To engineer a huge expansion in the chemicals business, the oil giant has put a woman in charge Nabilah Al-Tunisi was wrapping up an executive M.B.A. at Stanford University two summers ago when she got an urgent call from Saudi Aramco headquarters in Dhahran. Pack your bags, she was told, and move to Houston. Al-Tunisi had just been put in charge of the engineering on a new $25 billion refinery and petrochemicals plant--the Ras Tanura Integrated Project. Ras Tanura, a tiny finger in the Persian Gulf, already has a 550,000-barrel-per-day refinery, Aramco's biggest. Under a joint venture with Dow Chemical...