Method of using gelled fluids with defined specific gravity

1. A method of preventing ingress of an unwanted subsurface fluid into a subsurface apparatus which comprises introducing into the entire interior of the subsurface apparatus prior to introducing the subsurface apparatus to the subsurface a barrier fluid comprising a base fluid and a gellant system wherein the amount of base fluid in the barrier fluid is between from about 90 to about 99 volume percent wherein the barrier fluid, at subsurface environmental conditions, has a specific gravity within 0.01 of the specific gravity of the unwanted fluid.

2. The method of claim 1 , wherein the specific gravity of the barrier fluid is within 0.003 of the specific gravity of the unwanted fluid.

3. The method of claim 1 , wherein the subsurface apparatus is a pipe, pipeline, manifold or spool.

4. The method of claim 1 , wherein the amount of chloride in the barrier fluid is less than 50 ppm.

5. The method of claim 1 , wherein the unwanted fluid is fresh water, seawater or a hydrocarbon.

6. The method of claim 1 , wherein the barrier fluid is substantially free of a gas.

7. The method of claim 6 , wherein the barrier fluid does not contain a gas.

8. The method of claim 1 , wherein the gellant system contains a viscosifying polymer and a crosslinking agent.

9. The method of claim 8 , wherein the base fluid contains fresh water or a hydrocarbon.

10. The method of claim 8 , wherein the base fluid contains the combination of an alcohol and glycol.

11. The method of claim 10 , wherein the base fluid contains one of the following groups:

(a) methanol and ethylene glycol;

(b) methanol and propylene glycol;

(c) ethanol and ethylene glycol; and

(d) ethanol and propylene glycol.

12. The method of claim 8 , wherein the base fluid contains a weighting agent.

13. The method of claim 12 , wherein the weighting agent is a salt selected from the group consisting of an acetate and a dialkali hydrogen phosphate.

14. A method of preventing ingress of an unwanted subsurface fluid into a subsurface apparatus wherein the unwanted subsurface fluid is selected from the group consisting of fresh water, seawater and crude oil, the method comprising introducing into the subsurface apparatus a gelled barrier fluid wherein the barrier fluid, at subsurface conditions, has a specific gravity which is within 0.01 of the specific gravity of the unwanted subsurface fluid and wherein the entire interior area of the apparatus, after being filled with the gelled barrier fluid, is free of a gas.

15. The method of claim 14 , wherein the gelled barrier fluid is prepared by first preparing a base fluid having a specific gravity less than the unwanted subsurface fluid and then introducing to the base fluid a gellant comprising a viscosifying polymer and crosslinking agent.

16. The method of claim 14 , wherein the gelled barrier fluid is added to the subsurface apparatus above ground and before the apparatus is placed in the subsurface.

17. A method of preventing ingress of an unwanted subsurface fluid into a subsurface apparatus which comprises introducing into the entire interior of the subsurface apparatus a gelled barrier fluid wherein the barrier fluid, at subsurface environmental conditions, has a specific gravity within 0.01 of the specific gravity of the unwanted fluid and further wherein the barrier fluid is pumped into the subsurface apparatus after the subsurface apparatus has been placed into the subsurface.

18. The method of claim 17 , wherein the gelled barrier fluid is prepared from a base fluid having a specific gravity less than the unwanted subsurface fluid, a viscosifying polymer and a crosslinking agent and further wherein the crosslinking agent is added to the viscosifying polymer to form a crosslinked polymer while the base fluid is being introduced into the subsurface apparatus.

19. The method of claim 18 , wherein the crosslinked polymer is a crosslinked cellulosic derivative or guar derivative.