That is what should happen under normal conditions. The report states it. Since this is not normal...what is back filling and is it fast enough?
Ground water?
At first, the well is under pressure (not from being pushed in like a balloon, it’s PRESSURIZED). So the oil comes out on it’s own.
Later, when the pressure is less, they will pump water into the deposit to push oil out. Obviously we aren’t at that point yet, or they wouldn’t have to cap the well, it would have just stopped.
There’s no empty spaces to be filled by rocks or water; the water pressure pushing down couldn’t force the oil out into the water pushing down.