If they have that much reserves, wouldn’t they just sink into the hole they are creating at some point. Sand does move fast.
It takes water to take it out. Where they gonna get it?
Removing oil/gas doesn't leave a hole. It isn't a giant, buried lake with nothing but petroleum in an empty pocket.
Think of it this way. Take a bucket, pack it full of sand, tightly packed. You can pour water into that sand for a while without overflowing the bucket, if you go slowly. There are spaces between the grains to hold the water. If drill a tube down to the bottom and draw out the water, it will not subside the top, provided it was tightly packed to begin with.
The oil/gas is in porous rock the same way. It typically has less spaces and less connectivity between the spaces than the bucket of sand example, but it works the same way.
Also, not matter how hard you suction the tube in the bucket of sand example, you will never get out all the water you put in. Same way with an oil field; you never get it all out.