Abstract A new technique for zone isolation in horizontal wells has been developed and tested in the laboratory. The technique consists of three sequential stages:setting a chemical wellbore plug upstream of the problem zone,spotting and squeezing a gel into the problem zone, andwashing or drilling out excess gel and the wellbore plug to clean the borehole prior to production. Two main problems were investigated using laboratory horizontal wellbore models:placement of the wellbore plug so that slumping would not occur, andselection and testing of chemicals that could be used to make wellbore plugs with sufficiently high holding pressures. The horizontal wellbore models consisted of PVC pipes internally lined with sand. Three chemicals, used in the oil industry for gas and/or water shut-off, were selected for the study: a monomer, a polyacrylamide, and a plastic. The x-ray CT scanner was used to obtain cross-sectional images of the plug to help understand the shear mechanisms involved. Experimental results indicate that a plug could be placed in a horizontal wellbore with minimum slumping if the plug is introduced into a viscous completion brine pill. For the plastic plugs, the completion brine contained 100,000 ppm NaCl and 4 lb/bbl carboxyl methyl cellulose. Only the plastic plug had a sufficiently high holding pressure (343 psi for 37 in. long plug of 1 in. diameter). The experimental study indicates the proposed zone isolation technique to be viable. Research is being continued to evaluate the effectiveness of the wellbore plug during displacement of formation gels. P. 435