GEOL 115: The Earth’s Energy Resources

In this lecture we learned about reservoirs.  Reservoirs are the rocks from which we retrieve petroleum and natural gas. The term reservoir is based on economics and rock properties; different rocks can surrender oil and gas at different rates but a rock is a reservoir only if you can make an economic oil well by exploiting it.  Thus a rock that will be a reservoir under one set of oil prices will not be a reservoir at another set of oil prices.   If you can make money by extracting oil or gas from a rock then the rock is a reservoir, if not then the rock is neither reservoir nor seal.

The three primary variables are

porosity<φ>-the volume of void space (empty space) in a rock divided by the volume of rock

permeability-the ease with which a fluid can be transmitted through a rock

water saturation (S_w)-the fraction of pore space that is filled with water.

Porosity controls the volume of oil or gas that you can store in a reservoir. Permeability is a measure of the extent to which the pores are interconnected. Water saturation is a measure of the amount of water (and thus the amount of oil) in the reservoir.

Lets consider this image to be a slice of sandstone that has been greatly magnified. The yellow circles are sand grains. Between the sand grains we see some a blue layer coating each grain of sand. That blue represents the water that saturates the rock. The black is the oil that fills the remaining space.

If porosity decreases then we will have less space to store oil.

If we increase the fraction of pore space filled with water, there is also less space in which to store oil.

So the if we are exploring for oil or gas we want to find a rock in which a large fraction of the rock volume is occupied by pores.  We want most of that pore space to be filled with oil or gas rather than water.  We want the pore spaces to be interconnected.

In class we looked at these data from the San Joaquin Valley demonstrating that porosity decreases with increasing depth.  Similar relationships could be developed for clastic reservoirs in basins all over the world.  This porosity decline is important for two reasons.  1)This reduces the amount of storage space available for oil or gas and 2)it decreases the permeability of the rock.  Those two issues probably seem obvious.  What follows will not seem so obvious.  Consider the thin layer of water that is present on each sand grain.  The minimum thickness of that layer of water is basically fixed.  You can’t get rid of that water.  You can put more water into the pore space but you can’t reduce the amount of water in each pore below some irreducible value.  The thickness of each layer of water will depend on the properties of the reservoir, the water, and the oil in the area. 
It should become apparent that when porosity becomes very small the water saturation must increase (even if the sandstone is saturated with oil!).  This concept that there is a minimum value of water saturation is referred to as the irreducible water saturation (S_wirr).  The value of S_wirr must depend on porosity.  The diagram (go to the course documents section of Blackboard and download “Graph to go with Blog Post on Reservoirs” if the graph does not appear to your right) to the right is a cartoon but it illustrates the general principles. When porosity is high S_wirr will be low (the layer of water on each sand grain comprises a small fraction of the total porosity) whereas when porosity is low the S_wirr approaches one (if the volume of pore space is very small that layer of water can occupy almost all of the available space).

But there’s more (as they say on the commercials)! We have to consider permeability. It only makes sense that permeability (the ability to move a fluid through the reservoir) will decrease as porosity decreases. The reservoir will have a permeability for oil and a permeability for water. Water may flow through the reservoir at low values of porosity but if the little droplets of oil are not connected they can’t flow. Thus the permeability of the reservoir to oil will decrease as the water saturation increases.

Once you get below 25,000 feet or so you’ve had pretty much had it with regard to oil production. That’s not always the case but its pretty tough at those depths. Gas is easier of course.