The invention relates to a device for use in a wellbore, the device comprising a dissolvable material for being dissolved over a limited time period, to a tubing comprising the device, and to use of the device. In the oil and gas industry, situations arise where it is necessary or beneficial to block the flow of fluid in a tubing for a limited time. This is typically done by installing a plug in the tubing, where the plug comprises a material which will dissolve over time. In this way, the desired effect is achieved for a limited time. Dissolving material has been used in oil and gas wells for several years to create temporary barriers or activate downhole tools. The process takes place when a dissolvable material, typically a special alloy of magnesium, is in contact with chloride-containing water. However, the dissolvable material at times dis solves too quickly, at least partly because the plug is usually very small, whereby the time period of the effect is too short.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art. The object is achieved through features which are specified in the description below and in the claims that follow. The invention is defined by the independent patent claims, and the dependent claims de fine advantageous embodiments of the invention.

In a first aspect the invention relates more particularly to a device for use in a wellbore, the device comprising a dissolvable material for being dissolved over a limited time period, wherein the device further comprises an electrically insulating material covering a portion of the dissolvable material for keeping the dissolvable material out of contact with any more noble material when the device is in use. The dissolvable material and the more noble material may typically both be metals, and the dissolution of the dissolvable material may for example occur by corrosion such as oxidation. When in use, the device is typically in contact with a fluid, for example saltwater or a wellbore fluid, which may act as an elec trolyte. The invention is based on the observation that the dissolvable materials used in prior art are generally susceptible to galvanic corrosion and are positioned at locations in the well bore where they are in electrical connection with a more noble material. This will allow electrons to be transferred from the dissolvable material to the more noble material via the electrical connection, which will cause galvanic corrosion of the material which is less no ble, i.e. the one which has the lowest electrode potential. This may significantly increase the rate of dissolution of the dissolvable material when it is susceptible to galvanic corro sion, as it typically has a low electrode potential. To prevent the transfer of electrons, and thereby prevent or decrease the risk of galvanic corrosion, the device includes an electrically insulating material, which prevents or at least decreases electrical connection between the dissolvable material and any more noble material. This has been observed to considerably decrease the rate of dissolution of the dissolvable materials which are typically used, since the rapid dissolution of the dissolva- ble material by galvanic corrosion is avoided. The dissolvable material may thereby func tion for a much longer time, sometimes more than five times than if the electrically insulat ing material were not included. This effect is larger the smaller the dissolvable material part is. An important effect of the device is therefore to provide device comprising a dis solvable material, wherein the dissolution process is better controlled regardless of the dissolvable material used. For the dissolvable material to dissolve, a portion of the dis solvable material must be in contact with an electrolyte when the device is in use. There fore, the insulating material cannot cover the entire surface of the dissolvable material.

The electrically insulating material may be a structural part of the device, for example a ceramic structure configured to have a function in the wellbore after dissolution of the dis- solvable material. It may also be applied as a thin coating onto the dissolvable material, whereby its only function is to prevent electrical connection between the first and more noble material. The electrically insulating material may be any non-conducting material, for example a ceramic or a plastic material, depending on what is most suitable for the specific application of the device. The more noble material may be a part of the device itself, for example if the device is configured to block fluid flow for a limited time and the restrict fluid flow after the dissolva ble material is dissolved. The device may for example comprise a nozzle made of the more noble material which provides strength and resist long-term wear, a plug made of the dissolvable material for blocking fluid flow for a limited time, and an electrically insulat ing material between the two to prevent electrical connection, thereby increasing the time period in which the device functions as a plug. Alternatively, the more noble material may be a part of the surroundings, for example a metal tubing in the wellbore. In the example above, wherein the device is configured to initially block fluid flow and then restrict fluid flow after dissolution of the dissolvable material, the device may comprise a nozzle con structed in a strong, electrically insulating material, for example a ceramic material. Final ly, the more noble material may be a part of a complementary tool connected to the de vice. The more noble material may typically be steel or tungsten carbide, as these materials are often used in wellbores.

The dissolvable material may for example comprise magnesium or a magnesium alloy, which has been shown to be able to dissolve within a time period which is suitable for some applications in a wellbore. However, magnesium is susceptible to galvanic corro sion, and it is furthermore positioned at the bottom section of the galvanic series, i.e. it is one of the least noble metals. Dissolution of a dissolvable material comprising magnesium therefore occurs very rapidly if the dissolvable material is in contact with another metal. However, use of magnesium in the dissolvable material assures that it is the dissolvable material which undergoes galvanic corrosion in case of electrical connection, and not any other part of the device or the surroundings. The electrically insulating material may be arranged so that, when the device is in contact with a fluid, any distance between the dissolvable material and the more noble material measured through the fluid is longer than a predetermined value. The predetermined val ue may for example be selected to be large enough to avoid or significantly decrease the risk of an electrical connection being formed through the fluid by an electrically conducting component. During dissolution, a small portion of the dissolvable material may remain as debris close to the device. The debris may for example comprise zinc, aluminum, or a biproduct from the dissolution. If the distance between the dissolvable material and the more noble material of the device is short, particles from debris may form a bridge be tween the dissolvable material and the more noble material which may conduct electricity. This may cause galvanic corrosion and speed up the dissolution rate if the dissolvable material is susceptible to galvanic corrosion. Speeding up the dissolution rate is generally undesirable.

The electrically insulating material may be arranged so that a contact area between the dissolvable material and the fluid is selected to cause the dissolvable material to dissolve over a predetermined time period. Since the dissolution of the dissolvable material occurs when it is in contact with the fluid, the rate of dissolution will depend on the area of the dissolvable material which is in contact with the fluid. Therefore, for a specific fluid and dissolvable material, it will be possible to find a correlation between the contact area and time it takes for the dissolvable material to dissolve. This correlation may be exploited to select the contact area based on the time period over which it is desired that the dissolva ble material dissolves.

The device may typically be configured to block the flow of the fluid for a limited time peri od. Alternatively, the device may be configured to limit fluid flow for a limited time period.

In a second aspect, the invention relates to a tubing comprising the device according to the first aspect of the invention. The device may for example be installed and configured to block the flow of the fluid for a limited time in the radial direction of the tubing. A tubing comprising the device in such a configuration may be particularly useful in the completion step when the tubing is run in the wellbore. The tubing will in this situation allow circulation of fluid through the bottom of the liner to displace the entire wellbore with the desired fluid, function as a mean for well control in case of unexpected pressure build-up, or simply function as a means for fluid circulation and aid installation of tubulars. Additionally, if the tubing comprising the devices can be run in the wellbore with a closed end, it can be used to pressurize the tubing, set production packers, and perform barrier test of the tubing.

In a third aspect, the invention relates to use of the device according to the first aspect to block fluid flow in a wellbore for a limited time period.

In the following is described an example of a preferred embodiment illustrated in the ac companying drawings, wherein:

Fig. 1 shows a sectioned view of an embodiment of the device according to the invention; and

Fig. 2 shows a wellbore comprising a production tubing with the devices installed in the radial surface of the tubing.

In the drawings, the reference numeral 1 indicates a device according to the invention. Identical reference numerals indicate identical or similar features in the drawings. The drawings are presented in a simplified and schematic manner, and the features therein are not necessarily drawn to scale. Figure 1 shows a sectioned view of an embodiment of the device 1 according to the inven tion. The device 1 comprises a dissolvable material 3 in the form of a plug which blocks the flow of fluid until the dissolvable material 3 has dissolved. The device 1 also comprises a more noble material 5 in the form of a nozzle configured to restrict the flow of the fluid through an opening 7 after the dissolvable material 3 has dissolved. An electrically insulat ing material 9 covers a portion of the dissolvable material 3 to keep the dissolvable mate rial 3 out of contact with the more noble material 5. The device 1 comprises a further elec trically insulating material 11 to increase the distance between the dissolvable material 3 and the more noble material 5 measured through the fluid when the device 1 is in use in a wellbore. The device 1 has a first side 13 and a second side 15. When the device 1 is in use in the wellbore, the first side 13 will be in contact with the fluid which acts as an elec trolyte. The dissolution of the dissolvable material 3 will therefore occur on the first side 13, and the distance between the dissolvable material 3 and the more noble material 5 is longer on the first side 13 than on the second side 15. The device 1 is configured to be positioned within an opening complementing the more noble material 5 so that there is no direct fluid communication between the first side 13 and the second side 15 when the de vice 1 is in use before the dissolvable material 3 has dissolved.

Figure 2 shows a wellbore 17 comprising a production tubing 19 installed within. The well head 21 and casing 23 is also indicated. The production tubing 19 comprises a plurality of the devices 1 in the radial surface of the production tubing 19. In this way fluid can initially be pumped through the tubing 19 along the longitudinal axis, but flow through the side of the tubing 19 via the devices 1 is prevented. This may for example be beneficial during the completion step as described above. After a period of a few days, the dissolvable ma terial 3 has dissolved, and hydrocarbon flow from the reservoir 25 can be produced through the openings 7 of the devices 1.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodi ments without departing from the scope of the appended claims. In the claims, any refer ence signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.