Houston is on higher ground. The refineries aren't.
I am not saying the Houston area refineries could ride out a katrina like storm without serious damage. Just that after the storm the refineries would not be under water and the to and from roads would not be underwater either. Also keep in mind that Houston is 50 miles inland from the Gulf of Mexico.
Hopefully there will be no more Katrinas this year. Local folklore has it that every winter with snow is followed by a major huricane, Houston had a white Christmas last winter.