The ultimate purpose of this course is to build a firm knowledge of the phase behavior of fluids. With this understanding, we will be able to establish the basis and rationale upon which phase behavior applications in production systems are grounded. We are using the word production in a generic sense, that is, where it pertains to reservoir, pipeline, and the surface production (of any produced fluid).
So far, we have seen that much of the importance that we place on understanding phase behavior comes from the ability that it gives us to predict how a given system will behave at different conditions. We need phase diagrams to look at what the state of the system that we are dealing with is; that is, what is its original state. As a matter of fact, if we look at petroleum production, we often talk about a thermodynamic process that is taking place, involving a process path similar to one that we have seen in any basic thermodynamics course.
Just to give an illustration, consider Figure 6.1.
Production, as defined above, involves taking the reservoir from an initial condition (PA , Tf) to final state of depletion (PB , Tf), (PD , Tf), (PE , Tf) or even (PF , Tf). Once the end points of our thermodynamic path are fixed, the single most important question is determining the path that leads to such an end point. This path dictates whether or not you have the maximum recovery possible from the system.
When we talk about gas cycling, we are generally referring to the practice of injecting gas back into the reservoir. This is done in order to optimize the thermodynamic path we have chosen to take. In a typical condensate system, you generally produce a wet gas from the system with a high liquid yield at the surface. At the surface, you pass this gas through a series of separators; during this process liquid is going to drop out. The liquid that drops out will be rich in the heavier components. Hence, the gas that comes out of the separator will be dry (i.e., very light). If you inject this lean gas back into the reservoir, there will be a leaching process. All you are trying to do from the point of view of the phase diagram is to move the phase boundary and dew point towards the left (lower temperatures zones). Let me explain this in more detail.
Let us say that we have the phase envelope for the reservoir fluid shown in Figure 6.1, with the given path of production. If we were to follow the path from (A , Tf) to (E , Tf), we would enter the two-phase region and end up having liquid in the reservoir. However, you do not want liquid in the reservoir because its low mobility dictates that it would not be recovered! Next, you want to move that phase diagram to the left by injecting a lighter gas. When you inject a lighter gas, the phase envelope shifts to the left; your production path will be free of liquid dropout at reservoir conditions. By injecting the gas, we are making the overall composition of the reservoir fluid lighter. The effect of composition on phase behavior was discussed in the previous module (see Figure 5.2 in Module 5). This example demonstrates the importance of phase diagrams as tools that help us produce a reservoir in an optimal way.
So, we recognize that we needed phase behavior data for this particular system. The question now is how do we get the data? We can collect data in at least two ways: from laboratory measurements and from field measurements. Lab experiments are expensive, and we cannot hope to generate data for every foreseeable condition we may encounter. Just to give you an idea, generating a single phase envelope may cost at least $120,000. This is not something you want to be doing all the time. On the other hand, if you went to the field, you would lose valuable resources or have to stop operations to make your observations. On a routine basis, you don’t want to use the field or a lab as your main sources of phase behavior data. These options mean a lot of lost revenue and a great deal of expense. Is there a third option? Yes, indeed. We can rely on prediction, by which we produce a model that can do this work for us. In fact, we will be dealing with, and developing, this option in this course.
The basis for such a model is what is called an Equation of State (EOS). Hence, the central part of this course is EOS, since they are the basis of what we do in phase behavior.
There are several other examples that illustrate very vividly why we need to study equations of state. For instance, let us think about the concept of equilibrium.
In petroleum production, we generally make the assumption that, at every stage, the system is in equilibrium. When you think about equilibrium, you generally think about a system that is static, that is, not moving. When a system is moving, it cannot, in actuality, be in equilibrium. Nevertheless, the best approach we have so far is to describe it using equilibrium thermodynamics. While we usually assume equilibrium, we recognize that it is not a perfect assumption, but that it is a reasonable one.
This means that in the course of producing the reservoir, a process which always involves movement, I am assuming that everywhere the gas and the liquid are in equilibrium. With this assumption, we are free to use equilibrium thermodynamics, so we are able to employ EOS in describing the state of the system.
Consider the reservoir in Figure 6.1, in an entirely gaseous condition at (A, Tf), and having a known fluid composition zri (i=1,…n). As we produce this reservoir through a pipeline, we take the fluid from reservoir conditions through a battery of separators. Generally speaking, we deal with a series of separators, but for the sake of this discussion we will assume that we have just a single separator.
This separator does not care about the pressure and temperature of the reservoir. It only cares about its own pressure and temperature condition: Ps, Ts. The composition of the fluid at the separator inlet is assumed to be the same as that of the reservoir fluid, although this is strictly true only for single phase conditions.
The fluid exits the separator in two streams: a vapor stream and a liquid stream. As a petroleum engineer, we want to know how much gas, how much liquid, and the quality (compositions) of both streams. That is, we need quantitative and qualitative information. As we shall study in Module 12, we can perform a material balance around each separator to calculate the amount of vapor and liquid that is to be recovered. We will need the properties of both streams (such as density and molecular weight) in order to express flow rates in suitable field units.
How do we generate all this? We need a tool; that tool is an Equation of State! Why do we need an Equation of State? We need EOS to define the state of the system and to determine the properties of the system at that state. That is why it is called an equation of state. As you may have noticed, something critical in this series of lectures is the ability to establish links within all the material we are studying. We will not look at each topic simply as an isolated compartment, but instead, we must think in terms of how each piece of information fits into the overall picture that we are developing.