You should treat HepG2 cells with DMSO (1.5-2%). Treatment of HepG2 cells with DMSO and then incubating with HBV-positive human serum will help enhance and prolong HBV infection. You may want to refer to the attached paper below for the protocol. The investigators in the paper have reported that DMSO-treated HepG2 cells can be infected efficiently by HBV.
Article HBV infection of cell culture: Evidence for multivalent and ...