Expandable tubulars to isolate production casing

A method comprising:

• in a production well comprising a first production casing nested within a surface casing nested within a conductor casing, a liner downhole of the first production casing, the liner having an outer diameter smaller than an outer diameter of the first production casing:
• lowering a second production casing within the first production casing, the second production casing extending from a surface of the production well to a depth within the production well to which the first production casing extends and uphole of the liner, the second production casing having a smaller outer diameter than an inner diameter of the first production casing and larger than the outer diameter of the liner;
• from the surface of the production well to the depth within the production well to which the second production casing extends, forming a gas-tight seal between an outer wall of the second production casing and an inner wall of the first production casing by expanding the inner diameter of the second production casing from the surface to the depth to which the second production casing extends, wherein an inner diameter of the expanded second production casing is larger than the outer diameter of the liner;
• installing a production tubing within the second production casing from a surface of the production well to uphole of the liner, the production tubing extending a same length as the second production casing;
• flowing hydrocarbons in an uphole direction through the liner and into the expanded second production casing.