Method for sealing cavities in or adjacent to a cured cement sheath surrounding a well casing

1. A method for sealing cavities in or adjacent to a cured cement sheath surrounding a well casing of an underground wellbore, the method comprising the steps of:

providing an expansion device with edged expansion segments that is configured to be moved with the edged expansion segments in an unexpanded configuration up and down through a well casing which is surrounded by a cured cement sheath;

moving the unexpanded expansion device to a selected depth in the well casing; and

expanding the edged expansion segments at the selected depth, thereby pressing circumferentially spaced recesses into an inner surface of a selected casing section and expand an outer surface of the selected casing section into the cured cement sheath thereby sealing the cavities.

2. The method of claim 1 , subsequently comprising returning the expansion device to said unexpanded configuration before moving the unexpanded expansion device up or down through the wellbore.

3. The method of claim 2 , wherein the sealing of the cavities persists after returning the expansion device to said unexpanded configuration.

4. The method of claim 1 , wherein the cavities comprise micro-annuli in and/or adjacent to the cured cement sheath.

5. The method of claim 1 , wherein during the expansion step the expansion device is located at a substantially stationary depth within the wellbore, and the method further comprising returning the expansion device to said unexpanded configuration after expansion of the selected casing section, and subsequently moving the unexpanded expansion device up or down through the wellbore to another depth and expanding the edged expanding segments at said other depth whereby another selected casing section is expanded to seal micro-annuli and/or other cavities at that other depth.

6. The method of claim 5 , wherein the steps of expanding a selected casing section and moving the unexpanded expansion device up or down through the wellbore to another depth where another selected casing section is expanded are repeated several times to seal micro-annuli and/or other cavities at several depths along the length the wellbore.

7. The method of claim 1 , comprising:

moving the unexpanded expansion device to a selected first depth in the well casing;

expanding the edged expansion segments at the first selected depth, thereby pressing circumferentially spaced recesses into an inner surface of the selected casing section and expand the outer surface of the selected the expanded casing section into the surrounding cured cement sheath, while maintaining the expansion device located at a substantially stationary depth; followed by:

returning the expansion device to said unexpanded configuration;

moving the unexpanded expansion device to a selected second depth in the well casing which does not coincide with the first selected depth;

repeating said expanding step at said second selected depth; followed by:

returning the expansion device to said unexpanded configuration;

moving the unexpanded expansion device to one or more selected intermediate depths in the well casing between said first selected depth and said second selected depth; and

repeating said expanding and returning steps at each of said selected intermediate depths.

8. The method of claim 1 , wherein the expansion segments have a substantially V-shaped outer contour in longitudinal direction, and are configured to expand the selected casing section such that it has a ring of substantially V-shaped circumferentially spaced recesses.

9. The method of claim 8 , wherein the expansion segments have V-shaped edges with a segmented ring-shaped outer contour in circumferential direction, and are configured to expand the selected casing section such that the recesses have a substantially V-shaped inner contour in longitudinal direction, which section is connected to adjacent non-expanded casing sections by smoothly outwardly curved concave semi-expanded casing sections.

10. The method of claim 9 , wherein the length of the substantially V-shaped edge is less than 20 cm.

11. The method of claim 1 , wherein at least part of the outer surface of the expanded casing section and of the surrounding cured cement sheath is plastically deformed as a result of the expansion.

12. The method of claim 1 , wherein the expansion device comprises a hydraulic actuation assembly that radially expands and contracts the expansion segments.

13. The method of claim 1 , wherein the expansion device is suspended from one of the group consisting of: a tubular string, a wireline and an e-line; through which electric power and/or signals can be transmitted between the expansion device and a control assembly at the earth surface.