Chevron Corp. , Exxon Mobil Corp. and Royal Dutch Shell PLC spent more than $120 billion in 2013 to boost their oil and gas output—about the same cost in today’s dollars as putting a man on the moon. But the three oil giants have little to show for all their big spending. Oil and gas production are down despite combined capital expenses of a half-trillion dollars in the past five years. Each company is expected to report later this week a profit decline for 2013 compared with 2012, even though oil prices are high. One of the biggest problems: Costs are soaring for many of the new “megaprojects” to tap petroleum deposits needed to replenish depleting fields. Plans under way to pump oil using man-made islands in the Caspian Sea could cost a consortium that includes Exxon and Shell $40 billion, up from the original budget of $10 billion. […]