Recently someone asked the question, “Why is fluid characterization (FC) important, and how will this change the design of a field?” Almost all industry professionals will agree that the answer to this question is obvious: To design a production system correctly, an understanding of the produced fluid is required.

However, with such a clear answer, why then do so many projects have an issue with inadequate FC? The answer may stem from the fact that even with today’s advanced chemical–separation techniques, we cannot identify the hundreds of components found in reservoir fluids or gain a perfect understanding of how the fluid will perform during production.

Figure 1. A phase envelope helps identify the phase conditions of gas and condensate in a field. (All graphics courtesy of Amec Paragon)
Another reason for recurring FC issues may be related to conducting these analyses late in the project study. Conducting FC analysis early in the project is critical, and the industry has started realizing the importance and impact FC plays on facility design.

The goal here is to address the importance of FC, and provide abbreviated guidelines for successful FC. For the purpose of this article, FC is defined as the process to obtain a reservoir sample, perform laboratory tests, and use computer software to model the reservoir fluids.


Importance of characterization

Flow assurance and operability activities have always played a prominent role in the design and operation of production systems. However, as the industry’s focus on field development shifts to deep water, the importance of flow assurance has increased due to the high stakes involved. Deepwater developments face significant challenges, and an incorrectly designed field may become a financial nightmare to a production company. Therefore, proper planning is required in the design phase to ensure the system reliability in all modes of operation during the life of the field. To accomplish this, flow assurance analysis has become a cornerstone of the design process, and is used to correctly predict how the system will operate. It is used to identify required chemicals to minimize potential flow blockages due to hydrate formation or wax/asphaltenes deposition.

Deepwater exploration faces significant challenges from produced fluid hydrocarbon solids. These solids have the potential to disrupt production since they can deposit anywhere from the perforations to the well bore and pipelines. For typical flow assurance project execution, FC is the first and one of the most important steps in flow assurance activities. FC is completed to accurately represent the interaction between fluid flow and phase behavior. It is also a direct input in the process of modeling the system with process simulators. These process simulators determine long-lead items required for the project, such as pipe and equipment size. They also identify operating risks associated with transporting the product (fluid), which include hydrate, wax and asphaltene issues. FC should be completed with high importance, considering the fact that using incorrectly characterized fluid properties in the process simulators may lead to improperly sized equipment/pipe, incorrect designs, increased costs, and/or delays in schedule. FC also helps to define the depth at which equipment, like the surface controlled subsurface safety valve (SCSSV), should be installed in the tubing section. The SCSSV typically is, and should be, positioned in the tubing above the wax appearance temperature (WAT) in order for it to function effectively. Proper FC helps us to identify the height at which such equipment needs to be present in the tubing.

Phase behavior

Each reservoir fluid is unique, such that its phase behavior is different than any other reservoir fluid. Each reservoir composition has its unique set of properties, and the fluid behavior is related to these properties. Understanding this phase behavior and flow implications is critical in the design process. FC helps the operator gain understanding of the phase behavior and flow implications. Using this information, the project team should design a system that is optimized for that particular reservoir fluid.

A phase envelope identifies the behavior of the fluid at different pressure and temperature conditions. A phase envelope of a typical multiphase gas field fluid is presented in Figure 1. Inside the phase envelope, the fluid exists in a two-phase state, gas and liquid. At pressures and temperatures to the right of the dew point line, the fluid will be a single phase gas. If the temperature is greater than the cricondentherm, the fluid is a single-phase gas at all pressures. On the other side of the phase envelope, for pressures and temperatures to the left of the bubble point curve, the fluid will exist in a liquid state. Also illustrated in the phase envelope graph is a hydrate and wax curve. Pressures and temperatures on the left of the respective curves may encounter flow issues due to wax deposition and hydrate formations. However, it should be noted that crossing the thermodynamic conditions and moving into the hydrate or wax region does not necessarily imply that flow is blocked in the well bore or pipeline. Unless the solids deposit in the pipeline they do not pose any problem. Improper understanding of the different phases can result in a design that is not optimal for the flow and the processing of the reservoir fluid.

As mentioned above, FC helps in predicting the hydrate formation and wax deposition temperatures. These predictions may come from laboratory tests or from computer software. Using this data, operating procedures can be designed to effectively prevent/mitigate hydrate formation or wax deposition.

Method of fluid characterization

FC is a process that should be prioritized early in the project, to ensure that adequate samples are obtained and the correct laboratory tests are completed. Understanding the FC process and getting key people in the project involved can save significant time later in the project life. Inadequate foresight into the future operational issues may lead to additional fluid sampling and delays in schedule.

It is recommended that the key people in the project come together and discuss the process and steps of fluid characterization. The main steps in FC are:

· Fluid sampling and transport;

· Laboratory tests; and

· Using computer software to model and tune the fluid to correctly represent the fluid properties.

Each of these steps will be briefly discussed in the following sections.

Fluid sampling and transport

To understand the behavior of the reservoir fluid, samples are taken during the drilling process or during a production test. Bottomhole or separator samples can be obtained and stored in high-pressure containers.

Figure 2. Computer programs adjust fluid characterization parameters to match laboratory samples.
These reservoir fluid sample containers can be transported to laboratories for tests and used for composition analyses. The process of obtaining, storing and transporting the samples is critical to the accuracy of the test that will be completed in the laboratory. Therefore, procedures should be in place so that the sampling/transport process is completed to minimize any contamination or any irreversible phase changes. A common mistake in the sample collection process is not obtaining enough samples for all the required tests needed for field design. This problem can be minimized by insuring input from the project team regarding the required sample volume, and by reviewing sampling requirements of similar projects.

Laboratory testing

After the reservoir sample has been collected, the laboratory tests can be completed to gain an understanding of the fluid behavior. Laboratory tests include testing for hydrate formation and wax formation. Some of the steps for the lab testing process include obtaining key data for characterization by chromatography tests or true boiling point (TBP) distillations. The next step is defining the molecular weight (MW), specific gravity (SG) and boiling point (BP) for each of those fractions. The component distribution in a reservoir fluid, not the number of components, determines how close the fluid is to its critical state. A unique FC is required for each reservoir fluid to use the correct component properties in the equation of state. pressure/volume/temperature (PVT) computer simulation software can then be used to model the fluids, and align them with the lab data as discussed below.

PVT software

Using the laboratory test results, computer software can be used to tune the equilibrium of the fluid. The tuning process involves adjusting critical parameters in the equation of states

Figure 3. The impact of hydrocarbons with molecular weight heavier than hexane can have an effect on the phase envelope.
(EOS), so that the equilibrium predicted with the computer software matches the laboratory test. Usually, C7+ forms the plus component where all the carbon compounds higher than hexane are lumped together. Modeling of PVT properties consists of calculating physical properties of the carbon compounds using empirical co-relations.

Figures 2 and 3 present an illustration of how FC with computer software adjusts the default parameters in the equation of state to match the laboratory data. It is obvious that using a non-characterized fluid may lead to inaccurate designs. Figure 3 presents the impact of molecular weight of C7+ on the phase envelope of a rich natural gas mixture. As the C7+ molecular weight increases, the cricondentherm temperature and cricondenbar pressure will increase, and the two phase regions expand.

Viewing Figure 3, one can see that improper characterization of heavy ends may result in a bad design or excess liquid production. For example, a situation could occur in which a pipeline has been designed for dry gas when it should have been designed for two phase gas-liquid flow.

The way forward

The design of subsea systems and the modeling of reservoir fluids require accurate data describing the phase behavior of oil and gas mixtures. Early involvement with a well-defined procedure allows the most accurate results to be obtained with FC, and helps predict fluid behavior with a higher level of accuracy. The project team should be involved in this process, so that sufficient samples and laboratory test are performed to allow for the design process to be completed most effectively. Effective fluid characterizations will lead to better and more cost-effective field designs.