A gas separator assembly has a pump inlet that generates an artificial sump in a production casing. The assembly includes an inner casing in series with the production casing of the well and an outer casing supported externally of the inner casing. First and second ports at opposing top and bottom ends of the outer casing communicate from the production casing to an annulus between the inner and outer casings. A barrier supported in the primary passage between the first and second ports diverts flow through the annulus. One or more dissolvable sleeves within the annulus prevents the annulus from filling with material during construction of the casing but dissolves when the sleeves come in contact with wet wellbore fluid.