Patent Publication Number: US-9416643-B2

Title: Selective fracturing system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to United Kingdom Patent Application No. GB1204100.0 filed on Mar. 8, 2012, the contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a method of fracturing (fracing) a formation surrounding a wellbore. 
     BACKGROUND OF THE INVENTION 
     Typical fracturing systems move a fracture sleeve to uncover ports in a completion string. In a conventional fracture system, this is achieved by landing a dropped ball on a ball seat attached to the sleeve. Once a sufficient pressure differential is achieved across the ball, the seat and the attached fracturing sleeve are moved axially to uncover the ports. Fracturing fluid ibs pumped downhole and out into the surrounding formation through the ports. In order to fracture a multi-zone well, the ball seats decrease in diameter from the heel to the toe of the well. The smallest ball is dropped first and passes through all the larger ball seats until it lands on the seat closest to the toe of the well. Once the first zone has been successfully fractured, successively larger balls can then be dropped to initiate fracture port opening for each subsequent fracture zone. 
     The ever decreasing ball seats have several known disadvantages. The restrictions in inner diameter through the ball seats have a negative impact on the effectiveness of the fracture closest to the toe of the well. This disadvantage can be overcome by the use of powerful pumps to transmit fracturing fluid through the narrow bore, although this is costly. 
     Additionally, once fracturing operations are complete, the balls must be removed from the system before production can begin. Existing methods for removing the balls involve drilling out the balls and seats, flowing the balls back to surface and/or using dissolvable balls. Each of these methods is time consuming and pose a risk and/or limitation to production of hydrocarbons. 
     The present invention aims to alleviate at least some of the aforementioned disadvantages. 
     It is an object of at least one aspect of at least one embodiment of the present invention to seek to obviate or at least mitigate one or more problems and/or disadvantages in the prior art. 
     According to a first aspect of the present invention there is provided a method of fracturing a formation surrounding a well bore comprising the steps of:
         (i) providing a tubular including at least two portions, each portion comprising an annulus isolation means, a selective flow path between the interior and the exterior of the tubular and a throughbore isolation means;   (ii) running the tubular into the wellbore;   (iii) isolating an annulus between the exterior of the tubular and the wellbore to thereby create at least two isolated zones;   (iv) electing any zone to be fractured;   (v) remotely opening the flow path in the portion of tubular corresponding to the selected zone;   (vi) remotely isolating the throughbore of the tubular by closing the throughbore isolation means in the portion of tubular corresponding to the selected zone; and   (vii) fracturing at least part of the formation surrounding the well. The method can also include the steps of:   (viii) remotely closing the flow path in the portion of tubular corresponding to the selected zone; and   (ix) opening the throughbore of the tubular by remotely opening the throughbore isolation means in the portion of tubular corresponding to the selected zone.       

     At least steps (iv)-(vi) can be repeated to thereby fracture at least part of the formation surrounding a different zone of the well. 
     Step (iv) can include the step of selecting an uphole zone to be fractured before a downhole zone to be fractured. 
     In this context, “uphole” can be construed as meaning closer to either a heel of the wellbore or the surface and “downhole” can be taken to mean closest to a toe of the well distal from the surface. 
     The throughbore is preferably isolated downhole of the flow path. Preferably the throughbore isolation means in each portion of tubular is located immediately downhole of the selective flow path. The throughbore isolation means is preferably positioned proximate the selective flow path in each portion of the tubular. 
     The method can further include the steps of: 
     providing a tubular including a plurality of portions; 
     creating a plurality of zones; 
     selecting one zone at a time in any order; and 
     successively fracturing at least a portion of the formation surrounding each selected zone. 
     The method can include: 
     providing a tubular including a plurality of portions; 
     creating a plurality of zones; 
     selecting one zone at a time in a sequential manner; and 
     sequentially fracturing at least a part of the formation surrounding each selected zone. 
     The method can include: 
     selecting one zone at a time in a sequential manner from a heel of the well towards a toe of the well, and 
     sequentially fracturing at least a part of the formation surrounding each selected zone. 
     In a deviated well, the heel of the well typically refers to the part of the well closest to the point of deviation. The toe of the well typically refers to the part of the well distal from the deviated portion. 
     Alternatively, the method can include: 
     selecting one zone at a time in a sequential manner from a toe of the well towards a heel of the well; and 
     sequentially fracturing at least a portion of the formation surrounding each selected zone. 
     The method of the invention has the advantage that it allows fracturing of a formation surrounding a wellbore to occur in any sequence, e.g. zones created can be fractured in any order. This allows fracturing of the well to occur sequentially from the heel to the toe of the well or from the toe to the heel of the well. Alternatively, fracturing of the zones can occur out of sequence and in any order. 
     The method of the invention allows the fracturing operation to be performed remotely. Thus all tools can be actuated and controlled from surface with no mechanical intervention required. “Remotely” in the context of the invention can mean controlling operations from the surface of the well without direct mechanical intervention downhole. 
     Remote downhole actuation can be achieved by any method selected from the group including communicating actuation commands to the downhole tool using: pressure modulations (detector in tool), nuclear source (detector in tool), chemical source (tracer in tool), radio source (reader in tool), acoustic source (hydrophone in tool), and magnetic source (reader in tool). 
     These examples of methods, by which the tools making up the tubular can be remotely actuated, require some detector within the downhole tool. The detector (or equivalent) within the tool can be electrically connected to a circuit capable of recognising the unique signal, processing that information and an actuation command and initiating actuation of the tool. One example of such a detector and electronic circuit embedded within a downhole tool is disclosed in published patent GB 2 434 820 B. 
     The method can include remotely actuating the selective flow path by communicating an actuation command downhole using at least one of the following remote actuation means selected from the group consisting of: radio frequency source; pressure sequencing; and timed actuation. 
     The selective fluid flow path between the interior and the exterior of the tubular can be provided by a downhole tool such as a sleeve valve that is movable to selectively open and close ports extending through a sidewall of the tubular to selectively create and obturate a flow path respectively. 
     The method can include remotely isolating the throughbore of the tubular by communicating an actuation command to the throughbore isolation means using at least one of the following remote actuation means selected from the group consisting of: radio frequency source, pressure sequencing and timed actuation. 
     The throughbore isolation means can be an isolation valve that selectively seals the throughbore. The throughbore isolation valve can be a flapper valve pivotable between a stowed position in which the throughbore is unobstructed and a deployed position in which the flapper substantially obturates the throughbore. 
     The method can include remotely actuating tools downhole by circulating objects downhole said objects being communicable with the tools when the tubular represents an open system such that fluids are flowable within the throughbore. 
     The tubular represents an open system when the tubular has at least one opening such that fluids sent downhole flow within the throughbore. 
     Objects that may be circulated downhole, said objects being communicable with the tools include: nuclear source, chemical source, radio source or magnetic source. Objects can be communicated downhole by gravity, pumping, adding them to fluid flow or any combination thereof. 
     One object circulated downhole can be a radio frequency identification tag. The downhole tools can be provided with readers (such as an antenna) coupled to an 
     electronic circuit within the tool for detecting the presence of a radio frequency identification tag. Such a system is described in patent GB 2 434 820 B. 
     The method can include remotely determining actuation from surface by actuating the tools downhole using signals from surface or providing tools with pre-programmed timers to actuate the tools when the tubular represents a closed system. 
     The tubular represents a closed system when there are no openings within the tubular, such that fluids cannot flow freely within the tubular but instead back up within the throughbore. 
     The signals from surface for remote actuation of the tools can include pressure or acoustic signals. The signals from surface can include pressure sequencing. 
     Remote actuation by pressure sequencing can include modulated pressure sequencing. A distinctive profile of pressure modulations can be created at surface by modifying the pressure in the tubing. Transducers embedded within downhole tools can be pre-programmed such that the downhole tool is actuable in response to a distinctive pressure modulation profile. 
     The method can be a method of fracturing and producing hydrocarbons from a formation surrounding a wellbore including the steps of producing from the selected zones following the fracturing steps. 
     Hydrocarbons can be produced through the selective flow path. Alternatively, each portion of tubular can include a production flow path between the interior and the exterior of the tubular. The production flow path can be selectively actuable by movement of a sleeve valve to selectively cover ports extending through the sidewall of the tubing. The production ports can be provided with a mesh to restrict entry of particles above a predetermined size. The mesh can be a sand screen. 
     The method can include reselling to a primary configuration at the end of each fracturing operation, in which primary configuration the selective flow path(s) are closed and the throughbore isolation means are open such that the throughbore is unobturated. 
     The apparatus can be run into the wellbore in the primary configuration. 
     The method can include the step of automatically returning to the primary configuration after a predetermined period of time. 
     Downhole tools can be pre-programmed to a default configuration. The default configuration can be the primary configuration. All downhole tools can return to the default configuration after a certain or predetermined period of time, e.g. 6 hours, 12 hours, 24 hours or 48 hours. At least some downhole tools can be provided with a timer connected to the electronic circuit to return the downhole tool to the default configuration. 
     The method can include remotely actuating the tools to adopt a default configuration. 
     The method can also include providing all tools with a timer pre-programmed to remotely actuate the tools in their default configurations. 
     In the default configuration the throughbore isolation means can be open and the fluid flow paths can be closed. 
     Step (vii) can include pumping fracturing fluid through the tubular and directing fracturing fluid through the fluid flow path to fracture at least part of the surrounding formation. Step (vii) can include diverting fluid through the fluid flow path using the throughbore isolation means as a diverter. 
     Step (vii) can include fracturing at least part of the formation surrounding the well by pumping a fracturing fluid into the formation. The method can include different fracturing methods such as hydraulic fracturing or acid fracturing. 
     Step (vii) can include pumping fracturing fluid having particles suspended therein into the formation. 
     Suitable fluids having particulates suspended therein can be referred to as proppant fracturing fluids. Step (vii) can include pumping proppant fracturing fluid into the formation so that the method of the invention is a method of proppant fracturing a formation. The proppant fracturing fluid can include a mixture or gel of water, proppant and thickening agent in concentrations adjusted for the specific application. The proppant can include sand or ceramic beads. The thickening medium can include xanthum gel. 
     The method can include pumping fracturing fluid having particles suspended therein until the fractured part of the formation is full of particles and pumped fracturing fluid backs up within the throughbore of the tubular. 
     The method can include clearing particles within the throughbore by opening another selective flow path in a different zone, and pumping fluids within the throughbore, which fluids urge the particles into a different zone. 
     At least one clean-up (non-production) zone with an associated selective flow path and isolation means can be created for accepting particles to be cleared. A clean-up zone can be created at the end of the well closest to the toe. 
     This method maximises proppant packing in a zone by fracturing the formation until the fractured formation is full of proppant (a situation know as ‘sand out’). 
     Annulus isolation means can typically be provided on either side of the selective flow path in each portion of tubular. Isolating the annulus can be achieved by actuating annulus isolation means. The annulus isolation means can be a packer. 
     The method can include remotely actuating an annulus isolation means to isolate the annulus. 
     The method can include actuating the annulus isolation means by communicating actuation commands to the tool using a method selected from the group consisting of: radio frequency source; flow activation; timed activation; chemical actuation; and pressure signature actuation. 
     As an alternative, the annular isolation means can be mechanically actuated. 
     The method steps (i)-(ix) can be chronological. However, it will be appreciated that the method steps may not be necessarily chronological. For example, isolating the annulus to create zones can be achieved by flow actuable packers that are arranged to actuate by flowing fracturing fluid over the packer; thus step (iii) may occur simultaneously with step (vii). 
     The method can include anchoring the tubular in the wellbore prior to commencement of the fracturing operation. The method can include anchoring the tubular in the wellbore between method steps (ii) and (iii). The method can include anchoring the tubular in the wellbore towards an upper end of the tubular. The method can include anchoring the tubular in the wellbore in at least one other location along the length of the tubular. The method can include anchoring the tubular in the wellbore towards a toe end of the well. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, which are: 
         FIGS. 1 a  to 1 j    successive schematic side views of a tool string utilised in accordance with the method and system of the present invention; and 
         FIG. 2  a schematic view of the tool string of  FIGS. 1 a  to 1 j    located within a wellbore. 
     
    
    
     DETAILED DESCRIPTION OF DRAWINGS 
     All the described embodiments utilise a tool string  20  illustrated schematically in  FIGS. 1 a  to  j   . Three methods involving different fracturing sequences will be described: 
     sequential fracturing from a heel  87  to a toe  88  of the well; 
     out of sequence fracturing; and 
     sequential fracturing from the toe  88  to the heel  87  of the well. 
     Each tool within the tool string  20  is configured and pre-programmed for remote actuation from surface according to the anticipated zone fracturing sequence. 
     Alternatively, each tool can be pre-programmed to respond to unique instructions, which enable any method of selective zone fracturing. 
     An uphole (uppermost in use) end of the tool string  20  shown in  FIG. 1 a    incorporates an upper anchoring tool in the form of a liner hanger packer  30 . The liner hanger packer  30  is actuable to hang the tool string  20  from a liner hanger  70  ( FIG. 2 ) within the wellbore. 
     At its leading end, shown in  FIG. 1 j   , the tool string  20  has a guide shoe  60  that is located closest to the toe  88  of the well in use. Adjacent the guide shoe  60  is a throughbore isolation means in the form of a lowermost flapper valve  54 ; a lower anchoring tool  40  in the form of a RokAnkor™ (Petrowell product reference: RokAnkor™ Slip System 54-RK-A0); and a selective flow path controlled by a fracture sleeve  52  movable to selectively uncover ports extending through the sidewall of the tool string  20 . 
     Between the anchoring tools at either end, the tool string  20  is divided into several portions  1  to  5  corresponding to the areas or zones of the formation that are required to be fractured. The tool string  20  for each zone is made up from a fracture sleeve  102 ,  202 ,  302 ,  402 ,  502 , a flapper valve  104 ,  204 ,  304 ,  404 ,  504  immediately downhole of the fracture sleeve  102 ,  202 ,  302 ,  402 ,  502 , a production tool  103 ,  203 ,  303 ,  403 ,  503 , and a packer  101 ,  110 ,  201 ,  210 ,  301 ,  310 ,  401 ,  410 ,  501 ,  510  at each end of the zone. 
     Each fracture sleeve  102 ,  202 ,  302 ,  402 ,  502  comprises ports (not shown) that are selectively uncovered to provide a fluid flow path between the interior of the fracture sleeve  102 ,  202 ,  302 ,  402 ,  502  and the exterior of the tool string  20 . A suitable fracture sleeve  102 ,  202 ,  302 ,  402 ,  502  has Petrowell product reference RFID Operated Frac Sleeve 63-RF-50. The fluid flow path created by the open fracture ports allows fracturing fluid to be pumped into the surrounding formation. Each fracture sleeve  102 ,  202 ,  302 ,  402 ,  502  contains an electronics pack, a pressure transducer, an antenna for reading radio frequency identification (RFID) tags, a timer and a motor for driving the sleeve. The internal electronics are pre-programmed to enable each fracture sleeve  102 ,  202 ,  302 ,  402 ,  502  to be controlled by a uniquely programmed RFID tag, modulated pressure sequences and/or a timer to instruct selective opening and closing of the ports. 
     The timer within each fracture sleeve  102 ,  202 ,  302 ,  402 ,  502  can be pre-programmed to reset the fracture sleeve  102 ,  202 ,  302 ,  402 ,  502  in a default configuration so that after a predetermined period of time e.g. 48 hours, in the absence of other instructions, the ports are covered to close the fluid flow path to the exterior of the tool string  20 . 
     The production tool  103 ,  203 ,  303 ,  403 ,  503  is in the form of an inflow control device (ICD). The ICD comprises a sleeve slidable to selectively cover ports (not shown). The sleeve is movable between a closed position where the ports are blocked and an open position to uncover the production ports and enable production of well fluids therethrough. Ports allowing fluid communication between the interior of the production tool  103 ,  203 ,  303 ,  403 ,  503  and the annulus are covered by a sand screen to restrict the size of particulate matter that can be produced through the production ports. 
     Internally, each production tool  103 ,  203 ,  303 ,  403 ,  503  is provided with an electronics pack, a pressure transducer, an antenna for reading radio frequency identification (RFID) tags, a timer and a motor for moving the sleeve to selectively uncover the ports. Actuation of each production tool  103 ,  203 ,  303 ,  403 ,  503  is controllable by modulated pressure sequences, RFID tags and/or an internal timer. Each individual production tool  103 ,  203 ,  303 ,  403 ,  503  is designed or programmed to improve the drainage profile across horizontal portions of the well to reduce water coning and maximise hydrocarbon recovery. 
     The tubing isolation valve is provided in the form of a flapper valve  104 ,  204 ,  304 ,  404 ,  504  or reservoir isolation valve (RIV). A suitable valve is manufactured by Petrowell under product reference: Reservoir Isolation Valve  63  RIV 0 . Each flapper valve  104 ,  204 ,  304 ,  404 ,  504  contains a flapper that is pivotable between a stowed position in which the throughbore is open and unobturated and a deployed position in which the flapper extends across the throughbore of the tubing to contact a sealing seat. Once sealed in the deployed position, the flapper is able to withstand high pressures expected within the throughbore  80  of the tool string  20 . 
     Internally, each flapper valve  104 ,  204 ,  304 ,  404 ,  504  is provided with several components sealed within the housing: an electronics pack; a pressure transducer; an antenna for reading radio frequency identification (RFID) tags; a timer; and a motor for selective pivoting of the flapper within the throughbore  80 . When in the deployed position, the flapper valve  104 ,  204 ,  304 ,  404 ,  504  substantially obturates the throughbore to hold pressure and also to act as a diverter to divert fracturing fluid out through the adjacent fracture sleeve ports. The flapper valve  104 ,  204 ,  304 ,  404 ,  504  is controllable by RFID tags, modulated pressure sequences and/or the internal timer. 
     The timer within each flapper valve  104 ,  204 ,  304 ,  404 ,  504  can be pre-programmed to reset the flapper valve  104 ,  204 ,  304 ,  404 ,  504  in a default configuration so that after a predetermined period of time e.g. 24 hours, in the absence of other instructions, the flapper opens the throughbore  80  of the tool string  20 . 
     Annulus isolation means are provided in the form of packers  101 ,  110 ,  201 ,  210 ,  301 ,  310 ,  401 ,  410 ,  501 ,  510 . The packers delimit each zone and ensure zonal isolation by substantially sealing an annulus  85  between the exterior of the tool string  20  and the open hole  87 . An open hole packer used in the present embodiment has Petrowell product reference: CSI Open Hole Permanent Packer 52-CS10. Each packer  101 ,  201 ,  210 ,  301 ,  310 ,  401 ,  410 ,  501 , 510  is actuable in response to a unique pressure modulated sequence P 2  and once actuated provides annular isolation between reservoir zones. 
     Before use, the electronics within the downhole tools are pre-programmed as required and the tool string  20  is made up as previously described. The tool string  20  has a run-in configuration in which all ports in the production tools  103 ,  203 ,  303 ,  403 ,  503  and fracture sleeves  102 ,  202 ,  302 ,  402 ,  502  are closed so that each sleeve is positioned to obturate the associated ports extending through the sidewall of the tool string  20 . All the flapper valves  104 ,  204 ,  304 ,  404 ,  504  are open so that the throughbore of the tubing string  20  is unobstructed. The leading end of the tool string  20  can be open to allow fluid circulation during run-in if desired. Such a system allows full circulation and well control capabilities through the guide shoe  60 . The tool string  20  is then run into the open hole  90 . Since all ports are closed during run in a downhole motor or reamer shoe can be added at the end of the tool string  20  if desired. 
     According to the present embodiment the open hole  90  deviates at the heel  87 . When the tool string  20  has reached the desired location and each portion of the tool string  20  is aligned adjacent the formation to be fractured, an RFID tag, Tag  1  (not shown) is circulated downhole. Tag  1  is pre-programmed to communicate with the flapper valve  54  closest to the toe  88  of the well. Tag  1  is pumped downhole with fluid and on reaching the flapper valve  54 , Tag  1  passes within the throughbore  80  of the tool and the antenna within the flapper valve  54  reads the instructions from Tag  1 . The instructions are processed by the electronics pack and a motor drives the flapper from the stowed to the deployed configuration. Thus the flapper valve  54  is actuated to close off the throughbore  80  and the tool string  20  now represents a closed system that can be pressured up as required. 
     Once the flapper valve  54  has been closed, the tool string  20  must be anchored in the wellbore. This is achieved by pressuring up the throughbore  80 , which is now a closed system following the closing of the flapper valve  54  with Tag  1 . The RokAnkor™  40  and the liner hanger  30  are actuable in response to a threshold pressure. (Alternatively, the RokAnkor™  40  and the liner hanger  30  can be actuated in response to a unique pressure pulse signature P 1 .) At surface an operator pressures up the throughbore  80  to the required threshold setting pressure and sets the liner hanger  30  and RokAnkor™  40  to anchor the tool string  20  to the liner  70  and in the open hole  90  towards the toe  88  of the well respectively. This allows the tool string  20  to be set in the correct position relative to the zones of interest. The liner hanger  30  and RokAnkor™  40  both function to anchor the tool string  20  in the open hole  90  to restrict excess lateral movement of the tool string  20  and improve the effectiveness of the packers. 
     Simultaneously or as a separate operation, the packers  101 ,  110 ,  201 ,  210 ,  301 ,  310 ,  401 ,  410 ,  501 ,  510  are actuated also using a threshold setting pressure (or alternatively, a pressure pulse actuation sequence P 2 ). This packer  101 ,  110 ,  201 ,  210 ,  301 ,  310 ,  401 ,  410 ,  501 ,  510  setting operation creates and isolates individual zones  1  to  5  in preparation for the zone-by-zone fracturing operation. 
     Each of the three described embodiments below use the apparatus of  FIGS. 1 a  to 1 j    made up in a tool string  20  described above and shown in the well in  FIG. 2 . Although the apparatus describes five production zones any of the following embodiments may include unlimited production zones. 
     1. Heel to Toe Fracture 
     For the sequential heel to toe method of fracturing a formation, zone  1  is the first zone of interest to be fractured. In order to fracture zone  1 , the ports in the fracture sleeve  102  need to be opened. 
     All fracture sleeves  102 ,  202 ,  302 ,  402 ,  502 ,  52  are pre-programmed to open in response to a unique modulated pressure sequence, P 3 . A timer in each fracture sleeve  102 ,  202 ,  302 ,  402 ,  502 ,  52  actuates the fracture sleeve after a predetermined period of time, e.g. one hour following receipt of the signal. A transducer within the tool detects the pressure modulations and on receipt of the unique signal, P 3 , the electronics pack instructs the motor within each tool to axially translate the sleeve. All flapper valves  104 ,  204 ,  304 ,  404 ,  504 ,  54  are pre-programmed to move the flapper from the stowed to the deployed position in response to the same pressure sequence P 3 . A timer in each flapper valves  104 ,  204 ,  304 ,  404 ,  504 ,  54  actuates movement of the flapper after a predetermined period of time e.g. one hour following receipt of the signal. 
     The operator at surface controls the pressure within the tubing string in line with modulated pressure sequence P 3  to instruct the opening of the ports of all fracture sleeves  102 ,  202 ,  302 ,  402 ,  502 ,  52  and simultaneously instruct all flapper valves  104 , 204 , 304 , 404 ,  504 ,  54  to isolate the throughbore of the tubing string  20 . Thus in the region of zone  1 , the fracture ports are open and the throughbore is blocked immediately downhole of the fracture ports. 
     Fracturing fluid is pumped downhole through liner hanger running tools and is directed out by the deployed flapper  104  through the open ports of the fracture sleeve  102  to fracture the formation surrounding zone  1 . Proppant fracturing fluid (with suspended sand) is used for the fracture of the present embodiment. The pressurised fluid cracks the formation surrounding zone  1  and the sand is simultaneously packed within the cracks to prevent closure of the cracks. Fracturing fluid is pumped downhole until a pressure spike at surface indicates that the zone is full of sand (or after a predetermined proppant volume has been pumped downhole). At this point, the fracturing operation of zone  1  is complete. 
     With the ports of the fracture sleeve  102  open, the tool string  20  is an open system and objects can be circulated within the first portion of the throughbore  80 . Once the operator recognises that the formation of zone  1  is packed with sand, an electronic tag T 2  is pumped downhole. T 2  is pre-programmed to communicate with and instruct the fracture sleeve  102  to close the ports extending through the sidewall of the tubing string  20 . This closes the circulation path to zone  1  by closing the ports of the fracture sleeve  102 . Closure of the ports by the fracture sleeve  102  results in the tool string  20  once again reverting to a closed system that can be pressured as required. If excess sand has backed up within the interior of the tubing to substantially block the tubing  20  (a situation known as ‘sand-out’) such that the tag, T 2  cannot reach the fracture sleeve, a unique modulated pressure sequence can be controlled at surface to instruct closure of the fracture sleeve  102 . 
     An operator at surface then generates a unique pressure modulation sequence P 4  within the tool string  20  and the flapper  104  is pre-programmed to return to its stowed position and open in response to the pressure sequence P 4 . 
     Zone  1  has now been successfully fractured, the ports of the zone  1  fracture sleeve  102  are closed and the zone  1  flapper  104  is open. Zone  2  is the next zone of interest in a sequential heel  87  to toe  88  fracture. The same operation is now repeated for zone  2 . 
     The zone  2  fracture sleeve  202  is already open and the zone  2  flapper  204  is already closed in preparation for the fracturing of zone  2 . Fracturing fluid is pumped downhole to fracture the formation surrounding zone  2 . Any excess sand within the throughbore  80  following ‘sand-out’ of zone  1  is forced into the formation of zone  2 . Thus, the throughbore  80  around zone  1  is cleared by forcing sand into the formation at zone  2 . 
     Once the fracturing of zone  2  is complete (for example when a calculated proppant volume has been pumped downhole or if sand-out occurs), an electronic tag, T 3  is circulated downhole. T 3  is pre-programmed to instruct closure of the ports and flow path of the fracture sleeve  202 . The tool string once again represents a closed system. Another unique pressure sequence, P 5  initiated within the tubing gives a unique instruction to open the flapper  204  and unblock the throughbore  80 . Zone  2  has then been fractured successfully. 
     The same sequence can be repeated for all subsequent zones  3  to  5  sequentially from the heel to the toe of the well. Following fracturing of each zone a tag is circulated to close the fracture ports and another pressure sequence P(x+1) is used to open the flapper valve. This method can be used as many times as required and there is no limit to the number of zones that can be fractured in a well arranged to operate in this way. The fact that no bore restrictions are present means that any number of tools can be arranged in series to allow formation fracturing by the method of the invention. 
     Once the final zone has been fractured, (zone  5  according to the present embodiment) the zone  5  fracture sleeve  502  is instructed to close the ports and the flapper  504  is moved into the stowed position so that the throughbore of the tubing  20  has no obstructions. The final fracture sleeve  52  of the tool string  20  by the pressure sequence P 3 . Fluid is pumped downhole through the tubing string to entrain excess sand that has accumulated within the throughbore  80  of the tubing  20 . Excess sand is pushed into the formation surrounding fracture sleeve  52 . This sacrificial zone is used to clean the tubing string  20  of sand in preparation for the production of formation fluids through the production ports of the production tools  103 ,  203 ,  303 ,  403 ,  503 . Thus, the sand is cleared from the throughbore without any separate remedial action. A further tag, T 7  is dropped to instruct closure of the ports of the fracture sleeve  52 . 
     Once the sand has been flushed out of the throughbore  80  of the tool string  20 , the fracturing operation and subsequent clean up is complete and the upper completion can be installed in the well. 
     Ports of the production tool are now required to be opened in preparation for the production of hydrocarbons therethrough. 
     The production tools  103 ,  203 ,  303 ,  403 ,  503  can be pre-programmed to respond to the pressure sequence, P 9  to actuate the tool and cause opening of the production ports. Production of hydrocarbons is thus initiated. Ports of the production tools are surrounded by a sand screen mesh to restrict ingress of sand and larger particles with the hydrocarbons. The well can be produced one zone at a time, with the closure and opening of production ports achieved by predetermined time delays from receipt of the pressure sequence P 14 . 
     The method of the present invention maximises efficiency by allowing for ‘sand-out’ so that an operator knows with certainty that a particular zone is packed to capacity with sand. 
     One advantage of the method is that fracture sand from ‘sand-out’ of a higher zone is simply pumped into the subsequent fracture zone, meaning that no separate operation need be performed to clear the sand from within the tubing  20 . Thus, the above embodiments remove the requirement for any clean up. Further sacrificial zones can be spaced along the well if required where multiple zones are being successively fractured. These zones can capture accumulated sand so that no separate cleanup operation is necessary. 
     2. Toe to Heel Fracturing 
     According to a second embodiment of the present invention, the method enables the well to be produced sequentially from the toe  88  to the heel  87  of the well. The fracturing operation is controlled in the reverse direction with remote actuations controlled by pressure sequence when the tool string  20  represents a closed system and a tag where the tool string  20  represents an open system. 
     For the sequential toe to heel method of fracturing a formation, zone  5  is the first zone of interest to be fractured. In order to fracture zone  5 , the ports in the fracture sleeve  502  need to be opened. The fracture sleeve  502  is pre-programmed to open in response to a unique pressure sequence, P 3 . The flapper valve  504  is pre-programmed to close in response to the same pressure sequence, P 3 . The operator at surface controls the pressure within the tubing string in line with pressure sequence P 3  to open the ports of the fracture sleeve  502  and simultaneously close the flapper  504  to isolate the throughbore of the tubing string  20  in the region of zone  5 . 
     Proppant fracturing fluid is pumped downhole through liner hanger running tools and is directed out by the closed flapper  504  through the open ports of the fracture sleeve  502  to fracture the formation surrounding zone  5 . A calculated volume of fracturing fluid is pumped downhole. At this point, the fracturing operation of zone  5  is complete. 
     With the ports of the fracture sleeve  502  open, the tool string  20  represents an open system and objects can be circulated within the throughbore  80 . An electronic tag T 2  is pumped downhole. T 2  is pre-programmed to move the fracture sleeve  502  to close the ports extending through the sidewall of the tubing string  20 . This closes the circulation path to zone  5  by closing the ports of the fracture sleeve  502 . Closure of the ports by the fracture sleeve  502  results in the tool string  20  once again reverting to a closed system that can be pressured as required. If excess sand blocks the throughbore such that the tag, T 2  cannot be circulated downhole, a timer can respond to close the ports of the fracture sleeve after a predetermined period of time e.g. 48 hours. 
     Zone  5  has now been successfully fractured and an operator at surface generates a unique pressure sequence P 4  within the tool string  20  and the flapper  504  is pre-programmed to open in response to the pressure sequence P 4 . Zone  4  is the next zone of interest in a sequential toe  88  to heel  87  fracture. The same operation is now repeated for zone  4 . 
     The next fracture sleeve  402  is responsive to the same pressure sequence, P 4  and is pre-programmed to move to uncover the fracture ports. Flapper  404  is pre-programmed to close in response to the same pressure sequence, P 4  after a short time delay. The zone  4  fracturing operation begins. Once the operator has remotely actuated the opening of fracture ports and closing of the throughbore in zone  4  by initiating the pressure sequence P 4 . Fracturing fluid is pumped downhole to fracture the formation surrounding zone  4 . Once the fracturing of zone  4  is complete, an electronic tag, T 3  is circulated downhole. T 3  is pre-programmed to cause closure of the ports and flow path of the fracture sleeve  402 . Another unique pressure sequence, P 5  initiated within the tubing gives a unique instruction to open the flapper  404  and unblock the throughbore  80  and simultaneously open ports of the fracture sleeve  302  and close the zone  3  flapper  304  following a short time delay. 
     The same sequence can be repeated for all subsequent zones  3  to  1  sequentially from the toe  88  to the heel  87  of the well. For example pressure sequence P (X+1) is used to open the flapper from a previous fracture zone, open the fracture ports of the next zone to be fractured and close the flapper valve. Following fracturing of that zone a tag is circulated to close the fracture ports. This method can be used as many times as required and there is no limit to the number of zones that can be fractured in a well arranged to operate in this way. The fact that no bore restrictions are present means that any number of tools can be arranged in series to allow formation fracturing by the toe to heel method of the invention. 
     Once the final zone has been fractured, (zone  1  according to the present embodiment) the zone  1  fracture sleeve  102  closes the ports and the flapper  104  is opened so that the throughbore of the tubing  20  has no obstructions. 
     The final fracture sleeve  52  at the toe end  88  of the tool string  20  is pre-programmed to respond to a unique pressure sequence, P 9  to move the fracture sleeve  52  and open the ports to the surrounding formation. Fluid can then be pumped downhole and excess sand that has accumulated within the throughbore  80  of the tubing  20  is pushed into the formation surrounding fracture sleeve  52 . This cleans the tubing string  20  of sand in preparation for the production of formation fluids through the production ports. Thus, the next step in the operation clears up the sand without any separate remedial action. 
     Once the sand has been flushed out of the throughbore of the tool string  20  a tag, T 7  is sent downhole to instruct closure of the fracture ports by the sleeve  52 . The upper completion is then installed. 
     Ports of the production tools can be opened using a unique pressure sequence either individually or collectively with or without time delays. 
     3. Any Sequence Fracturing 
     The method of the invention also enables fracturing and production in any desirable sequence. One random out of order fracturing sequence will now be described as a second embodiment of the invention. 
     Run-in and set-up of the tool string  20  have been previously described with reference to the first embodiment of the invention. Again, the lowermost flapper valve  54  is closed and the RokAnkor™  40 , liner hangar  30  and packers  101 ,  110 ,  201 ,  210 ,  301 ,  310 ,  401 , 410 ,  501 ,  510  are set as described previously. 
     The zone selected to be fractured first, for example zone  3  is targeted by sending a unique pressure signal, Px, to open the zone  3  fracture ports by movement of the fracture sleeve  302  and close the zone  3  flapper valve  304  to divert flow of fracturing fluid out through the fracture ports into the formation surrounding zone  3 . An electronic tag, Ty, having a unique identification is circulated through the fracture tool  302  and the fracture tool  302  is pre-programmed to read tag, Ty, and respond to its command to close the fracture ports. A unique pressure signal, P (x+1), opens the zone  3  flapper valve  304 . 
     Any zone can be subsequently selected for the fracturing operation. When the system is closed, the fracture tools and flapper valve must be actuated using a unique predetermined pressure sequence. When circulation is possible within the tool string  20  (for example when the fracture ports are open) a suitably pre-programmed tag can be circulated downhole with a command for the appropriate tool. In this way any zone can be fractured in any chosen sequence. 
     The final step in the sequence is to clear excess sand from the throughbore  80  by moving the fracture sleeve  52  to open ports extending through the sidewall of the tubing string  20  using pressure sequence P 13 . Again, this enables excess sand to be pumped through the ports of the fracture sleeve  52  into the sacrificial zone surrounding the end of the tubing string  20 . 
     As a default, all tools for any of the described fracture sequences, can be provided with a timer and in the event that signals are not received or transmitted. Each tool can be pre-programmed to perform a certain function after a predetermined length of time. For example, the lowermost flapper valve  54  is pre-programmed to close off the throughbore  80  twenty four hours after initial run-in, so that in the event that the tag(s) fail to deliver the close command, the end of the tool string  20  can be closed off to allow the pressure operations to proceed that lead to fracturing and production of the well. Other flapper valves  104 ,  204 ,  304 ,  404 ,  504  can be provided with a timer to ensure that the flapper remains in the stowed position and they do not present an obstruction in the throughbore in the event that an actuation signal is not received. Each fracture sleeve  102 ,  202 ,  302 ,  402 ,  502  can be provided with a default timer so that the sleeve closes after a predetermined period of time. This allows pressure operations to proceed in the event that an actuation command is not properly received. 
     The above described method and apparatus is preferable to conventional methods since the full bore diameter is available for the subsequent production of hydrocarbons once the well is brought on-stream and there are no obstructions to choke the flow within the inner diameter of the tubing string. Lack of restrictions in the bore place no limits on the length of the well and the multiple zones that could potentially be produced. 
     Another advantage of the present invention is that the method allows greater flexibility. For example once the apparatus has been run into the well an operator can vary the fracturing sequence since the apparatus is not necessarily limited to a particular configuration at set-up or as run into the well. 
     The ability of the system to be operated remotely and the default operations that can be pre-programmed allow a high level of control over the well for both the fracturing and subsequent production of hydrocarbons, when compared with existing systems. 
     Although the methods described above use five zones, it will be appreciated that the methods can be used to fracture and produce from wells having any number of zones and is especially advantageous for wells having multiple zones. Additionally, the tool string  20  can contain as much blank tubing as required to space the zones according to the formation of interest. 
     All described embodiments are particularly advantageous to proppant fracturing where proppant such as sand suspended in fluid is pumped into the formation, although the method applies equally to other methods of fracturing, such as hydraulic fracturing and acid fracturing. Where hydraulic or acid fracturing methods are performed, the inflow control device can be omitted and fluids can be produced through the fracturing ports. 
     Modifications and improvements can be made without departing from the scope of the invention. The tool string  20  can be made up using additional tools and blank lengths of pipe to give the functionality desired for a particular application and provide a tool string  20  having zones of a length best suited to the characteristics of the particular formation to be fractured. The tool string  20  can be provided with a power reamer  61  at its leading end substituted in place of the guide shoe  60 . 
     Fracturing fluid can include sands or beads in suspension or any other suitable fluid for fracturing and packing out the fractured formation.