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1. (WO2007009247) A METHOD FOR DRILLING AND CEMENTING A WELL
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We claim:

1. A method for drilling and cementing a borehole to access at least a portion of a formation of interest, comprising: drilling a borehole using a drill string including an external packer and a drilling assembly to access at least a portion of a formation of interest; using a drilling fluid selected to minimize adverse effects on the at least a portion of the formation of interest when drilling into the at least a portion of the formation of interest; setting the packer to create a seal in the borehole so that (i) an upper drill string/borehole annulus above the packer is isolated from (ii) a lower drill string/borehole annulus below the packer, with the at least a portion of the formation of interest being open to the lower drill string/borehole annulus; and introducing cement above the external packer into the upper drill string/borehole annulus.

2. The method for drilling and cementing a borehole of claim 1 wherein the drilling fluid generates a hydrostatic pressure lower than the pressure in the formation of interest.

3. The method for drilling and cementing a borehole of claim 1 wherein the drilling fluid is substantially gaseous.

4. The method for drilling and cementing a borehole of claim 1 wherein the drilling fluid includes air.

5. The method for drilling and cementing a borehole of claim 1 wherein the drilling fluid includes foam.

6. The method for drilling and cementing a borehole of claim 1 wherein the external packer is set substantially without exposing the at least a portion of the formation of interest to a damaging hydrostatic head of fluid.

7. The method for drilling and cementing a borehole of claim 1 wherein the external packer is set by compression of the drill string.

8. The method for drilling and cementing a borehole of claim 1 wherein the external packer is set by use of a liquid-based fluid and the step of setting the external packer includes: sealing a lower end of the drill string and introducing a liquid-based fluid to hydraulically set the external packer.

9. The method for drilling and cementing a borehole of claim 8 wherein the step of sealing the lower end of the drill string includes displacing a plug through the drill string to a position at a lower end of the drill string.

10. The method for drilling and cementing a borehole of claim 1 wherein the step of introducing cement above the external packer includes opening a port through the drill string to access the upper drill string/borehole annul us.

1 1. The method for drilling and cementing a borehole of claim 1 wherein the drill string is at least in part formed of wellbore casing and wherein the method further comprises leaving the drill string downhole to case the wellbore.

12. The method for drilling and cementing a borehole of claim 1 1 wherein the drilling assembly is periodically retrieved and reintroduced through the wellbore casing during drilling.

13. The method for drilling and cementing a borehole of claim 1 wherein the external packer is set with its upper end above the formation of interest.

14. A method for drilling and cementing a borehole to access at least a portion of a formation of interest, comprising: drilling a borehole using a drill string including an external packer and a drilling assembly to access at least a portion of a formation of interest, the drilling process including tripping the drilling assembly to surface and reintroducing the drilling assembly during drilling; setting the packer to create a seal in the borehole so that (i) an upper drill string/borehole annulus above the packer is isolated from (ii) a lower drill string/borehole annulus below the packer, with the at least a portion of the formation of interest being open to the lower drill string/borehole annulus; and introducing cement above the external packer into the upper drill string/borehole annulus.

15. The method for drilling and cementing a borehole of claim 14 wherein the drill string is at least in part formed of wellbore casing and wherein the method further comprises leaving the drill string downhole to case the wellbore.

16. The method for drilling and cementing a borehole of claim 14 wherein the drilling proceeds during at least a period of time prior to setting the packer using a drilling fluid hydrostatic pressure lower than the pressure in the formation of interest.

17. The method for drilling and cementing a borehole of claim 14 wherein prior to setting the packer, the method further comprises: retrieving the drilling assembly to surface; and sealing the bottom of the drill string.

18. The method for drilling and cementing a borehole of claim 14 wherein after cementing, the method further comprises: drilling using a drilling fluid hydrostatic pressure lower than the pressure in the formation of interest.