Asphaltene Deposition Measurement and Modeling for Flow Assurance of Subsea Tubings and Pipelines

Abstract

Abstract High-pressure deposition cell is used to measure asphaltene deposition from asphaltenic crudes under realistic field conditions of pressure, temperature, composition, and shear. The laboratory-scale data is then used to fine-tune and validate a recently developed asphaltene deposition model for the high-pressure deposition cell. Sensitivity analyses are performed to indicate the impacts of changes in model parameters on the deposit predictions. Through this exercise, the number of model parameters is reduced from six to two. The model predictions are verified with the deposition experimental data that were not used for tuning. The identified parameters then become inputs to the asphaltene pipe deposition model for predictions of deposit thickness along the tubings and pipelines in the field. Introduction Deposition of asphaltenes in well bores and transportation pipelines as a result of reduction in pressure or change in composition of the reservoir fluid has been a flow assurance concern for the oil and gas industry. The large capital and operating costs associated with prevention and remediation of deposits have created the need for improved methods to measure and model for optimization of system design and operations while minimizing risk of deposition. A key to development of strategies for its prevention and mitigation is the proper measurement and modeling of the asphaltene deposition. Asphaltenes in crude oils that exhibit asphaltene precipitation and deposition behavior during primary depletion are typically undersaturated1. During reservoir production at a constant temperature, once pressure decreases to the asphaltene precipitation onset pressure, asphaltenes start to precipitate and potentially deposit in the wellbore region and flow lines. Typically, the amount of precipitated asphaltenes increases as the pressure decreases, and reaches a maximum at the bubblepoint pressure. Asphaltene precipitation is a thermodynamic process which is mainly a function of pressure, temperature, and fluid composition. Asphaltene deposition, on the other hand, is a much more complex process and also depends on flow shear rate, surface type and characteristics, particle size and particle-surface interactions. Asphaltene precipitation is a necessary condition for the formation of obstructions but it is not a sufficient condition for deposition. After precipitation, asphaltene particles must deposit and stick to a surface before they can become a flow assurance problem in straight flow lines. RealView, a high-pressure deposition cell based on the Taylor-Couette flow principles, is a laboratory tool for generating organic solids deposits under a wide variety of operating conditions. This equipment has been used to measure the deposition rate of waxes and asphaltenes from live fluids under laminar and turbulent flow conditions2–6. Although the high-pressure deposition cell is a capable tool in generating asphaltene deposits under a wide variety of operating conditions, the deposition rates obtained by this laboratory-scale equipment cannot be directly applied to the field. A deposition model can, however, fill this gap and link the laboratory data to the field environment. Typically, the data are used to fine-tune and validate deposition models and thereby identify their parameters. The refined models are then used to predict deposition in the field. Recently an asphaltene deposition model was developed by Eskin et al.7–9. The developed model has six parameters that need to be determined using experimental data.