A through-tubing multilateral system for downhole oil drilling operations includes a tubing extension positioned in a downhole end of a tubing string in a wellbore and an anchoring system configured and positioned to anchor the tubing extension in the wellbore. The tubing extension is dimensioned to accommodate the installation of a multilateral junction therein and has an outside diameter that is less than an inside diameter of the tubing string. The tubing extension has a body portion configured to be tubular in structure and a thin walled section attached to one end of the body portion. The thin walled section has a wall thickness that is less than a wall thickness of the body portion. A method of extending the tubing string in the wellbore includes running the tubing extension into the tubing string such that an uphole end of the tubing extension is overlapped by the downhole end of the tubing string, expanding the tubing extension such that the tubing extension is secured in position by the tubing string, and anchoring the tubing extension in the wellbore.

A multilateral well construction and sand control completion. In a described embodiment, a well completion includes first and second wellbores intersecting at an intersection; an assembly positioned in the second wellbore, the assembly including a packer and a well screen, the packer being sealingly engaged with the second wellbore; and a wellbore connector sealingly connected to the assembly, the wellbore connector also being sealingly engaged in the first wellbore on opposite sides of the intersection, and the wellbore connector isolating the intersection from fluid flow through the assembly in the second wellbore and from fluid flowing through the wellbore connector between the opposite sides of the intersection.

In a wellbore system, non-return valves placed in one or more well boreholes within the system. The valves are placed in the production string downstream from the producing zones, the valve having a closure member selectively biased to a closed position against flow from the producing zones. A fluid backpressure in the production string further operates to bias the valve to the dosed position. When the fluid backpressure is reduced, the force exerted by the flow from the producing zone against the closure member is sufficient to move the closure member in its open position. The closure member is maintained in its open position as a result of the force exerted by the flow from the production zone. An increase in fluid backpressure on the valve results in the movement of the closure member from the open position to the closed position.

The method and apparatus can create excavation cavities suitable for drilling multilateral wellbores through the excavation using a kick-off. One embodiment of the method comprises isolating a wellbore at a determined location, excavating a downhole cavity in a wellbore above the determined location, and removing the apparatus for isolating the wellbore and the apparatus for excavating the downhole cavity from the wellbore to provide full access to the wellbore and the excavation cavity.

A pack-off system for packing off an area of interest in a wellbore, the pack-off system comprising a body, two spaced-apart selectively settable packing elements on the body for sealing off the area of interest, selectively actuatable setting apparatus connected to the body for selectively setting the two spaced-apart selectively settable packing elements, the selectively actuatable setting apparatus actuatable by fluid under pressure introduced into the pack-off system. A method for packing off an area of interest in a wellbore, the method including installing a pack-off system as described herein in the wellbore to pack-off the area of interest. Such a method may also include flowing treatment fluid from the pack-off system to an area of interest in an earth formation and/or adjacent a wellbore in the earth.