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FIELD OF THE INVENTION 
     The invention relates to a multilateral well and electrical transmission system. 
     BACKGROUND OF THE INVENTION 
     Numerous electrical and non-electrical power and communication systems are known for use in unbranched or multilateral oil and/or gas production wells. 
     U.S. Pat. Nos. 5,706,892; 5,706,896 and 5,721,538 disclose that a multilateral well may be equipped with a hardwired electrical or with a wireless communication system and that such a wireless system preferably transmits acoustic waves through a string of well tubulars such as the production tubing. Disadvantages of the known system are that installation of a wire tree in a multilateral well is a complex and expensive operation and that a wireless acoustic transmission system will suffer from high transmission losses and background noise. These disadvantages are particularly significant if the well is equipped with an expandable casing and/or production tubing. Around such an expanded well tubular there is hardly any or no annular space left for housing of the electrical cables and as a result of the physical contact between the expanded tubular and the surrounding formation acoustic signals will be dampened to a high extent. 
     Numerous other hardwired or wireless power transmission and communication systems are known, which have in common that they require complex and expensive equipment and that they are not suitable for use in multilateral wells. 
     U.S. Pat. No. 4,839,644 and European patent No. 295178 disclose a wireless communication system known as “Tucatran” which generates antenna currents in an unbranched well where the production tubing and surrounding well casing are electrically insulated from each other. The requirement of electrical insulation between the tubing and the casing is often difficult to accomplish in e.g. curved borehole sections and areas where brine is present in the tubing/casing annulus. International patent application WO80/00727 discloses another signal transmission system which utilizes an electrical circuit formed by a production tubing and a surrounding well casing. 
     U.S. Pat. No. 4,484,627, UK patent application No. 2322740 and International patent applications Nos. PCT/GB79/00158; PCT/GB93/01272 and PCT/EP96/00083 disclose other downhole electric transmission systems which utilize an externally insulated tubing in an unbranched well. 
     The present invention aims to overcome the disadvantages of the known transmission systems and to provide a downhole power and/or signal transmission system which can be used to transmit electrical power and/or signals throughout a multilateral well system in a safe and reliable manner even if the well comprises expandable well tubulars and without requiring complex wire trees or production tubing that are electrically insulated from the surrounding well casings. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a multilateral well and electric transmission system, which comprises a primary wellbore in which a primary well tubular is arranged and a branch wellbore in which a branch well tubular is arranged, wherein the branch well tubular is connected in an electrically conductive manner to the primary well tubular such that the primary and branch well tubulars form a link for transmission of electrical power and/or signals between the primary and branch wellbore. 
     Preferably, the primary and branch well tubulars form a link for transmitting low voltage power from a first pole of an electrical power source, which is electrically connected to the primary well tubular, to electrically powered equipment within the branch wellbore which is electrically connected to the branch well tubular. An electrical circuit is created by electrically connecting a second pole of the electrical power source and the branch well tubular(s) to the earth. It is also preferred that said equipment comprises a re-chargeable battery which is trickle-charged by the low voltage electrical power transmitted via the well tubulars. 
     Suitably low voltage power is transmitted as a direct current (DC) having a voltage of less than 100 V, preferably less than 50 V through the casing or production tubing of the primary well, which is imperfectly insulated to the surrounding earth formation by a surrounding cement or other sealing material, such as an addition curing silicone composition. 
     At the same time pulsed electromagnetic signals are transmitted which involve changes of voltage level oscillating around the DC voltage level of the well tubular at very low frequency (VLF), between 3 and 20 kHZ, or preferably at extremely low frequency (ELF), between 3 and 300 HZ. 
     The surface power generator and the downhole equipment or battery may have an electrode which is connected to the earth so that an imperfect electric loop exists between the power generator and the downhole equipment or battery. 
     It is also preferred that the branch well tubular is a radially expandable tubular which is made of an electrically conductive material and which is radially expanded within the branch well during installation and wherein an electrically conductive receptacle is arranged at or near the branchpoint such that the expanded branch well tubular is pressed into electrical contact with the receptacle as a result of the expansion process. 
     A particular advantage of the use of expandable tubulars at least in the branch wellbore is that as a result of the radial expansion process a surplus expansion is created in the expanded tubular which will ensure an intimate electrical contact between adjacent well tubulars of which the ends coaxially overlap each other. Such an intimate electrical contact is also made at the branchpoint between the expanded branch well tubular and the receptacle which may be formed by the primary well tubular itself or by a branched bifurcation element. 
     Suitably the primary and branch well tubulars are made of a formable steel grade and the branch well tubular is expanded during installation such that the expanded branch well tubular has an inner diameter which is at least 0.9 times the inner diameter of the primary well tubular, so that a substantially monobore multilateral well system is created which may have any desired amount of branches and sub-branches. 
     Preferably the electrically powered downhole well equipment comprises measuring and/or control equipment which is powered by a rechargeable lithium-ion high-temperature or other battery and/or a supercapacitor and/or a downhole energy conversion system such as a piezo-electrical system, turbine or downhole fuel cell and is mounted on an equipment carrier module in the form of a sleeve which is removably secured within the branch well tubular such that one electrode of the battery is electrically connected to the branch well tubular and another electrode of the battery is electrically connected to the subsurface earth formation surrounding the branch wellbore. 
     Suitably the sleeve spans an inflow area of the branch wellbore where the branch well tubular is perforated, the expandable clamps consist of a pair of expandable packers which seal off an annular space between the branch well tubular and sleeve near each end of the sleeve and wherein the sleeve is provided with one or more fluid inlet ports which can be opened and closed by one or more valves which are powered by the rechargeable battery. The triggering can be done via a downhole or surface actuated control system. 
     In many lengthy multilateral well systems it is also preferred that at least one of the primary and branch well tubulars is equipped with at least one electrical booster station which station spans an electrically non-conductive section of the well tubular and which station is electrically connected to the electrically conductive parts of the well tubular at both sides of the electrically non-conductive section thereof. 
     The electrical booster stations may be distributed at regular intervals along the length of the primary and branch wellbores. If an electrical booster station is required at a location where the ends of two adjacent expanded well tubulars co-axially overlap each other, an electrical sealing material may be arranged between the overlapping tubular sections and the booster may be installed as a sleeve within the outermost tubular adjacent to the innermost tubular such that one electrode of the booster station is electrically connected to the innermost and another electrode thereof is connected to the outermost tubular. 
     It is observed that in some instances the booster station may be installed at a well junction, in which case the electrodes of the booster station will make the electric connection between the primary and branch well tubulars. 
     It is also observed that when used in specification the and the appended claims the term multilateral well system refers to a well system having a primary or mother wellbore which extends from a wellhead down into a surface earth formation and at least one branch wellbore which intersects the primary or mother wellbore at a subsurface location. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the system according to the invention will be described with reference to the accompanying drawings, in which 
     FIG. 1 is a schematic three-dimensional view of a multilateral well system according to the invention; 
     FIG. 2 shows how a well tubular is expanded using a conical expansion mandrel; 
     FIG. 3 shows a connection between two well tubulars where an electrical booster station is arranged; 
     FIG. 4 shows a branchpoint where a branch wellbore has been drilled through a window in the primary well casing; 
     FIG. 5 shows how an expandable well liner is expanded in the branch wellbore and electrically connected to the primary well casing; 
     FIG. 6 shows a branchpoint where the branch well casing and the primary casing underneath the branchpoint are expanded within a bifurcation element or splitter; 
     FIG. 7 shows a tubular equipment carrier sleeve in the open mode such that oil and/or gas flows via perforations in the sleeve into the wellbore; and 
     FIG. 8 shows the sleeve of FIG. 7 in the closed mode in which the perforations have been closed off. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1 there is shown a multilateral well and electric transmission system  1 , which comprises a primary wellbore  2  and two branch wellbores  3  and  4 . 
     The system  1  extends from an underwater wellhead  4  into the bottom  5  of a body of water  6 . Oil and/or gas processing equipment on an offshore platform  7  is connected to the wellhead  47  via an underwater flowline  8  and a power supply cable  9  extends from a first pole  10 A of an electrical power generator  10  at the platform  7  to primary well casing  11  which has been expanded against the wall of the primary wellbore  2  such that a thin annular layer (not shown) of cement or another sealing material such as an addition curing silicone formulation is present between the expanded casing  11  and borehole wall. 
     In the lower branch wellbore  4  a branch well liner  12  has been expanded and cemented in place, whereas in the upper branch wellbore  3  a branch well liner  13  is being expanded by pumping or pushing an expansion mandrel  14  therethrough towards the toe of the well. 
     As a result of the expansion process a surplus expansion is created in the expanded casing or liner which ensures that the expanded branch well liners  12  and  13  are firmly pressed against the inner wall of the primary well casing  11  at the branchpoints  15  and  16  so that an excellent electrical connection is established between the branch well liners  12  and  13  and the primary well casing  11 . 
     In the primary well casing  11  an electrical booster station  17  is arranged at a location where an electric insulation sleeve is mounted within the casing  11  and the casing has been milled away over a selected distance. The booster station  17  has one electrode  18  which is electrically connected to the casing section above the gap and another electrode  19  which is electrically connected below the gap. Likewise a similar booster station  17  is arranged in the lower branch wellbore  4  and has electrodes  18 , 19  which are connected to sections of the branch well liner  12  which co-axially overlap but which are electrically insulated from each other by an electric insulation sleeve  22 . Instead of using co-axial electrically insulated tubular sections the electrical insulation may be achieved also by using a pre-installed plastic section in the well tubular which plastic section is expanded in the same way as the steel parts of the tubular string. 
     For the sake of clarity the power booster stations  17  are shown outside the wellbore but in general these stations  17  will be mounted in an annular carrier sleeve within the well tubulars as is illustrated in FIG.  3 . FIG. 1 also shows schematically that a second pole  10 B of the electrical power generator  10  is connected to earth and that also the branch well liners  12  and  13  are connected to earth at one or more selected locations  21  and  23  so that the earth  5  forms an electrical return link, illustrated by phantom line  20 , from the well liners  12  and  13  and said second pole  10 B. 
     FIG. 2 shows how a lower well tubular, which is made of a formable steel grade  24 , is expanded inside the lower end of an existing well tubular  25  using an expansion mandrel  26  having a conical ceramic outer surface having a semi top angle A which is 10° and 40°, and preferably between 20° and 30°. The upper well tubular  25  has been cemented within the wellbore  28  and as a result of the expansion process the lower well tubular obtains a surplus expansion so that its inner diameter becomes larger than the outer diameter of the mandrel  26  and the expanded lower tubular  24  is firmly pressed against the overlapping lower part  27  of the upper tubular  25  so that a reliable electrical connection is created between the lower and upper well tubulars  24  and  25 . 
     FIG. 3 illustrates a location where a lower tubular  30  has been expanded within a widened lower end  31  of an upper well tubular  32  and an electrical insulation sleeve  33  is arranged between the co-axial tubular parts. 
     A ring-shaped electrical power booster station  34  is arranged within the widened lower end  31  of the upper tubular  32  just above the top of the lower tubular  30 . The station  34  is equipped with electrodes  35  which establish an electrical connection between the tubulars  30  and  32 . 
     FIG. 4 shows how a branch wellbore  40  is drilled away from a primary wellbore  41  through an opening  42  that has been milled in the primary well casing  43  and the surrounding cement annulus  44 . 
     FIG. 5 shows how an expandable branch well liner  45  is expanded in the branch wellbore  40  of FIG. 4 by an expansion mandrel  46  which is similar to the mandrel  26  shown in FIG.  2 . 
     As a result of the surplus expansion during the expansion process the branch well liner  45  is elastically pressed against the inner wall of the primary well casing  43  and to the rims of the opening  42  thereby establishing a firm electrical connection between the primary well casing  43  and the branch well liner  44  which connection remains reliable throughout the lifetime of the well. 
     FIG. 6 shows a branchpoint in a multilateral well system where a bifurcation element  50  or splitter is secured and electrically connected (optionally via an electric booster station as illustrated in FIG. 3) to an upper primary well casing  51 . 
     A lower primary casing section  52  and a branch well liner  53  are each radially expanded by an expansion mandrel  54  inside the primary and branch wellbores such that the upper ends of the lower primary casing section  52  and said liner are firmly pressed against the lower branches of the bifurcation element  50  which serve as an electric contact and receptacle. 
     FIG. 7 shows an inflow section of a branch wellbore  60  where the branch well liner  61  has perforations  62  through which oil and/or gas is allowed to flow from the surrounding oil and/or gas bearing formation  63  into the wellbore  60  as illustrated by arrows  64 . 
     An equipment carrier sleeve  65  is sealingly secured inside the liner  61  by means of a pair of expandable packers  66 . 
     The sleeve  65  has perforations  67  and is surrounded by a movable sleeve-type valve body  68  which has perforations  69  which are, in the position shown in fig. 7, aligned with the perforations  67  of the sleeve  65 . Because of the alignment of the perforations  67  and  69  oil and/or gas is permitted to flow into the wellbore  60 . 
     FIG. 8 shows how the sleeve-type valve body  68  is moved such that the perforations  67  and  69  are unaligned and flow of oil and/or gas from the formation  63  into the wellbore  60  is interrupted. 
     The motion of the sleeve type valve body  68  is achieved by an electrical actuator  70  which is powered by a rechargeable lithium-ion high temperature battery  71 , which has one electrode  72  which is electrically connected to the surrounding formation and another electrode  73  which is electrically connected to the liner  61 . 
     The electrical direct current (DC) power which is transmitted via the primary casing (not shown) to the branch well liner  61  is used to trickle charge the battery  71 . The battery  71  powers the valve actuator  70  and optionally also flow, pressure, temperature, composition, reservoir imaging and/or seismic equipment (not shown) carried by the sleeve  65  and signals generated by the equipment are transmitted to surface monitoring equipment by transmission of VLC or ELC pulsed electromagnetic signals which involve voltage level oscillations around the DC voltage level of the branch well liner  61  via the electrode  72  and said liner  61  to the primary well casing (not shown) and an electrical cable connected to the upper end of said casing (as is shown in FIG. 1) to surface monitoring and/or control equipment. 
     In the example shown in FIG. 7 the battery  71  is a tubular ceramic lithium-ion high-temperature battery and a series of reservoir imaging sensors  75  are embedded in the formation  63  surrounding the wellbore  60 . These sensors  75  transmit and/or receive signals via inductive couplers  76  which are connected to signal processing equipment (not shown) mounted on the sleeve  65 . Said processing equipment is able to actuate the valve body  68  and/or to transmit electric reservoir imaging data acquired by the sensors  75  via the wall of the well liner  61  and well tubulars in the primary or mother wellbore to production monitoring equipment at the platform or other surface facilities as illustrated in FIG.  1 .

Summary:
There is provided a multilateral well and electric transmission system comprising a branch well tubular in a branch wellbore which is connected in an electrically conductive manner to a primary well tubular in a primary wellbore such that the primary and branch well tubulars form a link for transmission of electrical power and/or signals between the primary and branch wellbores. Low voltage electrical power can be transmitted from the surface to a battery in the branch wellbore to trickle-charge the battery and signals from battery-actuated measuring and control equipment in the branch wellbore can be transmitted back to surface via the walls of the electrically interconnected primary and branch well tubulars.