HOUSTON -- On the outskirts of this sprawling city's core, Royal Dutch Shell PLC is in the midst of a major expansion of its corporate offices. About 672,000 square feet of new office space is being built just off Interstate 10 in west Houston for Shell's exploration and production arm, and more is rumored to be coming in the years ahead. Shell will have plenty of neighbors in this area dubbed the Energy Corridor District, where projects under way or kicking off this year will add about 3 million new square feet of office space. Projects totaling 2 million more square feet are proposed and will likely win approval.