A chemical packer composition having a polyurethane foam with nanoparticles or micron-sized particles for use as an annular chemical packer in openhole horizontal wells is provided. The chemical packer composition may be used in a horizontal well having a screen (for example, a gravel pack screen) and completed using an openhole completion. The chemical packer composition may include, for example, silica nanoparticles or sand micron-sized particles. The chemical packer composition may be placed in an annulus section defined by the wellbore and the screen that traverses one or more fluid producing zones. In some instances, a portion of the chemical packer composition may be selectively removed to open a fluid producing zone to the wellbore and form plugs blocking other fluid producing zones.