Downhole apparatus and methods

An assembly for location downhole, the assembly comprising:

• bore-lining tubing for location in a drilled bore;
• an inner tubing for extending from a distal end of the bore-lining tubing to a surface structure;
• a coupling at the distal end of the bore-lining tubing;
• a coupling at a distal end of the inner tubing for engaging and sealing with the coupling at the distal end of the bore-lining tubing;
• a proximal seal between the bore-lining tubing and the inner tubing;
• a sealed inner annulus between the distal ends of the bore-lining tubing and the inner tubing and the proximal seal;
• a selectively openable flow port in the distal end of the inner tubing for flowing fluid between the inner tubing and the inner annulus, and a volume of buoyant material retained within the inner annulus, the assembly having a first configuration and a second configuration, in the first configuration the flow port in the distal end of the inner tubing being closed and the volume of buoyant material being sealed within the inner annulus and the inner annulus being isolated from the inner tubing, and in the second configuration the flow port in the distal end of the inner tubing being open whereby fluid may flow between the inner tubing and the inner annulus.

A method of cementing bore-lining tubing in a drilled bore, the method comprising:

• isolating at least a portion of an inner annulus defined between a bore-lining tubing and an inner tubing extending through the bore-lining tubing, with the isolated portion of the inner annulus containing a compressible fluid;
• flowing cement slurry through the inner tubing and into an outer annulus surrounding the bore-lining tubing, with the cement slurry in the inner tubing at a first pressure;
• flowing a second fluid into the inner tubing whereby a column of the second fluid in the inner tubing generates a hydrostatic pressure while maintaining the compressible fluid contained within the isolated portion of the inner annulus at a second pressure lower than the first pressure and lower than the hydrostatic pressure generated by the column of the second fluid in the inner tubing;
• maintaining the compressible fluid contained within the isolated portion of the inner annulus at the second pressure at least until the cement slurry in the outer annulus has at least partially set, and then
• exposing the compressible fluid contained within the isolated portion of the inner annulus to the hydrostatic pressure generated by the column of the second fluid in the inner tubing once the cement slurry in the outer annulus has at least partially set.