Course of lections GAS PROPERTIIES

M. BRATAKH

Lection 1

Ideal gases

Common questions

1 Finding oil and gas

2 Why do we have the crude oil and gas on tubing head?

Special questions

3 Ideal gas law

1 Finding oil and gas

Drilling, as you know, is expensive. So oil companies plan carefully before they start drilling. First the scientists make 3D maps of the rock below the surface (figure 1) by the help of special computer programs, such as Surpac, Macromine, Datamine, Gems, Vulcan, and the famous for oil and gas sector – Petrel.

Figure 1 – 3D map of the rocks (examples)

Then they study these maps carefully. They look for possible oil traps (diagram 1)

Diagram 1: an oil trap

All oil and gas deposits are found in structural or stratigraphic traps.  

Most oil and gas deposits are found in sandstones and coarse-grained limestones.  A piece of sandstone or limestone is very much like a hard sponge, full of holes, but not compressible.  These holes, or pores, can contain water or oil or gas, and the rock will be saturated with one of the three.  The holes are much tinier than sponge holes, but they are still holes, and they are called porosity (figure 2).

Figure 2 - Oil Moving Through Pore Space In Sandstone

Oil and gas is situated between the layers of non-porous rocks since the gas can not go up to the surface.

There are two types of the traps.

Structural Traps

These traps hold oil and gas because the earth has been bent and deformed in some way.  The trap may be a simple dome (or big bump), just a “crease” in the rocks, or it may be a more complex fault trap like the one shown at the below figure 3.  All pore spaces in the rocks are filled with fluid, either water, gas, or oil.  Gas, being the lightest, moves to the top.  Oil locates right beneath the gas, and water stays lower (Figure 3).

Figure 3 – Structural traps

Once the oil and gas reach an impenetrable layer, a layer that is very dense or non-permeable, the movement stops.  The impenetrable layer is called a “cap rock.”

Stratigraphic Traps

Stratigraphic traps are depositional in nature.  This means they are formed in place, often by a body of porous sandstone or limestone becoming enclosed in shale.  The shale keeps the oil and gas from escaping the trap, as it is generally very difficult for fluids (either oil or gas) to migrate through shales.  In essence, this kind of stratigraphic trap is surrounded by “cap rock.” (figure 4).

Figure 4 – Stratigrafhic traps

2 Crude oil

But we can not tell that each of the layers of gas, oil and water is separated from the others. Gas saturates the oil; oil is saturated with water too. Oil, gas and water at reservoir conditions are in the equilibrium. During drilling and production we break the equilibrium and at the tubing head we have an oil emulsion: mixture of oil, gas and water known as crude oil. The same happens with the gas – in the starting point of your flow-line you have gas-condensate-water mixture, or a crude gas.

Our main task (within the step of gathering and processing) is to gather the oil and gas by the special pipeline (so called manifolds or flow-lines) and to separates the gas from oil, the water from oil and gas, and to extract the condensate from gas.

And a few words about seismic exploration. How do scientists make 3D maps of rocks.

The seismic waves help geophysicists to find out what is below the surface. Seismic waves are sound waves, and they can travel through the rock layers.

Most companies use vibrator trucks to make seismic wave. These heavy trucks make vibrations on the surface, and the vibrations send waves down to the rocks below (figure 5)

Figure 5 – Seismic exploration

Each rock layer depending on their penetrability reflects some of the waves. The reflected waves travel up to geophones on the surface. Geophones are like microphones: they convert the waves into electrical signals (diagram 2). A machine in the recording trucks records the signals. Computers can convert these signals into 3D maps.

Diagram 2: recording sound waves

Seismic reflection works at sea too. But the crews use hydrophones, not geophones. And they use an underwater gun to make seismic waves.

Figure 6 – Offshore seismic exploration

The exact position of field can be told you by the help of GPS (Global Positioning System) and the way on the map can be laid using sat nav (satellite navigation)/