There are different types of hydrocarbon fluids that can be found in a reservoir. Understanding the characteristics of these different fluid types will help in determining how best to deplete the reservoir. But that is a topic for another time; for now we will examine the different types of reservoirs.
Two Broad Types: Oil and Gas
First, petroleum reservoirs are broadly divided into oil reservoirs and gas reservoirs. Oil reservoirs around the world are not the same; the fluid composition, the prevailing temperature and the pressure (called the reservoir pressure) all vary. We can further break down oil reservoirs into different types based on the interaction between this reservoir pressure and the hydrocarbon fluids.
Classifying with Help from the Phase Diagram
To understand this classification, we’ll be looking at a very important chart; a pressure-temperature envelope or phase diagram. True, there are different types of crude oil, such as black oil, volatile oil, etc. For all oil reservoirs, the basic concept of the phase envelope is the same for each type of crude oil even though there are slight variations. Therefore, even though fluid types vary, we can study the three kinds of oil reservoirs by looking at a single phase envelope.
This is the phase envelope for an oil reservoir that contains black oil. The bubble point line on the envelope is our area of concentration for oil reservoirs. We already know that reservoir temperature and pressure for different oil reservoirs around the world varies. In this particular phase envelope, the numbers 1, 2 and 3 trace the different possible points the reservoir pressure can be. One reservoir in the North Sea can have the reservoir pressure at point 1 while another can have reservoir pressure at point 3. These two reservoirs may be oil reservoirs in the same region, but since they are found at different reservoir pressures, this pressure difference will affect the kind of oil reservoir and our depletion plan. Thus, our strategies to produce from a reservoir with a reservoir pressure at point 1 will be different from that of the reservoir whose reservoir pressure is at point 3 even though they are both oil reservoirs.
Undersaturated Oil Reservoirs
Take a look at the phase envelope. If the reservoir pressure is at point 1 then that reservoir is an undersaturated oil reservoir. What this means is that the reservoir pressure (point 1) is higher than the bubble point pressure.
The bubble point pressure (on the bubble point line) is a very important concept for every oil reservoir. This pressure is obtained at the laboratory from tests conducted on samples of the reservoir fluid. To conduct this test, a sample of the reservoir oil is placed in a chamber (container) at the reservoir pressure. We slowly lower the pressure exerted on this oil. It’s not difficult to imagine, just like your soft drink or beer, there is always some gas bubbled into the liquid under pressure. This pressure is sealed by the bottle cap. When you open the bottle, you release the pressure and expose the liquid to the atmosphere thereby lowering the pressure, so the gases bubble and come out of the liquid. Let’s assume that the bottling company did not force enough gases into this drink and the soft drink or beer could still have absorbed more gases if they were bubbled at that pressure. This assumption leads us to understanding undersaturated oil reservoirs.
The word “undersaturated” also means that this kind of oil reservoir is not saturated with gas bubbles. The saturation here is referring to gas bubbles, and so the pressure has kept all the lighter gaseous hydrocarbons (bubbles) inside the crude oil. At the same time, this undersaturated state tells us that assuming there were more gas bubbles, the crude oil in this reservoir would have comfortably absorbed them. An understanding of this will guide us in coming up with the best strategies to produce the hydrocarbons in this reservoir.
Saturated Oil Reservoirs
Any reservoir having a pressure that falls at point 2 on the phase envelope is a saturated oil reservoir. A saturated reservoir is fully saturated with lighter hydrocarbon gases at that reservoir pressure. Saturation means that the crude oil here is fully occupied with dissolved gases and will not take any extra gas bubbles at that pressure (point 2) unless we increase the pressure. Remember, it is the position of the reservoir pressure on the phase envelope that helps us distinguish what type of oil reservoir we are dealing with.
Back to our analogy with a bottle of beer or soft drink, in this case the bottling company kept forcing gas bubbles into the drink in the bottle until it could not take any extra gas. This state is the saturation state, and oil reservoirs found at this state are termed saturated oil reservoirs. It is not enough to know that we just drilled into an oil reservoir — knowing the kind of oil reservoir it is will help us choose the right methods to efficiently produce the hydrocarbons in the reservoir. (Related reading: The Different Ways of Perforating a Well.)
Point 3 on the phase envelope is called the 2-phase region; remember that at points 1 and 2 we only have crude oil in the reservoir with some form of gas saturation at that pressure.
In the beer and soft drink analogy, what happens the moment we open the bottle cap? Some gases come out of solution because we just disturbed the pressure equilibrium of the drink in the bottle. Gas-cap reservoirs are operating under similar conditions — some of the gases have come out of solution and are now occupying the top of the reservoir, whereas in the case of the soft drink or beer, the gases simply escape to occupy space in the room.
For an oil reservoir, because gas is less dense than oil, the gas will occupy the upper portions of the reservoir rock. If the reservoir pressure was higher (like at points 2 or 1) then the gases would have been forced back into solution. In other words, gas-cap oil reservoirs are supersaturated with gas. Gas-cap reservoirs have gases at the upper portions (cap) of the reservoir trap. Taking the time to know what kind of oil reservoir we are dealing with is never a waste of time, and this understanding could be the difference that allows us to maximize production from the reservoir.
It is not enough to know that we just drilled into an oil reservoir; an understanding of the kind of oil reservoir we are dealing with ensures efficient production from the reservoir. (For more information, see the article Understanding Reservoir Drive Mechanisms.) For example, if we know that the reservoir is a gas-cap oil reservoir, then we can engineer our flow rate in such a way that we do not produce too fast from the reservoir.
Producing too fast will make us produce the gases at the top sooner than we want to and we don’t want that. This is called a “blowdown”. The gas at the cap helps to push the oil down through the perforations into the drilled well. The longer we can keep the gas in the cap while producing economically the better. So an understanding of the kind of oil reservoir we are dealing with helps us make the best decisions to efficiently produce the hydrocarbons in the reservoir.