . 
     When the lower port  225   b  comes in fluid communication with the inlet channel  232 , the low pressure fluid  40   a  enters the lower chamber  236   b,  filling the lower chamber  236   b  and the lower plunger chamber  242   b.  The piston  235  moves upward, causing the upper plunger  240   a  to discharge the fluid from the upper plunger chamber  242   a  into the high pressure channel  248  at the high pressure. Thus, each rotation of the rotating sleeve  225  causes the piston  235  to stroke once upward and once downward, thereby supplying two pulses of the high pressure fluid  41   a  to the drill bit  170 . The low pressure fluid  40   a  is supplied continuously to the drill bit. 
     The high pressure line  249  supplies the high pressure fluid to the drill bit  170  via a suitable channel  162 . The low pressure fluid  40   a  from the outlet channel  231  discharges into the passage  164  in the drive shaft  166 , which rotates the drill bit  170 . The bearing assembly  160  includes radial bearings  168  and axial bearings  167 , which respectively provide radial and axial support to the drive shaft  166 . The high pressure fluid  40   b  is discharged at the drill bit bottom via a passage  162  while the low pressure fluid  40   a  is discharged via multiple passages  164 . 
     The pressure intensifier  100  described above and shown in FIGS. 2A-2G produces pressure pulses during each rotation of the housing  225  (FIG.  2 D). These pressure pulses normally interfere with mud pulse telemetry signals commonly utilized for transmitting data and signals from the downhole assembly  100  to the surface. This invention provides a novel method for transmitting signals uphole that are unaffected by the pressure pulses generated by the pressure intensifier  100 . In the preferred mode, this is accomplished by utilizing a pulse frequency control device or valve  110  to transmit signals from the downhole assembly  74  to the surface. The preferred pulse frequency control valve  110  includes a solenoid valve  101 , which contains a solenoid coil  102  with a conical end  111 . The solenoid coil is energized according to programmed instructions from a control circuit (not shown) in the downhole assembly  74  via conductors  103 . A valve poppet  108  having a compliant conical side  113  is disposed in the conical end  111 . The other end  114  of the valve poppet  108  seals an opening  115  in a seat  107 . The valve poppet seals the opening in the normal closed position, as shown in FIG.  2 A. When the solenoid coil  102  is energized, the valve poppet moves uphole, which unseats the valve poppet  108  from the valve seat  107  thereby allowing the low pressure drilling fluid  40   a  to pass from the passage  118  to the mud motor via the passage  115 . 
     As described above with reference to FIG. 1, data from the measurement-while-drilling devices and other sensors carried by the downhole assembly is transmitted to the surface. In the present invention, the signals are transmitted as pulse-modulated signals produced by the pulse frequency control valve  110  utilizing the pressure pulses produced by the pressure intensifier  100  as a carrier. To transmit a signal, for example a series of ones and zeroes, the solenoid is selectively activated and deactivated to increase or reduce the frequency to produce the required signal. For example a “one” may be defined as a first operating frequency of the pulse frequency control valve  110  and a zero as a second operating frequency. Thus, the signals are transmitted as a series of pulses. More than two frequencies may be utilized for special signals, such as the beginning and/or end of a signal series or for other special purposes. The above method provides for frequency modulated signals. Amplitude modulated pulses and other types of pulses may also be utilized to transmit signals. A processor or controller, preferably in the electronic section  106  (FIG.  2 A), controls the operation of the pulse frequency control valve  110 . This processor includes a microprocessor, memory and other related circuitry. One or more programs are stored in the memory downhole, which provide instructions to the microprocessor respecting the control of the valve  110 . The process also may include circuitry to receive command signals from the surface control unit  60  (FIG.  1 ), which may be programmed to send command signals to the downhole processor. The downhole processor controls the operation of the valve  110  according to the programmed instructions stored downhole and/or commands received from the surface control unit  60 . 
     The second preferred embodiment of the pressure intensifier  100 A that uses an alternative double-acting pressure intensifier/piston  300  is shown in FIG.  3 . This pressure intensifier  100 A includes a control valve sleeve  302  and a pressure intensifier sub  304 . The control valve sleeve  302  is the driving mechanism for the double-acting pressure intensifier/piston  300  and includes a valve piston  306  and an oscillating piston  308 . The valve piston  306  is slidably mounted in the control valve sleeve  302 . A valve spring  310  urges the valve piston  306  upwards into its open, biased position. The oscillating piston  308  also is slidably mounted within the control valve sleeve  302 . A main spring  312  urges the oscillating piston  308  upwards into its open, biased position. 
     An optional bypass nozzle  314  is used in the preferred embodiment to optimize the action of the drilling system  10 . The operation of the bypass nozzle  314  is well known in the industry and, therefore, is not discussed in detail. For ease of understanding, the following description assumes that the bypass nozzle  314  is in the closed position. 
     One cycle of the double-acting pressure intensifier/piston  300  includes four phases. In the first phase, the oscillating piston  308  is forced upward by the biasing action of the main spring  312 . At the end of Phase 1, a valve  316  is closed when a valve seat  318  contacts a valve body  320  of the valve piston  306  and the oscillating piston  308  comes to rest against the valve piston  306 . 
     In Phase 2, the valve  316  is closed and the drilling mud  40  cannot flow between the valve seat  318  and the valve body  320 . This creates flow pressure against both the valve piston  306  and the oscillating piston  308 , forcing the valve spring  310  and the main spring  312  to compress. This compression allows the valve piston  306  and the oscillating piston  308  to move downwards at the same rate, thus keeping the valve  316  in the closed position. When the valve piston  306  reaches the stop shoulder  322 , Phase 2 ends. 
     In Phase 3, the valve piston  306  stops its downward motion when the valve piston  306  reaches the stop shoulder  322  and the valve spring  310  forces the valve piston  306  to oscillate back upwards, pulling the valve body  320  away from the valve seat  318 . At the same time, due to high inertia, the oscillating piston  308  maintains its downward direction of movement, further widening the gap between the valve body  320  and the valve seat  318 , thereby opening the valve  316  which allows the mud  40  to flow downhole. This ends Phase 3. 
     The fourth and final phase starts (a few tenths of a second after the valve piston  306  reverses its direction) when the oscillating piston  308  stops due to the full compression of the main spring  312 . Because the mud  40  is flowing through the open valve  316  relieving the fluid pressure on the top of the oscillating piston  308 , the main spring  312  decompresses thereby forcing the oscillating piston  308  upward. The upward movement of the oscillating piston  308  is the beginning of Phase 1 and the cycle starts again. 
     The oscillating piston  308  of the preferred embodiment is designed as a sliding valve which connects the flow of drilling mud  40  to either a first actuator channel  324   a  or a second actuator channel  324   b.  The connection is made between the mud  40  and the first actuator channel  324   a  when the oscillating piston  308  is located towards the top of its upward path such that an aperture  326  in the oscillating piston  308  is adjacent a first inlet chamber  330   a  which is in fluid communication with the first actuator channel  324   a.  Similarly, when the oscillating piston  308  is towards the bottom of its downward path, the aperture  326  is adjacent to a second flow chamber  330   b  which is in fluid communication with the second actuator channel  324   b  thereby allowing the mud  40  to flow into the second actuator channel  324   b.    
     Pressure is created by the delta in the flow rate across the low-pressure nozzles  338   a-b.  If fluid is pumped into one of the low-pressure actuator channels  324   a,  then that flow rate is removed from the other low-pressure actuator channel  324   b  and a pressure differential is created. The double-acting piston  300  is driven by whichever channel (the first or second actuator channel  324   a-b ) is connected to the flow path of the drilling mud  40   a.  Driving pressure is established by the difference (drop) in pressure across the low-pressure nozzles  338   a-b.    
     An upper plunger  336   a  and a lower plunger  336   b  act as pumps in conjunction with four check valves  332   a-d  (two per plunger). The high pressure is created across the high-pressure nozzle  339  inside the drill bit  26 . The high-pressure fluid jet (not shown) is directed at the bottom of the wellbore  24  to support the drilling process. 
     Both low-pressure actuator channels  324   a-b  are connected to the double-acting pressure intensifier  300  and to the outlets (low-pressure nozzles)  338   a-b,  respectively. Part of the flow of low-pressure mud  40   a  from the first actuator channel  324   a  goes through a first low-pressure line  346   a  and exits the drill string  20  through the first low-pressure nozzle  338   a.  Due to high pressure forming in the double-acting pressure intensifier  300  by the action of high-pressure plungers  336   a-b,  another part of the low-pressure mud  40   a  flows into an upper chamber  342   a  of the double-acting pressure intensifier  300  through a first chamber line  340   a.    
     The final part of the low-pressure mud  40   a  flows into a first low-pressure inlet  328   a  in the pressure intensifier  304 . The first check valve  332   a  opens when the double-acting pressure intensifier/piston  300  is traveling downwards creating lower pressure in an upper plunger cavity  334   a.  This causes the upper plunger cavity  334   a  to equalize the pressure by sucking the low-pressure mud  40   a  from the first low-pressure inlet channel  328   a  through the first check valve  332   a  into the upper plunger cavity  334   a.  Continuing downward, the double-acting piston  300  forces the mud  40  in a lower plunger cavity  334   b  through a fourth check valve  332   d  at a higher pressure into a first high-pressure nozzle line  344   a.    
     As the double-acting piston  300  reaches its bottom stroke, it reverses direction whereby the mud  40  from the second low-pressure input channel  328   b  is sucked from a second low-pressure nozzle line  346   b  through a third check valve  332   c  into a lower chamber  342   b  in the pressure intensifier  300 . As the upper plunger  336   a  moves upwards, the pressure on the mud  40  in the upper plunger cavity  334   a  increases and keeps a second check valve  332   b  closed. 
     The low-pressure mud  40   a  that flows through the second actuator channel  324   b  passes through an aperture  326  into a second inlet chamber  330   b  and through a second low-pressure line  346   b  and exits the drill bit  26  through a second low-pressure nozzle  338   b.    
     A third preferred embodiment  100 B is illustrated in FIG.  4 . This embodiment uses a single-acting pressure intensifier  400 . A lower end  402  of the drill string  20  is connected to a pressure intensifier  404 . A valve piston  406  and a pressure intensifier piston  408  are slidably mounted inside the pressure intensifier sub  404 . The valve piston  406  and the pressure intensifier piston  408  are pushed back into their normal biased positions (up) by a valve spring  410  and a main spring  412 , respectively. 
     As in the double acting pressure intensifier  300  (as shown in FIG.  3 ), one cycle of the single acting pressure intensifier  400  includes four phases. In Phase 1, the pressure intensifier piston  408  is driven upward by the biasing action of the main spring  412 . When a valve seat  414  reaches a valve body  416  of the valve piston  406 , a valve  418  closes and Phase 1 ends. 
     At the start of Phase 2, the valve  418  is closed and the drilling mud  40   a  cannot flow between the valve seat  414  and the valve body  416 . This creates flow pressure against both springs (the valve spring  410  and the main spring  412 ) forcing them downward which allows the valve piston  406  and the pressure intensifier piston  408  to move downward until the valve piston  406  reaches a stop shoulder  420 . This is the end of Phase 2. 
     In Phase 3, the valve piston  406  stops its downward motion when the valve piston  406  reaches the stop shoulder  422  and the valve spring  410  forces the valve piston  406  to oscillate back upwards pulling the valve body  416  away from the valve seat  414 . At the same time, due to high inertia, the pressure intensifier piston  408  maintains its downward direction of movement, further widening the gap between the valve body  416  and the valve seat  414  thereby opening the valve  418  which allows the mud  40  to flow through. This ends Phase 2. 
     The fourth and final phase starts (a few tenth of a second after the valve piston  406  reverses its direction) when the pressure intensifier piston  408  stops due to the full compression of the main spring  412 . Because the mud  40  is flowing through the open valve  418  relieving the fluid pressure on the top of the pressure intensifier piston  408 , the main spring  412  decompresses thereby forcing the pressure intensifier piston  408  upwards. This upward movement of the pressure intensifier piston  408  is the beginning of Phase 1 and the cycle starts again. 
     The pressure intensifier piston  408  includes a plunger  422  which is guided inside a cylindrically-shaped passageway  424  and is protected by a bellows  426  which also acts as a means for pressure compensation. A high-pressure seal  428  separates a high-pressure channel  430  from a low-pressure channel  432  of the plunger  422 . To have clean drilling mud  40  in both channels (the high pressure channel  430  and the low-pressure channel  432 ), a high-pressure membrane  434  is positioned to separate the high-pressure drilling mud  40   b  from a pressure-transmitting fluid  436 . A ball-check valve  438  serves as a suction valve for the plunger  422 . 
     The up and down action of the plunger  422  in the passageway  424 , creates a pressure differential and low-pressure mud  40   a  in the low-pressure channel  432  is sucked through an inlet  444  into the ball-check valve  438 . The high-pressure mud  40   b  discharging through the ball-check valve  438  flows through the high-pressure channel  432  and exits the drill bit  26  as a high-pressure jet through the high-pressure nozzle  440  which is located inside the drill bit  26  and directed downwards towards the bottom of the wellbore  24 . 
     The remainder of the low-pressure mud  40   a  (that is not diverted through the inlet port  444  to the ball-check valve  438 ) continues flowing through the low-pressure channel  432  and exits the drill string  20  through a low-pressure nozzle  442  in the drill bit  26  where it circulates uphole through the annular space  54  (see FIG. 1) between the drill string  20  and the borehole  24  for discharge back into the mud pit  42  to complete the cycle. 
     While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.