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TECHNICAL FIELD 
       [0001]    The present invention relates to a valve, core sampling apparatus and method, and more particularly, but not exclusively, relates to a core sampling apparatus using said valve and method for use in the oil and gas exploration industry. 
       BACKGROUND OF THE INVENTION 
       [0002]    Core analysis is widely practised throughout the oil and gas exploration industry in order to determine various properties of the drilling formation. Analysis of the core generally involves removing the core from the formation and transporting it to the surface where it can subsequently be analysed. In order to obtain accurate and useful information from the analysis, it is important that the properties of the core at the surface are an accurate representation of the core properties downhole. In order to maintain core property measurements representative of the downhole conditions, it is important that, during removal to the surface, the surrounding pressure of the core sample is maintained at a pressure which is high enough to prevent any fluid present in the sample from escaping and thereby damaging the sample. Accordingly, it would be desirable to be able to retain the fluids within the sample. 
         [0003]    Current methods of extracting the sample from the downhole environment at relatively high pressure involve passing the core sample into an inner barrel (whilst downhole) which has a pressure valve at each end. Once the core sample has entered the inner barrel, the pressure valves are actuated in order to seal in the core sample at the surrounding downhole pressure. The inner barrel is then retrieved to the surface of the well and the highly pressurised (relative to the outside surface ambient pressure or atmospheric pressure) inner barrel is removed for analysis. This method has the disadvantage of being dangerous to persons operating the well since there exists the possibility of the high pressure, thin walled inner barrel exploding, thereby causing serious injury. 
       SUMMARY OF THE INVENTION 
       [0004]    According to the first aspect of the present invention there is provided a selectively operable downhole valve for use with core sampling apparatus, the valve comprising a throughbore through which a first body may pass, and an obstructing member which is capable of selectively obturating the throughbore, the obstructing member being arranged to open or close the throughbore by movement of the first body with respect to the valve. 
         [0005]    According to a second aspect of the present invention there is also provided a method of selectively operating a downhole valve comprising the steps of: 
         [0006]    passing a first member through a throughbore of the downhole valve, the downhole valve comprising a throughbore obstructing member; and 
         [0007]    opening or closing the throughbore by movement of the first member with respect to the valve. 
         [0008]    Preferably, the method of the second aspect further comprises the step of maintaining one side of the obturated valve at a higher pressure relative to the other side. More preferably, the said one side is the uppermost face of the downhole valve and the said other side is the lowermost face of the downhole valve. 
         [0009]    According to a third aspect of the present invention there is provided a method of obtaining a core sample from downhole, the method comprising: 
         [0010]    inserting a first member into a tubular string and providing for passage of the first member through a selectively operable downhole valve; 
         [0011]    permitting a core to be sampled to move into, and be held within, the first member; 
         [0012]    retrieving the first member back to surface from downhole; and 
         [0013]    characterised in that when the first member is withdrawn back through the downhole valve, the downhole valve is closed and the throughbore of the string of tubulars above the downhole valve is pressurised. 
         [0014]    Typically, the method according to the third aspect further comprises providing the valve with a throughbore through which a first body may pass, and an obstructing member which is capable of selectively obturating the throughbore, the obstructing member being arranged to open or close the throughbore by movement of the first body with respect to the valve. 
         [0015]    Typically, the downhole valve is incorporated into the tubular string which is run into the hole in a first step. Preferably, the first member is provided with a first portion of a retrieval mechanism and the first member is typically retrieved by running a second portion of a retrieval mechanism into the throughbore of the tubular string by an elongate member such as wireline until the first and second portions of the retrieval mechanisms engage and paying in the elongate member back to surface. Typically, the first member is retrieved to surface through the tubular string under pressure and is delivered into pressure retaining equipment at surface. 
         [0016]    Preferably, the obstructing member is biased into the obstructing position and more preferably is adapted to substantially remain in its obstructing position when a fluid pressure differential is applied across it such that fluid is typically prohibited from flowing through the valve. Typically, therefore, the obstructing position is essentially a closed position. 
         [0017]    Preferably, the valve is provided with a by-pass device adapted to allow fluid to flow through the valve when the pressure differential across the valve is at or exceeds a predetermined level when the obstructing member is in its obstructing position. 
         [0018]    Preferably, the by-pass device comprises an annular portion having flow apertures which are typically in fluid communication with a pressure relief device. 
         [0019]    Preferably, the annular portion has an inner diameter of greater diameter than the outer diameter of the first body. 
         [0020]    Preferably, the pressure relief device comprises a sealing mechanism held against one or more outlets of the flow apertures by a resilient member. More preferably the resilient member is a spring mechanism. 
         [0021]    Preferably, the obstructing member comprises a flap member which has a cross sectional area substantially similar to the cross sectional area of the throughbore of the downhole valve. More preferably, the flap member is provided with a hinged connection. 
         [0022]    Preferably, the hinged flap member is adapted to open or close the throughbore of the downhole valve by hinging toward the opposite direction to the direction of movement of the first member. 
         [0023]    Preferably, the downhole valve further comprises sensing mechanism which is preferably adapted to sense the presence of the first member at or near the valve. 
         [0024]    Preferably, the sensing mechanism comprises a cammed surface partially protruding into the throughbore of the downhole valve. 
         [0025]    Preferably, the downhole valve further comprises actuation mechanism, typically in communication with the sensing mechanism and the obstructing member. More preferably, the actuation mechanism comprises a lever connecting the sensing mechanism to the obstructing member. 
         [0026]    Preferably, the valve comprises a guiding block adapted to guide the first member through the throughbore via the sensing mechanism. 
         [0027]    Preferably, the guide block comprises a guide member having an inlet of greater diameter than its outlet. 
         [0028]    Typically, the guide member comprises apertures which allow fluid to flow from one side of the guide block, which may be an external side of the guide block, to the throughbore of the downhole valve on the other side of the guide block, and preferably the apertures allow such fluid to flow whether the first body is present or not present in the guide member. 
         [0029]    Preferably, the obstructing member is held in the obstructing position by resilient holding mechanism. More preferably, the resilient holding mechanism is a spring. 
         [0030]    Preferably, the first body is the inner barrel of a coring assembly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0032]      FIG. 1  is an isometric view of the apparatus according to the present invention; 
           [0033]      FIG. 2  is an exploded isometric view showing the components of the apparatus of  FIG. 1 ; 
           [0034]      FIG. 3  is a cross sectional view of the apparatus of  FIG. 1 ; 
           [0035]      FIG. 4  is cross sectional view of the apparatus taken through the line A-A of  FIG. 3 ; 
           [0036]      FIG. 5  is a schematic cut away view of the apparatus of  FIG. 1  in position downhole; 
           [0037]      FIG. 6  is a schematic cut away view of the apparatus of  FIG. 5  as an inner barrel arrives at the apparatus; 
           [0038]      FIG. 7  is a schematic cut away view of the apparatus of  FIG. 6  as the inner barrel passes through the apparatus; 
           [0039]      FIGS. 8A to 8G  are schematic diagrams showing an overshot device which is used to pull the inner barrel of  FIG. 7  out of the wellbore; 
           [0040]      FIGS. 9A and 9B  are partial cross sectional views of the overshot device of  FIGS. 8A to 8G  and the inner barrel of  FIG. 6 ; and 
           [0041]      FIG. 9C  is an isometric view of the on-rig recovery set-up used in conjunction with the apparatus of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0042]    An apparatus in accordance with the present invention is shown in  FIG. 5  in the form of a flap valve apparatus  10  which is positioned in an outer housing  12  of a tubular string such as a core barrel  13  between a pair of radially inwardly projecting shoulders  9  and  37  as shown in  FIG. 2 , the main body of the flap valve apparatus  10  comprises a lower substantially tubular body section  14  having a throughbore  15  connected to an upper substantially tubular body section  16  by a connecting pin  18 . The lower and upper body sections  14 ,  16  in use are located within an outer housing  12 . The apparatus  10  comprises a valve mechanism  20  between the lower and upper body sections  14 ,  16  and the valve mechanism  20  comprises a lever fork  22  and a flap  24 . 
         [0043]    As shown in  FIG. 1 , the lower body section  14  is tapered at  28  toward the longitudinal centre of the apparatus  10 . The upper substantially tubular body section  16  has a portion cut-away and so has a partially tubular extension  26 , extending in the direction of the longitudinal axis of the section  16  which provides a connection point to the lower section  14 . The cut away portion and tapered portion  28  together create a cavity  46 . throughbore 
         [0044]    Best shown in  FIG. 2 , the lever fork  22  of the valve mechanism  20  comprises an actuating cam  56  having a contact point  58  and a pivot point  60  provided by pivot pin  59 . The actuating cam  56  is connected to a pair of lever arms  62  which each comprise flap pivot points  64  at their lowermost ends. 
         [0045]    The flap  24  of the valve mechanism  20  consists of a planar oval member  66  having pivot points  68  provided by pivot pin  69  and slotted guide rails  70 . 
         [0046]    As shown in  FIGS. 5 to 7 , in use, the outer housing  12  is included, via conventional pin and box connections, in a string of tubulars having a core barrel  13  provided with a drill bit (not shown) at its lowermost end in the normal manner. The valve  10  is then inserted into the upper end of the outer housing  12  and is lowered until it comes to rest against the radially inwardly projecting shoulder  37 . An upper portion of the coring barrel  102  is then screwed onto the upper end of the outer housing  12  and the coring string  13 ,  102  and thus the outer housing  12  is positioned downhole in the borehole of a well being drilled by lowering the coring string  13 ,  102  into the borehole by means of drill pipe  106  (shown in  FIG. 9   c ). 
         [0047]    The flap  24  of the apparatus  10  will initially be in the closed position, as shown in  FIG. 5 . When the core sampling operation is to be performed, as shown in  FIG. 8A , an inner barrel  72 , having a spearhead  76  at its upper end, is dropped down the coring  102 /drill string  106  from the drilling rig. The purpose of the spearhead  76  will be described subsequently. Upon reaching the internal surface of the guide member  48  of the apparatus  10 , the inner barrel  72  progresses down the guide member  48  due to gravity. This is best shown in  FIG. 6 . Whilst passing through the guide member  48 , the outer diameter of the inner barrel  72  abuts against actuating cam  56  at its contact point  58 , thereby pushing the contact point  58 , and hence the actuating cam  56  outwardly toward the wall of the upper tubular section  16 . A longitudinal slot  54  provided in the wall of the tubular section  16  allows the actuating cam  56  room to actuate fully in this regard. As the actuating cam is moved outwardly by the presence of the inner barrel  72 , it pivots around the pivot point  60  thereby moving the lever arms  62  away from the central axis of the apparatus  10 . This outward movement of the lever arms  62  opens the flap  24  due to the flap pivot points  64  traversing along the guide rails  70 , as indicated in  FIG. 6 . The inner barrel  72  is now free to pass through the throughbore  15  of the lower body section  14 , thereby passing through the entire apparatus  10  as shown in  FIG. 7 . 
         [0048]    In this way, the core sample can be collected in the inner barrel  72  in the normal way, by rotating the coring string  102  from surface via the drill pipe  106 , such that the coring string  13  cuts into the formation and the sample moves into the inner barrel  72 . 
         [0049]    During operation of the coring assembly (not shown) it is normally necessary to circulate drilling mud through the well in order to move the core being cut into the inner barrel  72 . The apparatus  10  allows the drilling mud to flow through the upper body section  16  when the inner barrel  72  is in place by the provision of a guide member  48  having an internally tapered guide  50  and flow holes  52  provided in the throughbore of body section  16 . The flow holes  52  allow the drilling mud to flow from the annulus between the inner barrel  72  and the outer housing  12  and into the cavity  46 . The drilling mud may then pass from the cavity  46  through the annulus created between the outer diameter of the inner barrel  72  and the inner diameter of the throughbore  15  (which is substantially larger than that of the inner barrel  72 ) of the lower body section  14 . (Note the flap  24  will be in the open position due to the presence of the inner barrel  72  in the upper body section  16 ). 
         [0050]    When the core sample is to be brought to the surface, the inner barrel  72  is retracted (by means of the overshot device  74  as will be described subsequently) past the guide member  48 . The removal of the inner barrel  72  from the guide member  48  permits the flap  24  to close due to the action of a biasing mechanism in the form of a spring (not shown) provided on the pivot points  68  urging the flap  24  closed. This has the effect of immediately sealing off the inner bore of the apparatus  10  below the flap  24  from the inner bore of the apparatus  10  above the flap  24 . 
         [0051]    Sealing off the inner bore of the apparatus  10  in this way enables the upper section of the inner bore of the apparatus  10  (and thus the section of the inner bore of the coring/drill string above the apparatus  10 ) to be maintained at pressure by prohibiting dissipation of the pressure down the drill string  106  (which would normally happen if the apparatus  10 , in particular closed flap  24 , were not present). Since the apparatus  10  allows the upper section of the inner bore and thus the upper section of the coring barrel  102  and the drill string  106  to be maintained at a pressure value defined by the operator, the core sample pressure can be maintained as the sample is retrieved to the surface within the inner barrel  72 . Maintaining the coring barrel  102  and drill string  106  pressure has the great advantage (over maintaining the inner barrel  72  alone at pressure) that it is much more capable of safely withstanding high pressure differentials, when compared with the inner barrel  72  alone. It should be noted that although the ambient downhole pressure acting on the formation can typically be very high, it is only necessary to maintain the pressure of the sample at a value which inhibits any gas present in the formation from reaching its bubbling point. In this regard it is believed that a pressure of approximately 60 bar may be generally sufficient. 
         [0052]    Removal of the core sample, held within the inner barrel  72 , to the surface for analysis is typically done using an overshot device or fishing tool  74  as shown in  FIGS. 8A to 8G  and  FIGS. 9A and 9B . This is carried out by lowering the overshot  74  by wireline onto the inner barrel  72 ; for this purpose, the inner barrel is provided with a spearhead  76  at its upper end. The weight of the wireline and overshot  74  forces lifting dogs  78  on the overshot  74  onto the spearhead  76  by spreading the lifting dogs  78 . The lifting dogs  78  have hooked ends  80  and when the cone of the spearhead passes the hooked ends  80  of the lifting dogs  78 , the hooked ends  80  are locked onto the rear face of the spearhead cone  76  due to the action of return springs  82 . The secure grip of the lifting dogs  78  on the spearhead  76  allows the inner barrel  72  to be lifted into the barrel housing  84  of an on rig recovery set-up  96  (shown in  FIG. 9C ). 
         [0053]    The on rig recovery set-up  96  comprises a stuffing box  98 , an overshot housing  94 , an upper ball valve  87 , a barrel housing  84 , a lower ball valve  86 , and a pump in sub  100  which are connected to the coring barrel  102 , drill pipe string  106  and a rotary table  104  on the rig (not shown). Lifting the inner barrel  72  into the barrel housing  84  is performed by retracting the wireline through the stuffing box  98  until the lower end of the inner barrel  72  has passed the lower ball valve  86  on the recovery set-up  96 . The lower ball valve  86  is then closed and the locking dogs  78  are allowed to rest on a secondary cone  90  of the spearhead  76 . This spreads the locking dogs  78  further apart as shown in  FIGS. 8E and 8F . A locking sleeve  92  ( FIGS. 8E and 8F ) is then dropped onto the overshot  74  thereby locking the lifting dogs  78  in the wide opened position. The overshot  74  and locking sleeve  92  are then lifted into the overshot housing  94  by the wireline, through the stuffing box  98 , leaving behind the inner barrel  72  in the barrel housing  84  of the recovery set-up  96 . The upper ball valve  87  is then closed. 
         [0054]    The barrel housing  84 , containing the inner barrel  72  and core sample, can then be removed from between the upper and lower ball valves  87 ,  86  for analysis. 
         [0055]    During coring operations it is important to be able to circulate drilling mud, with very little notice, in the event of an emergency. This may be necessary when the apparatus  10  is in position downhole but the inner barrel  72  has not yet been dropped down the inner bore of the coring/drill string. As described previously, when the inner barrel  72  is not present in the apparatus  10 , the flap  24  will not be open and the drilling mud will be prohibited from flowing down the inner bore of the coring/drill string. This problem is tackled by the provision of the safety valve arrangement  30  located within the outer housing  12  immediately below the lower end of body section  14  in line with an outlet  32  of section  14 . As shown in  FIG. 4 , a number of bypass flow holes  34  are equi-spaced around the outer circumference of the lower section  14  and run from the tapered surface  28  (not shown in  FIG. 4 ) of section  14  toward an annular channel  36  formed around the lower end of the body section  14 . The safety valve arrangement  30  (not labelled in  FIGS. 3 and 4 ) includes an annular seal  38 , the upper face (left hand side in  FIGS. 3 and 4 ) of which is in fluid communication with the annular channel  36 . The lower face of the seal  38  abuts against an annular seal seat  40  which in turn abuts against a biasing mechanism in the form of disc springs  42 . An ‘O’ ring  44  is located in a groove formed around the circumference of the seal seat  40  such that the ‘O’ ring  44  provides a seal between the seal seat  40  and the inner wall of the outer housing  12 . 
         [0056]    As the differential pressure of the drilling mud is increased across the upper and lower section of the outer housing  12 , the mud (which passes through the flow holes  34  on the lower body section  16 ) pressure is exerted on the upper face of the annular seal  38  which in turn transfers this pressure to the annular seal seat  40 . The disc springs  42  are compressed due to the force being exerted upon them and the annular seal and seat  38 ,  40  are displaced away from the outlet  32  of the lower body section  14 . The displacement of the seal  38  away from the outlet  32  creates a small gap (not shown) between the seal  38  and the outlet  32  which allows the high pressure drilling mud present in the upper section of the outer housing  12  to flow into the lower section of the outer housing  12 . When the differential pressure is allowed to drop again, the spring urges the seal  38  back against the outlet  32 , thereby sealing off the upper and lower section of the outer housing  12 . In this way the safety valve arrangement allows drilling mud to flow through the apparatus  10 , in an emergency even if the inner barrel  72  is not present, without having to remove the apparatus  10  to the surface for servicing once the safety valve arrangement has been utilised. 
         [0057]    Modifications and improvements may be made to the foregoing without departing from the scope of the present invention.

Summary:
A selectively operable downhole valve for use with core sampling apparatus, the valve comprising a throughbore through which a first body may pass, and an obstructing member which is capable of selectively obturating the throughbore, the obstructing member being arranged to open or close the throughbore by movement of the first body with respect to the valve. A core sample of the formation may then be obtained and retrieved from the bore under pressure since the valve seals the bore downhole. Retrieving the sample under pressure maintains the integrity of the core sample so that subsequent analysis of the sample will more closely reflect the true characteristics of the formation.