Energy producers are throttling back their output, drivers are returning to the road and U.S. oil prices are roughly twice what they were a week ago, raising hopes—but not confidence—that the mounting fuel glut won’t overwhelm the world’s capacity to store oil. Since reaching a record in mid-March, daily U.S. crude production has declined by more than a million barrels and big producers are promising to push it lower yet by shutting in old wells, waiting to bring online the newly drilled and dialing down flows where they can. …