Abstract:
The present disclosure relates to a downhole casing expansion tool and to a method of expanding casings using the same. The expansion tool comprises an expansion module for expanding casings and a drive module for driving the expansion module. The diameter of the openhole can be kept consistent by the expansion tool according to the present disclosure, so that monohole well drilling and completion operations can be carried out. The expansion tool according to the present disclosure is particularly suitable for constructions in deep, ultra-deep and complex wells.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a drilling and completion tool, particularly to a downhole casing expansion tool. The present disclosure further relates to a method of expanding casings using the same. 
       TECHNICAL BACKGROUND 
       [0002]    In conventional oil and gas filed well drilling operations, it requires putting casings downhole to realize stratum blocking in order to ensure safety and favorable arrival in the target layer. In a wellbore casing structure sequence, as the number of casing layers in the well increases, the casing size and the borehole size are progressively reduced, and the borehole taper is progressively increased, so that the target depth cannot be reached in deep, ultra-deep and complex wells or the subsequent operations will be affected by too small sizes of boreholes. 
         [0003]    Therefore, it requires to improve wellbore structures and drilling and completion techniques to ensure a constant borehole size and completion inner diameter and thus to drill deeper wells. 
       SUMMARY OF THE INVENTION 
       [0004]    To solve the above technical problems in the prior art, the present disclosure discloses a downhole casing expansion tool, which can maintain a constant borehole size in the drilling operations and therefore is particularly suitable for the constructions of deep, ultra-deep and complex wells. The present disclosure further relates to a method of expanding casings using the same. 
         [0005]    According to a first aspect of the present disclosure, it discloses a downhole casing expansion tool comprising an expansion module for expanding a casing and a drive module for driving the expansion module to expand. 
         [0006]    The drive module comprises a central tube, a core tube and an outer tube successively arranged from the inside to the outside thereof, in which a downstream end portion of the central tube can be closed and communicate with a drill shaft, and the central tube is provided with a liquid transfer aperture; the core tube which is fixedly connected to the drill shaft comprises a group of upstream and downstream core tube segments fixedly connected to one another via a first connecting member, wherein the downstream core tube segment is provided with a liquid inlet, and the core tube is slidably connected to the central tube via a limit member; and the outer tube which is connected to the core tube via a fourth shear pin comprises a group of upstream and downstream outer tube segments fixedly connected to one another via a second connecting member. 
         [0007]    The first and second connecting members are arranged upstream and downstream of the liquid inlet respectively, and the first and second connecting members slidably contact with the outer tube and the core tube in a sealing manner respectively, so that the first connecting member, the second connecting member, the downstream core tube segment and the upstream outer tube segment together define a hydraulic chamber. 
         [0008]    The expansion module comprises a plurality of expansion cone sheets movably mounted at a downstream end portion of the downstream outer tube segment and a cone seat fixedly arranged downstream of the expansion cone sheets. 
         [0009]    In an initial state, the liquid transfer aperture does not communicate with the liquid inlet. As the casing expands, the central tube, with its downstream end portion being closed, moves downstream under the pressure of the liquid filled therein guided by the limit member, so that the liquid transfer aperture communicates with the liquid inlet, thus introducing the liquid into the hydraulic chamber to shear the fourth shear pin so as to drive the outer tube to move downstream, which in turn drives the expansion cone sheets to move downstream, wherein supported by the cone seat, the expansion cone sheets expand axially, so that the expansion of the casing is realized. 
         [0010]    According to the downhole casing expansion tool of the present disclosure, a downhole casing can expand under a hydraulic pressure exerted on the downhole casing expansion tool from on the ground, so that the operations are facilitated. 
         [0011]    In one embodiment, the limit member comprises a limit sleeve slidably disposed between the central tube and the core tube, wherein the limit sleeve connects to the core tube via a first shear pin and to the central tube via a second shear pin arranged downstream of the first shear pin, and wherein an inner wall of the core tube is provided with a radially and inwardly projecting first limit step and an outer wall of the limit sleeve is provided with a first limit ring that can engage with the first limit step. The first shear pin is sheared under the pressure of the liquid filled as the casing is expanding, so that the central tube and the limit sleeve move as a whole downstream until the first limit ring engages with the first limit step, at that time the liquid transfer aperture being communicating with the liquid inlet. In one preferable embodiment, the inner wall of the limit sleeve is provided with a radially and inwardly projecting second limit step, which can engage with a second limit ring arranged in the central tube. When the expansion cone sheets expand, the second shear pin is sheared under the increased hydraulic pressure, so that the central tube moves downstream until the second limit ring engages with the second limit step. At that time the liquid transfer aperture is communicating with the liquid inlet no longer. The hydraulic chamber is merely used for driving the outer tube to move downstream so as to enable the expansion cones to expand, and can be closed when the expansion cone sheets expand through a limit member of the same structure, so that the hydraulic chamber exerts acting forces to the first and second connecting members no longer, which is advantageous for improving the service life of the tool. 
         [0012]    In one embodiment, the cone seat comprises a support area upstream thereof for facilitating the expansion of the expansion cone sheets, a connection area downstream thereof and a transition area between the support area and the connection area. Preferably, the support area is constructed to form a cone with a small end facing upstream. A cone seat of such shape enables the expansion cone sheets to successfully arrive at above the cone seat, i.e., the cone seat can support the expansion cone sheets, so that the expansion cone sheets can successfully expand. 
         [0013]    In one embodiment, the downstream end portion of the central tube connects to an expansion aid for closing the central tube, the expansion aid comprising a rubber plug connecting to the downstream end portion of the central tube via a third shear pin and a rubber plug conduit fixedly connecting to the connection area of the cone seat. As the casing expands, the central tube is closed by throwing a drill shaft rubber plug therein which is capable of automatically engaging with the rubber plug. 
         [0014]    In one embodiment, the transition area of the cone seat is provided with a blasthole and the rubber plug conduit is sealably connected to a cementing accessory sealably connected to a downstream end portion of the casing, so that the rubber plug conduit, the cementing accessory, the casing and the cone seat enclose a first sealed expansion chamber. Preferably, the rubber plug conduit is provided with a rubber plug seat therein. During an expanding period of the casing, when the expansion cone sheets expand and the second shear pin is sheared, the third shear pin is sheared under the increased hydraulic pressure, so that the rubber plug together with the drill shaft rubber plug moves downstream to seal the rubber plug seat, and the liquid flows back to a chamber of the cone seat and then into the first sealed expansion chamber via the blasthole. With the above structure, when the casing expands, the friction between the expansion cone sheets and the inner wall of the casing is significantly reduced, which is beneficial for lifting and pulling the drill shaft to expand the whole casing. In one preferable embodiment, in the initial state, the blasthole is blocked, thus preventing the outside contamination from entering inside the cone seat through the blasthole to affect the construction. 
         [0015]    In an embodiment, a downstream portion of the blasthole in the transition area of the cone seat is provided with an annular platform which can be sealably connected to the casing. With this structure, when the blasthole is penetrated, the casing and the cone seat would enclose a small temporary sealed chamber, which can be filled by liquid, so that the expansion cone sheets and the casing are lubricated and the friction therebetween is reduced. Since the temporary sealed chamber has a relatively small volume, it can be quickly filled with liquid, reducing the time from when the liquid is to be filled to the expansion, thus improving the working efficiency. Moreover, when the temporary sealed chamber is defined by the annular platform, it can avoid cementing materials from flowing back to a contacting surface of the expansion cone sheets and the casing before the casing expands, which would render subsequent expansion construction difficult to be carried out. 
         [0016]    In one embodiment, a hollow fixing cone fixedly connected to the downstream end portion of the outer tube is further provided, the expansion cone sheets being movably connected to a downstream portion of the fixing cone. The fixing cone is arranged to facilitate the assembling of the expansion cone sheets. In one preferable embodiment, the fixing cone forms a truncated cone with a small portion thereof facing upstream, and the inclination of the fixing cone is smaller than the inclination of an outer surface of the expansion cone sheets. With this structure, when the casing expands, the fixing cone would first slightly expand the casing, and then the expanded expansion cone sheets would expand the casing to a required level, which promotes the successful construction of the expansion. 
         [0017]    In one embodiment, a downstream portion of the downstream core tube segment is fixedly provided with a locking connector slidably and sealably contacting with the outer tube, wherein a step portion with a reduced outer diameter which is constructed at an upstream portion of the fixing cone is inserted in between the locking connector and the outer tube to fixedly connect to the outer tube, the downstream end of which abuts a surface of the step portion. The engagement of the step structure of the fixing cone and the outer tube ensures that the fixing cone and the outer tube move as a whole, which in turn ensures that the outer tube can drive the expansion cone sheets to move. In one preferable embodiment, the locking connector is further provided with a locking member comprising a radially outward recess provided on the locking connector and a locking block connected to the recess via an elastic member. In the initial state, the step portion presses the locking block, and as the casing is expanding, under the action of the elastic member caused by the downstream movement of the step portion, the locking block radially and outwardly projects and then presses an upstream end portion of the step portion. The arrangement of such locking connector can effectively prevent the disengagement of the expansion cone sheets and the cone seat while the casing is expanding. Otherwise, the expansion construction cannot be normally carried out. 
         [0018]    In one embodiment, the core tube comprises a plurality of core tube segments fixedly connected to one another via the first connecting member; the outer tube comprises a plurality of outer tube segments fixedly connected to one another via the second connecting member; and the first and second connecting members are both disposed upstream of the locking connector. Each and every one of the core tube segments is provided with one liquid inlet; the first connecting member is provided adjacent to each liquid inlet upstream thereof and the second connecting member is provided adjacent to each liquid inlet downstream thereof. An interval is provided between each and every adjacent first and second connecting members. In one preferable embodiment, the first connecting members are alternately arranged with the second connecting members. The downhole casing expansion tool of such structure can produce greater driving forces thanks to the plurality of second connecting members capable of receiving hydraulic pressure, thus facilitating successful casing expansion constructions. 
         [0019]    A second aspect of the present disclosure discloses a method for expanding a casing using the above downhole casing expansion tool, comprising the following steps: 
         [0000]    Step I: drilling an openhole and putting a first level casing down into the openhole to carry out first well cementation, wherein a downstream portion of the first level casing is pre-expanded and a downstream end portion thereof is sealed;
 
Step II: putting a drilling tool into the downhole to pierce the first level casing and continue drilling, wherein a side wall of the pierced first level casing forms an overlapping portion downstream thereof;
 
Step III: putting a second level casing and expansion tools into the downhole, wherein a downstream end portion of the second level casing is sealably provided with cementing accessories and a downstream portion of the second level casing comprises a pre-expanded expansion promoter region, upstream of which is provided with an overlapping area, the upstream portion of which is disposed in the overlapping portion of the first level casing, the expansion tool being constructed that the fixing cone thereof is arranged in the expansion promoter region;
 
Step IV: carrying out well cementation and putting a drill shaft rubber plug into the downhole, wherein the drill shaft rubber plug engages with the rubber plug of the expansion tool so as to seal the downstream end portion of the central tube;
 
Step V: filling liquid in the downhole casing expansion tool, pressing so that the second level casing expands under the function of the expansion tool, and lifting the downhole casing expansion tool so that an outer diameter of the second level casing expands to be equal to an inner diameter of the first level casing and to enable the second level casing to fixedly connect to the first level casing;
 
Step VI: repeating Step II to Step V and connecting an upstream portion of the downstream casing to the overlapping area of the upstream casing so as to complete multilevel casing monohole expansion operations.
 
         [0020]    Monohole expansion of casings can be achieved by the method of the present invention, i.e., the casings expand without the inner diameter being reduced, so that well drilling with no loss in well diameter (i.e., monohole well drilling) can be realized. Moreover, drills of the same specification can be adopted throughout the construction because the borehole diameter is kept consistent, thus reducing the cost in well drilling and completion and the efficiency thereof is improved. 
         [0021]    In one embodiment, in Step III, the fixing cone of the expansion tool is constructed in such a way that an outer diameter thereof is smaller than an inner diameter of the expansion promoter region but larger than an inner diameter of the second level casing arranged upstream of the expansion promoter region, so that the expansion tool and the second level casing can be conveniently and favorably engaged with each other before the construction of the expansion. 
         [0022]    In one embodiment, in Step IV, an unexpanded portion in a downstream portion of the casing is removed after the expansion of each level of casing is completed, so that the next level casing can successfully enter into the downhole. 
         [0023]    In one embodiment, an outer side of the overlapping area of the second level casing is provided with a compressible filler for separating cement. In one specific embodiment, the filler is air, the compressibility of which is so large that working spaces are provided for expansion after well cementation and the problem of the overlapping area of the casing incapable of being expanded due to the restriction to the expansion caused by external cement is eliminated, thus facilitating the expansion construction after the well cementation. 
         [0024]    In one embodiment, a sealing member is further provided between the overlapped casings, so as to ensure that the overlapped casings are sealably connected. 
         [0025]    In the present disclosure, the term “upstream” refers to the direction towards the ground and the term “downstream” refers to the direction opposite to the upstream direction. The term “initial state” refers to a state before the expansion of the downhole casing expansion tool. 
         [0026]    Compared with the prior art, the present disclosure is advantageous in the following aspects. At the outset, by the downhole casing expansion tool of the present disclosure, multi-level casings are expanded under hydraulic pressure, so that the casings can expand without the inner diameter being reduced (i.e., monohole well drilling can be realized). A downhole casing can expand under a hydraulic pressure exerted on the downhole casing expansion tool from on the ground, so that the operations are facilitated. The filler composed of materials capable of being compressed is provided at the outer side of the overlapping area of the second level casing, so that working spaces are provided for expansion after well cementation and the problem of the overlapping area of the casing incapable of being expanded due to the restriction to the expansion caused by external cement is eliminated. Moreover, drills of the same specification can be adopted throughout the construction because the borehole diameter is kept consistent, thus reducing the cost in well drilling and completion and the efficiency thereof is improved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0027]    In the following the present disclosure will be described in detail in view of different examples and with reference to the drawings, wherein, 
           [0028]      FIG. 1  is a schematic drawing of a first embodiment of the drive module of the downhole casing expansion tool according to the present disclosure; 
           [0029]      FIG. 2  is a full view of part A of  FIG. 1 ; 
           [0030]      FIG. 3  is an enlarged view of the central tube limit member of the downhole casing expansion tool according to the present disclosure; 
           [0031]      FIG. 4  is a schematic drawing of a second embodiment of the drive module of the downhole casing expansion tool according to the present disclosure; 
           [0032]      FIG. 5  is a schematic drawing of the setting mode of the expansion cone sheets of the downhole casing expansion tool according to the present disclosure; 
           [0033]      FIG. 6  is a schematic drawing of the cone seat; 
           [0034]      FIG. 7  is another schematic drawing of the cone seat; 
           [0035]      FIG. 8  is an enlarged view of the locking member according to the present disclosure; and 
           [0036]      FIGS. 9 to 15  schematically show the steps of expanding a casing with the downhole casing expansion tool according to the present disclosure. 
       
    
    
       [0037]    In the drawings, the same component is indicated by the same reference sign. The drawings are not drawn in accordance with an actual scale. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0038]    In the following, the present disclosure will be further illustrated with reference to the drawings. 
         [0039]      FIG. 1  schematically indicates a first embodiment of a downhole casing expansion tool  10  (hereinafter referred to as an expansion tool  10 ) according to the present disclosure. As shown in  FIG. 1 , the expansion tool  10  comprises an expansion module for expanding a casing  11  and a drive module for driving the expansion module to expand, which will be described in detail in the following. 
         [0040]    The drive module comprises a central tube  101 , a core tube  102  and an outer tube  103  successively arranged from the inside to the outside thereof. The core tube  102  is fixedly connected to a drill shaft (not shown in the drawing) and the central tube  101  communicates with the drill shaft, wherein the drill shaft is a hollow shaft so as to facilitate injecting liquid in the expansion tool  10 . The central tube  101  with a downstream end portion capable of being closed is provided with a liquid transfer aperture  104  on a side wall thereof. The downstream end portion of the central tube  101  extends from the core tube  102  while a downstream end portion of the core tube  102  extends from the outer tube  103 , and the core tube  102  is connected to the outer tube  103  via a fourth shear pin  113  (see  FIG. 3 ). The central tube  101  and the core tube  102  are connected via a limit member, which is indicated in  FIG. 3 . 
         [0041]    The core tube  102  comprises an upstream core tube segment  105  and a downstream core tube segment  106  fixedly connected to each other via a first connecting member  107 . The downstream core tube segment  106  is provided with a liquid inlet  117 . The outer tube  103  comprises an upstream outer tube segment  109  and a downstream outer tube segment  110  connectedly to each other via a second connecting member  108 . In addition, the first connecting member  107  slidably contacts with the outer tube  103  in a sealing manner and the second connecting member  108  slidably contacts with the core tube  102  in a sealing manner. As shown in  FIG. 1 , a contacting surface of the first connecting member  107  and the outer tube  103  is provided with a first sealing member  114 , while a contacting surface of the second connecting member  108  and the core tube  102  is provided with a second sealing member  115 . These sealing members can be selected as O-rings so as to realize the sealable contact. The first connecting member  107  and the second connecting member  108  are respectively arranged upstream and downstream of the liquid inlet  117 , so that the first connecting member  107 , the second connecting member  108 , the downstream core tube segment  106  and the upstream outer tube segment  109  define a hydraulic chamber  111 . 
         [0042]    Still according to  FIG. 1 , the expansion module comprises a plurality of expansion cone sheets  201  movably arranged at a downstream end portion of the downstream outer tube  110  and a cone seat  202  fixedly arranged downstream of the expansion cone sheets  201 . In the embodiment as shown in  FIG. 1 , the central tube  101  extends from downstream of the cone seat  202 . The core tube  102  extends through the expansion cone sheets  201  and the cone seat  202  is fixedly connected to a downstream portion of the core tube  102 . When the expansion cone sheets  201  move downstream to contact with the cone seat  202 , the expansion cone sheets  201  would axially expand like an umbrella so as to expand the casing  11 .  FIG. 1  illustrates the state when the expansion cone sheets  201  expand. The structure of the cone seat  202  will be described in detail in the following. 
         [0043]    As shown in  FIG. 3 , the limit member comprises a limit sleeve  401  arranged between the central tube  101  and the core tube  102 . The limit sleeve  401  connects to the core tube  102  via a first shear pin  402  and to the central tube  101  via a second shear pin  403  arranged downstream of the first shear pin  402 . An inner wall of the core tube  102  is provided with a radially and inwardly projecting first limit step  404  and an outer wall of the limit sleeve  401  is provided with a first limit ring  405  that engages with the first limit step  404 . In the embodiment as shown in  FIG. 3 , the first limit ring  405  is formed by a radially and outwardly projecting upstream end portion of the limit sleeve  401 . Therefore, as the casing expands, when the first shear pin  402  is sheared, the central tube  101  and the limit sleeve  401  under the connection of the second shear pin  403  would move downstream as a whole until the first limit ring  405  engages with the first limit step  404 . At this time, the liquid transfer aperture  104  communicates with the liquid inlet  117 , so that the liquid would flow into the hydraulic chamber  111  to shear the fourth shear pin  113  to drive the outer tube  103  to move downstream. 
         [0044]    The limit sleeve  401  is provided with a second limit step, which in the embodiment as shown in  FIG. 3 , can be an inner edge of the first limit ring  405 . The central tube  101  is provided with a second limit ring  407  engaging with the second limit step. In the embodiment as shown in  FIG. 3 , the second limit ring  407  is actually formed by a radially and outwardly projecting upstream end portion of the central tube  101 . When the expansion cone sheets  201  expand, the hydraulic pressure is further increased to shear the second shear pin  403 , so that the central tube  101  moves downstream until the second limit ring  407  engages with the second limit step. At this time, the liquid transfer aperture  104  communicates with the liquid inlet  117  no longer and the liquid originally inside the hydraulic chamber  111  would be sealed therein. After the first limit ring  405  engages with the first limit step  404  and the second limit step engages with the second limit ring  407 , the central tube  101  and the core tube  102  would move as whole instead of being disengaged. 
         [0045]    After the expansion tool  10  is assembled and before it is expanded, the outer tube  103  is fixedly connected to the core tube  102  via the fourth shear pin  113 . The central tube  101  connects to the core tube  102  via the limit member and the liquid transfer aperture  104  is arranged upstream of the liquid inlet  117  but does not communicate with the liquid inlet  117 . 
         [0046]    When the casing  11  is to be expanded, first the downstream end portion of the central tube  101  is closed. Next, liquid, such as drilling liquid is filled into the central tube  101  via the hollow drill shaft. The liquid is compressed so as to shear the first shear pin  402 , which leads the central tube  101  and the limit sleeve  401  to move together downstream until the first limit ring  405  engages with the first limit step  404  and the liquid transfer aperture  104  communicates with the liquid inlet  117 . In this way, liquid would flow from the central tube  101  via the liquid transfer aperture  104  and the liquid inlet  117  into the hydraulic chamber  111 . Since a stress surface downstream of the hydraulic chamber  111  is the second connecting member  108  connected to the outer tube  103  and the core tube  102  connected to the drill shaft is fixed and stationary, the fourth shear pin  113  would be sheared under the downstream hydraulic pressure, so that the outer tube  103  moves downstream and drives the expansion cone sheets  201  to move downstream. When the expansion cone sheets  201  enter between the cone seat  202  and the casing  11 , or when the cone seat  202  is between the expansion cone sheets  201  and the core tube  102 , the expansion cone sheets  201  would radially expand (i.e., to form an umbrella), so that the expansion of the casing  11  is realized as shown in  FIG. 1 . When the drill shaft is lifted, driven by the core tube  102  and the cone seat  202  fixedly connected to the core tube  102 , the central tube  101 , the core tube  102  and the outer tube  103  would be lifted out as a whole, during which period, the expansion cone sheets  201  are still kept radially expanded, so that the expansion of the whole casing  11  is realized. 
         [0047]    In order to conveniently close the central tube  101 , an expansion aid is provided at the downstream end portion of the central tube  101  as shown in  FIGS. 1 and 2 . The expansion aid comprises a rubber plug  301  connected to the downstream end portion of the central tube  101  via a third shear pin  304 , a rubber plug conduit  302  fixedly connected to a downstream end portion of the cone seat  202 , wherein the rubber plug  301  is arranged within an region of the rubber plug conduit  302 . A rubber plug seat  303  is provided in the rubber plug conduit  302 . When the rubber plug  301  is separated from the central tube  101 , the rubber plug  301  would move downstream to close the rubber plug seat  303 . Before the expansion construction is carried out, a drill shaft rubber plug  305  (as shown in  FIG. 2 ) is first thrown into the central tube  101 , wherein the drill shaft rubber plug  305  would automatically engage with the rubber plug  301  so as to close the downstream end portion of the central tube  101 . 
         [0048]    It should be understood that, as shown in  FIG. 4 , the core tube  102  of the drive module of the expansion tool  10  can comprise a plurality of core tube segments  106 ′,  106 ″ fixedly connected to one another via first connecting members  107 ′,  107 ″, while the outer tube  103  can comprise a plurality of outer tube segments  110 ′,  110 ″ fixedly connected to one another via second connecting members  108 ′,  108 ″. Each and every core tube segment is provided with a liquid inlet, such as  117 ′,  117 ″ and the central tube  101  is provided with a plurality of liquid transfer apertures  104 ′,  104 ″, so that the drive module would comprise a plurality of hydraulic chambers  111 ′,  111 ″ (as shown in  FIG. 4 ), which can produce greater driving forces so as to facilitate the successful expansion construction of the casings. In the embodiment as shown in  FIG. 4 , it should be further noted that as a matter of fact, the plurality of hydraulic chambers are separately distributed, i.e., an empty cavity  112  always exists between two adjacent hydraulic chambers, so that during the expansion construction, a downstream driving force exerted on the second connecting member  108  would not be offset. In the embodiment as indicated in  FIG. 4 , the expansion module, the limit member and the expansion aid are respectively identical as have been recited above and will not be repeated for the sake of simplicity. 
         [0049]    As shown in  FIG. 6 , the cone seat  202  comprises three parts: a support area  601  upstream thereof, a connection area  602  downstream thereof and a transition area  603  between the support area  601  and the connection area  602 . The support area  601  is used for facilitating the expansion of the expansion cone sheets  201  during the expansion construction. In one preferable embodiment, the support area  601  is constructed to form a cone with a small end facing upstream, so that the expansion cone sheets  201  can successfully reach the cone seat  202  so as to successfully realize the expansion of the expansion cone sheets  201 . Moreover, the cone-shaped support area  601  is also a separate part fixed at the core tube  102 . 
         [0050]    In order to facilitate lifting and pulling the expansion tool  10  upward, a blasthole  204  is provided in the transition area  603  of the cone seat  202 , which is shown in  FIG. 6 . The rubber plug conduit  302  is sealably connected to a cementing accessory  12  sealably connected downstream of the casing  11 , so that the rubber plug conduit  302 , the cementing accessory  12 , the casing  11  and the cone seat  12  define a first sealed expansion chamber  306 . 
         [0051]    In an initial state, the blasthole  204  is blocked by a sheetbody (not shown in the drawing) such as a sheetmetal. During an expanding period of the casing  11 , when the expansion cone sheets  201  radially expand and continue to press the liquid to shear the third shear pin  304 , so that the rubber plug  301  together with the drill shaft rubber plug  305  moves downstream to seal the rubber plug seat  303 . The liquid flows back to a chamber of the cone seat  202  through an interval between the central tube  101  and the cone seat  202  and then into the first sealed expansion chamber  306  via the blasthole  204 . When the first sealed expansion chamber  306  is filled by liquid, a contacting surface between the expansion cone sheets  201  and the casing  11  will be lubricated, so that the friction between the expansion cone sheets  201  and the casing  11  is reduced, which is beneficial for pulling and lifting the expansion tool  10  upward. 
         [0052]    In one preferable embodiment, as shown in  FIG. 7 , a downstream portion of the blasthole  204  in a circumferential side wall of the cone seat  202  is provided with an annular platform  205 . In the initial state, the annular platform  205  is sealably connected to the casing  11 , which can be achieved by a third sealing member  206  provided on a side surface of the annular platform  205 . In this manner, the casing  11  and the cone seat  202  enclose a small temporary sealed chamber  207 . As the casing  11  expands, the liquid reflux would break through the blasthole  204  and enter into the temporary sealed chamber  207 . Since the temporary sealed chamber  207  has a volume smaller than the first sealed expansion chamber  306 , the liquid would fill the temporary sealed chamber  207  in a very short time, so that the contacting surface area of the expansion cone sheets  201  and the casing  11  can be lubricated and the time period from filling the liquid to lifting and pulling the expansion tool  10  is shortened and the working efficiency is thus improved. In addition, the temporary sealed chamber  207  formed by providing the annular platform  205  can prevent the liquid in the well (such as drilling liquid, cement slurry, etc.) and impurities from flowing back to the temporary sealed chamber  207  before the expansion construction of the casing, which would render it difficult to carry out subsequent expansion construction. In pulling and lifting the expansion tool  10  upstream, when the annular platform  205  enters into the expanded casing, the temporary sealed chamber  207  would form a whole with the first sealed expansion chamber  306 , so that the liquid would also fill the first expansion chamber  306  and continue to carry out the expansion of the casing. 
         [0053]    In order to facilitate the assembling of the expansion cone sheets  201 , as shown in  FIG. 5 , the expansion cone sheets  201  is connected to the outer tube  103  via a fixing cone  208 . The fixing cone can be constructed as a hollow truncated cone to be coupled to a downstream portion of the core tube  102  and the fixing cone  208  is configured with a small end facing upstream and connected to the downstream end portion of the outer tube  103  and a large end facing downstream and movably connected to the expansion cone sheets  201 . In one preferable embodiment, the inclination of a side surface of the fixing cone  208  is smaller than the inclination of an outer surface of the expansion cone sheets  201 . In the present disclosure, the term “inclination” refers to the dip angle formed by an element of the fixing cone  208  and a central axis of the casing  11 , or the dip angle formed by the outline of the expansion surface of the expansion cone sheets  201  as indicated in  FIG. 5  relative to the central axis of the casing  11 . The concept of “inclination” is well known by one skilled in the art. In addition, the largest diameter of the fixing cone  208  is still smaller than the largest diameter of the expanded expansion cone sheets  201 . The advantages of the structure of the fixing cone  208  will be described in the following. 
         [0054]    As shown in  FIG. 8 , in order to ensure that the outer tube  103  drives the fixing cone  208  to move, a downstream portion of the downstream core tube segment  102  is fixedly provided with a locking connector  801  slidably and sealably contacting with the outer tube  103 . And a step portion  209  with a reduced outer diameter which is constructed at an upstream portion of the fixing cone  208  is inserted in between the locking connector  801  and the outer tube  103  to fixedly connect to the outer tube  103 , the downstream end of which abuts a surface  210  of the step portion. In such a manner, when the expansion cone sheets  201  expand, the fixing cone  208  moves downstream to drive the expansion cone sheets  201  to move downstream so that the expansion cone sheets  201  expand. In lifting the drill shaft, the fixed connection of the outer tube  103  and the fixing cone  208  would drive the expansion cone sheets  201  to move upstream, while the cone seat  202  would move upstream at the same speed, so that the expansion cone sheets  201  would not disengage with the cone seat  202  and thus would not be closed. In one preferable embodiment, the locking connector  801  is further provided with a locking member. As shown in  FIG. 8 , the locking member comprises a radially outward recess  212  provided on the locking connector  801  and a locking block  214  connected to the recess  212  via an elastic member  213 . In the initial state, the step portion  209  would press the locking block  214 , and as the casing is expanding, the step portion  209  moves downstream and leaves the locking block  214 , which, under the action of the elastic member  213 , radially and outwardly projects and presses an upstream end portion of the step portion  209 , so that in lifting the expansion tool  10 , the locking block  214  would constraint the fixing cone  208  to avoid the expansion cone sheets  201  from disengaging with the cone seat  202 . 
         [0055]    In the following the method of expanding the casing  11  with the expansion tool  10  will be described according to  FIGS. 1 to 15 , comprising: 
         [0000]    Step I: as shown in  FIG. 9 , drilling an openhole  1101  and putting a first level casing  1102  down into the openhole  1101  to carry out first well cementation, wherein before the first level casing  1102  is put down into the openhole  1101 , a downstream portion  1103  of the first level casing  1102  is pre-expanded and a downstream end portion thereof is sealed;
 
Step II: as shown in  FIG. 10 , putting a drilling tool  1201  into the downhole to pierce the first level casing  1102  and continue the drilling, wherein according to actual conditions, openhole expansion operations can be carried out to provide an openhole space for realizing monohole well drilling, and wherein a side wall of the pierced first level casing  1102  forms an overlapping portion  1202  downstream thereof;
 
Step III: as shown in  FIG. 11 , putting a second level casing  1301  and the expansion tool  10  into the downhole and arranging an upstream portion of the second level casing  1301  in the overlapping portion  1202  of the first level casing, wherein a downstream end portion of the second level casing  1301  is sealably provided with cementing accessories  1302  and a downstream portion of the second level casing  1301  comprises a pre-expanded expansion promoter region  1303 , upstream of which is provided with an overlapping area  1304 , and the expansion tool  10  is configured that the fixing cone  208  thereof is arranged in the expansion promoter region  1303 , the cementing accessories  1302  being apparatuses such as float collars, float shoes, etc., which are all well known by one skilled in the art and will not be repeated.
 
Step IV: carrying out well cementation and putting a drill shaft rubber plug  305  into the downhole, wherein the drill shaft rubber plug  305  engages with the rubber plug  301  of the expansion tool so as to seal the downstream end portion of the central tube  101 ; in one embodiment, slow setting cement is selected for the well cementation, so that the expansion of the casing  11  can be ensured to be completed before the slurry thickening of the cement;
 
Step V: as shown in  FIG. 12 , filling liquid in the expansion tool  10 , and pressing the expansion cone sheets  201  to expand them, so that the second level casing  1301  is expanded by the expansion tool  10 , and as shown in  FIGS. 13 and 14 , lifting the downhole casing expansion tool  10  so as to expand an outer diameter of the second level casing  1301  to be equal to an inner diameter of the first level casing  1102  and to enable the second level casing  1301  to fixedly connect to the overlapping portion  1202  of the first level casing  1102 ;
 
Step VI: as shown in  FIG. 15 , repeating Step II to Step V and connecting an upstream portion of the downstream casing to the overlapping area of the upstream casing so as to complete multilevel casing monohole expansion operations.
 
         [0056]    It should be noted that, in the method according to the present disclosure, except that the downstream portion  1103  of the first level casing  1102  is pre-expanded, other casings are not pre-expanded before they are put into the downhole. Instead, they are expanded for once by the expansion tool  10 , in which the inner diameters thereof are expanded to be equal to the inner diameter of the first level casing  1102 . 
         [0057]    By the method of the present disclosure, monohole expansion of multi-casings is realized, i.e., as shown in  FIG. 15 , after expansion, the inner diameters of all the casings can be equal to the inner diameter of the upstream casings. Therefore, well drilling with no loss in well diameter (i.e., monohole well drilling) can be realized. Moreover, drills of the same specification can be adopted throughout the construction because the borehole diameter is kept consistent, thus reducing the cost in well drilling and completion and the efficiency thereof is improved. 
         [0058]    In one embodiment, the fixing cone  208  of the expansion tool  10  is constructed in such a way that the outer diameter thereof is smaller than the inner diameter of the expansion promoter region  1303  but larger than the inner diameter of the second level casing  1301  arranged upstream of the expansion promoter region  1303 , so that before the expansion construction, the expansion tool  10  can be conveniently engaged with the second level casing  1301 , which is beneficial for the expansion construction. In addition, as recited above, the inclination of the side surface of the fixing cone  208  is smaller than the inclination of the outer surface of the expansion cone sheets  201 , and the largest diameter of the fixing cone  208  is still smaller than the largest diameter of the expanded expansion cone sheets  201 . In carrying out the expansion operations, the fixing cone  208  would first slightly expand the casing  11  and the expanded expansion cone sheets  201  would expand the casing  11  to a required monohole size, so that the fixing cone  208  and the expansion cone sheets  201  actually realize a double-level expansion, which can facilitate the successful operations of the expansion construction. 
         [0059]    As shown in  FIG. 14 , to ensure that the construction can be successfully carried out, in Step V, after each level casing is expanded, an unexpanded portion  1305  at the downstream portion of the casings should be removed, which can be realized, for example by throwing milling tools in the downhole. In order to improve the connection tightness of multi-level casings, a fourth sealing member  1203 , such as a rubber tube or a soft metal member, etc. is further provided between overlapping casings. After being expanded and overlapped, the casing  11  is compressed between two casings to realize the sealing tightness. A filler  1306  for separating cement is provided on an outer side of the casing  11  in the overlapping area  1304 . In one embodiment, the filler  1306  is a compressible material, such as air (i.e., a cavity  1307  for separating cement is provided on an outer side of the overlapping area  1304  of the second level casing  1301 ). During the well cementing and expansion operations, the cavity  1307  can separate cement, i.e., no cement ring can be formed in the cavity  1307 , so that working spaces are provided for the expansion and overlapping of the second level casing  1301  and the overlapping area  1304  and outside cement rings are prevented from constraining the expansion thereof. 
         [0060]    Although the present disclosure has been discussed with reference to preferable examples, it extends beyond the specifically disclosed examples to other alternative examples and/or use of the disclosure and obvious modifications and equivalents thereof. The scope of the present disclosure herein disclosed should not be limited by the particular disclosed examples as described above, but encompasses any and all technical solutions following within the scope of the following claims.