In wells drilled for petroleum production, a plurality of well casings of different sizes are suspended from a wellhead.  A problem encountered in such wells is that of annular pressure control.  In the annulus between different casing sizes, pressure may develop due to leaks between strings of casing, tubing leaks, packer leaks, wellhead packoff leaks and a poor or failed primary cement job.  Currently, to control the annular pressure, a relatively heavy liquid is pumped into the annulus at the upper end of the well.  The heavy liquid migrates slowly downward, displacing lighter liquid.  This technique is expensive, time consuming and has yielded limited results.