What happens to the space remaining after oil or gas are removed?

Rocks are mostly solid. Most production of oil and gas comes from rocks that have an average porosity (percent of holes) of about 13%. That is also the average porosity of most concrete. So in 99.9995% of all wells drilled there is no effect of the fluid withdrawal. The particles of the rock hold the total rock together as if nothing has happened.

As well, when oil or gas is produced, something needs to fill the voids. As nature will not tolerate a vacuum, water fills in the space. All pore space in rocks are normally filled by fluid - either gas oir oil; gas is treated as if it were a fluid, since in many cases it exists as such underground. These rocks are under pressure of the overburden - the material above them, 1,000's of feet of rocks. So the fluid in the pore space also has the same pressure on it. As oil or gas is produced, the remaining fluid fills the space with a slightly lower pressure until the water begins to be produced

  MARIO RODRIGUEZ