Science Article

Myth #1: Oil is found in large pools in underground caverns, somewhere between the surface and the center of the earth.

Myth #2: You know you have tapped one of these pools when oil comes gushing out the top of your drilling rig.

Hollywood films and the conventional vocabulary of newspaper articles have embedded these myths in our collective consciousness. The truth is very different . . .

There are no hollow caverns deep in the earth, and oil does not exist in pools. Oil and gas, or hydrocarbons, are found within formations of sedimentary rock referred to as reservoirs: grains of sand cemented together, or relicts of ancient reefs. These rocks are porous, with the fraction of pore space (porosity) as high as 30%. The pores are usually quite small: 0.1 - 100 microns, compared to the 50 micron thickness of a human hair and can be filled with water, oil, gas, or a mixture of these fluids. Typically these sedimentary formations are found 1/2 to 6 miles (about 1-10 kilometers) underground since burial is essential to provide the temperature and pressure conditions needed to change plant or animal matter into oil and gas. When the depth is too great the porosity of formations is reduced to economically uninteresting levels; it costs more to extract the oil than it is worth.

So, how can we find oil that is worth extracting?

Sometimes oil does gush out of the ground when an oil-bearing formation is penetrated, but this is dangerous, wasteful and polluting, and every measure is taken to prevent it. The typical oil well is 8 inches (20 centimeters) in diameter, drilled with a drill bit screwed to the end of steel pipe that is rotated at the surface by the drilling rig. Wells are drilled under hydrostatic pressure provided by drilling fluid. The drilling fluid is water- or oil-based, with its density boosted by the addition of heavy minerals such as barium sulfate. In this way a positive pressure is kept on fluids underground during the drilling process, preventing these dangerous blowouts.

After a well is drilled, the driller has a hole 8 inches (20 centimeters) in diameter, as much as six miles deep, filled with a fluid he himself has put in and a list of questions: Did I strike oil? If so, how far down is it? How much oil is there? How fast can the well produce the oil?

Enter Physics

In the 1920's scientists realized that physical measurements made in well bores deep within the earth could answer many of the driller's questions. For example, measuring the electrical conductivity of a formation is a logical expedient, because water at great depth is salty and therefore a good electrical conductor, whereas oil and gas are poor conductors (good insulators). Thus low conductivity is an excellent, though not infallible, indicator of a hydrocarbon-bearing formation. Other families of measurements employ gamma ray scattering, natural gamma ray spectroscopy, neutron scattering and sonic wave propagation. Often, six or more measurement instruments will be used to determine the location, type and quantity of fluids in subsurface rock formations. However,the answer to one of the driller's questions has long been elusive: How fast can the well produce the oil?

The answer to this question depends on the permeability of the formation to fluid flow. And since an oil reservoir is worthless if the structure of the rock keeps the rate of production uneconomically low, permeability information is essential. Another type of measurement was discovered that could identify molecules, make images of the interior of the human body and measure how fast fluids can flow through porous rocks. It could even be used to make sure a cream cookie filling has just the right texture - now that's important! That measurement is called Nuclear Magnetic Resonance (NMR) and the pioneers of this field of study, fiddling with their old radar sets, had no idea how useful NMR would become.