Technique and apparatus to form a downhole fluid barrier

1. A method usable with a well, comprising:

communicating a slurry layer downhole into a stage of the well, the communicating comprising using a tool to engage the slurry layer and pumping the tool and the slurry layer together downhole into the well, wherein the tool comprises a screen and a closed chamber that has a lower pressure than a region outside of the chamber; and

opening the chamber to cause a continuous fluid phase of the slurry layer to separate from the slurry layer and flow through the screen into the chamber to create a plug in the stage.

2. The method of claim 1 wherein:

the slurry layer is pushed from behind by the tool downhole into a passageway of the well; and

the plug is formed in front of the tool in the passageway by dehydrating a portion of the slurry in the slurry layer.

3. The method of claim 2 , wherein dehydrating the portion of the slurry comprises removing a continuous fluid phase of the slurry to increase a viscosity of the portion of the slurry.

4. The method of claim 2 , wherein opening the chamber comprises:

communicating a signal downhole from an Earth surface of the well to the tool to cause the tool to open the chamber in response to the signal.

5. The method of claim 2 , wherein opening the chamber comprises:

opening the chamber in response to the tool autonomously sensing a position of the tool.

6. The method of claim 5 , wherein autonomously sensing the position of the tool comprises using the tool to autonomously sense features of the well and determine the position of the tool based at least in part on the sensed features.

7. The method of claim 6 , wherein determining the position of the tool comprises determining the position based on a recognized sequence of the sensed features or determining the position based on recognition of one the features as being associated with a target downhole location for the plug.

8. The method of claim 2 , further comprising:

using the plug as a boundary of the stage; and

performing a stimulation operation in the stage.

9. The method of claim 2 , wherein the slurry comprises a trimodal mixture of three particles.

10. The method of claim 2 , wherein the passageway comprises a casing string.

11. The method of claim 1 , further comprising:

deploying a perforating gun with the tool into the well;

firing at least one perforating charge of the perforating gun to perforate a formation associated with the stage.

12. The method of claim 1 , further comprising:

waiting for at least a predetermined time interval for the plug to form after the chamber is opened; and

after the waiting, performing a stimulation operation in the stage.

13. The method of claim 1 , further comprising removing the tool from the well, the removing comprising an act selected from the group consisting essentially of:

fishing the tool from the well;

milling to remove the tool;

degrading the tool; and

oxidizing the tool.

14. The method of claim 1 , further comprising:

in at least one other stage of the well, communicating another slurry layer downhole into the at least one other stage and opening a chamber of another tool to receive a fluid to create a plug in the other stage.

15. The method of claim 1 , wherein the slurry layer comprises a trimodal mixture of three particles.