| MadSci Network: Earth Sciences |
Catherine: Actually, both of the things you suggested happen. Exactly what happens in a given case depends chiefly on the nature of the reservoir rock. If the rock is not very rigid or tightly packed (or has even been dilated by buildup of fluid pressure), then removal of hydrocarbons can cause the rock to compact. One would not expect all the porosity (open space) in the reservoir to be squeezed shut. However, subsidence of several feet, and associated cracking of building foundations, is typical. The same thing can result from underground mining. Most reservoir rocks don't compact when oil or natural gas is removed. Instead, water moves in as the hydrocarbons leave. Below the water table, essentially all open spaces underground are filled with fluid. Water is by far the most common underground fluid, and so it is what usually enters any available space. In fact, because oil and natural gas are both lighter than water, they are generally found where high spots in porous rocks (rocks with holes) are right undernight tight rocks. Below the oil and/or gas is water, which can move up immediately if the hydrocarbons are removed. The holes in reservoir rocks are small. Most are less than 1 millimeter across, and many are far smaller. However, individual molecules of oil, gas, or water are tiny, and easily move in and out of holes that are way too small to see. David Kopaska-Merkel Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 FAX (205) 349-2861 www.gsa.state.al.us
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