During oil production, most pipelines are transporting a substantial amount of water together with crude oil and gas. The presence of water in the transporting pipeline may be due to the following:
- Water flowing out from the reservoir naturally. The % amount of water will become increasingly significant as the oil field ages.
- Conventional method of Enhanced Oil Recovery (i.e. water injection).
The immiscible mixtures of oil and water in the pipeline may result in the formation of emulsion. The major impact due to formation of stable emulsion is the increase in fluid viscosity, which can cause a high pressure drop in the pipeline. Failure to predict fluid viscosity and pressure drop accurately may lead to the under sizing of a subsea pipeline. This is a costly problem to oil production companies, as it causes production loss and decline in revenue.
Since pipeline pressure calculation is one of the principal aspects of the pipeline design, it is crucial to consider the possibility of emulsion formation using hydraulic and flow assurance studies.
The Science behind Emulsion
Emulsion occurs when one type of liquid is dispersed as droplets in the other continuous phase of immiscible liquid. The following types of emulsions are usually found in transporting pipeline:
- Water-in-oil emulsion
- Oil-in-water emulsion
- Multiple emulsion
Theoretically, viscosity of the fluid increases when water volume fraction increases until phase inversion point. Phase inversion is the point where water becomes the continuous phase and oil is dispersed in it. After this phase inversion point, viscosity of the fluid reduces with increasing water volume fraction. Figure 1 illustrates the effect of water volume fraction on viscosity. The phase inversion point depends on fluid sample and is different for each production fluids.
Figure 1: Fluid Relative Viscosity against Water Volume Fraction
The stability of emulsion can be affected by various factors, such as water volume fraction, temperature etc. With the aid of software, a flow assurance engineer will be able to determine the operating envelope of the transporting pipeline. The most accurate way is to allow the software to calculate the pressure based on emulsion viscosity obtained via laboratory study. Besides laboratory data, the software contains correlations to predict the emulsion viscosity. Some of the proven correlations available are Woelfin, Rønningsen, Pal & Rhodes, and etc.
Figure 2: Pressure Drop against Water Volume Fraction
As shown in Figure 2, the calculated pressure drop of a pipeline considering emulsion can be as high as 7 times (21 bar vs 3 bar). The high pressure drop results in high export pressure, in which the wells may not have sufficient pressure to push the fluid to the desired receiving platform. Hence, it is crucial to consider emulsion if it is foreseen to be an issue. Commercialized software which have the ability to consider emulsion in calculations are PIPESIM, OLGA, etc.
Key Features of Commercialized Software
- More precise prediction of fluid viscosity due to emulsion
- Accurate pipeline operating envelope
- Optimized pipeline size
- Able to minimize production loss