eli5: what happens to the areas that contain water and oil when we pump it all to the surface, why are there not sinkholes everywhere?
In: 7
The oil and water is trapped within rock, in the pore spaces between sand grains, or in the fractures and inter-crystalline structures in limes.
The short answer is that you often do get sinkholes or [subsidence](https://www.usgs.gov/special-topics/water-science-school/science/land-subsidence). That is, the overall level of the ground will drop. This can be a real problem for some cities and is one of the downsides of pumping excess water out of aquifers.
This is less of a problem for oil because it tends to be trapped much deeper down and supported by non-porous rock above and below. But, it’s a serious problem for pumping water.
While we picture petroleum as sitting in a massive well like bubble, that’s not really the case. It’s more like sucking water from soaked sand. The hole we drill allows the pressure from the surrounding rock to push the liquid petroleum our into the gap and up. The surrounding area is still stable rock. Now with all the pressure from the petroleum gone, some shifting can occur but it’s minor. On average the occasional drilling induced quake is only about 2-4 on the Richter scale. Which is only detectable on sensitive instruments. You wouldn’t even feel it.
Now this is with traditional oil drilling, not fracking. Fracking is a whole different process which brings its own set of problems.
The problem is thinking like an oil resivoir is an inflated balloon underground. Think of it like a soaked sponge and you are sucking the water out. The oil or water is coming out of the gaps between the rocks and sand and clay particles, but the “chunky bits” stay behind and help hold up the ground above.
If there are large empty pockets, like water dissolving a salt deposit, there could be a pocket that could collapse.
Sinkholes also can occur in areas where massive underground mining ( for coal etc) has been done. For example here in the German Ruhr area megatons of coal have been mined out of the ground and big cities have developed directly over the mines ( people are lazy) about 150 years ago. Todays those cities struggle, because more and more old mines collapse and the floor settles.
But those sinkholes can also have natural reasons. In the region of Schmalkalden, Germany for example, was a natural, loose layer of soil under the houses. Experts believe, due to ground water and rain it was washed out, a big cavity has formed which then collapsed and created a big sink hole.
https://www.theguardian.com/world/gallery/2010/nov/01/germany-sinkhole-crater-schmalkalden
It does happen with water being pumped from wells. California is a classic example with some areas subsiding by 1 foot per year according to the USGS. There are some great pics on Google showing how much the land has fallen.
Usually it’s not noticed as everything sinks over a large area and it’s not usually as dramatic as feet per year. So unless you have a fixed point that can’t sink that you can see the change against it’s hard to tell.
Latest Answers