Abstract:
A system is disclosed for positioning downhole tools in a borehole such as a cased or lined oilwell, wherein the system is adapted to control the vertical and rotational position of the tool within the borehole. The system uses a plurality of guide slots or ledges which form a spiral on the inner surface of a tubular.

Description:
This is a continuation-in-part of application Ser. No. 09/213,163 filed on Dec. 17, 1998, which claims priority from application Ser. No. 9726524.3 filed in Great Britain on Dec. 17, 1997, and application Ser. No. 9800344.5 filed in Great Britain on Jan. 9, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a wellbore positioning system for locating a downhole tool in a borehole. 
     2. Description of the Related Art 
     A common problem faced when drilling oil wells is how to locate a downhole tool such as a whipstock inside a casing section repeatedly at the same depth and at the same orientation, for example in order to drill a deviated side track through a window in the casing. Several solutions to this problem have been found. The most basic is a muleshoe and key (see FIG. 1) where the key is set inside the casing in a fixed point and a muleshoe profile is machined into the tool being run, or into a separate muleshoe attachment fitted to the string. The muleshoe profile can alternatively be machined into the inner face of the casing (e.g. upside down with the apex pointing upwards), and a spring loaded key can be provided on the locating tool. In each case, the key comes to rest at a key seat machined into the wall of the muleshoe. 
     While the muleshoe allows rotational orientation and depth control of the whipstock or other downhole tool on the string, it does not allow multiple vertical locations to be located automatically. In addition, the key can become snagged on the apex of the muleshoe rather than on the key seat. Also, if the muleshoe is machined into the inner face of the casing, which is preferred, there is a loss of effective diameter, and a consequent weakening in the casing. Furthermore, the key seat can become filled with accumulated debris which can prevent proper seating of the key. 
     In an effort to address the problem of identifying multiple vertical locations, location systems such as the selective key setting tool have been devised. In this tool (shown in FIG.  2 ), a number of locating profiles are machined into the casing, each having a unique shape, and a locating tool with laterally extending spring-loaded keys is run through the casing until the keys match with the desired locating profile, at which point the keys engage the profile and further downward movement is resisted. While this allows the identification of a particular vertical location corresponding to the key, it does not orient the locating tool or whipstock rotationally. 
     SUMMARY OF INVENTION 
     According to the present invention there is provided a wellbore system for selectively locating a downhole tool in a wellbore casing or tubing comprising a plurality of positioning guides spaced in the wellbore casing or tubing at predetermined depths wherein each positioning guide has a unique profile for selectively receiving a matching profile key of a downhole tool, the guides for each key having a configuration which controls the depth and rotational position of its matching key. The control of the vertical and rotational position of the key thereby controls the vertical and rotational position of the member. The member can be a downhole assembly such as a running tool or a whipstock etc. 
     The invention also provides a method of selectively locating a downhole tool in a wellbore casing or tubing comprising the steps of: 
     a) placing a plurality of positioning guides at desired depths in the wellbore casing or tubing wherein each guide has a unique profile for selectively receiving a matching profile locating key, 
     b) selecting the profile of a locating key for the downhole tool to match the profile of a desired positioning guide, and 
     c) running the downhole tool to the desired positioning guide wherein the locating key engages the matching profile of the positioning guide which controls the depth and rotational position of the locating key to orient the downhole tool in a desired direction. 
     Preferably each key guide is a slot or a ledge, which in particular embodiments extend downwards and laterally. In a preferred embodiment the key guides are spiral slots, optionally covering a minimum of 450° of circumference. 
     By casing, we mean any downhole tubular such as casing, liner, drill pipe, or any functional equivalents. 
     It is an important aspect of the invention that each key guide only allows engagement of a particular key, such that the profile of the key only fits within the profile or width of the key guide such as the slot. In particular cases, there can be more than two key guides, and/or more than two keys, so that multiple outlets spaced vertically and rotationally from one another in the casing string can be identified and particular tools in the work string can be located at different rotational positions and at different depths throughout the string. In one embodiment, the keys can be spring loaded and/or retractable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be described by way of example, and with reference to the accompanying drawings, in which: 
     FIGS. 1A,  1 B show a muleshoe arrangement of the prior art; 
     FIG. 2 shows a selective key locating system of the prior art; 
     FIG. 3 shows a casing tubular embodying a spiral groove in accordance with one aspect of the invention; 
     FIG. 4 shows a system of the invention being used to install a whipstock to deflect a drill string through a window in a casing tubular; and 
     FIGS. 5A,  5 B show the lower part of a groove in the FIG. 4 system. 
     FIG. 6 shows the invention in a wellbore with multiple positioning guides. 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, that will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     Referring now to FIGS. 3,  4  and  5 A and  5 B, a system in accordance with one embodiment of the invention comprises a casing tubular  1  having spiral grooves  5  and  5 ′ machined into its inner face. The grooves  5  and  5 ′ are spaced around the inner circumference of the tubular  1  so that they start at opposite sides at 180° spacing. The spiral grooves  5  and  5 ′ extend in a counter-clockwise direction in order to prevent the inadvertent backoff of threaded connections above during the impact of the key of the locating tool against the groove, and each has a predetermined width  5 w and profile. In the system of the invention shown in FIGS. 3,  4  and  5 A and  5 B, a locating tool  7  is mounted on a whipstock  10  at the lower end of a work string (not shown). A spring located key  15  is carried on the locating tool  7 , and comprises a laterally extending boss of a predetermined size to fit within the width  5 w of the spiral groove  5 . The spring loaded key  15  is received within a recess in the locating tool and is sprung laterally so that it abuts against the inner face of the casing  1  in normal operation of the tool. 
     In use, the whipstock  10  and locating tool  7  are inserted into a casing string by a conventional work string etc. (not shown) until the boss on the spring loaded key  15  engages the slot  5  in the casing  1 . When the boss is located in the spiral slot, the movement of the whipstock  10  and locating tool  7  is restricted to a downward spiral movement until the boss engages at the rest location  5 r at the end of the slot  5 . The whipstock is oriented in relation to the locating tool such that when the boss on the key  15  is at the rest location  5 r in the slot  5 , the whipstock  10  is positioned to deflect a drill string (not shown) from above through a window  1 w in the casing  1 . 
     Modifications and improvements can be incorporated without departing from the scope of the invention. For instance, three or more spiral grooves can be provided in the casing string at different rotational and/or vertical locations (i.e. at different depths). In the embodiment shown in FIG. 6, a plurality of positioning guides are spaced at different vertical depths in a wellbore casing. More paricularly, spiral slots  5 ,  5 ′,  5 ″ are spaced at different depths in casing  1 . Different spiral slots can have differing widths or profiles so as to allow the engagement of only predetermined keys  15 . By selecting the appropriate key on the location tool, the rotational and/or vertical positioning of the locating tool (and hence the whipstock, or other downhole tool) can be determined from the surface simply by inserting the locating tool  7  and whipstock  10  into the casing string and delivering the assembly to the appropriate depth until the key  15  engages in the slot. 
     Where separate slots are spaced vertically from one another, as shown in FIG. 6, any one slot will preferably be of narrower width or profile than the slot immediately below it, that the slot first engaged by the key on the locating tool being inserted will only be engaged if the profile of the key and slot fit, and larger keys intended for positioning further down the casing string will not engage with narrower slots at the upper levels of the casing string. 
     In certain embodiments (an example is shown in FIGS. 5A and 5B) the groove  5  has a profile at the key seat to facilitate self-cleaning of the groove  5 . In the example shown in FIGS. 5A and 5B, these comprise a narrower portion of the groove  5 n through which the key  15  cannot pass and/or a ramped profile  5 r to prevent accumulation of debris at the key seat. 
     It is of course possible to have different profiles or widths of slot at the same vertical position, but at different rotational positions as shown in FIG. 3, so that by engaging the appropriate key into the slot, the rotational resting position of the locating tool can be specified even if the depth is not altered. 
     While the present invention has been particularly shown and described with reference to various illustrative embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention. The above-described embodiments are illustrative and should not be considered as limiting the scope of the present invention.