Flow-Assurance Aspects of Subsea Systems Design for Production of Waxy Crude Oils
- Hussein Alboudwarej (Schlumberger) | Zhongxin Huo (Shell Global Solutions) | Elijah Charles Kempton (Multiphase Solutions, Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 24-27 September, San Antonio, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2006. Society of Petroleum Engineers
- 5.2 Reservoir Fluid Dynamics, 5.2 Fluid Characterization, 4.2 Pipelines, Flowlines and Risers, 4.1.2 Separation and Treating, 4.3 Flow Assurance, 5.2.2 Fluid Modeling, Equations of State, 4.1.9 Tanks and storage systems, 4.5 Offshore Facilities and Subsea Systems, 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements, 5.3.2 Multiphase Flow, 4.6 Natural Gas
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Development of deep offshore fields is costly. As such, accurate information is required before a decision can be made on the feasibility of prospect development. Such sets of information include the reservoir fluid characterization and flow assurance data. The subject of this paper is to demonstrate the importance of accurate and representative fluid characterization and resulting flow assurance data on optimum facility and production method design for development of offshore fields.
In this study, fluid characteristics and flow assurance aspects of a live waxy crude oil from offshore West Africa is investigated. Experimental work included determination of the wax appearance temperature (WAT) and rheological studies which included pour point, gel strength, and shear-dependent viscosity measurements under both dead and live oil conditions. The wax deposition tendency of the dead crude oil was also investigated. The experimental data were used for a case study to demonstrate the effect of live oil data versus dead oil data on the subsea system design and operating procedures.
The case study indicated if dead oil data was utilized, the subsea system design and operating procedures would be considerably more conservative than if live oil data was the basis of the design. For a marginal field, these differences could be the difference between an economical field and an abandoned field.
Waxy crude oils pose unique production and transportation related challenges1. Variation in temperature is the dominant factor affecting the waxy crude oil properties. Below a certain temperature, called the wax appearance temperature (WAT), the wax crystallizes out of the liquid solution. The precipitation of the wax components out of the oil is responsible for the changes in the waxy crude oil properties, including the gelation of oil and an increase in viscosity. When the fluid temperature falls below the WAT, there is the possibility of wax deposition on the tubing/pipelines. Wax deposition will reduce the effective flow area and may lead to complete pipeline blockage. Deposited wax will also increase the roughness of the solid-liquid interface, and thus increase the pressure drop. Therefore, it is important to understand waxy crude oil behavior and determine accurate properties so that design of the subsea system may be optimized and operational procedures may be outlined.
Flow assurance and phase behavior of waxy crude oils may be predicted; however, the prediction of these properties is often very difficult. The difficulty arises from the fact that heavy wax components of the crude oil are not properly characterized. Critical properties and interaction between these heavy wax components are often calculated by extending the correlations for lighter hydrocarbon components. As such, the model prediction is generally not accurate. Quality experimental data are required to "tune?? theoretical models. In addition to quality and accuracy, if flow assurance properties are measured at the representative temperature and pressure condition of the production, the model is more likely to represent realistic fluid behavior.
The objective of this paper is to demonstrate the variation in the measured flow assurance properties of a waxy crude oil at actual field conditions and at stock tank conditions. In particular, the impact of using live versus stock tank oil measurements in the design and operation of the subsea system will be discussed.
Flow Assurance Properties of Waxy Crude Oils
The experimental data generated in flow assurance studies are generally used to evaluate potential for solids deposition. This information is crucial for production facility design and operation. The utility of the waxy crude properties, from design/operation standpoint are mentioned in the Table 1. A more detailed description is provided hereafter.
Wax Appearance Temperature. Wax appearance temperature (WAT) is probably the most important flow assurance measurements for a waxy crude oil. WAT is the temperature at which first wax crystals start to form. Wax disappearance temperature (WDT) on the other hand, is the temperature at which the last wax crystals are in equilibrium with the wax liquid phase. Thermodynamically, these two points should be the same, however, subcooling and superheating of the sample causes differences between the two reported data points2. In flow assurance studies, generally the WAT is measured because it is of more critical concern than the WDT. The WAT of a waxy crude oil is affected by:
- Wax content
- Thermal history
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