Abstract:
A method of injecting a stream of treatment fluid into an earth formation in the course of drilling a borehole into the earth formation, using an assembly having a drill string provided with at least one sealing means arranged to selectively isolate a selected part of the borehole from the remainder of the borehole, the drill string further being provided with a fluid passage for the stream of treatment fluid into the selected part of the borehole, the method involving:
       operating the drill string, and stopping the drilling operation when a zone for which treatment is desired is arranged adjacent to the part of the selected part of the borehole;   isolating the selected part of the borehole using the sealing means so as to seal the drill string relative to the borehole wall; and,   pumping the stream of treatment fluid via the fluid passage into the selected part of the borehole and from there into the treatment zone.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an assembly and a method for injecting a stream of fluid into an earth formation using a borehole formed in the earth formation. 
     BACKGROUND OF THE INVENTION 
     During drilling of a borehole into the earth formation for the production of oil or gas, it frequently occurs that chemical treatment of the rock formation is required. For example in case of large losses of drilling fluid into fractures in the formation, shutting off of such fractures is necessary to prevent such further fluid losses. Such fractures may also lead to poor cementation of wellbore casing when drilling is done in overbalance mode, or to early breakout of reservoir water in case the fractures are connected to a water layer when the well is put on production. Similar problems as described above with regard to fractures can also be encountered when highly permeable zone of the earth formation are traversed during drilling, and the present invention is equally applicable to this situation. A highly permeable zone, wherein the permeability is for example at least 10 times higher than the average permeability of the earth formation that is traversed, is for example prone to early water breakthrough. Sealing off fluid communication between the borehole and the highly permeable region can therefore be desirable. 
     However, contamination of treatment fluid with drilling mud in the borehole during overbalanced drilling and the difficulty to place treatment fluid in the formation on the high side of the well, has negatively affected the treatment success. Injection of treatment chemical into the surrounding formation is normally avoided when drilling in the underbalance mode since such injection can only be achieved in overbalance mode, and switching to overbalance mode would necessitate the whole fluid column in the borehole becoming overbalanced. 
     Thus, there is a need to provide an improved method and assembly which allows placement of treatment fluid while drilling in the overbalance mode without mixing of treatment fluid with the drilling mud, and which allows placement of treatment fluid while drilling in the unbalance mode while the borehole outside the treatment zone still remains underbalanced. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a method of injecting a stream of treatment fluid into an earth formation in the course of drilling a borehole into the earth formation, so as to suppress fluid communication between the borehole and a fracture or a highly permeable region in the earth formation, using an assembly comprising a drill string extending into the borehole, the drill string being provided with at least one sealing means arranged to isolate a selected part of the borehole from the remainder of the borehole, each sealing means being rotatable relative to the drill string and being movable between a radially retracted mode in which the sealing means is radially displaced from the borehole wall and a radially expanded mode in which the sealing means is biased against the borehole wall so as to seal the drill string relative to the borehole wall, the drill string further being provided with a fluid passage for the stream of treatment fluid, the fluid passage having an outlet debauching into the selected part of the borehole, which method comprises the steps of:
         operating the drill string so as to progress the borehole until a treatment zone, which treatment zone includes the fracture or the highly permeable region, in the earth formation is reached for which treatment is desired;   stopping the drilling operation when the treatment zone is arranged adjacent to the part of the borehole which is selected by the arrangement of the sealing means on the drill string;   moving the sealing means from the retracted mode to the expanded mode thereof so as to seal the drill string relative to the borehole wall; and   pumping the stream of treatment fluid via the fluid passage and the outlet into the selected part of the borehole and from there into the treatment zone, wherein the drill string is rotated during pumping and/or during a curing period of the treatment fluid after pumping.       

     There is further provided a method of injecting a stream of treatment fluid into an earth formation in the course of underbalanced drilling of a borehole into the earth formation, so as to suppress fluid communication between the borehole and a fracture or a highly permeable region in the earth formation, using an assembly comprising a drill string extending into the borehole, the drill string being provided with at least one sealing means arranged to isolate a selected part of the borehole from the remainder of the borehole, each sealing means being movable between a radially retracted mode in which the sealing means is radially displaced from the borehole wall and a radially exdanded mode in which the sealing means is biased against the borehole wall so as to seal the drill string relative to the borehole wall, the drill string further being provided with a fluid passage for the stream of treatment fluid, the fluid passage having an outlet debouching into the selected part of the borehole, which method comprises the steps of:
         operating the drill string in underbalance mode so as to progress the borehole until a treatment zone in the earth formation is reached, which treatment zone includes the fracture or the highly permeable formation, for which treatment is desired;   stopping the drilling operation when the treatment zone is arranged adjacent to the part of the borehole which is selected by the arrangement of the sealing means on the drill string;   moving the sealing means from the retracted mode to the expanded mode thereof so as to seal the drill string relative to the borehole wall; and   pumping the stream of treatment fluid via the fluid passage and the outlet into the selected part of the borehole and from there into the treatment zone.       

     The assembly for injecting a stream of fluid into an earth formation as provided by the present invention comprises a drill string extending into the borehole, the drill string being provided with at least one sealing means arranged to isolate a selected part of the borehole from the remainder of the borehole, each sealing means being movable between a radially retracted mode in which the sealing means is radially displaced from the borehole wall and a radially expanded mode in which the sealing means is biased against the borehole wall so as to seal the drill string relative to the borehole wall, the drill string further being provided with a fluid passage for the stream of fluid, the fluid passage having an outlet debouching into the selected part of the borehole, wherein each sealing means includes an inflatable member movable between a radially retracted position when the sealing means is in the retracted mode and a radially expanded position when the sealing means is in the expanded mode, and wherein each inflatable member is arranged to be inflated by means of the pressure in the fluid passage when the stream of treatment fluid is injected, wherein each inflatable member includes a fluid chamber and an inflation channel having an outlet debauching into the fluid chamber, and wherein the drill string further comprises a means for selectively providing fluid communication between the inflation channel and the fluid passage, and wherein the means for selectively providing fluid communication comprises a tubular sleeve arranged on the inner surface of a tubular portion of the drill string, wherein the tubular sleeve is axially movable between a closing position and an opening position with respect to a port through the wall of the tubular portion, and wherein moving the tubular sleeve from the closing to the opening position opens fluid communication through the port, and thereby between the fluid passage, of which the interior of the tubular portion forms part, and the inflation channel. 
     The method of the present invention allows to selectively treat a treatment zone of the formation such as a fracture or a highly permeable zone, by pumping treatment fluid down the drill pipe. In particular, such a treatment zone can be sealed so as to suppress fluid communication between the borehole and the treatment zone after treatment, so that fluid losses into or water influx from the treatment zone are prevented. To this end, the treatment fluid is suitably a treatment chemical which can seal fractures or pores after curing or after a reaction with the formation rock. Cement can also be used. The present invention therefore allows such treatment to be conducted in the course of a drilling operation without the need to pull the drill string out of the borehole, if needed for a number of formation zones which may need to be treated at different depths. The method is both applicable for treatment in the course of overbalance and underbalance drilling. 
     By moving the sealing means from the retracted mode to the expanded mode, the selected part of the borehole is isolated from the remainder of the borehole, so that the treatment fluid which is pumped into the isolated borehole part is not mixed with the drilling fluid present in the remaining borehole part. Also, the pressure of the treatment fluid in the isolated borehole part is independent from the pressure in the remainder borehole part so that the remainder part can remain at underbalanced pressure during the injection process. The sealing means in the apparatus of the present invention comprises an inflatable member such as a packer, which is arranged to be inflated by means of the pressure in the fluid passage when the stream of treatment fluid is injected. In this way, a simple and fail-safe operation can be achieved, since the inflatable packer is inflated and kept inflated when the treatment fluid is injected. 
     Suitably the sealing means includes a primary sealing means arranged so that said outlet is located between the primary sealing means and the lower end of the drill string. 
     The sealing means can include a secondary sealing means arranged so that said outlet is located between the primary sealing means and the secondary sealing means. 
     To allow continued rotation of the drill string in the course of the injection process, i.e. during the injection and/or any curing period thereafter, suitably each sealing means is rotatable about the longitudinal axis of the drill string. In this way it can for example be prevented that the drill string gets stuck in the borehole after injection of a treatment chemical. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described hereinafter in more detail and by way of example with reference to the accompanying drawings in which: 
         FIG. 1  schematically shows a first embodiment of the assembly of the invention; 
         FIG. 2  schematically shows a second embodiment of the assembly of the invention; 
         FIG. 3  schematically shows an activation system of sealing means when in retracted mode; 
         FIG. 4  schematically shows the activation system of sealing means when in expanded mode; 
         FIG. 5  schematically shows an alternative activation system of sealing means when in retracted mode; 
         FIG. 6  schematically shows the alternative activation system of sealing means when in expanded mode; 
         FIG. 7  schematically shows a further activation system of sealing means when in retracted mode; and 
         FIG. 8  schematically shows another activation system of sealing means when in expanded mode. 
       In the Figures like reference numerals relate to like components. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1  there is shown a drill string  1  extending into a borehole  2  formed in an earth formation  4 , the drill string having a longitudinal axis  6 . The lower part of the drill string  1  includes, subsequently in upward direction, a drill bit  8 , a hydraulic motor  10  (also referred to as mud-motor) for rotating the drill bit  8 , a lower stabiliser  12  provided at the housing of the motor, a sealing means in the form of an inflatable packer  14 , an upper stabiliser  16 , and a measurement while drilling (MWD) tool  18 . The inflatable packer  14  is shown in inflated mode at the left side of the longitudinal axis  6 , and in deflated mode at the right side of the longitudinal axis  6 . 
     In  FIG. 2  is shown a drill string  1  extending into a borehole  2  formed in an earth formation  4 , the drill string having a longitudinal axis  6 . The lower part of the drill string  1  has substantially the same components as the lower part of the drill string of  FIG. 1 , the difference being that in  FIG. 2  the inflatable packer  14  is arranged on top of the MWD tool  18  rather than between the mud-motor  10  and the upper stabiliser  16  as in  FIG. 1 . Again, the inflatable packer  14  is shown in inflated mode at the left side of the longitudinal axis  6 , and in deflated mode at the right side of the longitudinal axis  6 . The fluid passage of the assemblies in  FIGS. 1 and 2  is formed by the interior of the drill string  1  and the outlet of the fluid passage by nozzles provided in the drill bit  8 . 
     In  FIG. 3  is shown the inflatable packer  14  and its activation system in more detail. The packer  14  includes an annular rubber packer element  30  connected to a sleeve  32  provided with holes  34 . The sleeve  32  is connected to a tubular portion  36  of the drill string  1  by means of bearings  38  so as to allow the sleeve  32  to rotate relative to tubular drill string portion  36 . An annular recess  40  in tubular portion  36  defines an annular fluid chamber  42  between the sleeve  32  and the tubular portion  36 . A port  44  is formed in the wall of tubular portion  36 , which port includes a nozzle  46  and provides fluid communication between the interior and the exterior of the tubular portion  36 . 
     A channel  48  extending from the port  44  in the wall of tubular portion  36  to an outlet debouching into the fluid chamber  42  provides fluid communication between the port  44  and the fluid chamber  42 . A tubular sleeve  50  is arranged at the inner surface  52  of the tubular portion  36 , which sleeve  50  is provided with an opening  54  in the wall thereof. The sleeve  50  is slideable in axial direction along the tubular portion  36  between a closed position ( FIG. 3 ) in which the port  44  is closed off by sleeve  50 , and an open position ( FIG. 4 ) in which the opening  54  is aligned with port  44 . Shoulders  56 ,  58  formed at the inner surface  52  of the tubular portion  36  define the respective end positions for axial movement of the sleeve  50 . A spring  60  is provided between the shoulder  56  and the sleeve  50  so as to bias the sleeve  50  to its closed position. The sleeve  50  has an inner surface  62  which tapers radially inward in downward direction. 
       FIG. 4  shows the inflatable packer  14  and activation system of  FIG. 3  when in inflated mode, whereby a flexible ball  64  seats on tapering inner surface  62  of slideable sleeve  50 , and whereby the earth formation  4  has a fracture  66 . The fluid passage for treatment fluid is formed by the interior of the drill string  1 , the opening  54 , the port  44  and the nozzle  46 . An inflation channel for the fluid chamber is formed by the opening  54 , part of the port  44 , and the channel  48 . 
     In  FIG. 5  is shown an alternative activation system of inflatable packer  14 . Here the rubber packer element  30  is directly connected to the outer surface of tubular drill string portion  70  whereby a fluid chamber  71  is formed between the packer element  30  and the outer surface of the tubular portion  70 . 
     A longitudinal channel  72  extending through the wall of tubular portion  70  provides fluid communication between the fluid chamber  71  and the inner surface  74  of tubular portion  70  via a first transverse channel  76  and second transverse channel  78  axially displaced from the first transverse channel  76 . A port  80  formed in the wall of tubular portion  70  at some axial distance from the second transverse channel  78 , provides fluid communication between the interior and the exterior of the tubular portion  70 . A tubular sleeve  82  arranged at the inner surface  74  of the drill string portion  70  is provided with an opening  84  in the wall thereof. The sleeve  82  is slideable in axial direction along the tubular portion  70  between a closed position ( FIG. 5 ) in which the first transverse channel  76  is closed off by sleeve  82 , and an open position ( FIG. 6 ) in which the opening  84  is aligned with first transverse channel  76 . Shoulders  86 ,  88  formed at the inner surface  74  of the tubular portion  70  define the respective end positions of axial movement of the sleeve  82 . A spring  90  is provided between the shoulder  86  and the sleeve  82  so as to bias the sleeve to its closed position. The sleeve  82  is furthermore provided with a recess  92  arranged to provide fluid communication between the second transverse channel  78  and the port  80  when the sleeve  82  is its closed position. The port  80  is closed off by sleeve  82  when the sleeve  82  is in its open position. 
       FIG. 6  shows the packer  14  and activating system of  FIG. 5  when in inflated mode, whereby a first dart  94  seats against the upper end of sleeve  82  by means of one or more shear pins  96  connected to the first dart  94 . The first dart  94  has a central opening in the form of flow restriction  97 , whereby a second dart  98  is seated against the first dart  94  so as to close off the flow restriction  97 . When the second dart  98  is not present, the fluid passage is formed by the interior of the drill string, the first dart, and an outlet into the borehole below the packer  14  (not shown). In  FIG. 6 , an inflation channel is formed by the opening  86 , the first traverse channel  76 , the longitudinal channel  72  debouching into fluid chamber  71 . 
     Referring to  FIG. 7  there is illustrated a further embodiment of an inflatable packer arrangement  100 . The packer  100  includes an annular rubber packer element  102  connected to a tubular drill string portion  104 . A ball valve  106  is arranged in the tubular portion  104  to open and close the bore  105  thereof. A turbine  108  is arranged in the tubular portion  104  to move a slideable rod  110  up or down via an actuating cam  112 , whereby the valve  106  is controlled by up- or downward movement of the rod  110 . The turbine  108  has a fluid inlet  114  provided with nozzle  116  and a fluid outlet  117 , both being in fluid communication with the bore  105 . The turbine is designed such that it is activated only when the mud flow rate in bore  105  is above a predetermined rate which is below the normal flow rate during drilling. The tubular portion  104  is provided with an inflation channel  119  providing fluid communication between the bore  105  and the annular chamber  121 . A valve  120  controlled by rod  110  is arranged in the channel  119 . The tubular portion  104  is further provided with a relief valve  122  arranged to provide fluid communication between the annular chamber  121  and the exterior of the tubular drill string portion  104  above the packer element  102  at a selected pressure difference across the relief valve  122 . The rod  110  is at its lower end provided with a double-acting piston  123  movable in a chamber  124 . The chamber  124  has a portion  126  at the lower side of the piston  123  filled with pressurized nitrogen, and a portion  128  at the upper side of the piston in fluid communication with the annular chamber  121  via a passage  130  provided with valve  132 . The valve  132  is designed to open only when the fluid pressure in the annular chamber  121  exceeds the nitrogen pressure in portion  126  of chamber  124  by a selected amount. The bore  105  is provided with a first receptacle  134  and a second receptacle  136 , both being connected to rod  110 . The first receptacle  134  is arranged to move the rod  110  upwardly when a dart is pumped onto the first receptacle, and the second receptacle  134  is arranged to move the rod  110  downwardly when another dart is pumped onto the second receptacle. 
     In  FIG. 8  is shown another embodiment of an inflatable packer arrangement  140 . This arrangement is largely similar to the embodiment of  FIG. 7 , except that the turbine  108  has been replaced by a solenoid  142  to control actuating cam  112 . Furthermore, solenoids  144 ,  146  are provided to respectively control valve  120  and valve  132 . In  FIGS. 7 and 8 , when the valve  106  is open, the fluid passage is formed by the interior of the drill string, valve  106 , and an outlet into the borehole below the packer  102  (not shown). 
     During normal operation of the embodiment of  FIG. 1 , when it is desired to inject a chemical treatment fluid into the borehole  2 , drilling is stopped and the packer  14  is inflated against the borehole wall, thereby selecting the part of the borehole below the packer  14 . 
     A batch of treatment fluid is then pumped down from the earth&#39;s surface (not shown) via the drill string  1  and the fluid nozzles (not shown) of the drill bit  8  into the selected part of the borehole  2 , and from there into the rock formation  4  surrounding the borehole  2 . Thus, the treatment fluid does not enter the section of the borehole  2  above the packer  14 , and the fluid pressure above the packer  14  is not affected by pumping of the treatment fluid. Depending on the characteristics of the treatment fluid, the packer  14  is deflated immediately after pumping the batch of fluid or a selected time period thereafter whereafter drilling can be resumed. The upper stabiliser  16  prevents inadvertent contact of the packer  14  with borehole wall during drilling, and centralizes the packer  14  in the borehole  2  when the packer is inflated. Instead of pumping the treatment fluid through the drill bit nozzles, the fluid can be pumped through a suitable opening (not shown) provided at the drill string  1 . In the arrangement of  FIG. 1  the packer  14  can be positioned close to the bit  8  so that a short section of the borehole can be isolated for treatment. Activation of the packer can in principle be achieved by means of darts or balls, however such darts or balls may not be able to pass the MWD tool  18 . Therefore activation of the packer  14  can be achieved by means of signals, e.g. pressure pulses from the MWD tool  18 . 
     Normal operation of the embodiment of  FIG. 2  is substantially similar to normal use of the embodiment of  FIG. 1  except that now darts or balls can be used for activation of the packer  14  since the MWD tool  18  is positioned below the packer  14 . 
     During normal operation of the embodiment of  FIGS. 3 ,  4  the flexible ball  64  is dropped onto the tapering inner surface  62  of the sleeve  50  when inadvertent drilling fluid losses into the fracture  66  occur. Treatment fluid is then pumped into the drill string  36 , resulting in an increase of the pressure in the drill string  36  to a level whereby the ball  64  induces the sleeve  50  to shift from its upper position ( FIG. 3 ) to its lower position ( FIG. 4 ) against the force of spring  60 . When the sleeve  50  comes into contact with shoulder  56 , further movement of the sleeve  50  is prevented. In this position the opening  54  is aligned with port  44  so that treatment fluid is allowed to flow through the fluid passage, i.e. from the central bore of the drill string via the port  44  into the borehole  2 , and from there into the fracture  66 . Treatment fluid also flows along the inflation channel, i.e. from the port  44  via the channel  48  and the holes  34  of sleeve  32 , into the annular fluid chamber  42  thereby inflating the packer element  30  against the borehole wall. The slideable sleeve arrangement therefore acts as means for providing fluid communication, both through the fluid passage, and between the fluid channel and the inflation channel. By virtue of the nozzle  46 , the pressure drop of fluid flowing from the drill string  36  via port  44  into the borehole  2  is larger than the pressure drop of fluid flowing from the drill string  36  into the annular chamber  42 . Therefore the inflation pressure of the packer  14  is higher than the fluid pressure in the borehole below the packer  14 , and no fluid will leak upwardly along the packer  14 . If desired the drill string  36  can be rotated during the injection process, whereby the inflated packer element  30  is allowed to remain stationary by virtue of bearings  38 . After the treatment process is finalised, a steel ball (not shown) is dropped into the drill string  36  to plug off opening  54  of the sleeve  50 . Upon arriving in sleeve  50 , the steel ball plugs off opening  54 . As a result a water hammer pressure pulse develops which causes the flexible ball  64  to be pushed through the seat of the sleeve  50 . The steel ball will follow the soft ball  64  and the sleeve will move to the closed position again. At the same time the packer starts to deflate by venting fluid via channel  48  and port  44  into the borehole  2 , which form a deflation channel. The balls are collected in a ball catcher (not shown). Several ball sets can be collected in the catcher to enable multiple injection jobs to be performed without having to make a roundtrip. 
     During normal operation of the embodiment of  FIGS. 5 ,  6  the first dart  94  is pumped into the drill string  70  to seat on sleeve  82  when a chemical treatment of the rock formation surrounding the borehole into which the drill string  70  extends, is required. By virtue of the flow restriction of the first dart  94 , continued pumping of fluid causes the dart  94  to slide the sleeve  82  from its closed position ( FIG. 5 ) to its open position ( FIG. 6 ) against the force of spring  90 . When the sleeve  82  comes into contact with shoulder  86 , further movement of the sleeve  82  is prevented. In this position the opening  84  is aligned with first transverse channel  76  so that fluid communication is provided between the interior of the drill string which forms part of the fluid passage and the inflation channel. Accordingly, treatment fluid is allowed to flow from the drill string  70  via the longitudinal channel  72  into the annular fluid chamber  71  thereby inflating the packer element  30  against the borehole wall. After the treatment process is finalised the second dart  98  is pumped into the drill string  70  to plug off the flow restriction of the first dart  94 . Continued pumping causes the shear pins  96  to be sheared off so that both darts  94 ,  98  are pumped through the sleeve  82  and collected in a suitable dart catcher (not shown). Simultaneously, the spring  90  moves the sleeve  82  to its closed position again, allowing the fluid present in the annular chamber  71  to be vented to the borehole via the deflation channel formed by channel  72 , second transverse channel  78 , recess  92  and port  80 . 
     During normal operation of the embodiment of  FIG. 7 , when a chemical compound is to be injected into the earth formation, the mud flow rate through the bore  105  of the drill string is increased above the predetermined flow rate in order to operate the turbine  108  which actuates the cam  112  so as to move the rod  110  upward thereby inducing the ball valve  106  to close the bore  105  and to open the valve  120 . Mud is now allowed to flow through the inflation channel  119  and into annular chamber  121  thereby inflating rubber packer element  102  against the wellbore wall. When a predetermined pressure is reached in the annular chamber  121 , mud flows from the annular chamber  121  via passage  130  and valve  132  into portion  128  of chamber  124  and pushes the piston  123  downward from its upper position to its lower position thereby compressing the nitrogen gas in chamber portion  126 . As the pressure in annular chamber  121  attains its final pressure the piston  123  reaches its lowest point whereby the sliding rod  110  closes valve  120  and opens ball valve  106 . It is expedient not to over-inflate the packer element  102  therefore any excess pressure in annular chamber  121  is relieved via the relief valve  122 . In case activation of the cam  112  with turbine  108  fails, a dart can be pumped or dropped onto receptacle  134  whereafter the bore  105  can then be pressurized to shift the rod  110  upwardly thereby closing ball valve  106  and opening valve  120 . With the ball valve  106  open, the treatment chemical is pumped through the drill string and via the nozzles of the drill bit into the lower well bore annulus where the chemical enters into the fracture treatment zone of the formation. After the injected chemical has cured in the formation, the packer element  102  is deflated by dropping and/or pumping a dart from the surface to seat in receptacle  136  whereafter the bore  105  can be pressurized so that receptacle  136  opens valve  120  thereby allowing mud to flow from annular chamber  121  via channel  119  into the drill string bore  105  while at the same time shearing the dart. The pumped dart also disengages the sliding rod  110  so that it can move from its lower position to its intermediate position as the mud in the annular chamber  121  flows into drill string bore  105 . A spring (not shown) retracts the deflated packer element  102  into its recess (not shown) in the tubular drill string portion  104 . When the sliding rod  110  reaches its intermediate position, the rod  110  closes the valve  120  and the cam  112  is reset to its original position. 
     Normal operation of the embodiment of  FIG. 8  is substantially similar to normal operation of the embodiment of  FIG. 7 , except that the actuating cam is controlled by solenoid  142 , and that the valves  120 ,  132  are controlled by respective solenoids  144 ,  146 . Power for the operation of the solenoids can conveniently be provided by a down-hole battery pack (not shown) arranged situated in the drill string. A signal-receiving unit (not shown) detects coded mud pulse signals, for instance shock waves transmitted through the mud column from the surface, to operate the solenoids  142 ,  144 ,  146 . This means of communication is already in use in the measurement while drilling (MWD) technology, whereby in the present application the coded mud pulse signals are based on information sent from downhole sensors to a surface detector and vice versa.