All of my 50+ year career has been involved with the science of Petrophysics, literally the physics of rocks, in some way or another. Petrophysics is a branch of Geoscience and intimately linked to geology, geophysics, and petroleum / mining engineering. There is no degree granted in pure petrophysics, so people in this field are often graduates of a closely related specialty and are self-taught from there.

Petrophysics is mainly used in petroleum exploitation, but also in defining mining and ground water resources.

To understand petrophysics, you need to understand rocks and the fluids they contain, how the earth's surface and subsurface change shape, and how pressure, temperature, and chemical reactions change rocks and fluids over eons of time. That's a tall order.

Rocks are formed in several ways, but usually end up as moderately flat layers, at least initially (mountain building comes later). As successive layers are laid on top of each other, the Earth builds a sequence of rocks with varying physical properties. Some layers will have open spaces, called pores or porosity, that contain fluids (water, oil, or gas). A rock on Earth with porosity cannot be "empty" -- they must contain something, even if it is only air.

Microphotograph of a rock -- black colour is the porosity where
 oil, gas, and water can be held inside the rock

Think of a porous rock as similar to a huge sponge full of holes that can soak up fluids. Although we often talk about "oil pools", these are not tanks of oil underground -- they are porous rocks. The porosity, or quantity of open space relative to the total rock volume, can range from near zero to as much as 40%. Obviously, higher values of this physical property of a rock are good news.

Some rocks have very little porosity and do not hold much in the way of fluids. These are often called "tight" rocks. Both tight and porous rocks can contain animal and plant residue that are ultimately transformed into hydrocarbons such as coal, oil, or natural gas that we can extract and use to power vehicles and heat our homes. As the plant and animal residues mature into oil or gas, they may migrate through porosity or natural fractures in the rock until trapped by a non-porous rock structure. Sometimes a rock only sources itself or an adjacent porous rock, so little migration occurs.

An anticline, the simplest form of petroleum trap

Rocks that are capable of holding  hydrocarbons in economic quantities are called reservoir rocks. Rocks in which the plant and animal residue has not been fully converted to useful hydrocarbons are called source rocks. Some rocks are both source and reservoir: others are barren of hydrocarbons, and some others may act as the trapping mechanism that keeps hydrocarbons from migrating to the surface and escaping.

A trap is what keeps oil and gas in the rocks until we drill wells to extract the hydrocarbons. Coal, being a solid, doesn't need a trap to be kept in place.

Reservoirs that contain oil or gas also contain water. The quantity of water relative to the porosity is called the water saturation. In the illustrations, the brown colour is solid rock grains and the space around the grains is the porosity. The black colour is the hydrocarbon and the white is the water, which forms a thin film coating the surfaces of each rock grain. This is a water-wet reservoir (left). In an oil-wet reservoir, the black and white colours are reversed (right).

Finding and evaluating the economics of such reservoirs is the job of teams of geoscientists and engineers in petroleum and mining companies. A petrophysicist, or someone playing this role, will be part of that team.

Once a useful accumulation has been found, drilling, completion, and production engineers take over to put wells on stream. Oil production may initially flow to surface due to the pressure in the reservoir. Some oil pools do not have enough pressure to do this and need to be pumped. Depending on the reservoir drive mechanism, some wells that start flowing will later need to be pumped. Water may be produced with the oil. It is separated and disposed of by re-injection into a nearby unproductive reservoir layer. You can't just dump the water in the nearest swamp.


   Aquifer Drive -- Before ... and After some production            Gas Cap Drive               Gas Expansion Drive

An aquifer drive mechanism usually maintains the reservoir pressure for some time but may drop off gradually. Recovery factors vary from 30 to 80% of the oil in place. The oil water contact rises as production depletes the oil. A gas cap drive pushes oil out as the gas expands. Recovery factor is similar to aquifer drive. There may or may not be some aquifer support. the gas oil contact drops as the oil is depleted. Gas expansion reservoirs do not have aquifer or gas cap support. Gas dissolved in the oil expels oil into the well bore because the pressure at the well bore is below the reservoir pressure. Recovery factor is awful - usually less than 10%, but this can be improved to maybe 20% by injecting water nearby to increase or maintain the reservoir pressure. Water floods, carbon dioxide injection, and re-injection of produced gas or water can be used in nearly any reservoir to improve recovery efficiency.

Gas wells do not need pumps, but if they also produce water, a special process called artificial lift is used to get the water out. That water is also disposed of legally.

The economics of a reservoir varies with improving technology. Bypassed reservoirs, discovered and ignored years ago, are now economic due to technical improvements in drilling practices and reservoir stimulation techniques. Horizontal wells and deep water drilling are now common. The use of heat or steam to assist production of heavy oil or bitumen, and multi-stage hydraulic fracturing to stimulate production in tighter reservoirs are relatively new techniques and relatively economic today. Obviously the specific price of oil or gas after delivery to the customer plays an important role in how much effort can be expended to recover oil and gas from underground.

There is controversy, of course, about new technology. Just as the Luddites resisted the weaving machines in the early 1800's, modern Luddites insist that the old ways of oil and gas extraction are best, while at the same time complaining loudly about the price of gasoline at the pumps or the cost of electricity for their air conditioners. You can't have low-cost and low-tech at the same time.

Green alternatives are 50 to 100 years away. Every green technology needs oil to make the required plastics and fuel the manufacturing and delivery systems. The electricity grid is far too fragile to fuel extensive use of electric vehicles anywhere, let alone everywhere. And where would all that electricity come from (coal?). Clean coal is more oxymoronic than military intelligence. So if you and the other 7 Billion people on this planet want to live a comfortable life, get used to oil and its risks. Staying in bed is risky too -- more people die in bed than anywhere else.

For the record, I've been off the grid with wind or solar since 1984. But I live in the middle of nowhere so the esthetics don't bother the neighbours. What have you done to green-up this world?

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