Royal Dutch Shell Plc will sell its stake in an offshore Thai gas field to a unit of Kuwait Petroleum Corp. for $900 million as the international energy giant continues hawking assets for cash in the midst of a years-long energy slump. Shell reached an agreement to sell two subsidiaries that own a combined 22.2 percent interest in the Bongkot field and adjoining offshore acreage to a subsidiary of Kuwait Foreign Petroleum Exploration Co., the unit known as Kufpec, Shell said in a statement Tuesday.