A depressurization tool is described for use downhole in depressurizing an isolated zone. A decompression chamber containing a compressible fluid volume is described. The opening of the chamber is sealed with a closure that is configured to open upon application of a pressure differential across the opening. When used downhole within an isolated and nonpermeable wellbore zone, excessive ambient pressure will cause the closure to open and allow the chamber to fill with fluid at increased pressure, depressurizing the wellbore zone. The tool is useful in wellbore completion systems that include sliding sleeves.