A downhole tool for use in sidetracking operations

Abstract

Described herein is a downhole tool for use in sidetracking operations comprising: a deflection element extending along the tool and having a deflection surface which is oriented at an angle to a longitudinal axis of the tool; an electrically conductive element positioned adjacent the deflection surface such that it extends along a length of the deflection surface; a coupling device for providing an electrical current to the conductive element; and a connection apparatus for establishing an electrical connection between the tool and a section of wellbore casing. A method for performing a sidetracking operation is also described.

Description

[0001] A Downhole Tool for Use in Sidetracking Operations

[0002] The present invention relates to a downhole tool for use in sidetracking operations, and in particular to a downhole tool comprising a deflection element having a deflection surface and a conductive element.

[0003] In the oil and gas industry it is fairly common that a wellbore needs to be extended at an angle to an initial vertical portion. The action of providing an angled wellbore section is referred to as “sidetracking”. Possible reasons to intentionally sidetrack a well are, for example, in order to bypass an obstruction in the original wellbore, to bypass a collapsed wellbore section, or to use the existing wellbore to reach a different, more productive, location in the reservoir.

[0004] A section of wellbore is typically lined with sections of metallic tubing referred to as casing (a cased hole). Intentional sidetracking can be performed in an open hole, where no casing is provided, or in a cased hole. In an open hole, sidetracking may be accomplished by first setting a cement plug that is harder than the surrounding formation and using known methods involving directional drilling tools and techniques.

[0005] In order to sidetrack where a casing is already present, however, it is necessary to cut through the casing and then to divert a drill bit to pass through the window formed in the casing to continue drilling at an angle to the vertical. This involves the use of a number of different milling tools and represents a complex drilling operation, as described in more detail below. Even so, it can bring many benefits such as lowering cost compared to drilling an entirely new well. It also allows oil companies to use existing infrastructure such as drilling pads or offshore platforms in order to enhance recovery of the maturing reservoir or to reach new neighbouring reserves. Both will help to maximize return on investment.

[0006] Diversion of the drill bit from a direction along the original wellbore to form the angled wellbore section can be achieved by use of a whipstock. The whipstock is a directional drilling tool that is used to deflect the drill bit and the entire BHA (Bottom Hole Assembly) towards the desired direction of the sidetrack. A typical whipstock contains a metallic deflection ramp designed to achieve a smooth radial displacement of the drill bit from the center of the wellbore to the side, and an anchor located below the deflection ramp to secure the whipstock from any movement during the subsequent milling and drilling operations.

[0007] A commonly used method for sidetracking from a cased hole begins by running the whipstock into the hole and placing it at the kick-off depth (the depth at which the angled well section is to begin). The whipstock is usually attached to a milling assembly by a pin, and the deflection ramp of the whipstock can be oriented by turning the drill pipe in the desired direction. An anchor located below the whipstock is then activated against the casing, locking the whipstock from any movement. The whipstock is detached from the milling assembly by shearing the connecting pin once the BHA begins to turn, at which point the milling assembly starts rotating and weight on bit is applied. The mill at the tip of the milling assembly slides along the ramp of the whipstock and is deflected, and the milling of the casing in the direction of the sidetrack commences. A watermelon mill is then used to further open the window in the side of the casing. After drilling through a few meters of formation, the milling assembly is pulled out of the hole. Several runs with different milling tools might be required to ensure that the oriented window in the casing is fully opened, after which the directional drilling assembly is run in hole and the sidetrack well can be drilled.

[0008] Depending on the downhole tools used and the complexity of the operation, the number of trips, cost, and time spent may vary. In addition, the lack of directional control when milling may result in the section of formation drilled during the milling operation being oriented in the wrong direction, which can lead to the angled wellbore eventually also extending in the wrong direction. This can make the drilling operation particularly challenging when sidetracking in zones where other wells are nearby, and will increase the risk of collision. When using a whipstock to sidetrack in a cased hole, this is done using a rig or a coiled tubing unit.

[0009] US-A-2020 / 0284115 and WO-A-2018 / 063003 each describe systems for removing whole sections of casing using a downhole tool equipped with conductive elements prior to plugging a wellbore section. Current is applied to the conductive elements from a power source, and a connection between the tool and the metal wellbore casing completes an electrical circuit including electrolyte within the well section. The metallic casing is corroded as a result of the applied current. In both cases, in order that the well can be plugged across its whole extent from the formation on one side of the well to the other (which is crucial when plugging a wellbore) the conductive elements are designed to achieve an even corrosion around the entire inner surface of the casing for removal of a whole section of the wellbore casing.

[0010] WO2017 / 138923 describes a method for side-tracking using pre-drilled windows within sections of casing. These are initially occluded or covered using degradable materials to provide a fluid seal. The degradable material is degraded or dissolved on exposure to a particular fluid, and this property is used to open the window prior to lateral drilling using a traditional whipstock mechanism. Such a system requires the positions of any lateral well sections to be predetermined.

[0011] In general, sidetracking operations for cased holes are complex and require milling tools and / or other equipment to be moved into and out of (or up and down) the wellbore several times during the process. A more efficient method for sidetracking, particularly in a cased hole, is desired.

[0012] According to a first aspect of the present invention, there is provided a downhole tool for use in sidetracking operations, comprising: a deflection element extending along the tool and having a deflection surface which is oriented at an angle to a longitudinal axis of the tool; an electrically conductive element positioned adjacent the deflection surface such that it extends along a length of the deflection surface; a coupling device for providing an electrical current to the conductive element; and a connection apparatus for establishing an electrical connection between the tool and a section of wellbore casing.

[0013] Each of the deflection element and the conductive element extend axially along the tool, and the two sit side-by-side. The deflection surface of the deflection element is angled with respect to the longitudinal axis of the tool, which also extends in the axial direction. The deflection surface is oriented at an acute angle with respect to a direction along the longitudinal axis of the tool, and from bottom towards the top of the tool (towards the surface when the tool is in place within a wellbore). The angle at which the deflection surface is oriented with respect to the downhole axial direction can, but does not necessarily, vary in a direction from the top to the bottom of the surface, and if this is the case will be smaller near to the top of the deflection surface than at the bottom to gradually turn a BHA away from the direction along the original wellbore and towards the direction that the lateral wellbore is to be drilled. The deflection element and conductive element are positioned side by side in a radial direction, at the same or overlapping axial positions within the tool. The section of wellbore casing may be a section of the casing adjacent the conductive element. This is the section of wellbore casing which will be corroded to form a window when the tool is in use and current is applied to the conductive element. This is therefore the section of wellbore casing that is closest to, or directly adjacent in a radial direction, an outer surface of the conductive element. The section of wellbore casing may be directly adjacent the outer surface of conductive element. Directly adjacent refers to the fact that there is no solid structure present between the section of casing and the conductive element. Only the electrolyte or brine will be present between the two when the tool is in use. The connection between the tool and the section of wellbore casing can carry current from the casing to the tool and back to a power source providing the electrical current to complete the electrical circuit. The same coupling device can be used to carry current from the tool to the power source as from the power source to the tool (in an embodiment it can comprise an electrical cable or similar).

[0014] In embodiments, the downhole tool is suitable for use in sidetracking operations in a cased hole or a cased wellbore.

[0015] The above mechanism allows for sidetracking without the use of a rig and without requiring several trips downhole to replace tools and to perform complex drilling operations. The deflection surface is oriented at an angle to the longitudinal axis of the tool, so that when the tool is placed within a wellbore a drill bit is lowered into the well the drill bit will bear on the deflection surface, and will be deflected or redirected by it towards a side of the well at the height of the deflection surface. The connection between the tool and the section of wellbore casing and the provision of current to the conductive element will result in corrosion of the section of wellbore casing. The positioning of the conductive element, adjacent the deflection element, with the two side-by-side on opposite sides of the device, will result in the formation of a window in the metal tubing corresponding to the section of wellbore casing due to corrosion of this section. The shape of the conductive element is such that a suitable drill bit can pass through the formed window without any contact with the wellbore casing. This window will be correctly positioned to begin sidetracking without readjustment of the position of the deflection surface, allowing a lateral wellbore to be drilled with minimal tripping of tools up and / or down the wellbore. The process is therefore greatly improved in terms of efficiency as compared to known sidetracking methods. The process also allows different, lighter, and cheaper, materials to be used for the whipstock, since this now only needs to be harder than the surrounding formation rather than harder than the (usually steel) casing.

[0016] The tool is elongate, so that when it is installed in a vertical wellbore it extends along the wellbore and has two ends, an upper end facing in a direction towards the surface and a lower end facing in a downhole direction. The sides of the tool extend between the lower and the upper ends. When the tool is in place within a wellbore, the longitudinal axis of the tool is parallel to and will typically extend along the central longitudinal axis of the tubular casing in which a window is to be opened (the tool will usually be orientated with its longitudinal axis substantially parallel to or coincident with the longitudinal axis of the wellbore and the casing). The tool comprises a deflection element and a conductive element positioned side by side, both being elongate and extending along the tool in the direction of its longitudinal axis (the axial direction). The deflection element has an outer surface which faces the wellbore casing when the tool is installed and a deflection surface (or internal surface) which is angled to deflect a drilling or milling tool from a vertical path to a path that extends at an angle to the vertical to drill a lateral wellbore. The conductive element similarly has an outer surface facing the casing and an internal surface which sits facing the deflection surface of the deflection element. The deflection element and conductive element are positioned at the same longitudinal distance along the tool, so that they are at the same depth within the well when the tool is installed within a vertical wellbore. The conductive element may be at least as long (and may also be at least as wide) as the deflection element, such that the two elements extend along the same length of the tool, measured in the direction along the longitudinal axis of the tool, when installed side-by-side as a compound element. In general, the conductive element will be at least as long and as wide as the deflection surface of the deflection element, but will preferably be longer and wider so that the pass of the bit through the window in the casing formed by the conductive element is guaranteed.

[0017] The conductive element may extend all of the way along the deflection surface from the base to the top, or from a lower end to an upper end of the deflection surface. References to upper, lower, side, etc., are to the relative positions of particular components or parts when the tool is installed within a vertical wellbore. The tool may obviously be used to drill a lateral bore from a wellbore that is not vertical, but is sloped. In this case references to up, down, and so on are relative to the direction in which the original wellbore extends. When the tool is installed within a wellbore, the conductive element can extend along the entire length of the deflection surface in a direction along the longitudinal axis of the tool. The deflection surface represents the surface with which the drill bit makes contact during the sidetracking operation, and which causes a change in direction of the drill bit. The top of the deflection surface is at the level of the first point of contact between the drill bit and the deflection element at which a sideways force is applied to the drill bit, and the bottom is at the level of the last point of contact between the drill bit and the deflection element.

[0018] In embodiments, the deflection surface is concave in the radial direction, to match the circular crosssection of a drill bit, which provides for more effective guiding of this part. The angle between the deflection surface and the longitudinal axis of the tool can change with distance along the surface from top to bottom (so that the surface is also concave in the longitudinal or axial sense), or can remain constant.

[0019] In embodiments, the tool comprises a circulation device for circulating brine within the wellbore. The fluid may be brine, and may be conditioned to have a desired conductivity for optimum performance. The brine can be present already within the wellbore, in which case it is usually preferable to add salt to adjust the conductive properties of the brine before use of the tool. Whether salt is added to the wellbore, and if so how much, will depend on the properties of the wellbore fluid and will vary from wellbore to wellbore. The circulation device can comprise a pump or a rotating element such as a propeller or impeller. The circulation device may additionally and / or alternatively help to remove byproducts from the electrolysis, for example by accelerating removal of the byproducts using the circulating brine. Nevertheless, the circulation device is not essential for removal of the byproducts.

[0020] In embodiments, the conductive element is positioned to cover at least a part of the deflection surface. Covering refers to the fact that when the conductive element and deflection element are coupled together as they will be within the tool, the part of the deflection surface is not externally visible (at least from the side), but is shielded from view by the conductive element.

[0021] In embodiments, the conductive element is positioned within the tool so that it covers the entirety / whole of the deflection surface. In such a case the deflection surface is not visible from the side when the conductive element and deflection element are coupled together. This helps to ensure that a window is opened that is large enough for a drill bit to pass through without contacting the casing. If the conductive element and deflection element do not contact each other and there is a hollow space between, parts of the deflection surface may obviously be visible from above or below (although not generally when additional tool components are present).

[0022] In embodiments, the conductive element is electrically isolated from at least an outer surface of the deflection element or at least the outer surface of the deflection element is formed of a non-conductive material. In embodiments, the whole of the deflection element is electrically isolated from the conductive element. The outer surface or the entirety of the deflection element can be isolated from the conductive element by way of an insulating layer (such as a coating) between the two, or by including a deflection element where at least the outer surface, and potentially the whole of the deflection element, is formed of a non-conductive material. This prevents current from passing through the deflection element to the casing adjacent the deflection element outer surface, which would result in an entire section of the wellbore casing being removed. Providing the electrical isolation allows a window to be opened at the desired location in the casing, leaving a more complete wellbore after the operation whilst improving efficiency and reducing waste. The outer surface of the deflection element or of the conductive element refers to a surface that is positioned adjacent and facing the casing when the tool is in use. The inner surface of the deflection or conductive element refers to a surface that faces away from the casing and towards the other element when the tool is in use.

[0023] In embodiments, the conductive element is positioned within the tool such that it extends along the whole length of the deflection surface. In embodiments, the conductive element is positioned within the tool such that it covers the whole of the deflection surface. This ensures that the window is at least as large as the deflection surface which will direct the drill-bit towards the window.

[0024] In embodiments, the conductive element extends above the top of the deflection surface. This ensures that the drill bit has ample space to pass at the upper end of the deflection surface to begin drilling the surrounding formation without needing to drill through any metal casing in the process. This helps to ensure that a longer drill bit will not contact the casing which could result in jamming of the bit. How far the conductive element should extend above the top of the deflection surface will depend on the shape and length of the drill bit to be used, but this distance may be between 10cm and 50cm, preferably between 20cm and 40cm. The conductive element can in some cases also (or instead) extend below the bottom of the deflection surface.

[0025] In embodiments, the conductive element extends outward of the sides of the deflection surface. This means that the conductive element has a width that is greater than the width of the deflection surface. The window in the casing will therefore be wider than the deflection surface, and a drill bit having a corresponding size to or a size larger than the deflection surface will be able to drill a lateral borehole without risk of contact with the casing. References to length and width are to measurements in an axial and radial direction respectively.

[0026] In general, the outer surface of the conductive element which faces the casing in use may be larger than the deflection element in either or both of the length and width direction, which provides some clearance for the drill bit being deflected by the deflection element after the window has been opened to ensure that no casing needs to be drilled.

[0027] In embodiments, the deflection element and the electrically conductive element form a compound unit that is substantially cylindrical. The figures show such a compound unit, in which the deflection and conductive elements include inner surfaces with corresponding shapes (i.e., one concave and one convex). The specific shapes of the two elements can be adapted, but in general the outer surface of the conductive element facing the casing in use will match the shape of the casing. This side surface will therefore be curved / shaped to generally match the cylindrical inner surface of the casing. In some cases, the outer surface of the conductive element can be slightly conical so that the outer surface is a bit closer to the casing at a one end, as described below.

[0028] In embodiments, the deflection surface and an inner surface of the conductive element have corresponding shapes such that the two surfaces fit together. The surfaces in this case can be described as interlocking. In some embodiments, the conductive element extends along and adjacent the deflection surface in use. There can be a gap between the deflection element and conductive element, with the two inner surfaces facing but not touching, or the two elements can contact one another when the tool is assembled along their respective inner surfaces. There can be an insulating layer provided between the two to prevent the flow of current from the conductive element to the deflection element, as mentioned above.

[0029] In embodiments, the tool comprises an attachment device for removable coupling of the conductive element to the deflection element. The attachment device functions to hold the elements in position adjacent one another with their inner surfaces facing. The attachment device can provide for a temporary coupling, meaning that the conductive element can be detached from the deflection element after the electrolytic process is complete. In some cases, the conductive element and components of the tool positioned above the conductive element and the deflection element will be removed from the wellbore together before drilling. This can involve detachment of the conductive element from the deflection element prior to removal. During subsequent drilling the deflection surface acts to direct the drill bit towards and through the window that has been opened in the casing.

[0030] In embodiments, the conductive element comprises a material having a hardness that is lower than the hardness of the material from which the deflection surface is formed. In embodiments the conductive element has a hardness on the Mohs hardness scale that is between 0.1 and 4, preferably between 0.5 and 3.

[0031] In embodiments, the conductive element is formed from graphite, copper, or any other material that can be drilled with existing drilling equipment, such as a polycrystalline diamond compact (PDC) bit. Whateverthe materials used, the relative softness of the material of the conductive element and at least the deflection surface of the whipstock can be selected so that the material of the deflection surface is harder than the material of the conductive element. The drill bit will then preferentially drill through the conductive element and will be deflected as desired. This is clearly an advantage since it dispenses with the need to pull the conductive element itself up the wellbore after corrosion and priorto drilling the formation to form the lateral / angled wellbore. This can also be useful in cases where the surrounding formation has caved in and has trapped the conductive element. In embodiments, the conductive element is detachable from components of the tool which are located above the deflection element when the tool is inserted in a wellbore. In such a case, the conductive element will remain downhole when the upper parts of the tool are disconnected and removed from the wellbore before drilling of the lateral wellbore. The conductive element will usually be fixedly or permanently coupled to the deflection element in such a case, to give a simpler configuration, although this is not necessarily the case. The drill can be tripped down the well to simply drill through the conductive element and then through the window in the casing, as directed by the deflection surface, to form the lateral wellbore. It is preferable for a detachable conductive element to be formed of a material having a hardness that is low enough that the drill used for forming the lateral well bore is able to drill through it. The softer the material from which the conductive element is formed, the greater the ease with which the conductive element can be drilled through. Copper and graphite are both good choices, as mentioned above. Softer materials also generally contribute to lower weight and cost for the tool as a whole.

[0032] In embodiments, the deflection surface is a concave surface. The surface can be concave in the radial direction (i.e. the cross section in a plane perpendicular to a direction along the length of the deflection surface is curved inwards towards the body of the deflection element). This helps to support and direct a drill bit having a circular cross-section. The radius of curvature of the concave surface can be selected to suit the drill bit that will be used to drill the lateral wellbore, and will therefore depend on the size of the wellbore to be formed. The radius of curvature of the concave deflection surface can be the same all of the way up the surface from the base to the top. The deflection surface can also (or instead) be concave in the longitudinal direction, so that the drill bit is more gradually redirected as it travels down along the deflection surface.

[0033] In embodiments, the conductive element and the deflection element are coupled together. The coupling may be such that an inner surface of the conductive element and the deflection surface are in contact, although this is not necessarily the case. The coupling can be permanent / fixed or temporary. In order to open a window rather than a section of casing, current should be prevented from flowing through the deflection element (or at least the outer surface of the deflection element closest to the casing). If this is not the case, corrosion will occur at the same rate in the casing close to this surface and an entire length of the casing will be removed, rather than just a window on one side. Where a non-conductive deflection element or an insulating layer between the deflection element and the conductive element is used, there will be some corrosion on the side of the casing facing the deflection element, but this will be negligible resulting in highly asymmetric corrosion and the opening of a window. Although the tool could still function to perform a sidetracking operation where a section of casing is removed, opening a window at the kick-off depth is more efficient, and the wellbore can be sealed more easily according to need. Current can be prevented from flowing through the deflection element in a number of ways. The whole of the deflection element itself, or at least the part facing the conductive element, can be formed of a non-electrically conductive material. Alternatively, an insulating layer can be provided between the facing surfaces of the conductive element and the deflective element to prevent current flow therebetween. An insulating layer can alternatively (or additionally) be provided between the deflection element and the facing surface of the casing. This will mean that an insulating layer is provided covering the external surface of the deflection element to prevent the flow of current between the deflection element and the casing. The insulating layer can be formed as a coating in all of the above examples.

[0034] A possible material for a non-conductive deflection element could be, for example, a plastic. Because steel does not need to be milled in order to complete the sidetracking operation, a heavy and hard material is not required. This is a clear advantage in terms of cost and efficiency. Manufacture of the whipstock can also be cheaper in this case.

[0035] In embodiments, the deflection surface is formed of a material that is harder than the formation surrounding the wellbore, but softerthan the material of the wellbore casing. The actual hardness range will vary depending on the formation being drilled and the type of casing being used, but measurements of the hardness of the casing and formation will be taken as part of preparation for the drilling and / or will be available from previous drilling operations. Methods and devices for taking the measurements will be known to operating personnel or to anyone with knowledge in the field.

[0036] In embodiments, the conductive element comprises a plurality of openings in an outer surface thereof, and the downhole tool comprises a fluid circulation system which causes fluid to flow into the conductive element and out through the openings to jet against the wellbore casing. The fluid circulation system include the circulation device of the tool, or the two may be separate. Reference to the outer and inner surfaces of the deflection element and the conductive element are to the surfaces which are exposed and / or which face the casing when the tool is assembled and is located downhole (outer) and which are not exposed and / or face away from the casing (inner). The inner surfaces of the two elements face each another, and the outer surfaces face the casing when the tool is assembled and installed downhole. The outer surfaces of the conductive element therefore include the side surfaces of the element adjacent the metal casing. The fluid system can operate to pump brine that is already present within the wellbore into the body of the conductive element and out through the openings at a relatively high pressure. Using fluid already present within the wellbore (i.e. brine) ensures that the conductivity of the fluid remains as desired. Of course, additional fluid can be added to the system for the purpose of jetting, but this may need to be conditioned to avoid affecting the corrosion process negatively. Jetting fluid can, conversely, be used to adjust the properties of the fluid within the wellbore if desired.

[0037] In embodiments, the tool comprises an ultrasonic transducer such as an ultrasonic transmitter, in order to remove byproducts such as growth of deposits at the metal tubing or casing.

[0038] In embodiments, the tool comprises at least one pressure sensor for monitoring a pressure of the fluid in the fluid circulation system. Since the pressure will be affected by the distance between the openings and the surrounding wellbore casing, monitoring the fluid pressure within the fluid circulation system for the jets is a simple and efficient way to monitor the progress of the corrosion. The at least one pressure sensor can be located anywhere within the fluid circulation system (pressure changes will be transmitted throughout the system). Some examples of suitable positions for the sensor or sensors are close to the pump which forces fluid into the conductive element and / or within the body of the conductive element itself.

[0039] In embodiments, the tool comprises an anchor activatable to prevent movement of the deflection element relative to the wellbore casing in a direction along the wellbore. The anchor can be configured to clamp against the wellbore casing, as described below. The anchor will at least prevent movement of the deflection element up and down the wellbore, but can in some cases prevent all movement of the deflection element, or can prevent all movement of the deflection element other than rotational movement due to activation of a positioning apparatus as described below.

[0040] The anchor can include clamping elements which extend when the anchor is activated to bear against the casing of a wellbore, and which retract to release the tool. In embodiments, the anchor comprises a plurality of clamps which are electromechanically or hydraulically activatable to bear against the wellbore casing to anchor the tool. The tool can comprise internal electromechanical or hydraulic actuators for the activation. The clamping elements can be activated by applying weight through hydraulics, however because the downhole tool can be deployed through a wireline the weight available might not be sufficient, in which case electromechanical means or internal hydraulic actuators will be required.

[0041] In embodiments, the downhole tool comprises a positioning apparatus for adjusting the position of the deflection surface. The positioning apparatus can be activated to rotate the deflection element within the well to determine the direction in which the lateral wellbore will extend from the main wellbore. When the conductive element is coupled to the deflection element, the positioning apparatus will also adjust the position of the conductive element, which will allow the positioning of the window in the casing to be selected. The depth of the window is selected by the depth to which the tool is initially lowered into the wellbore, and the rotational position (the angle within a plane perpendicular to the longitudinal axis of the tool and the wellbore) can be set using the positioning apparatus. The positioning apparatus may be a rotatable portion coupled to the base or the top of the deflection element and conductive element. In some cases, the positioning element may also hold the deflection surface in place once the position has been selected.

[0042] In embodiments, the conductive element is shaped to sit closer to the casing at an end that is located furthest from the connection apparatus. Where the connection apparatus is at the top of the tool, the outer surface of the conductive element therefore has a larger radius of curvature at its base than at its top end (where the shape of the outer surface is viewed in radial cross-section). In embodiments, the outer surface of the conductive element is provided with one or more non- conductive spacers, these spacers are configured to sit against the internal surface of the section of casing during the corrosion process. In embodiments, the spacers are formed of a porous material. Porous refers to the fact that the spacers have through-holes which can be regularly or irregularly spaced, and through which fluid can flow through the spacer. The porous (sometimes sponge-like) structure of the spacers allows brine to pass through and reach the casing behind the spacer. This allows corrosion to proceed even in regions of the casing against which the spacers sit, helping to prevent any large pieces of casing breaking off or being left behind.

[0043] In embodiments, the downhole tool is configured to receive electrical power from a power source, for example an external power source such as a high voltage electrical power source that can be located at the surface or if flow of mud is available, in the form of a downhole mud turbine generator that converts energy of the flowing mud into electricity. In embodiments, the downhole tool comprises: an electrical coupling that connects the assembly to the power source and can, for example, be in the form of a wireline, a wired drill pipe and / or a wire run through the drill pipe or coil tubing. In embodiments, the downhole tool is configured with a connection to a drilling BHA. In this way, efficiency of the sidetracking operations is further improved. For example, the window to be opened and the drilling of the sidetrack to be performed in one run or trip. A typical and known drilling BHA may be used for the drilling of the drillable components of the downhole tool and the drilling of the lateral wellbore. In embodiments, the coupling apparatus includes a mechanism to attach the BHA or drill bit to the downhole tool. The mechanism can be controlled to disconnect the BHA from downhole tool, or the mechanism can also be made of material that can be drilled by the drilling bit. The coupling apparatus can also direct the flow exiting the nozzles of the drill bit into the downhole tool to be circulated between the conductive element and the metal tubing and used for removal of the byproducts and retard or prevent deposition of byproducts such as growth of deposits on the metal tubing orcasing. The coupling apparatus can be a part of, for example an integral part of, or be included in the downhole tool. In embodiments, the power source is connected to a high current power unit. This power unit, which converts high voltage I low current AC or DC electrical power into low voltage I high current DC or pulsed DC electrical power, can be located above the drilling BHA, as part of the drilling BHA or more preferably inside the deflection ramp and / or below the deflection ramp. The more preferable location may permit a more efficient process as it may shorten the distance between the power unit and the conductive element and may increase the area to transfer the electrical current, decreasing the current density as well as decreasing the heat output, for example due to resistive heating.

[0044] In use, the entire assembly including the electrical coupling, the BHA, the coupling apparatus (if it is a separate component) and the downhole tool may be assembled together at the surface, run in hole and placed where the sidetrack operation is planned. After the anchoring and orientation process is completed, the opening of the oriented window can begin. The acceleration of the corrosion process to open the window begins when electrical power is provided. This electrical power can be provided from an external power source, such as directly from the surface via a wireline, a wired drill pipe or a wire run through the drill pipe or coil tubing or by starting the flow of mud, for example with the use of mud pumps. The mud flow will then be used to power a downhole mud turbine generator known in the industry and usually used to power MWD or LWD tools. The high voltage power reaches the high current power unit and the corrosion process begins. Once the oriented window in the metal tubing or casing is fully opened, the drilling BHA is detached from the coupling apparatus and / or the downhole tool. This can be done in a known way by rotating the BHA and shearing the connection, for example. Alternatively, a command can be sent to the downhole assembly via mud pulses, electromagnetic waves and / or another known method, to detach the drilling BHA. The BHA will first drill through the coupling apparatus (if included) and then through the drillable conductive element. Once these components are drilled, the drilling of the lateral wellbore can proceed. When the drilling of the lateral well is completed, remaining components of the downhole tool can remain in the well (i.e. in situ) and / or be retrieved.

[0045] In embodiments, the downhole tool is included in a Bottom Hole Assembly (BHA), for example as a part or a component thereof. In one example, there is provided a kit comprising a BHA and a downhole tool, as described herein.

[0046] According to a second aspect of the present invention, there is provided a method for performing a sidetracking operation, comprising: lowering a downhole tool into a well to a desired kick-off depth, the downhole tool comprising a deflection element extending along the tool and having a deflection surface which is oriented at an angle to the longitudinal axis of the tool, and an electrically conductive element positioned adjacent the deflection surface such that it extends along a length of the deflection surface; coupling the conductive element to a power source to provide an electrical current to the conductive element; activating a connection apparatus to establishing an electrical connection between the tool and the section of wellbore casing to cause it to corrode; monitoring the corrosion of the wellbore casing; and, once a window of a desired size has been opened in the casing, initiating a drilling operation wherein a drill bit bears against and is directed by the deflection surface through the window in the casing to drill a lateral wellbore. A lateral wellbore here refers to a wellbore that is oriented at an angle to the original wellbore (which will usually be vertical). The angle of the lateral wellbore will be the angle of the deflection surface at its base. The method can comprise disconnecting the conductive element from the power source prior to initiating the drilling operation.

[0047] In embodiments, the method comprises comprising, prior to activating the connection apparatus, orienting the deflection surface in the required the direction of the sidetrack and securing the downhole tool to the wellbore casing. The orienting may be carried out using a positioning apparatus which is part of the downhole tool, so that the orienting comprises activating the positioning apparatus to adjust the position of the deflection surface. The position is adjusted such that the deflection surface faces the region of the casing in which the window is to be opened. The method can comprise activating clamping elements after the positioning to lock the deflection element in place.

[0048] In embodiments, coupling the electrically conductive element to a power source comprises activating the connection apparatus to establish the electrical connection using clamps.

[0049] In embodiments, the method comprises disconnecting components of the tool from the deflection surface and removing these from the wellbore priorto initiating the drilling operation. In embodiments, the components removed from the wellbore do not include the conductive element. This provides for a simpler overall operation and reduces the number of components which need to be pulled during the operation. The conductive element is then simply drilled through by the drill bitthat then passes through the window opened by the element and afterwards through the formation to form the sidetrack.

[0050] In embodiments, the method comprises circulating fluid within the wellbore by a fluid circulation system at the same time as establishing the electrical connection. In embodiments, the fluid is a conductive fluid, such as a brine. Circulation of the fluid can begin before the electrical connection is established.

[0051] In embodiments, the method comprises: priorto lowering the downhole tool into the well, receiving data relating to a hardness of the wellbore casing in a region the window is to be opened; receiving data relating to a hardness of the formation behind the region; and selecting a deflection element having a deflection surface formed of a material that is harder than the formation surrounding the wellbore, but softer than the material of the wellbore casing. The deflection element used can be lighter and easier and cheaper to produce than would be required in a standard sidetracking operation where the casing needs to be drilled using a whipstock. In embodiments, the deflection surface of the deflection element is formed of plastic. In some cases, the method can comprise measuring the hardness of the formation and the hardness of the wellbore casing prior to the selecting.

[0052] In embodiments, the conductive element comprises a plurality of openings in an external surface thereof, and the downhole tool comprises a fluid system which causes fluid to flow into the conductive element and out through the openings to jet against the wellbore casing, and wherein monitoring the corrosion of the wellbore casing comprises monitoring a pressure of the fluid in the fluid system using at least one pressure sensor.

[0053] In embodiments, the shape of the conductive element and the drill bit used for the drilling operation are selected so that the drill bit can pass through the window without any contact with the wellbore casing. This can mean using a conductive element that extends above the top of the deflection surface of the deflection element. In some cases the window may also extend past the edges of the deflection surface in a sideways direction and / or a downwards direction. According to a third aspect of the present invention, there is provided a downhole tool comprising: a insulation element extending along the tool, wherein at least the outer surface of the insulation element is formed of a material that is not electrically conductive; an electrically conductive element positioned radially adjacent the insulation element and extending along the tool; a coupling device for providing an electrical current to the conductive element; and a connection apparatus for establishing an electrical connection between the tool and a section of wellbore casing adjacent an outer surface of the conductive element. Such a device can be used to efficiently open a window in a wellbore casing.

[0054] Embodiments of the present invention will now be described, by way of example only, with reference to the following diagrams wherein:

[0055] Figure 1 illustrates a downhole tool including a deflection element and a conductive element;

[0056] Figure 2 shows a deflection element;

[0057] Figure 3 shows a conductive element;

[0058] Figure 4 shows a conductive element with a smaller diameter at the top than at the base;

[0059] Figures 5 and 6 show parts of a downhole tool comprising a conductive element having a grooved surface;

[0060] Figures 7 illustrates a part of a downhole tool where the conductive element outer surface is provided with non-conductive spacers;

[0061] Figure 8A shows porous spacers with through-holes;

[0062] Figure 8B shows some possible configurations for the spacers;

[0063] Figures 9 and 10 show parts of a downhole tool where the external surface of the conductive element is provided with openings for jetting of fluid therethrough;

[0064] Figures 11 to 15 show possible surface structure for the conductive element of a downhole tool;

[0065] Figure 16 illustrates a downhole tool with a downhole power unit;

[0066] Figures 17 to 22 illustrate different methods for detaching components prior to drilling;

[0067] Figures 23 to 26 illustrate a possible shape of a window opened in a section of wellbore casing; Figure 27 illustrates a deflection element;

[0068] Figure 28 shows examples of conductive element for use with the deflection element of Figure 27; and

[0069] Figures 29 to 31 illustrates a downhole tool according to an exemplary embodiment.

[0070] Examples of a deflection element are shown in Figures 2 and 27. The deflection element 1 includes a deflection surface 11 , and can be anchored within a main wellbore by way of an anchor 2, which is shown in Figure 1. When the anchor is activated, the base 32 of the deflection surface will be located adjacent a region where a lateral wellbore is to be drilled. A lateral wellbore refers to a wellbore that extends away at an angle from the main wellbore. For a conventional whipstock, the material from which the deflection surface is formed must be hard enough to deflect a milling tool towards the wellbore casing in order open a window for a lateral wellbore or towards the formation at the side of a wellbore. Where a casing is present, this means that the material from which at least the deflection surface of the whipstock is formed must be harder than the material of the casing itself.

[0071] Figure 1 shows an example of a downhole tool 24 comprising a deflection element 1 and a conductive element 4, which are positioned so as to be at the same depth in the wellbore during at least the initial phases of a sidetracking operation, and which are coupled together either temporarily / removably or via a fixed coupling. The sidetracking device can also be equipped with an anchor 2, which in this case comprises clamping elements 27 which are activatable to bear against the formation or casing below the deflection element 1 and conductive element 4. The anchor can, for example, comprise expandable parts configured to apply a force outwards onto the interior surface of the metal tubing making up the casing 7 of the well. The anchor 2 functions to secure the deflection element 1 from any movement during subsequent drilling operations. It can be located below the deflection surface, as shown, or an any other position on or coupled to the tool. The anchor can be a separate device placed in the wellbore before the sidetracking device, or an additional mechanism placed in the wellbore after the downhole tool to lock the latter in place. In any of these cases, the downhole tool may comprise elements, such as teeth, which interlock with elements on the anchor to prevent rotational movement of the deflection element once the anchor is fixed in place within the wellbore.

[0072] Coupling to a power source is provided via a coupling device 3, in this case comprising cabling at the top of the element. This can be replaced with a connection to a self-contained power source within the sidetracking element itself, or both options can be used together (i.e. , with the power source within the element being used as a back-up).

[0073] The deflection surface 11 of the deflection element is shaped to achieve a smooth radial displacement of the drill bit from the center of the wellbore towards a side. In the example shown, the surface is concave and sized to correspond to the diameter of a drill bit which will be used to drill the lateral wellbore. The internal surface can be curved both in a radial direction to surround and support the drill bit, but also in the longitudinal direction to gradually angle the drill bit so that it points away from the vertical direction and towards the direction in which the lateral wellbore is to extend.

[0074] The concave deflection surface of the deflection element may be hardened such as by including hard components such as plurality of polycrystalline diamond inserts.

[0075] In some embodiments, the deflection element is electrically insulated from the conductive element using a covering or an insulation. If the part of the deflection element 1 located adjacent the casing 7 on the first side of the sidetracking device (the outer surface 36 of the deflection element) is electrically conductive, which is not always the case, then a covering or insulation can be provided to electrically insulate this part of the deflection element from any electrical current flowing through the conductive element. In some embodiments, the entirety of the deflection element is electrically insulated from the conductive element. This will limit the corrosion process almost entirely to the side of the casing on which the conductive element is located, and there will be very minimal corrosion adjacent the deflection element on the other (second) side of the device. This configuration will result in a window being opened up on one side only of the metal casing through which a lateral wellbore can be drilled, helping to maintain the integrity of the wellbore as a whole and making it easier to seal off one or other of the vertical wellbore and the lateral wellbore if desired. The electrically insulating layer or coating (not shown) may surround or cover the deflection element to insulate the deflection element 1 from the conductive element 4. This insulation both electrically insulates the deflection element 1 from the conductive element 4 and protects the deflection element 1 from the corrosion caused by the electrolytic process.

[0076] The choice of material for a standard whipstock is generally dictated by the hardness of the metal tubing orformation that must be milled in orderto open a window fordrilling the lateral wellbore. Ifthe hardness of the whipstock deflection ramp is greater than that of the metal tubing or formation, any milling tool will be diverted to cut into the casing or formation to form the desired window rather than passing through the material of the deflection surface. When a downhole tool 24 as described herein including a conductive element 4 is used, a window is opened in the casing priorto drilling, and only the hardness of the formation plays a part in whether the deflection surface is able to re-orientate the drill bit as desired. The material of the deflection surface 11 can in this case be lower than that of a conventional whipstock for use in a cased wellbore, which is potentially a big advantage in terms of cost and weight of the device. The deflection element can be made from softer materials and possibly non-electrically conductive materials such as hard plastics or polymers. The deflection element may be homogeneous in terms of the material used to form it, orthe deflection surface may be formed from a different (possibly harder) material than the rest of the element. The conductive element 4, shown in Figures 1 and 3, is used to corrode the metal tubing or casing of the wellbore 7 via an electrolytic process. The conductive element is formed of a conductive material such as steel, stainless steel, aluminum, copper, titanium, graphite, nickel, or another alloy or coating that reduces the overpotential between the conductive element and the electrolyte. The conductive element is positioned within the sidetracking device so that it sits beside and extends along the concave deflection surface 11 of the deflection element 1 . The two elements are therefore located adjacent one another. The inner surfaces of the two elements can contact one another or there can be an electrically insulating layer provided between the two as described above. The conductive element will usually extend above the top 30 of the deflection surface. In the examples shown in the figures where section 23 is present, the conductive element outer surface 42 extends above the deflection surface a distance equal to the height of the section 23.

[0077] During the corrosion process power is provided to the conductive element 4 via an electrical coupling device 3 which connects the conductive element so that a current flows to it from the power source. Current is then transferred by way of an electrolyte (or conductive brine) within the wellbore to the metal tubing forming the casing via transport of electrons within the brine. The electrical circuit is completed by a connection apparatus 13, which can be used to establish an electrical connection between the sidetracking device and the metal tubing. In the device shown in Figure 1 , this connection apparatus is formed of a number of electrical contacts which carry current from the metal tubing to the downhole tool 24. Current is carried back to the power supply, usually via the same coupling device 3.

[0078] The coupling device 3 may comprise a cable or wire for connecting the conductive element to a power source on the surface or subsea directly. Alternatively, the coupling device can be configured to provide a connection to a power source that is collocated with the sidetracking device, i.e. that is located downhole potentially as part of the same downhole tool. Power will always be provided from the power source via the coupling device 3, but the coupling device can take any form provided that it is suitable for transferring current from a power source to the conductive element. In one embodiment the power supplied to the conductive element is high current low voltage. This works well when performing operations fairly close to the surface, in which case any losses caused by the transfer of high current through a wire will not be too high. When operations are to be performed deeper within the wellbore, losses in the wire where high current is supplied are substantial. To address this, high voltage low current electrical power can be transferred along a cable to the tool. A local module can then be used to transform the high voltage low current to high current low voltage at the tool for provision to the conductive element.

[0079] The power source, whether it is located on the surface or downhole, may provide an electrical current between 0 and 15,000 Amps, more preferably between 0 and 6,000 Amps, and most preferably between 0 and 3,000 Amps. Where the power source is not collocated with the sidetracking device downhole and there is no module for conversion of low to high current, the device will preferably be used at depths between 0 and 3000 meters from the surface, more preferably between 0 and 1000 meters and most preferably between 0 and 300 meters.

[0080] Connection apparatus 13 can be placed above or below the conductive element 4, but is preferably located above, which allows easier access to a power source which is located at the surface or above the conductive element 4. The connection apparatus 13 may comprise at least one mechanical, hydraulic, or electro-mechanical clamping element that is movable (usually extendible) to electrically connect the downhole tool to the metal tubing, completing the electrical circuit. Said connection apparatus 13 can be fitted with a release mechanism to cause the clamping elements to return to their original position (to retract if extendible elements are used) should the tool fail, breaking the electrical connection with the metal tubing. The mechanical, hydraulic, or electro-mechanical element(s) can function to anchor and centralize the downhole tool in the casing 7 as in the example shown in Figure 1 , but it can also function solely to provide the required electrical connection.

[0081] The downhole tool 24, which comprises the deflection element and the conductive element, can be provided with a positioning apparatus 14, which is used to adjust the position of the deflection element so that the deflection surface faces the region of the casing where a window is desired to be opened, i.e. faces in the required direction of the sidetrack. The positioning apparatus 14 can be located below or above the deflection element. It can be located between an and the deflection element, above the deflection element (as shown in Figure 1), or as part of an additional downhole tool connected above the sidetracking device (as shown in Figure 16).

[0082] Where an anchor is also present, if the positioning apparatus 14 is located below the deflection element 1 when the sidetracking device is downhole then the anchor 2 will generally be activated to secure the sidetracking device to the casing 7, after which the deflection surface 11 of the deflection element 1 can be oriented towards the direction of the sidetrack by the positioning apparatus 14.

[0083] If the positioning apparatus 14 is above the deflection element 1 , as shown in Figure 1 , then the deflection element can be positioned correctly by the positioning apparatus 14 (which will also in this case rotate the anchor with respect to the casing) prior to securing the assembly to the casing 7 using the anchor 2.

[0084] The shape and position of the conductive element 4, which can be attached along the concave deflection face of the deflection element 1 , will result in the opening of a window in the wellbore casing which is oriented in the direction of the sidetrack. The shape and position of the window 22 in the casing is shown in Figures 23 to 26. The conductive element 4 can be shaped specifically to control the speed of corrosion at different positions on the casing, which will allow for a reduction in power consumption during the electrolytic process. It can be advantageous, for example, for the casing located furthest away from connection apparatus 13 to corrode faster than the casing that is closer to the connection apparatus 13. This ensures that the casing to be corroded is, at all times, electrically connected to the to the sidetracking device via the connection apparatus 13 until the section of casing where the window is to be formed is fully corroded. Failure to control the speed of corrosion in this way may result in portions of casing becoming disconnected early, meaning that they will not be corroded as desired.

[0085] The speed of corrosion is controlled in some cases by adjusting the shape of the conductive element. A preferred shape for the conductive element 4 is shown in Figure 3. As is clear from this Figure, along with Figure 2 which shows the deflection element 1 , the two have corresponding shapes so that they together form a substantially cylindrical sidetracking device. Although a cylindrical compound element is shown in the figures, other shapes are possible. The deflection element 1 in Figure 2 is shaped as a cylinder with a cut-out or missing portion of its volume exposing a deflection surface which is curved radially and extends at an angle to the longitudinal axis of the tool (and to the direction in which the original wellbore extends when the tool is installed). The deflection element is therefore thicker towards its base. The cross-section through the deflection element represents a thin crescent at the top end 30 of the deflection surface (as can be seen in Figure 2) and a circle at the base 32 of the deflection surface. The conductive element 4 is shaped correspondingly with a thicker region at the top end 38, tapering to a point-like structure at its base 40. This shape can be described as generally conical, although its cross-section will not be circular all the way along its length. The conductive element can include an additional section 23 extending above the top of the deflection surface 30 when the elements are installed as part of the downhole tool 24. This section is usually not conductive, but the conductive part of the element 4 (the outer surface 42) extends next to this and ensures that the window extends above the top 30 of the deflection surface.

[0086] Figure 4 shows an example of how the conductive element 4 can be shaped to provide for a faster rate of corrosion for casing lower down in the wellbore than higher up, the upper end being closer to the connection apparatus 13 in the example shown. In general, the shape is selected so that the distance between the surface of the conductive element facing the casing and the casing itself decreases along the length of the element from the end nearest the connection apparatus to the end furthest from it. If the connection apparatus is provided at the bottom of the tool, then the conductive element can be shaped so that the distance to the casing increases along the length of the element from the top to the base to provide for faster corrosion in a region of the casing furthest from the power connection.

[0087] In a specific example shown in Figure 4, the conductive element is shaped as a circular truncated cone, as described above, but the diameter 15 of the element at the top, which in this case is closer to the connection apparatus, is smallerthan the diameter required to exactly correspond to the concave shape of the deflection surface, which in this case is the diameter 16 of the deflection element at the base of the deflection surface. The important factor here is the radius of curvature of the outer surface 42 of the conductive element, which will be larger at the end furthest from the connection apparatus (here the bottom end) so that corrosion happens more quickly in this region.

[0088] These configurations for the conductive element result in a larger volume of electrolyte being present between the conductive element and the casing in the region of the smaller diameter part of the cone 15, which is closer to the connection apparatus 13 (in this case at the top of the device), than in the region having the larger diameter 16 which is located farther away from the connection apparatus 13 (in this case at the bottom or base of the device). This results in more current being diverted to the portion of the casing 7 furthest from the connection apparatus than parts closer to the connection apparatus, so that the more distant portion corrodes more quickly. This helps to prevent parts of the casing from being disconnected from the connection apparatus during the process 13.

[0089] Preferably, where the connection apparatus is located at a top of the tool, radius of curvature 15 of the conductive element outer surface at the top of the conductive element should be between 1 mm and 50 mm smallerthan radius of curvature 16 of the outer surface at the bottom of the conductive element. More preferably, this difference is between 1 mm and 10 mm and most preferably between 2 mm and 5 mm.

[0090] In addition, or as an alternative, the shape of the outer surface 42 of the conductive element 4 which faces the casing to be corroded can be adapted to control the speed of corrosion at different depths or to increase the speed of corrosion to minimize the time required to open the window. Figures 1 1 to 15 show examples of some surface shaping which can be applied to the conductive element, to adjust current density in different regions of the element as compared to a conductive element having a smooth surface. That is, the conductive element may have a smooth surface or may have surface shaping (i.e. a non-smooth surface).

[0091] The external surface of the conductive element (the surface which is exposed when the conductive element and the deflection element are coupled together) can be shaped with grooves 12, which can extend longitudinally or radially across the surface, or in any other direction. The internal surface will generally be smooth to sit flush with the deflection surface of the deflection element, but this can also include surface structure which may make manufacture of the conductive element simpler in some cases.

[0092] One or more of the distance between the grooves, the shape of the grooves, and the size of the grooves can vary across the outer surface. The grooves may be closer together at the base of the element, for example, which will result in a larger surface area in these regions and a faster rate of corrosion in portions of the casing located adjacent these regions. The distance between grooves, the depth of the grooves, and the width of the grooves can all be adjusted to control the speed of corrosion in different regions of the casing as desired. The grooves can be shallower at the top of the conductive element, for example. The grooves may be shaped to have a sinusoidal cross-section. Some examples of shaped surfaces which can be applied to the conductive element are shown in Figures 1 1 to 15. Frustoconical elements are illustrated, but the same surface structure can be applied to at least the outer structure of conductive elements shaped to fit with a deflection element, as described above.

[0093] The shape and materials of the conductive element 4 can also influence consumption of electrical power by the device. During electrolysis hydrogen bubbles may be formed at the surface of the conductive element 4, which will have the effect of decreasing its effective surface area. The bubbles can temporally isolate areas on the external surface of the conductive element 4 from the electrolyte, increasing the overall resistance and increasing the voltage drop and power consumption. When the effective surface area of the conductive element 4 decreases, the current density at the conductive element increases. Higher current densities will result in a higher power dissipation in the form of heat. The surface area of the conductive element 4 can be increased by machining grooves in its surface as described above. The provision of grooves can also therefore help to compensates for the effects of hydrogen bubble formation. The cross-sectional shape of the grooves can be sinusoidal in some embodiments.

[0094] Figure 15 shows a cross-section through a grooved surface of a conductive element 4 in one example. The distance between grooves 52 is preferably between 1 mm and 20 mm, more preferably between 2 and 10 mm and most preferably between 2 mm and 5 mm. Distance 44, which can be adjusted to adapt the depth of the grooves is preferably between 0 mm to 20 mm, more preferably between 0 mm and 15 mm, and most preferably between 0 mm to 5 mm. Where distance 44 is 0 mm, the grooves will have a sinusoidal profile. The radii of curvature for the peaks 46 and troughs 48 can be equal or different from each other. 50 represents the width of the troughs. Measurements 52, 44, 46, 48, and 50 can be selected so as to be different at one end of the conductive element from the other end.

[0095] Where patterning is applied to the surface of the conductive element 4, the surface area of the external surface of the conductive element is larger than the surface area of the internal surface of the metal casing facing the conductive element. In order to avoid sharp edges and an associated peak in current density, the conductive element 4 can be shaped to have curved edges at one or both of its upper or lower ends.

[0096] The conductive element can be provided with one or more non-conductive spacers 17 (show in Figure 7) which are configured to sit against the internal surface of the casing 7 during the corrosion process. The purpose of the spacers 17 is to maintain a physical separation between the conductive element 4 and the casing 7 so that a short circuit is prevented. The non-conductive spacers are sized to maintain a gap that is preferably between 0.05 mm and 20 mm, more preferably between 0.05 mm and 10 mm, and most preferably between 0.5 mm and 2 mm. The non-conductive elements can be shaped to minimize the area that is in contact with the casing. Figure 8B illustrates some possible configurations for the spacers. Spacers 21 have a triangular cross-sectional profile and extend in a longitudinal direction along the conductive element, spacers 20 have a square cross-sectional profile and extend in a longitudinal direction. Spacers 19 and 18 curve around the conductive element. Spacers will extend part or all of the way along the length of the element, and there can be many rows of spacers in some cases, as shown in Figure 7. Combinations of spacers of different shapes can be applied to the outer surface of the conductive element.

[0097] In some embodiments, the spacers comprise a network of small through-holes. They can in some cases be formed of a porous, sponge-like, material which allows brine to pass through, as mentioned above. Figure 8A illustrates an example of spacers 17 including through-holes 54. Any of the different configurations described above can be provided with through-holes, and the through-holes can be evenly spaced or not, depending on need.

[0098] In order to remove byproducts from the electrolytic process, circulation of fluid in the volume located between the conductive element 4 and the surrounding casing 7 is advantageous. The circulation of fluid can be controlled using a circulation device which may be located above or below the deflection element.

[0099] Circulation of the fluid within the wellbore can also be encouraged by jetting fluid through openings in the outer surface of the conductive element 4. Openings for this jetting of fluid are shown in Figures 9 and 10 as components 9, and these function to direct fluid through the surface of the element to form a “focused flow 10” from the outer surface of the conductive element 4 towards the casing 7 (jetting). These opening can have any shape, including circular, oval, or rectangular. The openings can also be sized differently at different positions along the length of the conductive element 4, and can be angled so that the direction of the flow is directed towards the bottom or towards the top of the wellbore as desired. The angle between the direction of flow 10 and the casing 7 can be perpendicular (90 degrees), near parallel (close to 0 degrees), or any angle in between. The apparatus for providing fluid to form the jets can be located anywhere within the tool, such as above the deflection element or below. A pump, propeller, or impeller can be used to move the fluid into the conductive element and out through the openings. This pump can be of any type, including axial flow pumps, rotary pumps, or pumps which work by way of centrifugal motion of parts of the fluid circulation system.

[0100] Instead of, or as well as, including openings and a pumping system for jetting fluid through the openings, fluid can be moved past the outer surface of the conductive element by positioning a propeller above or below this surface (or both), or even inside the conductive element. When the conductive element 4 includes openings 9 for fluid flow to remove byproducts, and there is a system for pumping or moving fluid from the wellbore, into the conductive element, and out through the openings for jetting, there will be a measurable pressure within the fluid circulation system. There will also be a pressure change when the fluid leaves the conductive element 4 and collides with the casing 7. This change in pressure depends on the distance between the outer surface of the conductive element 4 and the casing 7. Shorter distances between the conductive element and the casing will result in higher pressure while longer distance will result in a lower pressure. Therefore, by measuring these pressure variations the change in wall thickness of the casing can be deduced and the status of the dissolution process can be monitored. This method is effective both for monitoring how much corrosion has occurred in the casing, and for confirming when the window 22 in the casing is fully opened at a particular depth within the well. One or more pressure sensors can be placed within the fluid circulation system to measure the pressure of the fluid at one or more locations. Usually, a pressure sensor will be located at least close to the pump within the system and will measure variations in pressure at this point, but a pressure sensor located anywhere within the fluid circulation system will be able to measure the pressure change within the system due to a change in the distance between the openings and the casing.

[0101] In some embodiments, removal of byproducts may additionally and / or alternatively include use of sources of ultrasonic waves, for example ultrasonic transducers such as ultrasonic transmitters, in order to remove, retard or prevent deposition of byproducts such as growth of deposits and / or a depassivation layer at and / or on the metal tubing or casing 7.

[0102] At larger depths within a wellbore, it becomes more challenging to supply power to the tool using a cable to the surface because power losses due to high current passing through the longer cable increases. The downhole tool shown in Figure 16 includes an alternative downhole source of high electrical current in the form of a downhole power unit 5. As described below in more detail, Figure 29 and Figure 30 shows another alternative downhole power unit 31 located inside the deflection ramp 1 but which also can be located below the deflection ramp 1 or in both places. An electrical coupling device 3 providing an electrical connection to the power source is still used to provide high voltage I low electrical current from the power source to the downhole tool. The electrical coupling device may be in the form of a wire or a wired drill pipe. The voltage, for example DC voltage or AC voltage, provided to the downhole power unit 5 and / or 31 of the sidetracking device is preferably between 400 and 3,000 Volts, more preferably between 600 and 1 ,500 Volts and most preferably between 700 and 1000 Volts. Currents provided result in an available power ranging from 0 and 150 KVA, more preferably from 0 to 20 KVA and most preferably between 0 and 15 KVA. The voltage provided can be DC or AC, but it is preferable DC.

[0103] The downhole power unit 5 and / or 31 will then convert the high voltage / low current electrical power provided from the power source into a low voltage / high current electrical power. The converted electrical power is provided to the conductive element 4 to set up an electrical circuit to open a window in casing 7 via corrosion as described above. The current provided by the downhole power unit 5 and / or 31 can range between 0 to 15,000 Amps, more preferably between 0 and 6,000 Amps and most preferably between 0 and 3,000 Amps. Said output currents are preferably DC or pulsating currents and can contain high levels of electrical noise. The output voltage of the power unit 7 is preferably between 0 and 100 Volts, more preferably between 0 and 20 Volts and most preferably between 0 and 10 Volts.

[0104] Once the electrolytic process is complete, which will usually mean that a window has been opened in the casing adjacent the sidetracking device, some components of the device may be pulled out of the wellbore prior to drilling of the lateral wellbore in a sidetrack operation. In one example, the deflection element and components located below this in the well (usually the anchor and possibly the positioning apparatus 14) will remain in the well and other elements, including the conductive element, are detached from the rest of the device, and are pulled out of the well to the surface. In another example, the conductive element remains within the well and is drilled through as explained below.

[0105] Figures 17 and 18 show examples where the upper components of the downhole tool are pulled once the electrolytic process is complete. Here, the conductive element 4, the connection apparatus 13 if this is located above the conductive element, and the positioning apparatus 14 (if present) are pulled along with the electrical coupling 3 to a power source on the surface. The anchor 2, an additional positioning device (if present), and the deflection element 1 remain within the wellbore, coupled to the casing. In the example shown in Figure 19, a downhole power unit 5 is also present, and this is pulled along with the rest of the uphole components.

[0106] Figures 20 to 22 show examples in which the conductive element (including any non-conductive spacers provided thereon) remains within the well after the electrolytic process is complete. The conductive element is detachable from the uphole components, which include one or more of a circulation device, positioning apparatus 14, downhole power unit 5, connection apparatus 13, and coupling device 3, in this case a cable to a surface power source. Where the conductive element is detachable from the uphole components and is left within the well once corrosion is complete, this conductive element will preferably be made of a material that can be drilled by a conventional drill bit, such as graphite or copper among others.

[0107] When a sidetracking operation is to be carried out, the brine 6 contained in the well can first be conditioned to be of the preferred conductivity. This can be achieved by, for example, adding additional compounds (i.e. adding salt to increase the salinity of the brine). The downhole tool including the sidetracking device is then lowered into the wellbore as a conventional wireline or coiled tubing tool until it is positioned at the desired depth. Connection apparatus 13 is activated to close the electrical connection between the source of electrical power, the conductive element 4, the conductive brine 6 and the wellbore casing 7.

[0108] The conductive element 4 is then provided with optimal electrical current by the downhole power unit 5 and / or 31 as shown in Figure 16 and Figures 29 and 30 or directly from the surface as shown in Figure 1 , at which point the accelerated corrosion of the casing 7 starts. The conductive element 4 can also be vibrated at an optimal frequency to allow any gas bubbles forming on the outer surface of the conductive element 4 to escape more quickly. The fluid circulation system, which may include systems for jetting flow 10 through the openings in the conductive element helps to circulate the fluid present between the conductive element 4 and the casing 7. This circulation helps to remove the byproducts from the electrolysis.

[0109] The physical changes in casing 7, caused by corrosion, will result in changes in the conditions of the electrolytic process, affecting the voltage drop between the conductive element 4 and the casing 7 as well as the electrical current and power consumption. By monitoring these electrical properties, the operator, orthe tool itself, can decide if the casing has been fully corroded at a particular position. If jets are used to remove byproducts, there will be a measurable pressure within the fluid system carrying fluid into the conductive element body and forcing it out through openings 9 in the outer surface. There will also be a pressure variance when the fluid leaves the outer surface of the conductive element 4 and collides with casing 7. As described above, the status of the corrosion can also or alternatively be monitored by monitoring the pressure within the fluid system used for the jets.

[0110] Once it is confirmed that a window 22 of the desired size has been opened in the casing, the electrical power to the cathode is switched off and the electrolytic process ends.

[0111] The next step is to pull out of the wellbore the components that are blocking access to the recently opened window 22 and the deflection element 1 . To achieve this, some components are detached from the assembly and pulled out of the wellbore. As described above, in some cases the conductive element 4 can be detached from the deflection element 2, and this can be pulled out of the well along with all components located above it. In other examples the conductive element can be detached from uphole components and the uphole components can be removed while the conductive element remains coupled to the deflection element.

[0112] Elements located above the conductive element during use will most often include one, some, or all of the electrical components, coupling device to the surface 3 for coupling to a power source, the connection apparatus 13 for providing an electrical connection between the casing 7 and the sidetracking device, a positioning apparatus 14, and a circulation device 8. Components which are generally located downhole from the conductive element, and which will usually remain within the wellbore, are the deflection element 1 , an anchor 2, and possibly a positioning apparatus 14 (which may be present in place of or in additional to the uphole positioning unit 14). Where the conductive element remains coupled to the deflection element and remains downhole, this is preferably made of material than can be drilled by a conventional drilling or milling assembly.

[0113] After the desired components have been pulled out of the wellbore, the deflection element 1 and the window 22 are accessible to begin drilling of a lateral wellbore. If the conductive element 4 has been removed, then the formation external to the window is drilled in the required direction during the sidetracking operation. If the conductive element 4 remains in the wellbore, this will also be drilled when performing the sidetracking operation. The material of the conductive element will then be selected to be soft enough for a BHA to drill through.

[0114] The exemplary embodiment illustrated in Figures 29 to 31 further improves the efficiency of the sidetracking operations. This embodiment permits the window to be opened and the drilling of the sidetrack to be performed in one run or trip.

[0115] This embodiment comprises:

[0116] Power Source (not shown): a high voltage electrical power source, as described previously, that can be located at the surface or if flow of mud is available, in the form of a downhole mud turbine generator that converts energy of the flowing mud into electricity; and

[0117] Electrical coupling 3 that connects the assembly to the Power Source and can be in the form of a wired drill pipe 29, a wire run through the drill pipe or coil tubing, or if flow of mud is available, the connection can be to a downhole mud turbine generator.

[0118] A typical and known Drilling BHA 34 may be used for the drilling of the drillable components of the downhole tool 25 and the drilling of the lateral wellbore.

[0119] Coupling apparatus 36 includes a mechanism to attach the BHA 34 or drill bit 35 to the lower components of the embodiment. The mechanism can be controlled to disconnect the BHA from downhole tool 25, or the mechanism can also be made of material that can be drilled by the drilling bit 35. The coupling apparatus 36 can also direct the flow exiting the nozzles of the drill bit 35 into the lower assembly to be circulated between the conductive element 4 and the metal tubing 7 and used for removal of the byproducts and cooling of the downhole tool 25. The coupling apparatus 36 can be a part of, for example an integral part of, or be included in the downhole tool 25.

[0120] The downhole tool 25 shown in Figures 30 and 31 provides an example of a downhole tool 24 in which the high electrical current power unit 31 , is located inside the deflection ramp 1 and I or below the deflection ramp 1 . The power unit 31 receives electrical power from the power supply (not shown). The high electrical current is provided by the power unit 31 to the Conductive Element 4, made of drillable material. The downhole tool 25 also differentiates from the downhole tool 24, as described above, in that the connection apparatus 13 is located below the conductive element 4 and the deflection ramp 1 .

[0121] As the BHA 34 is between the electrical coupling 3 and the high electrical current power unit 31 , an electrical connection is implemented to transfer the high voltage power from the electrical coupling 3 to the high electrical current power unit 31 . There are many methods to provide this connection such as: a wire or cable outside the BHA 34 and / or a wire or cable may be integrated in the design and manufacturing of the BHA 34 components. As the electrical power is in the form of high voltage and low current, a low current capacity cable and therefore a cable having a relatively small cross-sectional area may be used. The use of a relatively small cross sectional area cable reduces the requirement for real estate when integrating the electrical connection into the BHA 34, or using existing electrical connection between the BHA components. If the wire of cable is left outside the BHA 34, a relatively smaller encapsulation to protect the wire is required or, if left unprotected, the small cable or wire would not affect the drilling operation if it is separated from the BHA 34. The power may also be transferred wirelessly via electromagnetic induction. If electromagnetic induction is used, a preferred method is to use AC voltage from the power source through the coupling 3.

[0122] In use, the entire assembly including the electrical coupling 3, the BHA 34, the coupling apparatus 36 (if it is a separate component) and the downhole tool 25 are assembled together at the surface, run in hole and placed where the sidetrack operation is planned. After the anchoring and orientation process is completed, the opening of the oriented window can begin.

[0123] The acceleration of the corrosion process to open the window begins when electrical power is provided. This electrical power can be provided directly from the surface or by starting the flow of mud, for example with the use of mud pumps. The mud flow will then be used to power a downhole mud turbine generator known in the industry and usually used to power MWD or LWD tools.

[0124] The high voltage powerfrom the power supply reaches the high current power unit 31 and the corrosion process begins. Monitoring of the process can be done by establishing communications to the downhole tool in a known way, for example via the electrical coupling, a separate wire and / or fiber optic, electromagnetic waves and / or by mud pulses. If mud flow is available, the monitoring of the process to open the window can also be done by monitoring the pressure in the mud line. As the mud flow is directed into the downhole tool, said flow is jetted through openings in the outer surface of the conductive element 4. Openings for this jetting of fluid are shown in Figures 29, 30 and 31 as components 9, and these function to direct fluid through the surface of the element to form a “focused flow 10” from the outer surface of the conductive element 4 towards the casing 7 (jetting). Since the pressure will be affected by the distance between the openings and the surrounding wellbore casing 7, monitoring the fluid pressure within the fluid circulation system for the jets is a simple and efficient way to monitor the progress of the corrosion. Once the oriented window 22 in the metal tubing or casing 7 is fully opened, the drilling BHA 34 is detached from the coupling apparatus and / or the downhole tool 25. This can be done in a known way by rotating the BHA and shearing the connection, for example. Alternatively, a command can be sent to the downhole assembly via mud pulses, electromagnetic waves and / or another known method, to detach the drilling BHA 34.

[0125] The BHA 34 will first drill through the coupling apparatus 36 (if included) and then through the drillable conductive element 4. Once these components are drilled, the drilling of the lateral wellbore can proceed.

[0126] When the drilling of the lateral well is completed, remaining components of the downhole tool 25 can remain in the well (i.e. in situ) and / or be retrieved.

Claims

Claims1 . A downhole tool for use in sidetracking operations comprising: a deflection element extending along the tool and having a deflection surface which is oriented at an angle to a longitudinal axis of the tool; an electrically conductive element positioned adjacent the deflection surface such that it extends along a length of the deflection surface; a coupling device for providing an electrical current to the conductive element; and a connection apparatus for establishing an electrical connection between the tool and a section of wellbore casing.

2. A downhole tool according to claim 1 , wherein the conductive element is electrically isolated from at least an outer surface of the deflection element or at least the outer surface of the deflection element is formed of a non-conductive material.

3. A downhole tool according to any of claims 1 and 2, wherein the conductive element is positioned within the tool such that it extends along the whole length of the deflection surface.

4. A downhole tool according to claim 3, wherein the conductive element is positioned within the tool such that it covers the whole of the deflection surface.

5. A downhole tool according to any of claims 3 and 4, wherein the conductive element extends above the top of and / or outward of the sides and / or below the deflection surface.

6. A downhole tool according to any of claims 1 to 5, wherein the deflection surface and an inner surface of the conductive element have corresponding shapes such that the two surfaces fit together.

7. A downhole tool according to any of claims 1 to 6, comprising an attachment device for removable coupling of the conductive element to the deflection element.

8. A downhole tool according to any of claims 1 to 7, wherein the conductive element is detachable from components of the tool that are located above the deflection element when the tool is inserted in a wellbore.

9. A downhole tool according to any of claims 1 to 8, wherein the conductive element comprises a material having a hardness that is lower than the hardness of the material from which the deflection surface is formed.

10. A downhole tool according to any of claims 1 to 9, wherein the deflection surface is formed of a material that is harder than the formation surrounding the wellbore, but softer than the material of the wellbore casing.11 . A downhole tool according to any of claims 1 to 10, wherein the conductive element comprises a plurality of openings in an external surface thereof, and the downhole tool comprises a fluid system which causes fluid to flow into the conductive element and out through the openings to jet against the wellbore casing.

12. A downhole tool according to claim 11 , comprising at least one pressure sensor for monitoring a pressure of the fluid in the fluid system.

13. A downhole tool according to any of claims 1 to 12, wherein the conductive element is shaped to sit closer to the casing at an end that is located furthest from the connection apparatus.

14. A downhole tool according to any of claims 1 to 13, comprising: a positioning apparatus for adjusting the position of the deflection surface; an anchor activatable to prevent movement of the deflection element relative to the wellbore casing in a direction along the wellbore; and / or means for disconnecting components of the tool from the deflection surface.

15. A downhole tool according to any of claims 1 to 14, wherein the outer surface of the conductive element is provided with one or more non-conductive spacers formed of a porous material and configured to sit against the internal surface of the section of casing.

16. A downhole tool according to any of claims 1 to 15, wherein the high current power unit providing the electrical current to the conductive element is located inside and / or below the deflection element.

17. A downhole tool according to any of claims 1 to 16, wherein the connection apparatus for establishing an electrical connection between the tool and a section of wellbore casing is located above and / or below the deflection element.

18. A downhole tool according to any of claims 1 to 17, comprising: a connection for a drilling BHA and / or a drilling bit, and / or means to disconnect a drilling BHA from the downhole tool.

19. A method for performing a sidetracking operation, comprising: lowering a downhole tool into a well to a desired kick-off depth, the downhole tool comprising a deflection element extending along the tool and having a deflection surface which is oriented at an angle to the longitudinal axis of the tool, and an electrically conductive element positioned adjacent the deflection surface such that it extends along a length of the deflection surface;coupling the conductive element to a power source to providing an electrical current to the conductive element; activating a connection apparatus to establishing an electrical connection between the tool and the section of wellbore casing to cause it to corrode; monitoring the corrosion of the wellbore casing; and, once a window of a desired size has been opened in the casing, initiating a drilling operation wherein a drill bit bears against and is directed by the deflection surface through the window in the casing to drill a lateral wellbore.

20. A method according to claim 19, comprising, prior to activating the connection apparatus, orienting the deflection surface in the required direction of the sidetrack and securing the downhole tool to the wellbore casing.21 . A method according to any of claims 19 and 20, comprising disconnecting components of the tool from the deflection surface and removing these from the wellbore prior to initiating the drilling operation.

22. A method according to claim 21 , wherein the components to be removed from the wellbore do not include the conductive element.

23. A method according to any of claims 19 to 22, comprising: prior to lowering the downhole tool into the well, receiving data relating to a hardness of the wellbore casing in a region the window is to be opened; receiving data relating to a hardness of the formation behind the region; and selecting a deflection element having a deflection surface formed of a material that is harder than the formation surrounding the wellbore, but softer than the material of the wellbore casing.

24. A method according to any of claims 19 to 23, wherein the conductive element comprises a plurality of openings in an external surface thereof, and the downhole tool comprises a fluid system which causes fluid to flow into the conductive element and out through the openings to jet against the wellbore casing, and wherein monitoring the corrosion of the wellbore casing comprises monitoring a pressure of the fluid in the fluid system using at least one pressure sensor.

25. A method according to any of claims 19 to 24, wherein the shape of the conductive element and the drill bit used for the drilling operation are selected so that the drill bit can pass through the window without any contact with the wellbore casing.

26. A method according to any of claims 19 to 25 wherein a downhole tool for use in sidetracking operations is attached to a drilling BHA below the drilling bit.

27. A method according to claim 19 to 26 wherein the power is provided to the downhole tool via a wire, a wired drill pipe and / or by using mud flow to power a downhole turbine.

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