Patent Publication Number: US-6712158-B2

Title: Apparatus and method for coring and/or drilling

Description:
FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for selective coring or drilling, with particular application to recovering core samples from potential water, oil or gas reservoirs. 
     BACKGROUND OF THE INVENTION 
     Extracting core samples from downhole wells is an important aspect of the drilling process to provide geological and geophysical data to establish reservoir models. 
     Conventionally, core samples of a borehole are recovered from the bottom of a borehole during the drilling phase by means of a bit attached to the lower end of a core barrel which is further attached to the lower end of the drill string. 
     Sidewall cores may also be recovered during or after the logging phase, and a known method for obtaining side wall cores is described in our UK Patent No 2305953B. The conventional method of recovering borehole core samples typically produces long undisturbed samples which are preferred to the short, often highly fractured samples produced by the sidewall coring method, and it is desirable to increase the quality of the sidewall samples. 
     The accurate positioning of known coring apparatus is also difficult, frequently resulting in samples of limited value being recovered from geological zones of little interest. 
     Moreover, the equipment currently available to remove sidewall core samples tends to be somewhat cumbersome and expensive. 
     A further limitation of the prior art is the method of piercing the well bore lining to allow ingress of production fluids. Wells are conventionally lined with a section of metal tubing which is perforated to allow fluid to enter into the borehole. 
     These perforations are normally formed in a violent manner by setting off an explosive charge to fire projectile(s) through liner or by the explosive charge itself being designed to blast through the material. The lining is thereby ruptured and perforations are thus formed. However, such a method results in compression of the rock formation surrounding the perforation, reducing its pore size and creating a local barrier to fluid flows around, and significantly, into the borehole. The lining rupture caused by the explosive charge is also relatively uncontrolled and creates a random shape which is not streamlined and requires higher fluid energy to negotiate the perforation. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided apparatus for creating a hole in a subsurface formation. The apparatus includes an inner assembly adapted for connection to an elongate member. The inner assembly is adapted to be raised and lowered within a borehole. The inner assembly includes a member capable of engaging either an outer assembly or the borehole. 
     According to a second aspect of the invention there is provided a method for creating a hole in a subsurface formation. The method comprises the steps of: 
     connecting an inner assembly to an elongate member, said inner assembly including a member capable of engaging either of an outer assembly or a borehole; 
     lowering the inner assembly within the borehole; 
     engaging the member with either of the outer assembly or the borehole to resist substantially vertical movement of at least a portion of the inner assembly with respect to at least one of the outer assembly or the borehole; and; 
     driving a cutting member into said subsurface formation to create a hole. 
     Preferably, the method is performed using the apparatus according to the first aspect of the invention. 
     The subsurface formation may be a casing, liner or subterranean formation. 
     Preferably, the method further comprises drilling a hole in a casing of a borehole, typically prior to drilling a hole in the subterranean formation. 
     The cutting member may be a drill bit. Preferably, the drill bit engages the lining of the borehole at a point proximate to the producing zones. Alternately or in addition, the drill bit preferably engages the borehole and punctures a hole therein. 
     In one embodiment the inner assembly also comprises a coring barrel. 
     A rotation resistance mechanism is preferably further provided to prevent rotation of at least a portion of the inner assembly with respect to at least one of the outer assembly or borehole. 
     Preferably, the member capable of engaging either the outer assembly or the borehole is an expandable member. 
     Preferably the outer assembly is incorporated into a tubular string comprising a side exit mandrel. Preferably the outer assembly is secured in said borehole before the inner apparatus is lowered therein. Typically, the tubular string is a drill string. 
     Preferably the expandable member engages the outer assembly. Preferably the expandable member is an inflatable member. Typically the expandable member is formed from rubber and metal and preferably has a high friction coefficient. 
     Preferably the inner assembly comprises a piston cylinder and preferably a piston rod member. Preferably the piston rod member extends through the piston cylinder, then typically through a rotation resistance mechanism and may connect to spacers below the rotation resistance mechanism. The coring barrel is preferably connected to the lower (opposite) end of the spacers if used or the rotation resistance mechanism if no spacers are used. Typically a drill bit is connected to the coring barrel to engage the geological formation. 
     Preferably, the rotation resistance mechanism comprises a locking mechanism which locks the piston rod member in a rotational direction with respect to the piston cylinder. 
     Preferably the elongate member is attached to a wireline head. Preferably the wireline head comprises a sacrificial weak link between the elongate member and the wireline head. Preferably the elongate member comprises electrical conductors and cable. Preferably the electrical conductors transfer communication and/or power from the surface of the borehole to the wireline head, or from the wireline head to the surface. 
     Preferably the wireline head is attached to a housing. Preferably the housing comprises a valve block, a hydraulic pump, power pack and fluid reservoir. Preferably the housing is also attached to the piston rod member. 
     Preferably the power pack comprises an electric motor, most preferably a low amperage electric motor. Preferably the electric motor is connected to electrical conductors of the elongate member. Preferably the housing also has an electronics carrier which is also attached to electrical conductors of the elongate member. Typically, the elongate member is a wireline. 
     Preferably the motor is activated from the surface, through the electrical conductors, to drive the hydraulic pump to transfer fluid from the reservoir into the piston cylinder. 
     Preferably the cylinder and inflatable member are connected by two fluid flow control means which may be valves. Typically, one valve permits fluids to transfer from the cylinder to the inflatable member and the second valve permits fluids to travel in the opposite direction, that is from the inflatable member to the cylinder. Typically either valve may be closed to resist transfer of fluids. Optionally the valves may be opened by actuation thereof, or alternatively when a specified fluid pressure is attained. 
     Preferably the main hub part of the piston cylinder is separated into two portions, typically by a piston attached to the piston rod assembly. 
     Preferably fluids can be injected or rejected from each portion of the main hub part of the piston cylinder. Preferably a first hydraulic line connects to the first, upper, portion of the main hub part of the piston cylinder and a second hydraulic line connects to the second, lower, portion of the main hub part of the piston cylinder. Typically each hydraulic line connects to the hydraulic pump and fluid reservoir. Typically fluid flow control means are provided to control the fluid travelling in the hydraulic lines between the reservoir/pump and each portion of the main hub part of the piston cylinder. Preferably the rate and direction of the fluid may be controlled by the fluid control means. Preferably the fluid control means are valves. Preferably there are four valves. 
     Preferably the first valve is provided on the first hydraulic line. Preferably the first valve is a two way valve, that is it may be set to allow fluid to travel from the reservoir to the cylinder or in the opposite direction, from the cylinder to the reservoir. 
     Preferably the second hydraulic line connects to the reservoir and pump via the other valves which are connected in parallel. Typically the second valve may transfer fluid from the reservoir to the lower portion of the cylinder. Typically the third and fourth valves allow fluid transfer from the lower portion of the cylinder to the reservoir. Preferably the third valve can accurately regulate the amount of fluid passing therethrough. Typically, a further valve is provided in series with the third valve to resist the flow of fluid therethrough below a specified pressure. 
     Typically, each valve can be set to resist flow of fluids therethrough. 
     Preferably the inner assembly has drive means for rotating said core barrel about its longitudinal axis. Preferably the drive means comprises a hydraulic motor, most preferably a positive displacement drilling motor or mud motor. 
     Preferably the inner assembly comprises flow diverter means, and most preferably the expandable member functions as the flow diverter means. 
     Preferably a rod assembly comprises the housing containing the power pack which comprises the hydraulic pump, electrical motor, electronics carrier and reservoir; and typically the rod assembly further comprises the piston rod member, piston, spacers (if used), mud motor, coring barrel and drill bit. 
     Preferably a packer assembly comprises the piston cylinder, the rotation resistance mechanism and the member capable of engaging the borehole or outer assembly. 
     The inflatable member may be inflated by injecting pressurised hydraulic fluid therein which expands and engages the borehole or outer assembly. 
     Preferably the expandable member frictionally engages the borehole or outer assembly, insodoing providing a reaction force for said coring barrel to engage an oil and gas reservoir below. 
     Preferably, fluid is injected into the piston cylinder of the inner assembly to move the piston with respect to the upper portion of the piston cylinder, thereby moving the rod assembly with respect to the packer assembly. The rod assembly includes the coring barrel and is thereby pushed down towards the oil and gas reservoir below wherein the reactive force is typically provided by the expandable member engaging the outer assembly. 
     Preferably the inflatable member is then disengaged from the outer assembly by appropriate means e.g. deflation of the member. 
     Typically the rod assembly is held by the wireline, and the piston and the top of the piston cylinder are pushed together by injection of hydraulic fluids which enter the lower portion of the piston cylinder thereby moving the packer assembly downhole. 
     In this position the rod and packer assembly are typically in the start position with respect to each other; but lower (e.g. 5 ft) with respect to the borehole, than the position in which they started collecting the core sample. 
     The method may be repeated as many times as necessary to complete the core sample. Typically the length of the main hub part of the piston cylinder is 5 ft, but could suitably be longer or shorter. Typically the length of the core barrel is 25 ft, but could suitably be longer or shorter. Therefore the method will normally be repeated five times, although this may be varied depending on the cylinder size, core barrel size, length of core required or for other reasons. 
     To extract the inner assembly from the borehole, the expandable member may be disengaged and the inner assembly may be winched up to the surface. Alternatively where winching cannot retrieve the inner assembly, because, for example, the coring barrel is jammed in the geological formation, the method of coring may be adapted to remove the inner assembly from the borehole. 
     In such a case, the expandable member may engage the borehole or outer assembly. Hydraulic fluid is injected into the lower portion of the cylinder to push the piston and complete rod assembly in an upwards direction. Optionally, the winch may also be used to assist this operation. 
     Preferably, the expandable member then disengages the borehole and the inner assembly is held on the wireline. The rod and packer assembly may then be separated by injecting hydraulic fluid into the upper portion of the piston cylinder, causing the packer assembly to move in an upwards direction. 
     The rod assembly may then be raised by engaging the expandable member with the outer assembly and injecting pressurised fluid into the lower portion of the cylinder, forcing the piston and rod assembly in an upwards direction. 
     This process may be repeated as necessary until the inner assembly may be retrieved by winching alone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the apparatus and methods of the present invention are described, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a simplified schematic view of an apparatus according to the present invention, showing the outer and inner assemblies; 
     FIG. 2 is a sectional view of a first embodiment of the inner assembly, and a portion of the outer assembly, in accordance with the invention; 
     FIG. 3 a  is a schematic view of a hydraulic valve network, which forms part of the inner assembly according to the present invention; 
     FIG. 3 b  is a sectional view of the inner assembly; 
     FIG. 4 is a schematic view of a prior art and conventional method of perforating tubing with explosive detonation; and, 
     FIG. 5 is a schematic view of drilling a perforation in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The apparatus in accordance with the invention generally comprises an outer assembly  51  which is incorporated in a drill string  52 . An inner assembly  50  includes a core barrel  6  and which is run into the outer assembly  51  by a wireline  18 . 
     FIG. 1 shows an apparatus according to first and second embodiment of the invention being operated from a drilling platform  53 ; differences in the two embodiments will be subsequently detailed. The outer assembly comprises a side exit mandrel  2 , positioned above a rock bit  1 . The side exit mandrel  2 , also known as a whipstock, consists of a steel tube approximately 7.6 m (25 ft) long with a hole of approx. 63.5 mm (2½″) diameter starting centrally at the top, and exiting from one side at the lower end. The hole forms a long tapered side exit with an angle of approximately 1°. The lower end of the mandrel  2  is fitted with three centralising blades, preferably straight, with the side exit hole exiting along the top of one blade. Both ends of the barrel are threaded with standard API connections. 
     The side exit mandrel  2  is connected at its upper end to a drive sleeve  3 , consisting of a tube approximately 9.1 m (30 ft) long, machined with an internal bore of approximately 63.5 mm (2½″) diameter. The bore contains two opposing key slots of approximately 12.7 mm (½″) width, travelling the length of the tube. Standard API connections are applied to both ends. 
     The upper end of the drive sleeve  3  is connected to a load control housing  4 , consisting of a steel tube approximately 6.1 m (20 ft) long with a 76.2 mm (3″) smooth bored hole through the centre and API connections top and bottom. 
     A first embodiment of the inner assembly  50  is shown in FIG.  2 . The inner assembly  50  is coupled to, and suspended from, an electric wireline  18  which extends from a circulating head  19  of the drilling platform  53 . The wireline  18  is a standard electric wireline  18  with the capability to raise and lower the inner assembly  50  with respect to the outer assembly  51 , and provides electrical conductors, within itself, for power and communication purposes as will be subsequently discussed. 
     The wireline  18  is connected to the inner assembly via a conventional slimline wireline head (not shown), many of which are available from a variety of suppliers, one example of which is a Reeves™ wireline head. The wireline head typically has the capacity to connect the inner assembly  50  to seven electrical conductors provided within the wireline  18 . The wireline head typically also provides a sacrificial weak link to the wireline  18 . 
     The wireline head is in turn connected to a cylindrical rigid tubular housing  20  which contains a miniature single direction hydraulic pump  70 , a valve block manifold  90  (as shown in schematic in FIG. 3 a ), a power pack  91  and electronics carrier  92  all described below. 
     The power pack  91  typically comprises a high voltage (400 v-500 v), low amperage electric motor, the rotational output of which is coupled directly to the hydraulic pump  70 . The pump  70  is capable of producing a small volume of typically 0.5-0.8 l/m at a pressure of 3000 psi and will be capable of starting under load. Both the electric motor and hydraulic pump  70  are immersed in a flexible reservoir or tank  75  that is in turn located inside the rigid tubular housing  20 . A cable head (not shown) is attached to the top of the housing  20  and electric wires leading from the wireline head protrude through and into the reservoir  75 ; five or six of these electric wires are attached to the electric motor and the others continue down, protrude through and out of the reservoir  75 , and connect to the electronics carrier  92 . The power pack  91  attaches directly to the valve block  90  utilising a series of “O” rings (not shown) to provide pressure integrity. 
     The valve block  90  typically comprises up to 4 individual electrically operated solenoid valves into the side of the housing  20 , aligned horizontally. The valves are retained on the housing  20  with cap nuts (not shown) and oriented so that their working exits correspond directly with a specially drilled port system that has been manufactured through the valve block. The valve block  90  also comprises a drilled conduit which provides a passageway for the electrical conductors to pass into the electronics carrier located below the valve block  90 . 
     Additionally, various relief and check valves are provided within the valve block, and are designed to direct the hydraulic fluid through an electronics carrier to a cylinder  23  and packer  30 . An oriented non-rotating coupling connects the valve block to the carrier with suitably positioned “O” rings providing pressure integrity. 
     The electronics carrier  92  has a drilled bore (not shown) which retains an electronics board (not shown) and an electric conduit to provide the electrical connection to a linear transducer (not shown). The linear transducer is a standard component which senses the position of the piston  24  in the cylinder  23 . The linear transducer is provided within a piston rod member  15  and senses the existence of a magnet (not shown) in a top cylinder gland  89  (not shown in FIG. 2 but shown in FIG. 3 b .) The electronic board is a standard board designed to provide digitised communication via a restricted number of electrical conductors between the down hole valves/transducer and the surface  59 . The carrier  92  also has drilled ports which allow the hydraulic fluid to flow into the cylinder  23  and packer  30  below. 
     A control panel unit (not shown) is provided at the surface to manipulate the apparatus. The control unit includes four control switches (not shown) which are linked to the valves  71 - 74  via wireline conductors (not shown) in the wireline  18 . A progressive switch (not shown) controls the electric power to the motor. Gauges (not shown) are provided on the control unit, one for monitoring the amperage supplied to the electric motor and one to monitor the position of the piston  24  with respect to the cylinder  23 , as indicated by the linear transducer. 
     The housing  20  is attached to the rod  15  which carries an upper piston  24 . Hydraulic lines  21 ,  22  shown in FIG. 2 connect the hydraulic pump  70  to the inside of a slimline piston rod member hydraulic cylinder  23 . The cylinder  23  defines a chamber  28 ,  29  therein which is split into two portions  28 ,  29  by the piston  24 . 
     The packer  30  is a standard third party supplied packer, typically used without an exterior rubber cover. It is typically manufactured from a combination of metal and rubber and has a high friction coefficient. A weight gauge (not shown) is provided below the packer  30 . 
     As shown in FIG. 2, the piston rod member  15  has a key  42  in the region above the packer  30 . In use, the key  42  engages slots  43  milled into the inner circumference of a torque tube  40  and act to prevent rotation of the piston rod member  15  with respect to the torque tube  40  about its longitudinal axis. The rod  15  extends through a seal in the bottom of the torque tube  40 . The torque tube  40  may be integral with the cylinder  23 , or could be a separate component secured to the cylinder  23 . In alternative, and preferred embodiments there are 4 keys welded onto the inner circumference of the torque tube  40  and four corresponding slots milled into the outer circumference of the rod  15 . 
     Spacer rods (not shown) may be attached to the bottom of the rod  15  shown in FIG.  2  and have a coring or drilling assembly, as the case may be, attached to their opposite end. The spacers allow the inner assembly to extend below the side exit mandrel  2 . 
     The coring/drilling assembly is powered by a conventional positive displacement mud motor (not shown). Mud is directed into inflow ports  45  via the lower portion of the inner bore of the rod  15  and the inner bore of the spacers to the mud motor. Typically a dump sub (not shown) is provided to control the mud flowing through the mud motor, excess mud being disposed into the annulus  17  between the inner  50  and outer  51  assemblies. 
     The core barrel  6  can be a mining style barrel with bearing suspended inner tubes that are supplied in multiples of 5 of 10 ft (or other multiple to correspond with the piston&#39;s stroke). The inner tubes are typically standard steel versions suitable for recovering core of 1.4″ diameter. The outer barrels are typically thin wall models that enable higher than average flow rates and offer little resistance to the high bending loads introduced when passing over the side exit mandrel  2 . 
     To operate the apparatus, the side exit mandrel  2  is attached directly to the bottom of the heavy weight drill pipe used when drilling the original well. The outer assembly  51  and drill string  52  is lowered into the well to the required depth and landed into the slips. 
     The inner assembly  50  is assembled on the surface and run down to the required level on the wireline  18 . The packer  30  is then inflated, by activating the electric motor to operate the hydraulic pump  70  to inflate the packer  30 . 
     The packer  30  abuts against the outer assembly  51  to form a frictional connection therebetween and resist vertical movement of the packer  30  with respect to the outer assembly  51 . 
     The connection between the packer  30  and the outer assembly  51  can be checked by lowering the wireline  18  and monitoring the weight of the inner assembly  50 —a reduced weight confirms that the packer  30  is supporting the inner assembly  50 . 
     Typically, 5 ft of wireline  18  is lowered into the drill string  52  before the circulating head  19  is closed. Alternatively, where the stroke of the piston  24  is larger or smaller than 5 ft, the appropriate amount of wireline  18  is inserted. 
     To perform the drilling/coring operation, the mud pumps are activated by energising the electric motor from the surface via the electrical conductors, the pressure of hydraulic fluid and weight of the inner assembly being continually monitored. 
     When operating the piston action of the inner assembly  50 , the piston rod member  15  and cylinder  23  move with respect to each other, as will be described below. When the piston rod member  15  and cylinder  23  move, the other components in the inner assembly  50  either move along with the piston rod member  15  or along with the packer  30 . The housing  20  containing the electronics carrier, hydraulic pump, valve block, tank and power pack; the spacers (if used), and the piston  24  move with the piston rod member  15  and are defined as the “rod assembly”. The wireline  18  is attached to the rod assembly as previously described. 
     The cylinder  23  and the torque tube  40  move with the packer  30  and are defined as the “packer assembly”. 
     As previously described the packer  30  is inflated and hydraulic fluid is then directed into the area  28  by an operator controlling the valves in the valve block  90  from the surface via the electric cable  18 . The hydraulic fluid pushes the piston  24  down—reactive force being provided by the packer  30  engaging the outer assembly  51 —which in turn moves the attached rod assembly (which includes the drill bit) down to engage and drill or core the geological formation below. 
     Once the rod assembly has completed its stroke, and the required drilling, cutting or coring has been completed, the packer  30  is then deflated and disengaged from the outer assembly  51 . Hydraulic fluid is directed into the lower portion of the cylinder  29  and the piston  24  and the top of the cylinder  23  are pushed together. This results in the piston  24  and rod assembly remaining static while the packer assembly moves down towards the rod assembly until the piston  24  abuts against the top of the cylinder  23 . 
     The above described process may then be repeated to recover a further portion of rock formation into the core barrel  6 . 
     The drill bit may be raised at any time. This is achieved by engaging the packer  30  with the outer assembly  50  as previously described. Hydraulic fluid is directed into the lower portion  29  of the cylinder  23  which forces the piston  24  upwards along with the piston rod member  15 , spacers and drill bit. 
     A second embodiment of packer assembly and rod assembly in accordance with the present invention is shown in FIG. 3 b . The second embodiment shares many common features with the first embodiment, and where this is the case, common reference numerals have been used; where this is not the case, the differences are described below. 
     The second embodiment of the inner assembly  50  comprises a hydraulic cylinder  23 , a torque tube  40  and a packer  30  all referred to as the “packer assembly”. The inner assembly  50  further comprises a rod  15  and associated components, such as an electronics carrier, hydraulic pump, valve block (which houses the valves  71 - 74 ), tank and motor, spacers and a coring barrel (not shown) all previously described with respect to the first embodiment and referred to as the “rod assembly”. The rod  15  extends through the cylinder  23  and has an attached piston  24  which divides the cylinder  23  into an upper  28  and lower  29  portion. 
     The two-way valve  71  is connected to the upper portion  28  of the cylinder  23  via hydraulic line  84 . Valves  72 ,  73  and  74  are connected to the lower portion  29  of the cylinder  23  via the hydraulic line  83 . An insert gland  88  seals the lower  29  portion of the cylinder  23  and the top cylinder gland  89  seals the upper  28  portion of the cylinder  23 . A pressure release valve  81 , in the insert gland  88 , connects to a hydraulic line  82  to transfer hydraulic fluid from the lower  29  portion of the cylinder  23  to the packer  30 . 
     In the start position, the piston  24  is positioned at the top of the cylinder  23  (as shown in FIG. 3 b ) with all valves  71 - 74  closed. To operate the inner assembly  50 , valve  72  is opened to allow hydraulic fluid to travel through the hydraulic line  83  into the lower portion  29  of the cylinder  23 . The pressure in the lower portion  29  of the cylinder  23  increases until it exceeds that of the pressure release valve  81  causing the hydraulic fluid to continue through the hydraulic line  82  and into the packer  30 . Continued injection of hydraulic fluid into the packer  30  causes the packer to inflate and engage the outer assembly or borehole (not shown in FIG. 3 b ) as appropriate. A pressure release valve  85  is provided between the valve  73  and the inner assembly  50  to ensure that the pressure in the packer  30  does not fall below the required level to maintain it inflated and engaged with the outer assembly or borehole, as the case may be. 
     The operation continues as described for the previous embodiment; wireline  18  is lowered into the drill string  52  and the circulating head  19  is closed and the mud pumps are activated. 
     When the packer  30  is fully inflated, valve  72  is closed and valve  71  is opened, and hydraulic fluid is pumped into the upper portion  28  of the hydraulic cylinder  23  via the hydraulic line  84 . Valve  73  is opened so that only the pressure release valve  85  prevents the hydraulic fluid in the lower portion  29  of the cylinder  23  draining through the hydraulic line  83  back to the tank  75 . This ensures that a minimum level of pressure is maintained in the lower portion  29  of the cylinder  23  and in the packer  30 . Once the pressure in the lower portion  29  of the cylinder  23  exceeds that of the pressure release valve  85  due to the continued injection of the hydraulic fluid into the upper portion  28  of the cylinder  23 , the fluid in the lower portion  29  drains through the hydraulic line  83 , pressure release valve  85  and valve  73  back to the tank  75 . 
     Piston  24 , core barrel and all other components included in the rod assembly are thus forced downwards towards a geological formation below whereas vertical movement of the cylinder  23  and other components of the packer assembly are resisted by the engagement of the packer  30  with the outer assembly or borehole as the case may be. An increase in pressure of drilling mud within the drill pipe  52  (i.e. the standpipe pressure) will signify that the motor is encountering resistance, i.e. that the bit has started cutting. The progress of the bit into the reservoir is controlled by opening valve  73  to its maximum extent without stalling the motor  70 . 
     At the end of its stroke (normally 5 ft), the piston  24  abuts against the insert gland  88  and so further downward movement of the piston and therefore the rod assembly is resisted. After the full stroke of the piston  25  has been completed, valve  73  is closed and the hydraulic pump  70  shut down. The mud pumps are stopped and the standpipe pressure vented off. The circulating head  19  is released and tension is applied to the wireline  18 . At this point, the core barrel, may contain a core sample, the length of the core sample corresponding to that of the piston stroke. Valve  74  is opened to drain the hydraulic fluid from the packer  30  through line  83  back to the tank  75 , insodoing deflating and disengaging the packer  30  from the outer assembly. 
     The packer assembly is then moved towards the geological formation below by closing valve  74  and opening valve  72 . Hydraulic fluid is pumped through hydraulic line  83  into the lower portion  29  of the hydraulic cylinder  23  forcing the rod and packer assemblies apart. As the rod assembly comprises components which weigh approximately three times that of the packer assembly, the latter will be forced down towards the geological formation below until the piston  24  abuts against the top cylinder gland  89  of the cylinder  23 . 
     The packer  30  can then be inflated as previously described and an additional section of core cut. The process may continue until the core barrel is full or has removed the required amount of core. Thus embodiments of the invention allow core samples e.g. 25 ft long to be recovered by apparatus comprising a single piston stroke of 5 ft; the limitation on the size of the core sample depends only the length of the core barrel  6  and not on the length of the piston stroke. 
     The bit may be retrieved from the geological formation when it is jammed therein or when the drilling or coring is complete and the inner assembly  50  is to be removed to the surface, by applying an upwardly directed force. 
     To apply the necessary upward force, valve  71  is placed in the return position and valves  72 ,  73  and  74  are all closed, leaving the packer  30  inflated. The hydraulic pump  70  is switched on. When the coring or drilling is complete the piston  24  will normally be abutting against the lower end  26   b  of the cylinder  26 . 
     Opening valve  72  will allow hydraulic fluid to enter the lower portion  29  of the cylinder  26 , which acts to push the piston  24  and the whole of the rod assembly in an upwards direction, insodoing removing the bit from the geological formation. 
     Hydraulic fluid in the upper portion  28  of the cylinder  26  is drained through valve  71  back into the tank  75 . If necessary, extra upward force may be exerted on the rod assembly by loosening the circulating head  19  at the surface and pulling the wireline  18  with any suitable winch (not shown). 
     The piston  24  is moved to the upper end of the cylinder  26  and the drill bit or core barrel  6 , being attached to the piston rod member  15  and piston  24  via spacer rods etc is removed from the geological formation by the length of the piston stroke (normally 5 ft). 
     Thus certain embodiments of the invention benefit from the ability to conveniently release the bit from the rock formation by applying an upward force via the hydraulic valve network  70 - 76 . 
     Drilling or coring may optionally be continued as previously described. 
     To remove the inner assembly  50  to the surface, the hydraulic pump  70  is switched off and the load is placed on the wireline  18 . The packer  30  is deflated by opening valve  74  with valves  72  and  73  closed and valve  71  in the return position. The inner assembly  50  may then be winched to the surface. 
     In the event that more effort is required to retrieve the inner assembly  50  to the surface, the packer assembly may be forced towards the surface by hydraulically activating the piston  24  in the cylinder  26 ; resulting in the rod assembly including the piston  24  remaining static and the packer assembly including the cylinder  26  and packer  30  rising towards the surface. The packer  30  can then be inflated to hold it in the higher height, and the rod assembly raised to the level of the packer assembly by action of the piston  24  in the cylinder  26  working in the opposite direction, with the packer  30  anchoring the packer assembly at the higher height. 
     To achieve this valve  74  is opened, the hydraulic pump is started and valve  71  is placed in the open position. Hydraulic fluid will enter the upper portion  28  of the cylinder  26  forcing the piston  24  and the cylinder  26  apart. Hydraulic fluid in the lower portion  29  of the cylinder  26  is drained through valve  74  to the tank  75 . As the piston  24  is attached to the piston rod member  15  which is in turn attached to the wireline  18 , the result of the hydraulic fluid entering the upper portion  29  of the cylinder  26  is to raise the packer assembly towards the surface. 
     When fully stroked (normally 5 ft) the piston  24  will be in its lower position, that is, abutting against the lower end of the cylinder  26 . 
     The packer  30  is then inflated by opening valve  71  to pressure, closing valves  73  and  74  and opening valve  72  to inject hydraulic fluid into the lower portion  29  of the cylinder  26  which will in turn inflate the packer  30  as previously described. The piston  24  is held static throughout this operation by the pressure exerted into the upper portion  29  of the cylinder  26  through valve  71 . When the packer  30  is inflated, valve  72  is closed and valve  71  is set in the return position. 
     The rod assembly may then be raised as described previously, that is by opening valve  72 , allowing hydraulic fluid to enter into portion  29  of the cylinder  26  (fluid in portion  28  of cylinder  26  being drained through valve  71  to the tank  75 ) insodoing pushing the piston  24  towards the upper end of the cylinder  26 . 
     This process may be repeated as necessary to move the inner assembly  50  up the outer assembly  51  until it is possible to remove it by winching in the wireline  18 . When this is possible the hydraulic motor  70  is shut down, valves  72 ,  73  and  74  are closed and valve  71  is set to the return position. The circulating head  19  is pressured up to allow the wireline  18  to be retrieved without mud loss and the inner assembly  50  is pulled to the surface. 
     The apparatus may also be used to drill side-tracks from wells, and also perforations into wells, and in this scenario, a drill bit of up to 3″ (normally 2.5″) diameter would be used. 
     FIG. 5 shows a perforation  310  formed in a lined borehole  300 . FIG. 4 shows the perforations  210  formed in a similar borehole  200  using apparatus and method common in the art, namely explosive detonation. 
     The density  305  of the rock formation around the perforation  310  in FIG. 5 is much less compared with the density  205  of the perforations in FIG. 4 utilising known technology. This scenario has the advantage over existing methods of perforating wells because the perforated area of the well is not compressed. Indeed, the perforated area may optionally be removed in an attached the core barrel, and thus increased production rates are experienced. A further advantage is the streamlined perforation formed in the borehole lining. 
     Changes and modifications may be made to the embodiments without departing from the scope of the invention.