The present invention relates to a method for stabilizing a cavity in a well.

It is known to stabilize open annuli in production and injection wells in order to avoid sand production. Today, this is usually done by means of so-called gravel-packing. Gravel and/or sand is packed around a sand screen or a perforated casing to function as a sieve by preventing finer sand from the formation from being carried in petroleum into the well. Another alternative has been to stabilize formation sand by supplying resinous materials to "glue" the formation together.

Gravel-packing is connected with a high risk of not succeeding in placing the sand/gravel pack, especially in long horizontal wells. It may be challenging to place sand and gravel packs in production and injection wells in which packers divide the annulus along the well path into several production or injection intervals. In addition, there are not any good solutions, either, for stabilizing the annulus for several production or injection intervals when there are inflow or outflow valves along the well path and different pressure conditions in the different formations that divide the well into several zones. Today, these are cemented and perforated and cannot be completed with sand screens in the entire production or injection interval. Further, it is only the lowermost part of the well that is gravel-packed . There is also a great risk of erosion on pipes and equipment in the well if the sand/gravel pack leaks through the sand screen or a perforated casing. If the annulus is closed naturally by the formation sand in one or more places along the well path, the entire well length cannot be sand- /gravel-packed in a satisfactory manner and the gravel-packing will be incomplete. When the formation sand is glued, it is then to be fractured to enable production. This method is time-consuming and the directions of the fracture systems are not predictable. This means that a risk arises that the well does not produce/inject in the right formation intervals. In sum, the known methods are generally expensive, complicated and not very flexible.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art or at least provide a useful alternative to the prior art.

The object is achieved through features which are specified in the description below and in the claims that follow.

The invention relates, more specifically, to a method for stabilizing a cavity at a production or injection zone in an underground well, the method including the steps:

(A) providing a filtering element in the well at the cavity which is to be stabilized, the filtering element being formed with openings; and

(B) injecting a first fluid including expandable particles through the filtering element into the cavity, the expandable particles, in a non-expanded state, having a diameter which is smaller than the diameter of the openings of the filtering element, characterized by the method further including the step:

(C) injecting a second fluid through the filtering element, the second fluid being a rranged to react with the expandable particles in such a way that the expandable particles are expanded to a diameter that is larger than the diameter of the openings in the filtering element, whereby the expanded expandable particles and the filtering element form a filter at the production or injection zone in the well.

In one embodiment, the steps (B) and (C) may include injecting the first and/or second fluid(s) through a fluid-carrying string. In an alternative embodiment, the fluids may be pumped down into the well from the wellbore opening.

The cavity to be stabilized may include various types of cavities, annuli and formation fractures in an underground well.

The expanded particles may thus function as a filter together with a filtering element such as a sand screen and/or a perforated casing or an inflow control device or an outflow control device.

The expandable particles may for example include an elastomer. The particles may further include one or more layers of organic and/or inorganic materials. It is known that some elastomers can expand on contact with hydrocarbon-containing fluids and/or with water containing various added chemicals. The second fluid may thus be a fluid including hydrocarbons and/or water.

In one embodiment, the method may include injecting a mixture of expandable and porous particles. This may be beneficial if expandable particles are used that, on expansion, attach to each other and thereby do not allow sufficient flow through the expanded particles. The porous particles may, for example, be taken from a group i n- eluding : macroporous silica, macroporous carbon, macroporous polymer particles, volcanic rocks, for example pumice, diatomite (diatomaceous earth), zeolites, sintered ceramic materials and sintered metallic materials.

In one embodiment, the method may, as an alternative or in addition, include injecting a mixture of expandable particles and non-porous particles like glass spheres, polymer spheres and mineral particles. The non-porous particles may prevent the expandable particles from attaching to each other in such a way that sufficient flow is obstructed.

The above-mentioned particles, both porous and non-porous ones, may have a diameter which is smaller than the diameter of the filtering element. After expansion of the expandable particles, said porous and non-porous particles will be locked into the mixture so that they will not escape back out through the openings in the filtering element, in spite of their size.

The openings in the filtering element and the expandable particles may have diameters in the micrometre range. The final composition of expandable particles and any porous or non-porous materials must allow a flow of hydrocarbons through the filter, that is to say through the expanded particles and the filtering element, into or out of the well.

The method may further, before step (B), include setting one or more packers sealing- ly around the fluid-carrying string within a casing in the well. This may be appropriate in order to isolate the annulus outside the fluid-carrying string so that the expandable particles are carried towards the cavity which is to be stabilized and will not flow up the annulus around the fluid-carrying string.

The filter that is provided in step (A) may, for example, include one or more filtering elements. It may be, for example, a casing with perforations and/or slots. In addition, the filter may include a filtering element placed on the outside of the casing. The filtering element on the outside of the casing may be, for example, a sand screen, of a kind known per se.

Compared with the above-mentioned known methods for stabilizing a cavity at a production or injection zone in an underground well the present invention provides a substantially simplified method which will save much time and which, in addition, gives increased flexibility. That will, among other things, enable annulus-packing of an almost unlimited number of production or injection intervals along the well path. In addition, annulus-packing will be possible independently of local pressure conditions in the well. Annulus-packing will be possible in long horizontal wells, wells with inflow and outflow valves and multilateral wells. The present invention will also reduce the risk of erosion in/on pipes and equipment in the well.

In what follows, an example of a preferred embodiment is described, which is visua lized in the accompanying drawings, in which :

Figure 1 shows a well as used in an embodiment of the present invention, in a side view; and

Figure 2 shows a portion of a well as used in the present invention, in a side view and on a larger scale than figure 1.

In what follows, the reference numeral 1 indicates a well as used in the method of the present invention. The figures are shown in a simplified and schematic manner, and like reference numerals indicate like or corresponding elements. A fluid-carrying string 2 extends down into the well 1, the well 1 being cased, in the portion shown, with a casing 9. In some portions, the casing 9 is provided with sand screens 7. A cavity in the form of an annulus 5 outside the casing 9 is provided with permanent packer elements 3. Packer elements 4 are used to seal an annulus 10 between the fluid-carrying string 2 and the casing 9. The packer elements 4 may be temporary or permanent. A fluid, not shown, including expandable particles 8, see figure 2, is carried down the fluid-carrying string 2, into the annulus 10 between the fluid-carrying string 2 and the casing 9 via openings 21 in the fluid-carrying string 2, further through perforations, not shown, in the casing 9, through a sand screen 7 and into the annulus 5 between the casing 9 and a formation 6 as indicated by arrows in figure 1.

Another fluid, not shown, is then carried through the fluid-carrying string 2 and out to the expandable particles 8. The expandable particles 8 thus expand to a diameter which is larger than the diameter of openings in the sand screen 7, see figure 2, so that the expanded particles 8 cannot escape back into the annulus 10 between the fluid-carrying string 2 and the casing 9. Thus, the expandable particles 8 together with the sand screen 7 form a filter which prevents undesired sand production in the well 1, but which allows the production of hydrocarbons or injection of water, and which supports the formation 6.

Figure 2 shows an enlarged portion of the annulus 5 after the expandable particles 8 have been injected through the sand screen 7 and expanded to a diameter which is larger than the diameter of openings in the sand screen 7.