Home >> Science & Nature >> Science

What Happens When You Pump Too Much Oil Out of the Earth?

Whether it's oil and natural gas or groundwater that is being pumped, rapid and uncontrolled removal of fluids from reservoirs deep underground can sometimes have unexpected effects at the surface. Subsidence, the sinking of the earth's surface relative to a fixed elevation, can result from any of several causes, one of which is the withdrawal of oil or water from subsurface sediments and rocks. The phenomenon has been observed in many oil fields, as well as in areas where groundwater is in heavy use.

Subsurface Fluid Basics
- Petroleum and groundwater do not collect in underground lakes and rivers. Instead these fluids fill tiny spaces between grains of rock, called pores. Rocks below the surface are subjected to pressure resulting from the weight of the material above them, called overburden. Without liquid filling the pore spaces, the weight of the overburden crushes the empty spaces, making the porous rock layers thinner through compaction. Liquids do not compress, a property that keeps pore spaces open as long as liquid is present.
Examples
- Researchers first suggested that the extraction of oil can cause land subsidence in the 1920s based on studies in the Houston, Texas, region. Subsidence has since been observed at other oil fields around the world. The Wilmington Oil Field, which underlies a portion of Long Beach, California, has been particularly well studied since subsidence was first described in the 1940s.
Amount of Subsidence
- At Wilmington Field in California, subsidence related to the extraction of oil and water occurred over an area of approximately 50 square kilometers (19 square miles). In the areas of greatest subsidence, surface elevation fell almost 9 meters (about 30 feet) before regulatory action helped stabilize the underground reservoir. Such dramatic effects are not typical, however. Subsidence over oil fields is most likely to take place when the field is large, the producing zone is shallow and the reservoir rock is not well cemented; these conditions were present in the fields studied in Houston and California.
Mitigating Subsidence
- Government departments charged with overseeing the production of oil and gas are also responsible for preventing or minimizing subsidence caused by the pumping of hydrocarbons from underground reservoirs. Efforts made to reverse subsidence at Long Beach have met with limited success. Maintaining the fluid pressure in the subsurface through waterflooding -- pumping water into a reservoir to offset the produced fluids -- has been effective in stabilizing oil fields, including the oil field underlying Long Beach.

Science