Abstract:
A method is disclosed of connecting a first borehole to a second borehole, the boreholes being formed in an earth formation and extending at a mutual distance. The method comprises inserting a volume of hardenable fluidic material into a space in the earth formation extending between the first and second boreholes, and allowing the hardenable fluidic material to harden so as to form a body of hardened material between the first and second boreholes. At least one fluid channel is created in the body of hardened material, each fluid channel providing fluid communication between the first borehole and the second borehole.

Description:
PRIORITY CLAIM 
       [0001]    The present application claims priority from PCT/EP2008/053617, filed 27 Mar. 2008, which claims priority from EP Application 07105066.0, filed 28 Mar. 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a method of connecting a first borehole to a second borehole, said boreholes being formed in an earth formation and extending at a distance from each other. 
         [0003]    In operations for the production of oil or gas from a subterranean reservoir at a remote location, such as an offshore location, it is general practice to produce hydrocarbon fluid from one or more wells to a production platform located at the site of the wells. The production platform can be fixedly installed on the seabed, such as a jack-up platform or a gravity based platform, or it can be floating at the sea surface, such as a floating production storage and offloading (FPSO) vessel. Generally, one or more wells are drilled into the reservoir from directly below the platform, and hydrocarbon fluid is produced from the wells through risers extending between the seabed and the platform. Most offshore fields also involve one or more satellite wells located at a distance from the platform and tied to the platform by pipelines on the seabed. 
         [0004]    Offshore platforms, especially those in deep water, attribute considerably to the costs of exploiting offshore hydrocarbon reservoirs. In some instances, installing an offshore platform may even be prohibitive to economical exploitation of the reservoir. In view thereof it has been proposed to use relatively small subsea production systems instead of fixed or floating platforms for producing oil or gas from offshore fields. Such subsea systems are arranged to receive hydrocarbon fluid from one or more wells to initially separate the produced stream into a gas stream and a liquid stream, and to pump the separated streams to an onshore production facility. Alternatively the produced fluids can be transported in multi-phase flow from the subsea system to an onshore facility through a single pipeline, hence without initial separation of gas from liquid. 
         [0005]    Although conventional technologies can be applied for the exploitation of some remote hydrocarbon fluid reservoirs, a variety of applications require improved systems and methods to produce hydrocarbon fluid in an economical way. For example, the production of hydrocarbon fluid from reservoirs located below Arctic offshore waters can prove difficult, if not impossible, with conventional technologies. Generally Arctic conditions prohibit continued operation of offshore facilities throughout the year, for example because the sea is frozen a large part of the year. For this reason, conventional offshore drilling and/or production platforms are considered inadequate for continued operation throughout the year in Arctic conditions. Moreover, exposure of pipelines to scouring from floating ice and/or hazards associated with unstable permafrost, can be prohibitive. 
         [0006]    US patent application 2004/0079530 A1 discloses a method of interconnecting subterranean boreholes, whereby a first borehole extends into an offshore hydrocarbon reservoir, and whereby a second borehole is drilled from a surface location horizontally displaced from the surface location of the first borehole such that a lower, substantially horizontal, section thereof intersects the first borehole to provide fluid communication between the first and second boreholes. 
         [0007]    A problem of the known method of interconnecting subterranean boreholes relates to the difficulty to drill the second borehole such that it intersects the first borehole. Moreover, the two boreholes can be unaligned at the point of intersection so that it becomes difficult, or impossible, to install a liner at the location of the intersection. Also, the two boreholes may have to be drilled at an undesirably high inclination angle relative to each other to create the intersection. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore an object of the invention to provide an improved method of interconnecting first and second boreholes formed in an earth formation, which method overcomes the problems of the prior art. 
         [0009]    In accordance with the invention there is provided a method of connecting a first borehole to a second borehole, said boreholes being formed in an earth formation and extending at a mutual distance, the method comprising: 
         [0010]    inserting a volume of hardenable fluidic material into a space in the earth formation extending between the first and second boreholes, and allowing the hardenable fluidic material to harden so as to form a body of hardened material between the first and second boreholes; and 
         [0011]    creating at least one fluid channel in said body of hardened material, each fluid channel providing fluid communication between the first borehole and the second borehole. 
         [0012]    With the method of the invention it is achieved that there is no longer a need to drill the boreholes exactly so that one borehole intersects the other borehole. Moreover it is achieved that there is no abrupt change of direction of the boreholes at the location where the connection is made, so that a liner (or casing) can be installed more easily at said location. Also, due to the relative hardness of the body of hardened material, there is a reduced risk of erosion at the location of the connection during continued production of hydrocarbon fluid through the fluid channel(s) formed therein. 
         [0013]    Suitably, said space provides fluid communication between the first borehole and the second borehole. For example, said space can include a plurality of pores of the earth formation. 
         [0014]    In a preferred embodiment, the method of the invention comprises creating a cavity in the earth formation, said cavity forming at least a part of said space. 
         [0015]    To reduce the size of the cavity, suitably the cavity extends between a selected location of the first borehole and a selected location of the second borehole, and wherein said mutual distance of the boreholes is minimal from the selected location of the first borehole to the selected location of the second borehole. 
         [0016]    An exemplary way of creating the cavity in the earth formation, is to create at least one flow passage in the earth formation, each flow passage providing fluid communication between the first borehole and the second borehole. Such flow passage can be created, for example, by perforating the earth formation using a shaped charge. To enlarge the diametrical size of the flow passage, suitably fluid is induced to flow through the flow passage so as to erode the earth formation surrounding the flow passage to form the cavity. 
         [0017]    Each fluid channel is preferably formed by perforating the body of hardened material. 
         [0018]    In an advantageous embodiment of the method of the invention, the first borehole extends into a reservoir zone of the earth formation containing hydrocarbon fluid. Suitably the reservoir the first borehole extends substantially parallel to a boundary of the reservoir zone. 
         [0019]    To prevent an undesired high drawdown of reservoir fluid at the location of the connection of the two boreholes, it is preferred that the first borehole is provided with a liner passing from outside the body of hardened material to within the body of hardened material. 
         [0020]    The hardenable material can be selected, for example, from cement and resin such as a phenolic-based thermoset plastic resin. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention will be described hereinafter in more detail and by way of example, with reference to the accompanying drawings in which: 
           [0022]      FIG. 1  schematically shows an embodiment of two wellbores interconnected with the method of the invention; 
           [0023]      FIG. 2  schematically shows a detail of the embodiment of  FIG. 1 ; 
           [0024]      FIG. 3  schematically shows cross-section  3 - 3  of  FIG. 2  during an initial stage of the method of the invention; 
           [0025]      FIG. 4  schematically shows cross-section  3 - 3  of  FIG. 2  during a subsequent stage of the method of the invention; 
           [0026]      FIG. 5  schematically shows cross-section  3 - 3  of  FIG. 2  during a further stage of the method of the invention; 
           [0027]      FIG. 6  schematically shows cross-section  3 - 3  of  FIG. 2  during a final stage of the method of the invention; and 
           [0028]      FIG. 7  schematically shows cross-section  7 - 7  of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    Referring initially to  FIG. 1 , there is shown a first wellbore  1  and a second wellbore  2  formed in an earth formation  3  that includes a reservoir zone  4  containing hydrocarbon fluid. First wellbore  1  extends from a drilling rig  6  at surface into the earth formation  3  such that a lower section  8  of the first wellbore  1  extends inclined into the reservoir zone  4 . Second wellbore  2  extends from a hydrocarbon fluid production facility  9  at surface into earth formation  3  whereby a lower section  10  of the second wellbore extends substantially horizontally, or deviated, into reservoir zone  4 . Lower sections  8 ,  10  of the respective first and second wellbores  1 ,  2  do not directly intersect each other, but extend at a distance from each other whereby the shortest distance therebetween is about one or several meters. The area in which first and second wellbores  1 ,  2  cross each other, is indicated by reference sign ‘A’. 
         [0030]    The area ‘A’ is shown in more detail in  FIGS. 2 and 3 , wherein  FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 2 . First wellbore  1  is provided with a casing  12  extending to about the bottom of wellbore  1 , and second wellbore  2  is provided with a liner  14  extending in lower wellbore section  10 . Liner  14  has a plurality of inlet openings (or perforations)  16  to allow hydrocarbon fluid from the reservoir zone  4  to flow into liner  14 . However a portion  18  of liner  14  extending near first wellbore  1  is solidly formed, that is, the liner portion  18  is not provided with inlet openings (as shown in  FIG. 2 ). Furthermore, a portion of casing  12  nearest second wellbore  2  is provided with a plurality of primary perforations  20 . Primary perforations  20  extend further through the earth formation surrounding casing  12  and liner  14  so as to provide fluid communication between wellbore  1  and wellbore  2 . 
         [0031]    In  FIG. 4  is shown the area ‘A’ after a cavity  22  has been formed in the earth formation. Cavity  22  encloses a portion of liner  14  and extends to casing  12 , at the location thereof where primary perforations  20  are formed. 
         [0032]    In  FIG. 5  is shown the area ‘A’, in the view along line  3 - 3  of  FIG. 2 , after cavity  22  has been filled with a body of cement  24  or other substantially impermeable material. 
         [0033]    In  FIGS. 6 and 7  is shown the area ‘A’ after a series of secondary perforations  26  have been formed in casing  12 , which extend further through the body of cement  24  and liner  14  so as to provide fluid communication between wellbore  1  and wellbore  2 . 
         [0034]    During normal operation, first wellbore  1  is drilled such that the lower section  8  thereof crosses lower section  10  of second wellbore  2  at a relatively short distance, for example a distance between 0.2-2 meters. A perforating gun (not shown) may then be lowered into first wellbore  1  and operated so as to form primary perforations  20  which extend through casing  12 , earth formation  3  and liner  14  so as to provide fluid communication between first wellbore  1  and second wellbore  2  (as shown in  FIGS. 2 and 3 ). 
         [0035]    In a subsequent step, a stream of liquid, such as brine or drilling fluid, is pumped from surface into the first wellbore  1 . The stream of liquid passes into the lower wellbore section  8 , and flows from there via the primary perforations  20  into the lower section  10  of the second wellbore  2 . The stream of liquid is then discharged from the second wellbore  2  through the surface production facility  9 . The stream of liquid flows at high velocity through the primary perforations  20  and thereby erodes the rock material around the perforations  20 . Upon continued pumping of the stream of fluid, virtually all rock material around the primary perforations  20  erodes away so that, as a result, the cavity  22  is formed in the earth formation  3  (as shown in  FIG. 4 ). 
         [0036]    During a next phase, cement is pumped into the lower section  8  of the first wellbore  1 , and thence via the primary perforations  20  of the casing  12  into the cavity  22 . Upon hardening of the cement, the body of hardened cement  24  forms in the cavity  22  (as shown in  FIG. 5 ). 
         [0037]    A perforating gun (not shown) is then lowered into the first wellbore  1  and operated so as to form the secondary perforations  26  which extend through the casing  12 , the body of hardened cement  24 , and the liner  14  so as to provide fluid communication between the first wellbore  1  and the second wellbore  2  (as shown in  FIG. 6 ). 
         [0038]    The sets of primary perforations  20  and the sets of secondary perforations  26  can be shot with the same perforating gun, however it may be preferred to use different perforation guns depending on the hardness of the rock to be penetrated (for the primary perforations  20 ) and the hardness of the cement to be penetrated (for the second perforations  26 ). 
         [0039]    Alternatively, a suitable abrasive jetting tool may be used to create the primary perforations and/or the secondary perforations by jetting a fluid stream containing abrasive particles against the rock formation and/or the body of cement. 
         [0040]    In this manner it is achieved that hydrocarbon fluid produced from the reservoir zone  4 , can flow from the second wellbore  2  to the first wellbore  1 , or vice versa, via the secondary perforations  26 . For example, if the second wellbore  2  extends below the sea, and the first wellbore  1  extends to an onshore surface location, produced hydrocarbon fluid can flow from the lower section  10  of the second wellbore  2 , via the secondary perforations  26 , into the lower section of the first wellbore  1  and from there to the onshore surface location. Also, both wellbores can be formed below the seabed. 
         [0041]    It should be noted that, by virtue of the absence of inlet openings in the liner, hydrocarbon fluid can only flow into the liner  14  at some distance from the body of cement  24 . It is thereby achieved that undesired high drawdown of hydrocarbon fluid from the reservoir zone  4  in the region near the body of cement  24 , is prevented. 
         [0042]    Instead of pumping cement into the cavity, a hardenable resin can be pumped into the cavity. Upon hardening of the resin, a body of hardened resin is formed in the cavity, whereafter the secondary perforations are formed in the body of hardened resin.