Selective hydraulic fracturing tool and method thereof

1. A selective downhole tool comprising:

a tubular having a longitudinal bore enabling passage of fluids there through and having a valve opening in a wall of the tubular;

an expandable ball seat selectively movable between a first size sized to trap a ball to block flow through the tubular and a larger second size sized to release the ball through the tubular; and

a valve cover longitudinally movable within the tubular, the valve cover including a dissolvable insert;

wherein the insert covers the valve opening in a first condition and is longitudinally movable within the tubular to expose the valve opening in a second condition, and the insert re-covers the valve opening in a third condition.

2. The selective downhole tool of claim 1 wherein the valve cover cooperates with the ball seat and is longitudinally movable with the ball seat in response to a pressure change within the tubular.

3. The selective downhole tool of claim 1 wherein the ball seat has the first size in the first and second conditions, and the second size in the third condition.

4. The selective downhole tool of claim 3 , wherein the insert is dissolved in the fourth condition.

5. The selective downhole tool of claim 1 , wherein the expandable ball seat includes a collet having a plurality of fingers, the free end of the fingers moving from the first size to the second size, a base connecting a fixed end of the fingers.

6. The selective downhole tool of claim 1 , further comprising a shear pin fixedly connecting the valve cover to the tubular in a run-in condition of the tool.

7. The selective downhole tool of claim 1 , wherein the dissolvable insert includes a selectively degradable material including a sintered powder compact formed from electrochemically active metals.

8. A selective downhole tool comprising:

a tubular having a longitudinal bore enabling passage of fluids there through and having a valve opening in a wall of the tubular;

an expandable ball seat selectively movable between a first size sized to trap a ball to block flow through the tubular and a larger second size sized to release the ball through the tubular;

a valve cover longitudinally movable within the tubular, the valve cover including a dissolvable insert disposed in an aperture in the valve cover, and the aperture is selectively alignable with the valve opening; and

an indexing apparatus engageable with the expandable ball seat, the expandable ball seat lockable in one of the first size and the second size by the indexing apparatus.

9. The selective downhole tool of claim 8 , wherein the indexing apparatus includes an indexing sleeve having an indexing path, an indexing pin movable with respect to the indexing sleeve, and at least one spring biasing member acting on the indexing pin.

10. The selective downhole tool of claim 9 , wherein the at least one spring biasing member includes a compression spring on one side of the indexing pin and a compression spring on an opposite side of the indexing pin.

11. The selective downhole tool of claim 9 , wherein the indexing path includes a uphole extending first section to lock the ball seat in the second size, a downhole extending second section allowing movement of the indexing pin, and an uphole extending third section shorter than the first section to lock the ball seat in the first size.

12. The selective downhole tool of claim 11 , wherein the indexing path is a continuous path around a diameter of the indexing sleeve and includes a second section interposed between every first section and third section.

13. A method of operating a downhole tool, the method comprising:

running the downhole tool in a bore hole, the tool including a tubular having a valve opening covered by a valve cover;

moving the valve cover longitudinally to expose the valve opening;

re-covering the valve opening with the valve cover subsequent an operation through the valve opening; and

dissolving a portion of the valve cover to re-expose the valve opening.

14. The method of claim 13 , further comprising repeating exposing the valve opening, performing an operation through the valve opening, and re-covering the valve opening for a plurality of valve openings and corresponding valve covers, and subsequently dissolving a portion on the valve covers to expose the valve openings.

15. The method of claim 14 , wherein the operation is a fracturing operation performed on a plurality of zones of the borehole, and further comprising allowing entry of production fluids through the valve openings after dissolving a portion on the valve covers.

16. The method of claim 14 , wherein an order of operations performed through the valve openings is a top-down order where a first operation is performed through an upholemost valve opening and a last operation is performed through a downholemost valve opening.

17. The method of claim 14 , wherein an order of operations performed through the valve openings is a center encroaching order where successive operations are performed alternatingly through downhole and uphole valve openings closing in on a center valve opening.

18. The method of claim 13 further comprising:

dropping a ball in the tubular into an expandable ball seat;

catching the ball within the ball seat;

building pressure within the tubular and forcing the ball and ball seat in a downhole direction; and,

bleeding pumping pressure;

wherein moving the valve cover longitudinally occurs with the building of pressure within the tubular and re-covering the valve opening with the valve cover occurs with the bleeding of pumping pressure.

19. The method of claim 13 , wherein the valve cover is fixedly attached to the tubular via a shear screw while running the downhole tool in the bore hole, and further comprising shearing the screw after the valve opening is aligned with a target zone in the bore hole.